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Archean tectonics and crustal evolution of the Biligiri Rangan Block,southern India
Abstract
The Southern Granulite Terrain in India is a collage of crustal blocks ranging in age from Archean to Neoproterozoic. This study investigate the tectonic evolution of one of the northernmost block – the Bili-giri Rangan Block (BRB) through a multidisciplinary approach involving field investigation, petrographic studies, LA-ICPMS zircon U–Pb geochronology, Hf isotopic analyses, metamorphic P–T phase diagram computations, and crustal thickness modeling. The garnet bearing quartzofeldspathic gneiss from the central BRB preserve Mesoarchean magmatic zircons with ages between 3207 and 2806 Ma and positive Hf value (+2.7) which possibly indicates vestiges of a Mesoarchean primitive continental crust. The occurrence of quartzite-iron formation intercalation as well as ultramafic lenses along the western boundary of the BRB is interpreted to indicate that the Kollegal structural lineament is a possible paleo-suture. Phase diagram computation of a metagabbro from the southwestern periphery of the Kollegal suture zone reveals high-pressure (∼18.5 kbar) and medium-temperature (∼840 • C) metamorphism, likely during eastward subduction of the Western Dharwar oceanic crust beneath the Mesoarchean BRB. In the model presented here, slab subduction, melting and underplating processes generated arc magmatism and subsequent charnockitization within the BRB between ca. 2650 Ma and ca. 2498 Ma. These results thus reveal Meso-to Neoarchean tectonic evolution of the BRB. The spatial variation of crustal thickness, derived from flexure inversion technique, provides additional constraints on the tectonic linkage of the BRB with its surrounding terrains. In conjunction with published data, the Moyar and the Kollegal suture zones are considered to mark the trace of ocean closure along which the Nilgiri and Biligiri Rangan Blocks accreted on to the Western Dharwar Craton.
Supplementary resources (4)
... The Western and Central Cratons are separated by the Chitradurga schist belt (e.g., Hokada et al. 2013;Lancaster et al. 2014;Sreehari and Toyoshima 2020), and the Central and Eastern Cratons by the Kolar suture zone (Jayananada et al. 2013;Yang and Santosh 2015). Published geochronological results and Nd model ages indicate a magmatic age range for the Western Dharwar Craton of 3400 to 2900 Ma (e.g., Jayananda et al. 2000Jayananda et al. , 2015Jayananda et al. , 2018Jayananda et al. , 2023Dey 2013;Mohan et al. 2014;Maibam et al. 2016); a magmatic age range of 3400 to 2500 Ma for the Central Dharwar Craton (e.g., Peucat et al. 2013;Ratheesh-Kumar et al. 2016); and a magmatic age range of mostly between 2700 and 2500 Ma for the Eastern Dharwar Craton (Clark et al. 2009;Gireesh et al. 2012;Glorie et al. 2014;Manikyamba et al. 2015;Hazarika et al. 2015;Maibam et al. 2016;Jayananda et al. 2020). ...
... The southern margin of the Dharwar Craton is divided by ductile shear zones into a set of tectonic blocks of granulite grade (Chardon et al. 2008), which are dominated by orthopyroxene-bearing TTG gneisses, and mutually separated by shear/suture zones ( Fig. 1; Ghosh et al. 2004;Chardon et al. 2008;Peucat et al. 2013;Ishwar-Kumar et al. 2013;Collins et al. 2014;Ratheesh-Kumar et al. 2016, 2020. They are sometimes given other names or several of these blocks are grouped into one block (e.g. the Salem Block of Glorie et al. (2014) which consists of the Shevaroy, Kolli, and Madras Blocks in Fig. 1). ...
... They are sometimes given other names or several of these blocks are grouped into one block (e.g. the Salem Block of Glorie et al. (2014) which consists of the Shevaroy, Kolli, and Madras Blocks in Fig. 1). In Fig. 1, following Ishwar-Kumar et al. (2013) these blocks are termed the Coorg Block Ishwar-Kumar et al. 2013Santosh et al. 2015); the Nilgiri Block (Samuel et al. 2014(Samuel et al. , 2019, the Biligiri Rangan Block Ratheesh-Kumar et al. 2016); the Shevaroy Block (Li et al. 2018;Harlov et al. 2022); the Kolli Block (named after the Kolli Massif but also known as the Namakkal Block) (George and Sajeev 2015;Behera et al. 2019;Gou et al. 2022); and the Madras Block (Rameshwar Rao et al. 1991a, b;Braun and Kriegsman 2003). These granulite facies blocks, which extend from west to east across southern India (Fig. 1), are separated from the Madurai Block to the south by the Palghat-Cauvery shear zone (Plavsa et al. 2012Peucat et al. 2013;Santosh et al. 2009). ...
Metamorphosed banded iron formation (BIF) in granulite-amphibolite facies, tonalitic orthogneisses from a series of locations in the Kolli Massif of southern India are described and analysed with regard to their lithologies, whole rock chemistry, mineral reaction textures, and mineral chemistry. On the basis of their mineral reaction textures along magnetite-quartz grain boundaries these BIFs are grouped according to their predominant silicate mineralogy: 1) amphibole; 2) orthopyroxene; 3) orthopyroxene–clinopyroxene; 4) orthopyroxene-clinopyroxene-garnet; 5) clinopyroxene-garnet-plagioclase; and 6) Fe-Mg silicates are absent. Two-pyroxene and garnet-pyroxene Fe-Mg exchange thermometry, coupled with thermodynamic pseudo-section modelling of whole rock data from one of the magnetite-quartz-orthopyroxene-clinopyroxene-bearing lithologies, indicates that the magnetite-quartz-orthopyroxene-clinopyroxene-garnet assemblages formed at ~900 to 1200 MPa and 750 to 900 °C under relatively low H2O activities. Magnetite-quartz-orthopyroxene reaction textures were experimentally replicated at 800 and 900 °C and 1000 MPa in a synthetic BIF using isolated magnetite grains in a quartz matrix to which was added a hypersaline Mg- and Al-bearing fluid (approximately 1% by mass), which permeated along all the grain boundaries. The fact that Fe-Mg silicate reaction textures did not form in one of the BIF samples, which had experienced the same P-T conditions as the other BIF samples, suggests that, unless a BIF initially incorporated Mg, Al, and Ca during formation with or was infiltrated from the surrounding rocks by Mg-, Al-, and Ca-bearing saline fluids, these silicate minerals could not and would not have formed from the inherent magnetite and quartz during granulite-facies and amphibolite-facies metamorphism.
... The southern tip of Peninsular India, popularly known as Southern Granulite Terrane (SGT) comprises differentcrustal scale granulite blocks ranging from Archean to Neoproterozoic in age.These crustal blocks are Northern Granulite Block(NGB), Madurai Granulite Block (MGB) and Trivandrum Granulite Block (TGB)or Kerala Khondalite Belt (KKB) from north to south respectively ( Fig. 1; Bhaskar Rao , 1991Harris , 1994;Janardhan and Anto, 1997;Ghosh , 2004;Vijaya Rao and Rajendra Prasad, 2006;Yellappa , 2012 Ratheesh-Kumar , 2016, 2020. The NGB and MGB are separated by means of a very significant crustal scale E-W trending tectonic unit namely Cauvery Suture Zone (CSZ) with its northern boundary as Moyar-Bhavani-Salem-Attur shear zone (MBSASZ)and southern boundary as Chennimalai Noyil Shear Zone (CNSZ) also known as Palghat-Cauvery shear zone (PCSZ) (Chetty, 2017). ...
... The transpressional behavior of CSZ within the convergent tectonic setting has fashioned thrusting of Charnockites/ granulites over NGB, which form a channel of supply for various Alkaline and Mafic/Ultramafic intrusions within NGB. Recently it has been proposed that magmatism associated with Nilgiri Block and thrusting of Charnockites/granulites over NGB is a result of deep subduction and continental collision of Nilgiri block under BRB along Kollegal Suture Zone which is identified as paleao-suture zone along eastern margin of BRB (Ratheesh-Kumar , 2016, 2020. These granulite facies rocks include, two-pyroxene granulites, charnockites, migmatites and retrogradedrocks likeamphibolitesare considered as the basement gneisses for the younger intrusions. ...
... These highly elevated charnockite massifs are surrounded by granitic orthogneisses, paragneisses and migmatites and are exposed in the areas of lower elevations, which has undergone amphibolite to granulite facies metamorphism (Janardhan , 1982;Ghosh , 2004). Recent developments on the understanding of the NGB is all about the occurrences of charnockite massifs, ortho and para gneisses, shonkinites and other associated mafic-ultramafic rocks and younger intrusions (He , 2015;Ratheesh-Kumar , 2016;Chetty 2017Chetty , 2021.The highland areas towards west of NGB, like BR hill is recently studied by Ratheesh Kumar (2016, 2020) and their results are described in terms of suture zone tectonics including subduction, slab dehydration, partial melting and exhumation during the Neo-Archean period. They have suggested about two closely associated crustal reworking processes beginning with arc magmatism followed by crustal thickening. ...
Southern Granulite Terrane (SGT) of India preserves extensive high-grade granulite facies rocks of Archaean and Proterozoic age. The SGT is divided into number of blocks by several suture/shear zone. Structural investigations on the basement gneisses and younger mafic/ultramafic dikes have been carried out within the Salem block which is part of Northern Granulite Block (NGB), north of the Cauvery Suture Zone (CSZ). The present work emphasizes various scale fold styles and other structural patterns of the area, which includes regionally metamorphosed high-grade rocks as basement for the multiple ultramafic intrusions to the north of Cauvery Suture Zone (CSZ) which highlights the finite strain geometry, complex deformation pattern and high-grade metamorphism. Structural map of the study area is prepared showing two generations of folding, namely F1 whose axial trend is NE-SW, subparallel with general trend of gneissic foliation and are tight isoclinal folds while F2 which are open folds with axial trend NW-SE.E-W structural cross section across the foliation planes, characterizes antiformal and synformal fold patterns of the basement due to varying dip directions which also reflects type-3 interference pattern of folding. Mesoscopic scale shear zones of dextral kinematics in response to E-W collision during Paleo-Meso Archean time, delta type porphyroclasts, S-C fabrics with the dextral movement of CSZ system, Riedel shear, thrust imbricates implying duplex structures, rotation of mafic boudins along shear zones are the most prominent ductile structural features of this area. Brittle structures like different sets of cross cutting joints and faults indicate younger deformation as well. Petrography of major lithologies has classified them into amphibolite gneiss, migmatite gneiss, charnockites, granulites and mylonites as basement rocks to the younger pyroxenite intrusions. Typical textures like, perthite, granulose, reaction rims, sieve textures and microstructures like S-C fabrics, kink bands, rotated porphyroclasts, etc are observed within the basement rocks. Coarse grained textures with fractured porphyroclasts of garnets indicating the water interactions and retrogradations within the granulite facies rocks. Reaction rims observed in charnockites and granulites are indicative of retrogression during shearing. The coarse grained cummulate nature of pyroxenites neither represent deformation nor metamorphism. Keywords: Southern Granulite Terrane, Salem, Structural Analysis, Mafic/Ultramafics, Dikes, Petrography
... Recently, Taylor et al. (2020) demonstrated that suprasolidus metamorphic temperatures of lower continental crust could persist for 200 Ma or more during the Neoarchean and suggested that the persistence of meltbearing lower crust for hundreds of millions of years might be the norm during the Archean. A nearly continuous spread of metamorphic zircon ages from ca. 2550 to 2440 Ma has been reported in the northern SGT (Anderson et al., 2012;Clark et al., 2009;Glorie et al., 2014;Li, Zhang, Wei, et al., 2018;Mohan et al., 2013;Peng et al., 2019;Peucat et al., 2013;Ratheesh-Kumar et al., 2016Saitoh et al., 2011;Samuel et al., 2014;Samuel et al., 2015;Sato et al., 2011), which is consistent with prolonged high-grade regional metamorphism of the lower continental crust during the late Neoarchean to early Paleoproterozoic. However, the close to isobaric cooling path (Harley, 1989;Mezger et al., 1991;Mitchell et al., 2019;Möller et al., 2000) presently is poorly constructed for the granulite-facies metamorphic rocks in the northern SGT. ...
... SHRIMP zircon U-Pb dating yielded a weighted mean 207 Pb/ 206 Pb age of 2520 AE 27 Ma for the garnet pyroxenite, which also records an anticlockwise P-T path, with peak P-T condition of $900-1000 C at $18-20 kbar (Samuel et al., 2018). The Billigiri Rangan Block mainly consists of charnockite, mafic granulite, granite, felsic gneiss, amphibolite, meta-granite, and migmatite Ratheesh-Kumar et al., 2016. Zircon U-Pb dating obtained crystallization ages of ca. ...
... Ga for the charnockite protoliths Ratheesh-Kumar et al., 2020). The metamorphic ages of charnockites range from ca. 2.53 to 2.50 Ga Ratheesh-Kumar et al., 2016, which are slightly younger than those (ca. 2.55 Ga) of the mafic granulites (Ratheesh-Kumar et al., 2020). ...
This study presents the results of petrology, SHRIMP zircon U–Pb analysis, conventional Fe–Mg geothermobarometry, REE (rare earth element)‐based geothermobarometry and phase equilibrium modelling, for newly discovered ultramafic‐mafic granulites (garnet pyroxenite with minor plagioclase and garnet‐bearing granulite) in the Namakkal Block of the southern granulite terrain (SGT), India. Results from this study shed light on the growth, evolution, and thermal state of lower continental crust during the Neoarchean, as a cross‐section of the Archean lower continental crust is well‐exposed in the SGT. SHRIMP zircon U–Pb dating of a garnet pyroxenite sample (18ID‐41) yielded a weighted mean 207Pb/206Pb age of 2531 ± 6 Ma, which is the same within uncertainty as one weighted mean SHRIMP metamorphic zircon 207Pb/206Pb age of 2519 ± 9 Ma from mafic granulite sample (18ID‐43). In addition, zircon U–Pb analyses also yielded an apparent 207Pb/206Pb age of 2474 ± 3 Ma (1σ) for the garnet pyroxenite, and a weighted mean 207Pb/206Pb age of 2489 ± 8 Ma for the mafic granulite. Thus, the timing of metamorphism of the ultramafic‐mafic granulites was constrained to be ca. 2530–2470 Ma. The peak metamorphic phase assemblage of the garnet pyroxenite is grt–cpx–pl–ilm–liq, which was modelled to be stable at 1058–1172 °C/7.8–11.6 kbar, whereas the retrograde mineral assemblage of grt–cpx–hbl–pl–qz–ilm–liq occurred at 877–888 °C/9.9–11 kbar. The mafic granulite was demonstrated to record similar peak and retrograde metamorphic P–T conditions to those of the garnet pyroxenite, but have a relatively small P–T range of the peak mineral assemblage. The ultrahigh temperature metamorphism of these ultramafic‐mafic granulites was further confirmed by the temperature results (1057–1087 °C) from the REE‐in‐two‐pyroxene thermometer. As a result, a retrograde metamorphic P–T path characterized by near‐isobaric cooling from ~1080 °C at ~10 kbar to ~900 °C at ~10 kbar was constructed for these rocks, which is consistent with an extremely high thermal state that persisted for ca. 60 Ma in the lower crust.
... The nearest major source regions for Palaeoarchean to Neoarchean rocks in southern India are the western Dharwar Craton and the Coorg Block (e. g., Jayananda et al., 2000;Santosh et al., 2015). In addition, the Nilgiri, Bilgiri-Rangan, Namakkal and Madras Blocks contain some Paleoarchean to Neoarchean gneisses and protoliths to gneisses (e.g., Samuel et al., 2014;Ratheesh-Kumar et al., 2016;George et al., 2019;Glorie et al., 2014). Ages >3000 Ma are not typical of the rocks of the SGT. ...
... Neoarchean crustal growth is widely recorded from various blocks to the north of the SGT, especially the Nilgiri, Shevaroy, Bilgiri Rangan and Madras Blocks (Clark et al., 2009a;Clark et al., 2009b;Samuel et al. 2014;Ratheesh-Kumar et al., 2016;Li et al., 2018). Further south, the Madurai Block is composed of a relatively juvenile Archean-Paleoproterozoic domain, a Palaeoproterozoic central domain and a Neoproterozoic domain to the south . ...
... Simplified geological map of southern India compiled with bathymetry (data obtained from GEBCO), ocean currents (data obtained from global ocean physics analysis and forecast product (Global_Analysis_Forecast_PHY_001_030) during 1 June to 31 August 2017) and sediment thickness (data obtained from NOAA) data. A summary of geochronological results from the previous studies are shown in this map (Collins et al., 2007a, b;Taylor et al., 2014;Taylor et al., 2015;Whitehouse et al., 2014;Shabeer et al., 2005;Harley and Nandakumar, 2014;Harley and Nandakumar, 2016;Liu et al., 2016;Kröner et al., 2012;Kröner et al., 2015;Johnson et al., 2015;Vijaya Kumar et al., 2017;Ghosh et al., 2004;Sato et al., 2010;Plavsa et al., 2012;Plavsa et al., 2014;Plavsa et al., 2015;Tang et al., 2018;Santosh et al., 2012;Santosh et al., 2015;Santosh et al., 2016;Santosh et al., 2017;Brandt et al., 2011;Brandt et al., 2014;Saitoh et al., 2011;Li et al., 2017;Li et al., 2018;Teale et al., 2011;Clark et al., 2009a;Clark et al., 2009b;Clark et al., 2015;Clark et al., 2020;Yellappa et al., 2012;Samuel et al., 2014;Samuel et al., 2015;Samuel et al., 2016;Samuel et al., 2018;Yang et al., 2015;Peucat et al., 2013;Ratheesh-Kumar et al., 2016;Glorie et al., 2014;Jayananda et al., 2006). southwest India yielded ages ranging from 650 to 450 Ma, 1100 to 650 Ma, 2300 to 1600 Ma, 2800 to 2300 Ma and 3500 to 2800 Ma, which can be compared with ages obtained from various crustal fragments of southern India. ...
The southwestern coast of India is known for its rich beach placer deposits. So far, there have been no substantial attempts to determine the provenance of these deposits on a regional scale. Here we present for the first time geochronology, trace element chemistry and Hf isotope systematics of zircon and monazite from representative beach placer deposits along the southwestern coast of India to constrain provenance. Detrital zircon grains display a wide variety of ages, with modes in the ranges of 650–450 Ma, 1100–650 Ma, 2300–1600 Ma, 2800–2300 Ma and 3500–2800 Ma. A high proportion of ages fall within the Neoproterozoic–Late Cambrian range (1066–490 Ma). Growth zoning in zircon grains shows magmatic growth of many generations with ages affected by Neoproterozoic Pb loss, along with overgrowths typical of those formed during high-T metamorphism. Grains of metamorphic zircon, along with grains of monazite from the same samples of sediment, have strong modes between 570 and 470 Ma, indicating derivation from sources in the Southern Granulite Terrane affected by a major Late Neoproterozoic to Cambrian tectonothermal event. Mineral geochemistry of zircon grains indicates continental derivation, especially with older zircon from TTG-type sources. The geochemistry of late Neoproterozoic–Cambrian zircon and monazite are consistent with derivation from amphibolite to granulite-facies source rocks. Zircon ages and Hf isotope systematics can be ascribed to provenance from various source terranes exposed along the Western Ghats. This young rift-flank escarpment formed along the coast at a high angle to the distribution of different Precambrian terranes in the Indian Peninsula. Erosion promoted by a tropical climate and a steep coastal geomorphology, in combination with strong southward along-shore ocean currents, led to the concentration and distribution of heavy minerals from multiple sources along southwestern coast of India. The relative intensity of age modes in detrital zircon differs between localities, demonstrating differing inputs from proximal sources (from rivers dissecting steep terrain) versus distal sources from more northerly cratons and terranes brought to deposition sites by along-shore drift.
... The transition zone is characterized by the occurrence of migmatitic gneisses with orthopyroxene-bearing patches formed by dehydration melting in the presence of CO 2 -rich fluids (incipient charnockites) from lower crustal or mantle sources (Pichamuthu, 1960;Janardhan et al., 1979Janardhan et al., , 1982Hansen et al., 1984aHansen et al., , b, 1995Stähle et al., 1987;Newton, 1990Newton, , 1992Srikantappa et al., 1992;Peucat et al., 2013). Jayananda et al., 2018;Ratheesh Kumar et al., 2016). Sample locations are indicated by filled stars. ...
... The southern boundary of the Dharwar Craton is girdled by a series of complexly amalgamated microblocks. From west to east, these blocks include the Coorg Block, the Nilgiri Block, the Biligirirangan Block, and the Shevaroy Block (including Salem) (Samuel et al., 2014;Santosh et al., 2015;Ratheesh Kumar et al., 2016;Vijaya Kumar et al., 2017;Li et al., 2018a,b). They were welded together along a number of crustalscale suture zones such as the Mercara, Moyar, Mettur, and the Nallamalai shear zones (Drury et al., 1984;Raith et al., 1999;Chetty et al., 2012;Santosh et al., 2015;Li et al., 2018b). ...
... The Biligirirangan Block comprises large charnockitic massifs with typical TTG affinities emplaced between 3.4 and 3.0 Ga. The charnockites record peak P-T conditions of 6.0-8.5 kbar and 750-850°C (Janardhan et al., 1982;Buhl, 1987;Peucat et al., 1989a,b;Mahabaleswar et al., 1995;Jayananda et al., 2000;Bhaskar Rao et al., 2003;Friend and Nutman, 1991;Mojzis et al., 2003;Chardon et al., 2011;Peucat et al., 2013;Ratheesh Kumar et al., 2016, Vijaya Kumar et al., 2017. The 2.56-2.51 ...
The Western Dharwar Craton in southern India is underlain by Paleoarchean to Neoarchean granitoids. Here, we use major-trace element chemistry and zircon U-Pb-Hf isotopic composition to identify major components of the crust, constrain the timing of juvenile crust extraction, and discuss the implications for Archean tectonic processes. The granitoids are metaluminous to weakly peraluminous, magnesian, and calcic. They were derived from basaltic protoliths with minor components sourced from pre-existing felsic crust. Low La/Yb and Sr/Y indicate shallow garnet-free plagioclase-bearing amphibolitic sources. The granitoids display large variations in concentration of trace elements, attributed to plagioclase accumulation or fluid-assisted mobilization of REEs during metamorphism. Zircon ages help to constrain four major episodes of granitoid crust formation at 3.43–3.41 Ga, 3.36–3.34 Ga, 3.29–3.25 Ga, and 2.66–2.65 Ga. The 3.43–3.41 Ga, 3.36–3.34 Ga, and 3.29–3.25 Ga granitoid suites have positive εHfi (2.7–4.5) and plot on a common εHfi vs. time trend consistent with repeated granitoid extraction at 3.43–3.41 Ga, 3.36–3.34 Ga, and 3.29–3.25 Ga from mafic sources that separated from model depleted mantle between 3.55 Ga and 3.35 Ga. The εHfi (0.4–0.69) of the 2.66–2.65 Ga Neoarchean granitoids can be explained by melting of similar 3.35 Ga mafic crust or by mixing between juvenile magmas and preexisting granitoids. Uranium-Pb ages from metamorphic zircons indicate polyphase metamorphism of the granitoids at 3353–3329 Ma, 3264–3256 Ma, 3187–3141 Ma, 3083–3062 Ma, and 2574–2526 Ma. Hf-isotopic data from zircons in granitoids from several cratons indicate that prior to c. 3.5 Ga most granitoids have chondritic or crust-like εHfi explained by repeated granitoid extraction from long-lived mafic crusts with limited interaction with juvenile magmas. Juvenile εHfi and short protolith residence times of the Western Dharwar Craton Paleoarchean granitoids is suggestive of a tectonic setting with rapid recycling of basalts as in subduction zones. In contrast, greater protolith residence times and crust-like signature of granitoids older than 3.5 Ga in the crustal record indicate a tectonic setting where basalts persisted for prolonged periods of times such as in an oceanic plateau.
... The zircon UePb ages presented in this study, together with published zircon ages from the Central and Eastern Dharwar Craton, show episodic juvenile crustal accretion and multi-stage continental growth at ca. 3360-3200 Ma, 3000-2960 Ma, 2700-2600 Ma and 2570-2520 Ma (Friend and Nutman, 1991;Peucat et al., 1993Peucat et al., , 2013Balakrishnan et al., 1999;Rogers et al., 2007;Chardon et al., 2011;Bidyananda et al., 2016;Anand and Balakrishnan, 2010;Anand et al., 2014;Dey et al., 2014Dey et al., , 2016Jayananda et al., 1995Jayananda et al., , 20002013aKhanna et al., 2014Khanna et al., , 2016Ratheesh Kumar et al., 2016;Mohan et al., 2013Mohan et al., , 2014Nasheeth et al., 2015). These juvenile accretion events are spatially associated with crustal reworking, including metamorphism at ca. 3200 Ma, 3000 Ma, 2620 Ma, 2520 Ma (Mahabaleshwar et al., 1995;Peucat et al., 2013;Jayananda et al., 2011Jayananda et al., , 2013a. ...
... Furthermore, detrital zircons from metapelite from the western (Jayananda et al., 2013a) and eastern margins of the Closepet batholith record UePb ages ranging from 3350 to 3200 Ma (Jayananda unpub data). In the southern part of the Central block, UePb zircon ages for the migmatitic gneisses from the Bangalore-Kolar and Kabbaldurga e B.R. Hills region reveal two stages of magmatism that contributed to significant crustal growth at ca. 3360-3200 Ma and 3000-2960 Ma (Friend and Nutman, 1991;Jayananda et al., 2000;Peucat et al., 2013;Mahabaleshwar et al., 1995;Bidyananda et al., 2011Bidyananda et al., , 2016Ratheesh Kumar et al., 2016). A few younger UePb ages documented from rims on old zircons at ca. 2700 Ma, ca. ...
... 2700 Ma greenstone volcanism, 2600 Ma transitional TTG accretion and 2540 Ma sanukitoid magmatism, respectively (Balakrishnan et al., 1999;Jayananda et al., 2000Jayananda et al., , 2013aMojzsis et al., 2003;Yang and Santosh, 2015;Peucat et al., 2013). In situ Hf isotope [εHf (T) ¼ þ5 to À7)] data for zircon and whole-rock Nd isotope data [εNd (T) ¼ þ3 to À1.6] data suggest significant continental growth at ca. 3360-3150 Ma and 3000-2960 Ma involving juvenile addition as well as minor ancient crustal contamination (Bhaskar Rao et al., 2003;Peucat et al., 2013;Ratheesh Kumar et al., 2016;Bidyananda et al., 2016). In summary, the UePb/PbePb ages of magmatic and detrital zircons from the oldest basement in the Central block show two stages of continental growth at ca. 3360-3150 Ma and 3000-2960 Ma, prior to Neoarchean juvenile accretion. ...
The Dharwar Craton is a composite Archean cratonic collage that preserves important records of crustal evolution on the early Earth. Here we present results from a multidisciplinary study involving field investigations, petrology, zircon SHRIMP U–Pb geochronology with in-situ Hf isotope analyses, and whole-rock geochemistry, including Nd isotope data on migmatitic TTG (tonalite-trondhjemite-granodiorite) gneisses, dark grey banded gneisses, calc-alkaline and anatectic granitoids, together with synplutonic mafic dykes along a wide Northwest – Southeast corridor forming a wide time window in the Central and Eastern blocks of the Dharwar Craton. The dark grey banded gneisses are transitional between TTGs and calc-alkaline granitoids, and are referred to as ‘transitional TTGs’, whereas the calc-alkaline granitoids show sanukitoid affinity. Our zircon U–Pb data, together with published results, reveal four major periods of crustal growth (ca. 3360-3200 Ma, 3000-2960 Ma, 2700-2600 Ma and 2570-2520 Ma) in this region. The first two periods correspond to TTG generation and accretion that is confined to the western part of the corridor, whereas widespread 2670-2600 Ma transitional TTG, together with a major outburst of 2570–2520 Ma juvenile calc-alkaline magmatism of sanukitoid affinity contributed to peak continental growth. The transitional TTGs were preceded by greenstone volcanism between 2746 Ma and 2700 Ma, whereas the calc-alkaline magmatism was contemporaneous with 2570–2545 Ma felsic volcanism. The terminal stage of all four major accretion events was marked by thermal events reflected by amphibolite to granulite facies metamorphism at ca. 3200 Ma, 2960 Ma, 2620 Ma and 2520 Ma. Elemental ratios [(La/Yb)N, Sr/Y, Nb/Ta, Hf/Sm)] and Hf-Nd isotope data suggest that the magmatic protoliths of the TTGs emplaced at different time periods formed by melting of thickened oceanic arc crust at different depths with plagioclase + amphibole ± garnet + titanite/ilmenite in the source residue, whereas the elemental (Ba–Sr, [(La/Yb)N, Sr/Y, Nb/Ta, Hf/Sm)] and Hf-Nd isotope data [εHf(T) = −0.67 to 5.61; εNd(T) = 0.52 to 4.23; ] of the transitional TTGs suggest that their protoliths formed by melting of composite sources involving mantle and overlying arc crust with amphibole + garnet + clinopyroxene ± plagioclase + ilmenite in the residue. The highly incompatible and compatible element contents (REE, K–Ba–Sr, Mg, Ni, Cr), together with Hf and Nd isotope data [εHf(T) = 4.5 to −3.2; εNd(T) = 1.93 to −1.26; ], of the sanukitoids and synplutonic dykes suggest their derivation from enriched mantle reservoirs with minor crustal contamination. Field, elemental and isotope data [εHf(T) = −4.3 to −15.0; εNd(T) = −0.5 to −7.0] of the anatectic granites suggest their derivation through reworking of ancient as well as newly formed juvenile crust. Secular increase in incompatible as well as compatible element contents in the transitional TTGs to sanukitoids imply progressive enrichment of Neoarchean mantle reservoirs, possibly through melting of continent-derived detritus in a subduction zone setting, resulting in the establishment of a sizable continental mass by 2700 Ma, which in turn is linked to the evolving Earth. The Neoarchean geodynamic evolution is attributed to westward convergence of hot oceanic lithosphere, with continued convergence resulted in the assembly of micro-blocks, with eventual slab break-off leading to asthenosphere upwelling caused extensive mantle melting and hot juvenile magma additions to the crust. This led to lateral flow of hot ductile crust and 3D mass distribution and formation of an orogenic plateaux with subdued topography, as indicated by strain fabric data and strong seismic reflectivity along an E-W crustal profile in the Central and Eastern blocks of the Dharwar Craton.
... The southern part of the Dharwar Craton is a complex amalgamation of several microblocks much smaller in size than the WDC, CDC, and EDC terranes, including the Neoarchean Nilgiri Block, Biligirirangan Block, Shevaroy Block, and Madras Block from west to east (Samuel et al., 2014;Kumar et al., 2016). These microblocks were welded together during the Neoarchean-Paleoproterozoic transition along multiple crustal-scale suture zones termed the Mercara, Moyar, Mettur and Nallamalai . ...
... Widespread continental arc magmatism resulting from this subduction is recorded by Neoarchean granitoids developed within the CDC, particularly the Closepet batholith. Gabbro-anorthosite suites from the eastern flank of this suture zone probably formed via crystallization from high-alumina basaltic Peucat et al. (2013) and Kumar et al. (2016). magmas produced by slab melting and underplating beneath older arc crust, followed by fractional crystallization within a mid-crustal magma chamber. ...
... The EDC, to which the newly defined CDC belongs (see Manikyamba & Kerrich, 2012), is notably younger than the WDC, and does not contain equivalent Sargur Group basement gneisses or greenstone successions of Mesoarchean age. The core of the EDC comprises TTG gneisses with crystallization ages of c. 2Á80-2Á70 and c. 2Á55-2Á53 Ga (Hokada et al., 2013;Nasheeth et al., 2016), and lithologies from the northern region near Hutti (Fig. 1b) have produced a magmatic monazite electron microprobe analysis (EPMA) emplacement age of 2Á56 Ga for tonalitic gneiss and metamorphic overprinting ages of 2Á51-2Á48 Ga from greenstone sequences (Glorie et al., 2014;Kumar et al., 2016;Li et al., 2018). Widespread charnockitic metamorphic overprinting in the south of the EDC (Fig. 1b) has been dated at 2Á51 Ga, synchronous with that in the WDC. ...
Numerous tectonic scenarios have been proposed for terrane growth and accretion within the Archean Dharwar Craton, southern Peninsular India. Previously accepted interpretations involving a two-terrane model-comprising a Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) block-have invoked west-dipping subduction and ocean closure, leading to arc magmatism and accretionary orogeny in the WDC, followed by metamorphic overprinting and collisional orogeny in the EDC. However, recent field investigations have revealed the existence of a previously unrecognized 'central' block (Central Dharwar Craton; CDC) within the craton, which requires revision of this model and reinterpretation of metamorphic and magmatic age data. Five samples of high-pressure, upper amphibolite- and granulite-facies meta-igneous and metasedimentary rocks from the southern portion of the Chitradurga Suture Zone, which divides the WDC and CDC, record minimum peak metamorphic conditions of ~820-875°C at ~10 kbar, indicating equilibration at the base of thickened continental crust. U-Pb zircon and Pb-Pb monazite geochronology indicates crystallization of parent mafic magmas at c. 2·61-2·51 Ga and subsequent regional metamorphism of these intrusions to garnet-amphibolite and garnet-granulite facies at c. 2·48-2·44 Ga, bracketing the timing of microblock accretion to the Archean-Proterozoic boundary. Light rare earth element enrichment within these zircon grains indicates magma generation in a suprasubduction-zone environment. In addition, detrital magmatic zircon grains with ages of c. 3·10-3·03Ga and c. 2·97- 2·86 Ga imply contamination of these magmas with Mesoarchean material sourced from the Western Dharwar Craton continental nucleus. Comparison of these metamorphic and magmatic age data with those recorded in the EDC shows that westward-directed subduction is implausible, and that all three terranes (the WDC, CDC, and EDC) must have accreted synchronously, driven by two separate eastward-dipping ocean-continent convergent plate margins. These data further support a recent abundance of observations from the geological record supporting the hypothesis that subduction-driven plate tectonics had initiated on Earth before c. 2·5 Ga, as opposed to a Neoproterozoic onset (c. 0·8-0·9 Ga) reported by numerous studies.
... To the south of Dharwar Craton are several crustal blocks including Coorg, Biligiri Rangan, Shevaroy (including Salem) and Madras Kumar et al., 2016;Peucat et al., 2013;Glorie et al., 2014). The Coorg Block is located along the southern margin of the WDC, the core of which comprises > 3.3 Ga to 3.1 Ga charnockites, TTG gneisses and granitoids, in the absence of the 2.5 Ga thermal event and hence considered as an exotic block . ...
... The Biligiri Rangan Block is mainly composed of charnockite at is core, surrounded by felsic orthogneiss and granitoids in the northern part of the block. Two stages of magmatic events are recorded at 3.3-3.1 Ga, and at 2.5 Ga (Peucat et al., 2013;Kumar et al., 2016). The Shevaroy Block is composed of the Krishnagiri area and part of the Salem Block (previous classification) and is dominantly composed of charnockites and felsic orthogneisses. ...
... Yang et al. (2016) reported detrital zircon grains in a quartzite from the Wynad region which show εHf(t) both positive and negative values of +5.24 to −4.86, when calculated by corresponding spot ages of 3043-3306 Ma, indicating both juvenile and reworked continent crustal components. Kumar et al., (2016) also reported similar data from quartzite in the Biligiri Rangan Block, immediately west of the Nallamalai Suture Zone, where zircon εHf(t) values show both positive and negative values of +5.1 to −6.4 and mostly positive when calculated by corresponding spot ages of 2972-3315 Ma, suggesting mixed juvenile and reworked sources. However, the granitic rocks from this block with ca.2.5 Ga emplacement age yield negative εHf(t) values of −8.2 to −2.3 suggesting magma derivation from reworked older crust (Kumar et al., 2016) (Fig. 22). ...
A collage of crustal blocks ranging in age from Mesoarchean to Neoarchean accreted to the southern margin of the Dharwar Craton in Peninsular India preserve distinct evolutionary history, but share a common metamorphic record during Archean-Proterozoic transition. Here we investigate the tectonic boundary between two of these microblocks, the Shevaroy Block to the west and the Madras Block to east, termed as the Nallamalai Suture Zone (NLSZ). We present integrated field, petrological, geochemical and zircon U-Pb and Lu-Hf data from a suite of meta-igneous and metasedimentary units along the NLSZ and its flanks. Zircon grains from the meta-monzogranite, hornblende-biotite gneiss, amphibolite, granodiorite, diorite, charnockite, BIF (banded iron formation) and BMQ from this area show magmatic emplacement ages clustering around 2.50 to 2.56 Ga, except for the meta-monzogranite showing ages up to 3.2 Ga, correlating with long-lived convergent margin magmatism through multiple slab melting episodes. All the rocks show tightly constrained early Paleoproterozoic (ca. 2.46–2.48 Ga) metamorphic ages, including the metamorphic zircon in the BIFs, marking the timing of collision of the two continental blocks with consumption of the intervening oceanic lithosphere. The εHf(t) values of magmatic zircon grains from the different rock types range from −4.1 to +5.7, and together with TDMC Hf model ages of 2672–3247 Ma, a dominantly juvenile crust growth is indicated, which initiated around 3.3 Ga and continued to 2.7 Ga, followed by crustal reworking in a continental arc towards the end of Neoarchean. The geochemical data suggest that tholeiitic to calc-alkaline parental melts for the mafic-intermediate-felsic suite were generated by low degree partial melting of a peridotitic mantle wedge metasomatized by subduction-derived fluids and sediments and continuous slab melting ensued by intracrustal fractional crystallization of melts with reworking of older continental crust. The BIF and BMQ samples display positive Eu anomalies, negative to negligible Ce anomalies, and superchondritic Y/Ho ratios suggesting their formation in an oceanic realm proximal to an active continental margin setting. The magmatic and tectonic attributes are consistent with eastward oceanic subduction and ocean closure along the NLSZ which is defined here as the trace of a suture welding the Shevaroy and Madras Blocks. We envisage that multiple subduction and amalgamation of several microblocks occurred in the Dharwar Craton and its southern domains, amalgamating several microblocks during the Archean – Paleoproterozoic transition.
... The Southern Granulite Terrain (SGT) of India provides a natural window for understanding the Precambrian crustal accretionary processes owing to its prolonged tectonic evolutionary history from the Archean to the Neoproterozoic (Praveen et al., 2013;Ratheesh-Kumar et al., 2016;Samuel et al., 2014;Santosh et al., 2013Santosh et al., , 2014bSantosh et al., , 2015Santosh et al., , 2016Sato et al., 2011). The Palghat-Cauvery shear zone, PCSZ (Fig. 1), alternatively the Palghat-Cauvery Shear System, represents a major tectonic break (Chetty and Bhaskar Rao, 2006;Santosh et al., 2009 and references there in) and separates the Southern Granulite Terrain into a northern and southern domain that are characterized, respectively, by the Neoarchean-Paleoproterozoic Santosh et al., 2012) and Neoproterozoic-Cambrian (Collins et al., 2007(Collins et al., , 2014Plavasa et al., 2012;Santosh et al., 2009Santosh et al., , 2013 peak crustal accretionary events. ...
... Samuel et al. (2014) documented a Neoarchean active margin process along the Moyar and Bhavani shear zone and crustal growth by arc magmatism for the Nilgiri Block. Ratheesh-Kumar et al. (2016) suggested that the Nilgiri Block and the adjoining Biligiri Rangan Block (BRB, Fig. 1) evolved together by vertical addition and lateral accretion processes (Xiao et al., 2010(Xiao et al., , 2013 during the Neoarchean SE-ward subduction of the Dharwar craton beneath the Nilgiri and Biligiri Rangan Blocks. However, a geological database pertaining to the Neoarchean accretionary tectonic processes along the entire stretch of the Moyar shear zone, which runs for more than 200 km from the eastern flank of the Coorg Block to the eastern side of the Nilgiri Block, is still incomplete. ...
... Recent studies across the Nilgiri Block (Samuel et al., 2014(Samuel et al., , 2015 and the Biligiri Rangan Block (Ratheesh-Kumar et al., 2016) documented Neoarchean (2539.2 ± 3 Ma) to Paleoproterozoic (2458.9 ± 8.6 Ma) subduction-accretion tectonics leading to arc magmatism and subsequent high-grade granulite facies metamorphism. Ratheesh-Kumar Fig. 13. ...
We present mesoscale structural development across the Nilgiri Block and the flanking Moyar and Bhavani shear zones in south India, and detailed mineral-chemical and geothermobarometric studies of a garnet-bearing quartzofeldspathic gneiss from the easternmost part of the Moyar shear zone. Barring a narrow (< 100μm) rim domain, major element distribution within garnet porphyroblasts reveals complete chemical homogenization. The absence of growth zoning in garnet porphyroblasts may suggest a protracted post-garnet growth residence period of the rock at elevated temperatures. Chemical zoning near garnet rim reflects the signature of both retrograde net-transfer (ReNTR) and retrograde exchange (ReER) equilibria. The ReNTR-equilibrium is recognized by prominent Mn kick-up in garnet, whereas the ReER-equilibrium is identified by divergence of Fe and Mg between garnet and biotite. Diffusion modelling, though qualitative, of the observed chemical zoning in garnet suggests an initial phase of rapid (~ 150 °C/Ma) cooling, which may have been achieved by tectonic-extrusion-induced exhumation. Pressure-temperature conditions for peak, ReNTR and ReER are constrained, respectively, at 900⁰C; 9–11 kbar, 735⁰C; 8 kbar and 685⁰C; 7.8 kbar. Analyses of structural fabrics establishes oppositely verging nature of the Moyar and Bhavani shear zone and may suggest a doubly vergent orogenic development, with the former as prowedge and the latter as retrowedge. The presence of the Nilgiri Block as a topographically elevated region between these oppositely dipping thrust faults indeed corroborates a doubly vergent orogenic setup. The tectonic scenario is comparable with a continent-continent collision type accretionary tectonics. Peak high-P granulite facies metamorphism and post-peak long residence period of the studied quartzofeldspathic gneiss at deep crustal level suitably fit into the Neoarchean crustal dynamics resulting in crustal thickening, in the order of ~ 41 km, within the Nilgiri Block.
... The Southern Granulite Terrane (SGT) provides a critical window to understand the tectonic evolution of Precambrian crust as it preserves the geological records of continent reconstruction, subductioncollision and terrane accretion (Sato et al., 2011;Praveen et al., 2014;Samuel et al., 2014;Santosh et al., 2015;Ratheesh-Kumar et al., 2016;Santosh and Li, 2018;Santosh, 2020). The SGT consists, collage of Neoarchean to Neoproterozoic terranes/ tectonic blocks, which are characterized by distinct lithological assemblages, grades of metamorphism and isotopic age (Behera et al., 2019). ...
... The SGT consists, collage of Neoarchean to Neoproterozoic terranes/ tectonic blocks, which are characterized by distinct lithological assemblages, grades of metamorphism and isotopic age (Behera et al., 2019). The Meso to Neoarchean terrane adjacent to Dharwar Craton is termed as Northern Granulite Block (NGB; Clark et al., 2009;Brandt et al., 2014;Collins et al., 2014;Ratheesh-Kumar et al., 2016, Chetty, 2017Kumar et al., 2020a), which is made up of orthogneisses, metasedimentary rocks and charnockities. The NGB includes several crustal blocks ( Fig. 1) such as Coorg Block (CB), Nilgiri Block (NB), Biligiri Rangan Block (BRB), Shevaroy Block (SB) and Madras Block (MdB). ...
The gravity and magnetic surveys were carried out across
E-W trending Salem-Attur Shear Zone (SASZ), Southern
Granulite Terrane (SGT), India. The study envisages detail
crustal architecture/ fabric across the SASZ from the analysis
of potential field data. The Bouguer gravity map shows a
circular gravity high closure over ultramafics complex of
Chalk hill. Gravity model indicates that the complex is a
spherical plug type body with depth extension of ~7-8 km.
The SASZ is reflected by dominantly E-W trending elongated
isogals with low gravity values and depth extension
of 3-4 km. The gravity high anomaly at the southeastern
part of the SASZ indicates a high-density crustal unit that
might have accreted along the southern margin of the shear
zone. The magnetic map shows a high anomaly zone in
the north-western block of the study area and bordered by
a predominantly low anomaly zone at the southeastern
boundary. The gravity-magnetic joint modelling reveals
that a plane in south of the SASZ separates two upper crustal
unit of contrasting density and susceptibility. The study
indicates evidence of accretional tectonic in the southern
margin of Dharwar Craton (DC) through arc magmatism.
The SASZ has a shallow vertical extension (~4 km) and is
characterized by diamagnetic rock.
... In the Biligiri Rangan massif, TTG orthogneisses and charnockites reveal zircon U-Pb ages of ca. 3362-3315 Ma, 3207-3100 Ma and 2985-2972 Ma marking successive stages of accretion (Peucat et al., 2013;Ratheesh Kumar et al., 2016). Along the western flank of BR hills, Vijaya Kumar et al. (2013) obtained zircon U-Pb ages between 3391 and 3309 Ma, with εHf (t) between +4.1 and -2.5 for tonalitic orthogneisses. ...
... 2.70-2.50 Ga) accretionary processes, broadly contemporaneous with those in the WDC and EDC (Praveen Kumar et al., 2014;Ratheesh Kumar et al., 2016;Santosh et al., 2016;Shan-Shan Li et al., 2018). The entire Dharwar craton (WDC, EDC and the northern parts of the SGT) was finally shaped by the end Archean thermo-tectonic event(s) (D 2 M 2 ) around ca. 2.56-2.50 ...
An emerging view is that Earth’s geodynamic
regime witnessed a fundamental transition towards
plate tectonics around 3.0 Ga (billion years). However,
the manifestations of this change may have been
diachronous and craton-specific. Here, we review
geological, geophysical and geochronological
data (mainly zircon U-Pb age–Hf isotope compositions)
from the Dharwar craton representing over a billion
year-long geologic history between ~3.5 and 2.5 Ga.
The Archean crust comprises an oblique section of ~12
km from middle to deep crust across low- to mediumgrade
granitegreenstone terranes, the Western
and Eastern Dharwar Cratons (WDC and EDC), and
the highgrade Southern Granulite Terrain (SGT).
A segment of the WDC preserving Paleo- to
Mesoarchean gneisses and greenstones is characterised
by ‘dome and keel’ structural pattern related to vertical
(sagduction) tectonics. The geology of the regions with
dominantly Neoarchean ages bears evidence for
convergent (plate) tectonics. The zircon U-Pb age–
Hf isotope data constrain two major episodes of
juvenile crust accretion involving depleted mantle
sources at 3.45 to 3.17 Ga and 2.7 to 2.5 Ga with
crustal recycling dominating the intervening period.
The Dharwar craton records clear evidence for
the operation of modern style plate tectonics since
~2.7 Ga.
... In the Biligiri Rangan massif, TTG orthogneisses and charnockites reveal zircon U-Pb ages of ca. 3362-3315 Ma, 3207-3100 Ma and 2985-2972 Ma marking successive stages of accretion (Peucat et al., 2013;Ratheesh Kumar et al., 2016). Along the western flank of BR hills, Vijaya Kumar et al. (2013) obtained zircon U-Pb ages between 3391 and 3309 Ma, with εHf (t) between +4.1 and -2.5 for tonalitic orthogneisses. ...
... 2.70-2.50 Ga) accretionary processes, broadly contemporaneous with those in the WDC and EDC (Praveen Kumar et al., 2014;Ratheesh Kumar et al., 2016;Santosh et al., 2016;Shan-Shan Li et al., 2018). The entire Dharwar craton (WDC, EDC and the northern parts of the SGT) was finally shaped by the end Archean thermo-tectonic event(s) (D 2 M 2 ) around ca. 2.56-2.50 ...
An emerging view is that Earth’s geodynamic regime witnessed a fundamental transition towards plate tectonics around 3.0 Ga (billion years). However, the manifestations of this change may have been diachronous and craton-specific. Here, we review geological, geophysical and geochronological data (mainly zircon U-Pb age–Hf isotope compositions) from the Dharwar craton representing over a billion year-long geologic history between ~3.5 and 2.5 Ga. The Archean Dharwar crust comprises an oblique section of ~12 km from middle to deep crust across low- to medium-grade granitegreenstone terranes, the Western and Eastern Dharwar Cratons (WDC and EDC), and the highgrade Southern Granulite Terrain (SGT). A segment of the WDC preserving Paleo- to Mesoarchean gneisses and greenstones is characterised by ‘dome and keel’ structural pattern related to vertical (sagduction) tectonics. The geology of the regions with dominantly Neoarchean ages bears evidence for convergent (plate) tectonics. The zircon U-Pb age–Hf isotope data constrain two major episodes of juvenile crust accretion involving depleted mantle sources at 3.45 to 3.17 Ga and 2.7 to 2.5 Ga with crustal recycling dominating the intervening period. The Dharwar craton records clear evidence for the operation of modern style plate tectonics since ~2.7 Ga.
... Based on its different geological characteristics and tectono-thermal structure, South India is divisible into several tectonic units: (1) the Archean Western and Eastern Dharwar Craton and associated high-grade granulite (charnockite) terranes, which includes from west to east the following Blocks: the Coorg, Nilgiri, Biligiri Rangan, Shevroy, and Madras (e.g., Ratheesh- Kumar et al., 2016Kumar et al., , 2020 and (2) the Southern Granulite Terrain (SGT) that mostly comprises the Proterozoic granulite facies Madurai Block and Trivandrum-Nagercoil Blocks (Brandt et al., 2011(Brandt et al., , 2014Braun & Kriegsman, 2003;Chetty et al., 2006;Harris et al., 1994;Kröner et al., 2015;Santosh, 1996;Santosh et al., 2003). The E-W trending Palghat Cauvery Shear Zone (PCSZ) has been widely recognized as the northern boundary of the Madurai Block, regardless of its controversial tectonic evolution as a late Neoproterozoic or late Archean suture (Clark et al., 2009;Collins et al., 2007;Santosh et al., 2009Santosh et al., , 2012. ...
... We suggest that potentially high Te (>50 km) and high Tc (>35 km) Archean provinces in and around the Dharwar Craton can each be attributed to an old, cold, and stable core of the continental lithosphere, which was amalgamated by subduction, collision, and accretionary processes in the Mesoarchean to Neoarchean and remained unaffected by later tectono-thermal events (Ratheesh- Kumar et al., 2020). A relatively low-Te (<50 km) and thinned (Tc < 35 km) crustal structure of the Shevaroy Block may indicate its distinct evolution as a thermally weakened lithosphere that possibly resulted in association with the back-arc evolution of the Mettur Shear Zone (Ratheesh- Kumar et al., 2016Kumar et al., , 2020. However, the low-Te and crustal thinning patterns over the Coorg Block and part of the Nilgiri Block may indicate delamination-related weakening of the lithosphere. ...
Geophysical studies of the tectonic links between Sri Lanka and India are limited, and consequent paleo‐fit configurations differ and remain uncertain. Here, we present first‐order constraints for an optimum link between Sri Lanka and southern India by using high‐resolution maps of elastic thickness (Te) and crustal thickness (Tc) derived from a flexure inversion method that based on the space‐domain convolution technique for a data window covering integral continental‐oceanic lithospheric setting. We find that the spatial variations of Te and Tc over different crustal provinces in southern India and Sri Lanka agree well with their tectonic classifications proposed by geological studies. Importantly, this study provides much‐needed confirmation of the rifting and associated lithospheric deformation along the mirrored continental margins of India and Sri Lanka and in the intervening transitional lithosphere. The two margins are marked by patterns of low Te and thinned crust, which suggests that an anticlockwise rift (eastward rift) of Sri Lanka away from India resulted in the creation of the Mannar Basin, and a secondary N‐S rifting gave rise to the Cauvery Basin. However, these developments led to a final failed rift, which we attribute to a mechanically strong continental lithospheric bond (a NW‐SE zone of high‐Te and relatively thick crust) that was present in the intervening Palk Strait. We matched the identical zones of Te variations along the two mirrored continental margins and thus obtained a coherent paleo‐fit configuration, and a step‐by‐step evolution of these margins was constructed by using magnetic anomaly‐based paleo‐fit reconstruction models.
... In the central block, TTGs accreted during ca. 3300-3000 Ma whilst protoliths of transitional TTGs were emplaced at ca. 2700-2560 Ma (Balakrishnan et al. 1999;Chardon et al. 2011;Jayananda et al. 2000Jayananda et al. , 2013aJayananda et al. , 2019aPeucat et al. 2013;Maibam et al. 2016;Ratheesh Kumar et al. 2016). On the other hand, the gneisses of the eastern block accreted during 2700-2550 Ma (Krogstad et al. 1991;Peucat et al. 1993Peucat et al. , 2013Dey et al. 2016), which show compositions like transitional TTGs of the central block ). ...
... In the Kabbaldurga-B.R. Hills corridor U-Pb zircon ages of TTGs reveal three significant episodes of crustal accretion ca. 3360-3300, 3200-3150 and 2965-2960 Ma (Friend and Nutman 1991;Peucat et al. 2013;Ratheesh Kumar et al. 2016). Immediately north in Bangalore-western margin of Kolar greenstone belt zircons from TTG indicate ages ranging 3140-3265 Ma (Krogstad et al. 1991;Jayananda et al. 2000;Maibam et al. 2016). ...
Understanding coupled evolution of the crust-mantle system, building up of habitable continents and tectonics of evolving Earth constitute a major focus of research in Earth and Planetary Sciences. This contribution reviews the processes of the evolution of early Earth, including thermal records, mantle evolution, crustal growth, craton formation and tectonics in the first part, followed by the evolution of individual cratonic blocks in Peninsular India and their assembly into shield framework in the second part. Closely scrutinized global geochronologic and isotope database show that remnants of the Hadean-Eoarchean terrestrial record preserved in the core of cratons provide invaluable insights into planetary evolution. Multidisciplinary studies on the preserved earliest crustal remnants reveal unique features such as distinct lithological associations (tonalite-trondhjemite-granodiorite (TTG)-komatiite dominated greenstones), steeper geothermal gradients, hotter mantle, high rates of crustal growth, dome-basin patterns and plume-dominated tectonics and absence of high-pressure mineral assemblages compared to Phanerozoic Earth.
... The detailed tectonic setting of the Dharwar Fig. 1. Regional geology and tectonic framework map of southern India showing the major shear/ suture zones and crustal blocks (modified after Ishwar- Kumar et al., 2013;Ratheesh-Kumar et al., 2016). The rectangular box marks the location of the present study area. ...
... BRB were mostly based on strain fabric analysis (Chardon et al., 2008), and on limited studies of lithologies (Janardhan et al., 1994;Peucat et al., 2013;Basavarajappa and Srikantappa, 2014;Ratheesh-Kumar et al., 2016), and hence they failed to explain the spatial variations of data necessary for regional tectonic interpretations, and that has created significant ambiguities in understanding the evolution of the Dharwar Craton. Peucat et al. (2013) broadly divided the lithologies in the BR-Hill, namely charnockites, enderbites, mafic granulites, and meta-sediments, into three age groups including > 3300 Ma, ca. ...
This study provides a new perspective into the Archean accretionary tectonic evolution of the Dharwar Craton and the adjacent high-grade granulite blocks of southern India. The necessary constraints for this tectonic assembly were derived using new petrochemical and zircon U-Pb age data from the granulite facies Biligiri Rangan Block (BRB), which occupies a central key position in the Archean collage of southern India. The BRB, composed largely of charnockites and mafic granulites, is a vestige of ca. 3400–3200 Ma cratonic fragment that was amalgamated with the Western Dharwar Craton (WDC) at ca. 2700–2500 Ma. The spatial variations in composition and evolution of the charnockites (i.e. low- and high-Sr/Y variants) are attributed to arc magmatic processes at different crust-mantle depths accompanied by a flat- or shallow-dipping subduction geometry. Different stages in the subduction processes such as exhumation and magma underplating (ca. 2800–2950 Ma), initial arc magmatism (ca. 2700–2650 Ma), and peak magmatism together with metamorphism (ca. 2700–2500 Ma) were identified from zircon age populations of different rock-types. Our new results integrated with all published geological and geophysical data lead to the conclusion that the Kollegal and Mettur Shear Zones on the western and eastern sides of the BRB respectively were sited on an east-dipping Archean suture and a closed back-arc basin. We propose a tectonic model for southern India according to which subduction-accretion of the Coorg Block in the Mesoarchean (ca. 3300–3100 Ma) and of the Nilgiri Block, Biligiri Rangan Block and the Eastern Dharwar Craton in the Neoarchean (ca. 2700–2500 Ma) led to closure of the Paleo-Dharwar Ocean and to terminal collision with the Western Dharwar Craton.
... In spite of their chemical coherence on some variation diagrams (figure 8a-d), the plutons collectively exhibit incoherent trends for some major and trace elements. In the Rb vs. (Y+Nb), Nb vs. Y, Rb vs. (Yb+Ta) binary variation diagrams (figure 8ac), some of the granites (Vaniyambadi, Bargur, Krishnagiri, Kangayam, Palladam, Udhayur, Pulliyampatti and Palani are confined to the volcanic arc to syncollisional granite fields, and their genesis might have been associated with the Neoarchean and Neoproterozoic subduction-accretion tectonics described in northern margin of terrain and that of CSZ and south of Madurai block Santosh et al. 2009Santosh et al. , 2012Santosh et al. , 2013Santosh et al. , 2015Santosh et al. , 2017Ratheesh Kumar et al. 2016;Bhadra and Nasipuri 2017). It is also described that the basement gneisses and intrusive granitic rocks near the transition zone around Krishnagiri as Itype and S-type (Chandrasekaran and Subrmanian 1996). ...
... The granites of Northern block including Veppanapalli, Vaniyambadi, Jakkampatty, and Bargur from the present study are possible equivalents of Krishnagiri of Neoarchean emplacements. Their emplacement sequence appears to be related the Neoarchean subduction-collision and accretionary tectonics (Saitoh et al. 2011;Yellappa et al. 2012;Santosh et al. 2012Santosh et al. , 2013Noack et al. 2013;Glorie et al. 2014;Samuel et al. 2014;Ratheesh Kumar et al. 2016) followed by crustal extension associated juvenile addition of magmas. In the Salem Block, the Neoproterozoic granites like Kullampatti granite is 534 ± 15 Ma and in Nilgiri block, the Ambalavayal granite of 595 ± 20 Ma, Kalpatta granite of 553 ± 5 Ma Peralimali granite of 750 ± 40 Ma are described by several workers (Santosh et al. 1986(Santosh et al. , 1989Pandey et al. 1993;Miller et al. 1996). ...
Granitoid intrusions occur widely in the Southern Granulite Terrain (SGT) of India, particularly within the Cauvery Suture Zone (CSZ), which is considered as the trace of the Neoproterozoic Mozambique ocean closure. Here we present the petrological and geochemical features of 19 granite plutons across the three major tectonic blocks of the terrain. Our data show a wide variation in the compositions of these intrusions from alkali feldspathic syenite to granite. The whole rock geochemistry of these intrusions displays higher concentrations of \(\hbox {SiO}_{2}\), FeO*, \(\hbox {K}_{2}\hbox {O}\), Ba, Zr, Th, LREE and low MgO, \(\hbox {Na}_{2}\hbox {O}\), Ti, P, Nb, Y and HREE’s. The granitoids are metaluminous to slightly peraluminous in nature revealing both I-type and A-type origin. In tectonic discrimination plots, the plutons dominantly show volcanic arc and syn-collisional as well as post-collisional affinity. Based on the available age data together with geochemical constrains, we demonstrate that the granitic magmatism in the centre and south of the terrain is mostly associated with the Neoproterozoic subduction–collision–accretion–orogeny, followed by extensional mechanism of Gondwana tectonics events. Similar widespread granitic activity has also been documented in the Arabian Nubian shield, Madagascar, Sri Lanka and Antarctica, providing similarities for the reconstruction of the crustal fragments of Gondwana supercontinent followed by Pan-African orogeny.
... The geochemical studies imply a subductionrelated tectonic settings across the AKSZ. This subduction-related geodynamic settings obtained from the geochemical characterization is consistent with the observations by several researchers (Sreejith and Ravindra Kumar 2013;Amaldev et al. 2016;Ratheesh-Kumar et al. 2016;Ravindra Kumar and Sreejith 2016;Santosh et al. 2017;Yellappa and Rao 2018). ...
Madurai Block (MB) and Trivandrum Block (TB), composed of similar high-grade gneisses separated by the Achankovil shear zone (AKSZ), represent two different crustal domains with distinct crustal evolutionary history. Geochemistry of garnet-biotite gneisses and charnockites from both the blocks shows that the studied rocks are magnesian and calcic to calc-alkaline in nature, indicating magmatic-arc settings. Both the litholunits from the TB and MB displayed an enrichment of light rare earth elements, depletion of high field strength elements and flat heavy rare earth elements pattern. In addition to these, the garnet-biotite gneisses of the TB shows a strong negative Eu anomaly, suggesting the crustal contamination of the source and post-Archean granitoid origin. In the Nb vs. Y diagram, most of the charnockites and garnet-biotite gneisses from the MB and TB fall within the syn-collisional granite and volcanic-arc field. Rb vs. (Y+Nb) and Ta vs. Yb diagrams show a volcanic-arc granite trend for all the lithounits. In the modified La/Yb vs. Th/Yb plot, most of the MB rocks fall within the island-arc field and the TB rocks fall within the continental margin-arc field. Such chemical signatures, characteristic of arc magmatism in active continental margins, suggest subduction-related geodynamic settings along the AKSZ.
... The Northern Block is dominated by charnockite and mafic granulite, which underwent crystallization and subsequent metamorphism spanning c. 2.65 Ga-2.49 Ga (Clark et al., 2009;Ratheesh-Kumar et al., 2016). The Hogenakkal carbonatite complex consists of clinopyroxenite dykes and carbonatites emplaced along NNE-SSW faults within granulite country rocks. ...
... In comparison to the WDC, the SGT is a high-grade crustal domain made up of a collage of crustal blocks traversed by a network of regional scale shear zones ( Fig. 1) (Drury et al. 1984;Sacks et al. 1997;Rajesh et al. 1998;Braun and Brocker 2004;Santosh et al. 2009Santosh et al. , 2015Santosh et al. , 2016Collins et al. 2014;Plavsa et al. 2014;Praveen et al. 2014;Yang et al. 2015;Ratheesh-Kumar et al. 2016, 2019. The north-western part of the SGT is composed of the Coorg Plavsa et al. 2012 andManikyamba et al. 2017). ...
The South Indian Granulite Terrane is traversed by several crustal scale shear zones, however the tectonic significance of the shear zones are poorly understood. The tectonic relevance of the Bavali Shear Zone (BSZ) ‒ in the WNW extremity of the Moyar Shear Zone ‒ at the interface between the Paleoarchean to Neoarchean Western Dharwar Craton (WDC) in the north and the late Neoarchean Nilgiri block in the south is poorly constrained. The most conspicuous feature in the WDC is a set of N-striking gently-plunging upright folds and N-striking dextral shear zones (deformation D4). These D4 structures are superposed on a shallowly-dipping D3 recumbent folds and gently-dipping mylonite fabrics in a suite of anatectic gneisses, lower-grade supracrustal rocks and foliated granitoids. In regional scale, the D3 fold axes curve into the WNW-striking BSZ (D5 deformation), a steep-dipping transpressional shear zone with dextral kinematics. The BSZ is characterized by steeply-plunging stretching lineations sub-parallel to the hinges of reclined folds on the pre-shearing fabrics in the lithologies of the adjacent cratons. Syn-D5 charnockite veins suggest the BSZ formed at T > 850 °C. Existing U–Pb (zircon) dates and monazite chemical dates (this study), indicate that the deformation-metamorphism-magmatism in the WDC and the Nilgiri block occurred between 3400 and 2500 Ma; by contrast the high-T D5 oblique crustal shortening in the BSZ contemporaneous with multiple felsic emplacements was active between 830 and 720 Ma. The BSZ collision orogeny possibly preceded the eventual integration of the Greater India landmass with the Gondwanaland during the early-Palaeozoic.
... Error bars(Tables 2 & 3)are removed for clarity. ɛ Hfi values of granitoids from the western Dharwar Craton are represented by filled grey circles(Ao et al., 2021) and filled grey circles with darker rimsRanjan et al., 2020); Central Dharwar Province (RamMohan et al., 2014; Yang & Santosh, 2015;Ratheesh Kumar et al., 2016) by filled pink circles. The light blue circles represent ɛ Hfi values of felsic rocks from different cratons around the world are compiled from literature(Bauer et al., 2017;Chaudhuri et al., 2018;Dey et al., 2017;Fisher & Vervoort., 2018; Gardiner et al., 2017;Gardiner et al., 2019;Geng et al., 2012;Hiess & Bennett, 2016;Iizuka et al., 2009;Kaur et al., 2014;Kaur et al., 2016;Kaur et al., 2019;Kemp et al., 2009;Kemp et al., 2010;Kröner et al., 2013;Kröner et al., 2014;Kröner et al., 2016; Liu et al., 2009;Laurent & Zeh, 2015;Naeraa et al., 2012;Ao et al., 2021;Pandey et al., 2019;Reimink et al., 2016; Saha et al., 2016;Upadhyay et al., 2019;Wan et al., 2014;Wang et al., 2015;Wu et al., 2005;Zeh et al., 2009;Zeh et al., 2011). ...
The combination of UPb zircon ages with Hf-Sr-Pb isotopes of different intrusive and extrusive felsic and sedimentary rocks provides constraints on the petrogenetic evolution of the continental crust in the western Dharwar Craton, India. The oldest detrital zircon preserved formed at ~3.6 Ga and represents a relic of the oldest felsic crustal material in the region. The dominant granitoid units of the western Dharwar Craton contain zircon grains with magmatic ages between 3.4 Ga and 3.0 Ga that indicate the formation of major felsic continental crust during this interval.
Trace element abundances of the granitoids indicate that the oldest members of the intermediate to felsic suite derived by partial melting of mafic material at ~3.6–3.4 Ga. The initial bulk rock Hf isotope compositions of these granitoids are consistent with their formation by melting of even older mafic material that was slightly enriched relative to the depleted mantle composition. This mafic and slightly enriched material formed by mantle melting at ~ < 3.8 Ga. The Hf isotope compositions of individual zircon grains, obtained by two different analytical techniques (in-situ and complete dissolution followed by chromatographic separation) give evidence for the presence of such older mafic material (<3.8 Ga) that formed the immediate precursor of their granitoid host rocks. Such a mafic source for the granitoids is consistent with PbSr isotope systematics of these that shows no indication of Eoarchean enriched/evolved material in the western Dharwar Craton. The mafic source material of the granitoids thus represents an intermediate stage of crust formation that started after 3.8 Ga with the formation of mafic crust by mantle melting. The combined geochronological and isotopic constraints suggest that the Mesoarchean felsic crust of the Dharwar Craton formed by differentiation of melts derived from an amphibolite/eclogite source rock and included increasing contributions of reprocessed crustal material with time from ~3.6 to 3.0 Ga. The major interval of growth of felsic continental crust was from 3.4 to 3.0 Ga. The younger generation of granitoids formed mostly by reworking of older intermediate to felsic crust. These different felsic magmatic bodies with distinct petrogeneses and sources, that include the depleted mantle, older mafic crust and the evolved continental crust, became essential elements of the stable continental crust of the western Dharwar Craton, the majority of which was generated from 3.4 to 3.0 Ga.
... Santosh et al., (2009b) suggested that the subduction process, arc magmatism, and accretion along with high-grade metamorphism would explain several things in the crustal blocks and intervening sutures. Slab melting along with vertical addition was also basic to the growth of the continent in SGT (Ratkheesh Kumar et al., 2016). ...
This paper elucidates the petrology, geochemistry, petrogenesis and tectonic aspects of nepheline syenites of Sivamalai alkaline pluton, emplaced into the northern margin of Madurai block, Southern Granulite Terrane (SGT) of peninsular India. This pluton consists of a variety of mesocratic to leucocratic nepheline syenites. Petrographically, amphibole, biotite, K-feldspar (perthite), nepheline, and plagioclase with subordinate pyroxene are the dominant minerals in mesocratic nepheline syenites in decreasing order of abundance. Whereas, K-feldspar, nepheline, biotite, amphibole, and plagioclase are among the major minerals that form the leucocratic variety. Geochemically, these rocks are metaluminous and show positive trends for total alkalis, Al2O3, and negative trends for MgO, MnO, CaO, Fe2O3, TiO2, and P2O5 with increasing silica, indicating the dominant role of crystal fractionation. On the primitive normalized trace elemental diagram, they display enriched signatures for LILE and depletion for HFSE. Similarly, the chondrite normalized REE patterns indicate the LREE enrichment over HREE with a significant positive Eu anomaly. The LILE and LREE enrichment of mesocratic variety is attributed to the source enrichment by carbonate metasomatism and depletion of the same in leucocratic variety refers to the high degree fractionation of the LILE and LREE host mineralogy. Depletion of HFSE like Nb, and Ti in both varieties signifies the imprint of subduction or crustal contamination in the genesis of the nepheline syenites of the Sivamalai. The positive K, Pb further substantiates this observation. The HFSE depletion coupled with mobile elemental enrichment signatures was possibly inherited from the mantle sources to Sivamalai nepheline syenites by the earlier subduction events on the northern segment of the Madurai block. Geochemical interpretations of the present study indicate that the primary melts of the Sivamalai are sourced by an OIB-like mantle, equilibrated under the presence of garnet. In terms of tectonic discrimination, the studied rocks exhibit volcanic arc/syn-collisional affinity. Alkaline magmatism within the Indian subcontinent and elsewhere is mainly operated by crustal scale-shear zones or paleao-rifts, which were subsequently subjected to multiple deformation and metamorphic events. These later events mask up the earlier tectonic history thus making it difficult to trace the exact tectonic control of the alkaline magmatism elsewhere in the present-day context.
... The study area forms an undulating terrain. The northern part of the area is occupied by Biligir Rangan hills (Figure 1a,b) and represented by charnockites associated with felsic gneiss, ultramafic, magnetite quartzite and granite (Samuel et al. 2014;Ratheesh-Kumar et al. 2016). The geochronological study of charnockites, granite and mafic granulite from Biligiri Rangan hill indicate magmatism during ca. ...
We present a combined analysis of remote sensing, gravity and magnetic data across Moyar Bhavani Shear Zone (MBSZ), Southern Granulite Terrane (SGT), India to study crustal architecture and tectonics. The horizontal gradient of gravity data shows strong density inhomogeneity across the MBSZ. The magnetic data and residual gravity analysis shows corroboration of a NE–SW trending linear magnetic anomaly zone and residual gravity high strip along the MBSZ. The low magnetic anomalies dominate the northern part of the MBSZ, while the southern part is dominated by moderate to high anomalies. The integration of potential data and remote sensing data clearly reveals that ENE–WSW is the dominant structural trend that coincides with the Mettur Shear Zone (MeSZ) trend. The joint gravity and magnetic modelling suggest that the MBSZ, is characterized by the disposition of mélange of moderate to high-density rocks of diamagnetic nature.
... Peucat et al., 1993;Jayananda et al., 2000Jayananda et al., , 2015Jayananda et al., , 2018Bhaskar Rao et al., 2003;Dey, 2013;Mohan et al., 2014;Maibam et al., 2016); a magmatic age range of 3400 to 2500 Ma for the Central Dharwar Craton (e.g. Peucat et al., 2013;Ratheesh-Kumar et al., 2016); and a magmatic age range of mostly between 2700 and 2500 Ma for the Eastern Dharwar Craton (Radhakrishna & Naqvi, 1986;Rogers & Giral, 1997;Clark et al., 2009;Gireesh et al., 2012;Glorie et al., 2014;Maibam et al., 2016;Jayananda et al., 2018Jayananda et al., , 2020. ...
Systematic changes in whole-rock chemistry, mineralogy, mineral textures, and mineral chemistry, are seen along a ca. 95 km traverse of late Archean granitoid orthogneisses in the Shevaroy Block, Eastern Dharwar Craton, southern India. The traverse passes from amphibolite-grade gneisses in the north to granulite-grade rocks (charnockite) in the south. Changes include whole-rock depletion of Rb, Cs, Th, and U in the granulite grade rocks as relative to the amphibolite grade gneisses, and oxidation trends regionally from highly oxidized granulite-facies rocks near the magnetite-hematite buffer to relatively reduced amphibolite-facies rocks below the fayalite-magnetite-quartz. Rare earth elements show limited mobility and are hosted a variety of minerals whose presence is dependent on the metamorphic grade ranging from titanite and allanite in the amphibolite-facies rocks to monazite in the vicinity of the orthopyroxene-in isograd to apatite in the granulite-facies charnockite.
Cathodoluminesence (CL) and back scattered electron (BSE) sub-grain imaging and Sensitive High-Resolution Ion MicroProbe (SHRIMP) analysis of zircon from 29 samples of dioritic, tonalitic, and granitic orthogneiss from the traverse reveals magmatic zircon cores that record the emplacement of the granitoid protoliths mostly about 2580 to 2550 Ma, along with a few older mid to late Archean tonalites. Protolith zircon was modified during metamorphism by overgrowth and/or replacement. Relative to igneous cores, U-enriched metamorphic zircon, dominant in the amphibolite-grade gneisses, formed at ca. 2530 Ma, predating retrograde titanite growth at ca. 2500 Ma. Uranium-depleted mantles grew on zircon between 2530 and 2500 Ma in granulite-grade samples south of the orthopyroxene-in isograd. In some of these samples, the U-depleted metamorphic zircon is preceded by mantles of U-undepleted zircon, indicating a progression of metamorphic zircon growth with increasing depleted compositions between 2530 and 2500 Ma .
With increasing metamorphic grade (from amphibolite to granulite) and oxidation state, allanite and monazite disappear from the assemblage and zircon became depleted in U and Th. Whole-rock U-Th compositions became decoupled from relict magmatic zircon compositions, reflecting the development of U-depleted magmatic zircon and indicating that whole-rock chemical differences along the traverse were produced during metamorphism, rather than just reflecting differences in dioritic vs. granitic protoliths. Although in situ anatexis and melt extraction may have played a role, whole-rock and zircon depletion of trace elements can be explained by the action of externally-derived, oxidizing, low-H2O activity hypersaline fluids migrating up through the mid to lower crust. Fluids and element migration during metamorphism may be the end result of subduction related processes that cumulated in the collision and concatenation of island arcs and continental blocks. These tectonic processes assembled the Dharwar Craton at the end of the Archean.
... Steep gravity gradients were reported in the area where the Coorg block is bounded with the Dharwar craton to the north by the Mercara Shear Zone (Sunil et al., 2010), suggesting the presence of underplated high-density material in the lower crust. In previous studies (Bernard-Griffiths et al., 1987;Raith et al., 1990;Bhaskar Rao et al., 1992;Peucat et al., 1993;Janardhan et al., 1994;Raith et al., 1999;Bhaskar Rao et al., 2003;Devaraju and Janardhan, 2004;Ghosh et al., 2004;Clark et al., 2009;Sato et al., 2011;Ratheesh-Kumar et al., 2016), the Coorg-Nilgiri-Salem (or Biligiri Rangan)-Madras Blocks were considered to have witnessed a common Neoarchean (ca. 2500 Ma) history and granulite facies metamorphism. ...
As one of the oldest crustal blocks in Southern Peninsular India, the Coorg Block has been the focus of investigations related to crustal evolution in the early history of the Earth with implications on Mesoarchean plate tectonic processes. The Coorg Block is dominantly composed of arc magmatic rocks and bordered on the north by the Mercara Suture Zone, a Mesoproterozoic subduction-collision zone hosting extruded high-pressure and ultra-high temperature metapelitic and meta-mafic rocks. Here, we report the occurrence of a dismembered gabbro-anorthosite complex corresponding to a layered intrusion from the northern margin of the Coorg Block. We present results from an integrated study on the petrology, mineral chemistry, P-T phase equilibria, whole-rock geochemistry, and zircon and monazite U–Pb geochronology to understand the magmatic and metamorphic evolution of the layered intrusion and associated rocks. The geochemical data suggest that the parental magma was generated within a suprasubduction zone setting. Phase equilibrium modelling of garnet-bearing metagabbro from the Coorg Block suggests metamorphic P–T range of 10.5-11 kbar at 1000-1100°C corresponding to ultra-high temperature conditions. The zircon U-Pb data from the various rock types of the mafic-ultramafic suite yield weighted mean ages at 3176 Ma, 3174 Ma, 3143 Ma, and 3124 Ma whereas the associated charnockite shows a slightly older age of 3319±12 Ma. Metamorphic zircon from charnockite yield an age of 3101 Ma, close to the monazite age of 3110±24 Ma from the same rock, constraining the timing of metamorphism as Mesoarchean, and marking the collisional event between the Coorg Block and the Dharwar Craton along the Mercara Suture Zone. We propose a tectonic model involving southward subduction of the Western Dharwar Block beneath the northern margin of the Coorg Block that can explain the extensive arc magmatism and suprasubduction zone rock suites, followed by ocean closure and collision along the Mercara Suture Zone, accompanied by high P-T metamorphism.
... the Neoproterozoic, covering a time span of more than 2 billion years (Brandt et al., 2014;Collins et al., 2014;Harris, Santosh, & Taylor, 1994;Santosh, Morimoto, & Tsutsumi, 2006). The major crustal blocks include the Mesoarchean Coorg Block , Meso-to Neoarchean Nilgiri, Biligirirangan hills, Shevaroy, and Madras blocks from West to East (Li, Santosh, & Palin, 2018;Ratheesh-Kumar et al., 2016;Samuel, Sajeev, Hokada, Horie, & Itaya, 2014;Samuel, Santosh, Yang, & Sajeev, 2016) collectively referred to as the Northern Granulite Terrane, and the Neoarchean-Neoproterozoic Madurai Block (Brandt et al., 2014;Collins et al., 2014;Ghosh, Maarten de Wit, & Zartman, 2004;Plavsa et al., 2012), Late Neoproterozoic-Cambrian Trivandrum Block (Blereau et al., 2016;Harley & Nandakumar, 2016;, and Palaeoproterozoic Nagercoil blocks to the south collectively referred to as the Southern Granulite Terrane (SGT) Drury et al., 1984;Santosh, 2020; Figure 1). ...
The determination of metamorphic pressure–temperature (P–T) conditions of granulites provides us a natural window into the composition, structure, and P–T conditions of the lower crust and process of continental growth. Here, we constrain the P–T evolution of a suite of mafic granulite from the eastern part of Madurai Block. Integrated results from mineral reactions, conventional thermobarometry, and pseudosection analysis suggest that the studied mafic granulites, with a peak assemblage of coarse‐grained garnet + coarse‐grained clinopyroxene + plagioclase + quartz + [rutile], were metamorphosed under granulite facies conditions of 830 ± 50°C and 9.5 ± 1 kbar. Subsequently, the rocks underwent near isothermal decompression leading to the formation of symplectic assemblages of clinopyroxene + orthopyroxene + plagioclase and orthopyroxene + ilmenite, within a P–T range of 9.0–5.5 kbar and 750–800°C. The estimated peak temperature conditions are somewhat lower than those obtained using characteristic ultra‐high temperature (UHT) mineral assemblages and non‐conventional thermometers (~1,000°C at ~10 kbar) from the co‐metamorphosed metapelitic granulites of the Madurai Block. This may be attributed to the high diffusion rate of divalent Fe and Mg, which often results in retrograde re‐equilibration leading to considerable underestimation of peak temperatures in mafic granulites. To overcome this problem, we have additionally determined peak metamorphic temperatures of the studied rocks using a recently developed garnet–clinopyroxene rare earth element (REE)‐based thermometer that takes advantage of much lower diffusion rate of trivalent REEs. Using the REE thermometer, we have obtained a peak temperature of 1,050 ± 50°C for the studied rocks, which is in reasonable agreement with the peak metamorphic temperature obtained from previous studies. The constrained P–T conditions and the clockwise P–T path suggest that the studied rocks were buried to lower crustal depths at UHT conditions in a convergent margin setting. The near isothermal decompression suggests rapid exhumation, most likely due to an extensional event. The peak UHT metamorphic conditions could have been attained at the core of a long‐lived orogenic plateau, as suggested by other studies.
... The Coorg Block represents an exotic Mesoarchean micro-continent with major lithological units comprising arc-magmatic rocks (Santosh et al., 2015. The Biligiri Rangan Block is proposed as a Mesoarchean continent, which is dominated by gneisses, charnockites, minor mafic granulites, and high-grade metasedimentary rocks (Li et al., 2018;Ratheesh-Kumar et al., 2016). The Shevaroy Block is composed of charnockites and felsic orthogneisses with magma emplacement at 2.76-2.53 ...
The Palghat Cauvery Shear Zone (PCSZ), which dissects the Southern Granulite Terrane (SGT), evoked much attention for last two‐three decades as it offers an enticing opportunity to understand Precambrian tectonics. Many studies suggest that it is a suture zone or a terrane boundary, but tectonic evolution is mainly derived from structural and geochronological evidences. The present gravity‐magnetic study provides insight into both shallow and deeper crustal architecture and addresses the issues related to validation/modification of the existing tectonic realm. A total of 3,650 gravity and magnetic measurements were made covering ~8,640 km² area, represented by high‐grade metamorphic rocks and acid intrusives of Archean to Neoproterozoic age. Representative rock samples were also collected for determination of petrophysical properties (density and susceptibility) which have augmented the understanding of gravity‐magnetic signature. The gravity anomaly map and subsequent modelling suggest an occurrence of a high‐density batholith at mid‐crustal level at the central part of the area. The magnetic anomaly map reveals that ENE‐WSW trending linear magnetic low zone is coincident with a residual gravity high, which probably indicates the fossil of an oceanic crust. The horizontal gradient of gravity‐magnetic data reveals the shallow crust, which is bounded by Palghat Cauvery Shear Zone (PCSZ) and Dharapuram Shear Zone (DSZ), and separates two terranes in terms of density and susceptibilities. The study suggests N‐S verging compressional tectonics had led to closure of an ocean, imprints of which is well manifested in gravity and magnetic maps.
... This chapter follows the earlier two-fold classification of the Dharwar Craton into the WDC and the EDC, with the granulite along their southern margin, making the SGT. The Dharwar Craton exhibits the following main components: (i) polyphase TTGtype gneisses of the WDC (Jayananda et al. 2015;Guitreau et al. 2017), minor TTGs, abundant transitional TTGs and composite granitoids in the EDC (Jayananda et al. , 2020, (ii) Southern Granulite Terrain to the south of the orthopyroxene isograd (Peucat et al. 2013;Ratheesh Kumar et al. 2016), (iii) Volcano-sedimentary greenstone belts (Swami Nath and Ramakrishnan 1981), (iv) Major Shear Zones, (v)~2.5 Ga granitoid belts, (vi) Proterozoic mafic dikes, (vii) deformation patterns, (viii) metamorphism, and (ix) crustal evolution. ...
The Indian Subcontinent is comprised of the Archean-Proterozoic cratons, the Aravalli Craton, the Bundelkhand Craton, the Mehghalaya Craton, the Bastar Craton, the Singhbhum Craton and the Dharwar Craton.
... In the northern part of CSZ, the evolution of Biligiri Rangan Hill granulites and its protoliths described as partial melting of young hydrated oceanic crust in a subduction zone-like environment based on geochemical and isotopic ratio (Janardhan et al., 1994). The Neoarchean subduction-accretionary tectonics and 2650 Ma and 2490 Ma metamorphism of charnockites, pink granite and mafic granulites have been described from Biligiri Rangan Block (Ratheesh Kumar et al., 2016). The occurrence of arc-related volcano magmatic suite along the northern periphery of Nilgiri © 2018 JETIR July 2018, Volume 5, Issue 7 www.jetir.org ...
The granulite facies rocks of Southern Granulite Terrain (SGT), India represent lower crust-upper mantle products of different tectonothermal events of different origins. They include charnockites, two pyroxene granulites of mafic-felsic, dismembered ophiolite sequence, mafic-ultramafic intrusions and granitoids. The southern part of the Cauvery Suture Zone (CSZ) around Aniyapuram-Mohanur, near Namakkal is dominated by a wide range of mafic-felsic granulites. These are well exposed in close association with dismembered mafic and ultramafic rocks including peridotite, pyroxenite, amphibolite, gabbro-gabbronoritic rocks, metapelite, metachert with younger intrusives of pegmatite and granitoids. The petrology of these granulites is characterized by sub-idioblastic fine to coarse-grained clinopyroxene (40-50%) and orthopyroxene (10-20%), idioblastic garnets (20-30%), subhedral to anhedral amphiboles (10-20%) and plagioclases (5%) with accessory phases of apatite, zircon, biotite and opaque minerals. Whole rock geochemistry of 14 representative samples reveals, that the granulites are mafic to slightly intermediate in composition with higher SiO2 (47-53 wt%) and A12O3 (10-16 wt%) with low K2O contents (<0.4wt%). Trace element ratios are extremely variable with very high K/Rb ratios (540-13447) reflected in very low Rb contents (0.2-7 ppm) and varied Ba/La (1.8-43.40), Rb/Sr (0.0001-0.13) and Sr/Nd (3.23-34.25) concentrations. The geochemical variation plots (Na2O+K2O-FeOt-MgO and Zr vs. Y) reveal that these granulites belong to calc-alkaline to tholeiitic signatures. On various tectonic discrimination plots (MnO-TiO2-P2O5, Ti vs. V, Ti vs. Zr, and Cr vs. Y) these show Island arc origin. Spider diagrams with normalized MORB show enrichment of LILE (Sr, K, Rb, Ba) and depletion of HFSE (Ti, Nb, Ta, Hf) with -ve Nb anomalies. The above results with available age data suggest that the protoliths of the mafic granulites are tholeiitic basalts developed in an island arc environment related to Neoarchean to Paleoproterozoic suprasubduction zone tectonic setting, consistent with dismembered ophiolite sequence of the region and have been subjected to granulite facies metamorphism during Neoproterozoic arc magmatism in the terrain.
... This study redefines the maximum pressure in the northern thrust belt as 1.8 to 2 GPa. This thrusted mélange zone formed during the suturing of the Nilgiri Block to the Dharwar Craton in an arc-continent collision setting (Peucat et al., 1989;Raith et al., 1999;Ratheesh-Kumar et al., 2016;Samuel et al., 2014Samuel et al., , 2015Samuel et al., , 2016Santosh et al., 2009;Srikantappa, 1993). An anticlockwise P-T path defines the characteristics of final exhumation of these rocks. ...
Southern peninsular India preserves records of Late Neoarchean−Early Paleoproterozoic continental building and cratonization. A transect from the Paleoarchean Dharwar Craton to the Neoarchean arc magmatic complex in the Nilgiri Block across the intervening Moyar Suture Zone reveals an arc-accretionary complex composed of banded iron formation (BIF), amphibolite, metatuff, garnet-kyanite schist, metagabbro, pyroxenite and charnockite. Here we investigate the petrology, geochronology and petrogenesis of the pyroxenite and garnet-clinopyroxenite. The pyroxenite is mainly composed of orthopyroxene and clinopyroxene with local domains/pockets enriched in a clinopyroxene-garnet assemblage. Thermobarometric calculations and phase equilibria modeling suggest that the orthopyroxene- and clinopyroxene-rich domains formed at 900–1000 °C, 1−1.2 GPa whereas the garnet- and clinopyroxene-rich domains record higher pressure of about 1.8−2 GPa at similar temperature conditions (900−1000 °C). Zircon U–Pb SHRIMP dating show weighted mean ²⁰⁷Pb–²⁰⁶Pb age of 2532 ± 22 Ma, with metamorphic overgrowth at 2520 ± 27 Ma and 2478 ± 27 Ma. We propose a tectonic model involving decoupling and break-off of the oceanic plate along the southern flanks of the Dharwar Craton, which initiated oceanic plate subduction. Slab melting eventually built the Nilgiri volcanic arc on top of the over-riding plate along the flanks of the Dharwar Craton. Our study supports an active plate tectonic regime at the end of the Archean Era, aiding in the growth of paleo-continents and their assembly into stable cratons.
... Further south in the Biligiri Rangan Hills, gneisses and granulites reveal zircon U-Pb ages of ca. 3362-3315 Ma, 3207-3100 Ma and 2985-2972 Ma marking successive stages of accretion (Peucat et al., 2013;Ratheesh Kumar et al., 2016). ...
The generation, preservation and destruction of continental crust on Earth is of wide interest in understanding the formation of continents, cratons and supercontinents as well as related mineral deposits. In this contribution, we integrate the available field, petrographic, geochronologic, elemental Nd-Hf-Pb isotope data for greenstones, TTG gneisses, sanukitoids and anatectic granites from the Dharwar Craton (southern India). This review allows us to evaluate the accretionary processes of juvenile crust, mechanisms of continental growth, and secular evolution of geodynamic processes through the 3600–2500 Ma window, hence providing important insights into building of continents in the Early Earth. The Dharwar Craton formed by assembly of micro-blocks with independent thermal records and accretionary histories. The craton can be divided into three crustal blocks (western, central and eastern) separated by major shear zones. The western block contains some of the oldest basement rocks with two generations of volcano-sedimentary greenstone sequences and discrete potassic plutons whereas the central block consist of older migmatitic TTGs, abundant younger transitional TTGs, remnants of ancient high grade supracrustal rocks, linear volcanic-dominated greenstone belts, voluminous calc-alkaline granitoids of sanukitoid affinity and anatectic granites. In contrast, the eastern block comprises younger transitional TTGs, abundant diatexites, thin volcanic-sedimentary greenstone belts and calc-alkaline plutons. Published geochronologic data show five major periods of felsic crust formation at ca. 3450–3300 Ma, 3230–3150 Ma, 3000–2960 Ma, 2700–2600 Ma, and 2560–2520 Ma which are sub-contemporaneous with the episodes of greenstone volcanism. U-Pb ages of inherited zircons in TTGs, as well as detrital zircons together with Nd-Pb-Hf isotope data, reveal continental records of 3800–3600 Ma. The U-Pb zircon data suggest at least four major reworking events during ca. 3200 Ma, 3000 Ma, 2620–2600 Ma, and 2530–2500 Ma corresponding to lower crustal melting and spatially linked high grade metamorphic events. The TTGs are sub-divided into the older (3450–3000 Ma) TTGs and the younger (2700–2600 Ma) transitional TTGs. The older TTGs can be further sub-divided into low-Al and high-Al groups. Elemental and isotopic data suggest that the low-Al type formed by melting of oceanic island arc crust within plagioclase stability field. In contrast, the elemental and isotopic features for the high-Al group suggest derivation of their magmatic precursor by melting of oceanic arc crust at deeper levels (55–65 km) with variable garnet and ilmenite in residue. The transitional TTGs likely formed by melting of composite sources involving both enriched oceanic arc crust and sub-arc mantle with minor contamination of ancient crustal components. The geochemical and isotopic compositions of granitoids with sanukitoid affinity suggest derivation from enriched mantle reservoirs. Finally, anatectic granites were produced by reworking of crustal sources with different histories. In the light of the data reviewed in this contribution, we propose the following scenario for the tectonic evolution of the Dharwar Craton. During 3450–3000 Ma, TTGs sources (oceanic arc crust) formed by melting of down going slabs and subsequent melting of such newly formed crust at different depths resulted in TTG magmas. On the contrary, by 2700 Ma the depth of slab melting increased. Melting of slab at greater depth alongside the detritus results in enriched melts partly modified the overlying mantle wedge. Subsequent melting of such newly formed enriched oceanic arc crust and surrounding arc-mantle generated the magmatic precursor to transitional TTGs. Finally at ca. 2600–2560 Ma, eventual breakoff of down going slab caused mantle upwelling which induced low degree (10–15%) melting of overlying enriched mantle at different depths, thereby, generating the sanukitoid magmas which upon emplacement into the crust caused high temperature metamorphism, reworking and final cratonization. The crustal accretion patterns in the Dharwar Craton share similarities with those in other Archean cratons such as the Bundelkhand Craton in Central India, Pilbara-Yilgarn Craton in Western Australia, Southern Africa (Swaziland and Limpopo belt), North China Craton, Tanzania Craton, Antongil Craton, NE Madagascar.
... Finally, our results suggest that the NCC records multi-stage crustal growth/reworking and craton stabilization during the Neoarchean to early Paleoproterozoic, and the magnitude and spatial distribution of these processes were inferred to be stronger than those operated in other cratons in the world (e.g., west Greenland Craton, Friend and Nutman, 2005). Therefore, this study provides evidence for rapid crustal growth and evolution during the Archean-Paleoproterozoic boundary, quite similar to those reported from a very few Precambrian terrains such as the Dharwar Craton in India (e.g., Clark et al., 2009;Ratheesh Kumar et al., 2016), the Rum Jungle complex in northern Australia (e.g., Drüppel et al., 2009), and the Napier complex in Antarctica (e.g., Choi et al., 2006), together revealing the complex and heterogeneous evolutionary processes of the early Earth. Whalen et al., 1987;Frost et al., 2001). ...
The Archean-Paleoproterozoic Taihua Complex, extending from east to west along the southern margin of the North China Craton (NCC), is an ideal window to understand the tectonic evolution of the early Earth. Here we present new data on petrology, zircon geochronology, whole-rock geochemistry and isotopes of TTGs, granitic gneisses and felsic leucosomes of migmatites from the Taihua complex in the Xiaoqinling area. Zircon LA-ICP-MS analyses of rocks yield the formation ages of 2564-2503 Ma for TTGs, 2481-2476 Ma for felsic leucosomes and 2359-2334 Ma for granitic gneisses, respectively. The zircon Lu-Hf isotopes suggest that crustal growth occurred in the late Neoarchean and crustal reworking occurred in the end of Neoarchean and the early Paleoproterozoic. The TTG gneisses are characterized by high Na2O/K2O, La/Yb, Sr/Y and low Nb/Ta ratios, and they were likely generated by partial melting of subducted juvenile oceanic crust with garnet or amphibole in residuals, a mechanism similar to the slab melting model. Both the felsic leucosomes and granitic gneisses have high SiO2 contents and various Hf isotopic compositions, and they probably were formed in an extensional setting, attributing to partial melting as well as mixing of the preexisting TTG suite.
Statistically, we report a major transitional phase in the tectonic milieu of the southern NCC from compression to extension during the late Neoarchean (ca. 2.57-2.42 Ga). The compressional tectonics can be attributed to amalgamation of micro-continental blocks during the late Neoarchean (ca. 2.57-2.5 Ga) as the leading period of crustal growth and associated multistage TTG magmatism. In contrast, the extensional phase occurred during the end of Neoarchean (ca. 2.5-2.42 Ga) that may represent the end of cratonic stabilization period and formation of large amounts of K-rich granitic rocks, widespread high-grade metamorphism and migmatites. Additionally, the formation of a large amount of crust-derived granites and mafic dykes during the early Paleoproterozoic in southern NCC reflect another stage (ca. 2.36-2.24 Ga) of crustal extension, which represents an episode of initial worldwide rifting events probably linking to the great oxygen event (GOE). Therefore, the present study proposes a new profile on multi-stage crustal growth/reworking and craton stabilization during Neoarchean-Paleoproterozoic period along the southern NCC, and provides useful insights on complex and heterogeneous evolutionary processes of early Earth.
... Tectonic framework of the southern part of Peninsular India showing the major crustal blocks and intervening shear/suture zones (after Collins et al., 2014;Santosh et al., 2015Santosh et al., , 2016. et Bhaskar Rao et al., 1992;Peucat et al., 1993;Janardhan et al., 1994;Raith et al., 1999;Bhaskar Rao et al., 2003;Devaraju and Janardhan, 2004;Ghosh et al., 2004;Clark et al., 2009;Sato et al., 2011;Ratheesh-Kumar et al., 2016), recent investigations have revealed that the Coorg Block is an exotic Mesoarchean (ca. 3.2-3.5 ...
The Southern Peninsular India preserves a number of Archean cratonic nuclei among which the Coorg Block is considered to represent a Mesoarchean microcontinent. Here we investigate the southern margin of this block where remnants of rare ultramafic rocks in association with arc magmatic rocks and accreted metasediments occur in the Wynad region. We present petrology, mineral chemistry, whole rock geochemistry and zircon U-Pb and Lu-Hf data from a suite of meta ultramafics and associated diorite, orthopyroxene-bearing felsic charnockite, and quartz-mica schist. The peak P-T conditions of metamorphism of these rocks are estimated at 720-840°C and 6.5-8.7 kbar. The geochemical features of the ultramafic suite provide evidence for melting and metasomatism of lithospheric mantle in a suprasubduction zone (SSZ) setting with the involvement of both silicate melts and slab-dehydrated fluids. The parent melts were generated at varying depth levels extending from spinel to garnet stability fields. A subduction-modified, LREE replenished, metasomatically enriched mantle source endorses arc-like chemical signatures in the melts that were acquired through interaction between the depleted sub-arc lithospheric mantle and subduction components.
... In contrast, the Manamedu Ophiolite Complex (Yellappa et al., 2010; Chetty Fig. 1. Generalized geological and tectonic framework of the Southern Granulite Terrane showing major crustal blocks and their boundaries (after Collins et al., 2014; Santosh et al., 2015b; Ratheesh-Kumar et al., 2016). The study area (Wynad) is also shown Santosh et al., 2012 ) in the PCSZ is an example of Neoproterozoic ophiolite with evidence for Cryogenian magmatic event and subsequent amphibolite-facies metamorphism. ...
The Wynad region is located along the confluence of the Mercara and Moyar Suture Zones in southern India which mark major zones of amalgamation of microcontinental blocks during Neoarchean to Paleoproterozoic. Here we report garnet- and clinopyroxene-bearing metagabbros from this region and present petrological, geochemical, and zircon U-Pb and Lu-Hf data. The prograde, peak, and retrograde mineral assemblages of the metagabbro are garnet + clinopyroxene + plagioclase + ilmenite + hornblende + quartz, garnet + clinopyroxene + plagioclase + ilmenite, and garnet + clinopyroxene + plagioclase + ilmenite + hornblende + orthopyroxene, respectively. The geochemical data showing nearly flat or slightly depressed LREE and constant HREE patterns, with the absence of negative Nb anomaly in primitive mantle-normalized plots suggest N-MORB affinity for the protolith, indicating remnants of oceanic lithosphere accreted during the subduction-collision event associated with the microblock amalgamation. The metamorphic conditions of the rock were estimated using phase equilibria modeling in the system NCFMASHTO which yield P-T conditions of >960°C and >12.8 kbar, suggesting high-pressure and ultrahigh-temperature conditions. The prograde P-T conditions are inferred as 7.8-8.3 kbar at 600°C and 10.6-11.8 kbar at 900°C, whereas the retrograde P-T conditions computed based on hornblende- and orthopyroxene-bearing mineral assemblages show <660°C and <10.4 kbar. A hairpin-type clockwise P-T path is inferred from the P-T phase equilibria modeling, suggesting rapid heating and cooling in the lower crustal level. This P-T path is in contrast to those reported from other meta-mafic rocks in southern India. The absence of any decompression texture involving garnet also supports near-isobaric cooling of the rock. Our zircon U-Pb data suggest Neoarchean (2.51 Ga) magmatism and Early Paleoproterozoic (2.36-2.22 Ga) high-grade metamorphism. Zircon Lu-Hf data indicate that the protolith magma was derived from Meso- to Neoarchean juvenile components with very limited reworked crustal materials. Our study reveals Paleoproterozoic UHT metamorphism associated with the hot orogen formed during the collision between the Coorg and Nilgiri Blocks.
The roots of the tectonically uplifted Archean continental crustal blocks containing various greenstone-granite terranes are the preserved crustal lithologies exposed in various Archean cratons of the world. The trace element geochemical signatures of some of the preserved lithologies of these magmatic belts reflect the primary melt compositions in the mantle which help in understanding the petrogenesis of the rocks and the involvement of subduction zone processes, if recognized, in the generation of these melts. The Dharwar craton (DC), one of the major cratons of the world located in the Indian subcontinent, is made up of different tectonic blocks amalgamated along various suture zones. Here we review the current understanding about the division of the DC, the polarity of subduction of the different blocks and the petrogenesis of the magmatic rocks of the greenstone belts, and present a synthesis based on the trace element characteristics of the primary basalts. Our study validates the heterogeneous nature of the Archean mantle during the Dharwar crustal evolution. Collation of geophysical, structural, lithological, geochemical and geochronological data imply that the Dharwar craton was accreted by the juxtaposition of at least three major cratonic blocks with noticeably distinct tectonic and magmatic histories. The continued preservation of siliciclastic sedimentary rocks in the greenstone belts of the western Dharwar craton combined with the dominant presence of the >3.0 Ga old gneissic crust, as compared to the central and eastern cratons, could be significant in constraining the involvement of continental crustal detritus in the generation of the magmatic rocks of the greenstone belts. In the central and eastern Dharwar cratons, granitoid crust older than 3.0 Ga was probably destroyed to different extents for the generation of magmatic rocks. The existence of such an older crust is implied by the available Nd model ages. The scattered records of bimodality in the greenstone belt rocks of the central and eastern Dharwar cratons attest for the presence of heterogeneous mantle during Neoarchean. Subduction processes had become predominant by Neoarchean as evidenced from the chemical composition of primary basalts of the greenstone belts of the Dharwar craton. In all likelihood, the polarity of subduction was directed westward with the WDC acting as the foreland and the amalgamation of distinct cratonic blocks were along suture zones represented scatteredly by and as greenstone belts.
In this study we analysed layers of metamorphosed banded iron formation (BIF) in granulite-amphibolite facies tonalitic orthogneisses in the Kolli Massif of southern India with regard to their lithologies, whole rock chemistry, mineral reaction textures, and mineral chemistry and experimentally replicated the formation of pyroxene and garnet. On the basis of their mineral reaction textures along magnetite-quartz grain boundaries these BIF layers are grouped according to their predominant silicate mineralogy: 1) amphibole; 2) orthopyroxene; 3) orthopyroxene–clinopyroxene; 4) orthopyroxene-clinopyroxene-garnet; 5) clinopyroxene-garnet-plagioclase; and 6) silicate-absent. Two-pyroxene and garnet-pyroxene thermometry coupled with thermodynamic modelling of the magnetite-quartz-orthopyroxene-clinopyroxene-bearing lithologies indicates that the magnetite-quartz-orthopyroxene-clinopyroxene-garnet assemblages formed at ~ 10 kbar and 750 to 900°C under relatively low H 2 O activities. Magnetite-quartz-orthopyroxene textures were experimentally replicated at 800 and 900°C and 10 kbars using isolated magnetite grains in a quartz matrix to which was added small amounts of a hypersaline MgCl 2 - and Al 2 O 3 -bearing fluid, which permeated along all available grain boundaries. The fact that silicate reaction textures did not form in one of the BIF samples, which had experienced the same P-T , whereas a variety of orthopyroxene +/- clinopyroxene +/- garnet textures formed in the other BIF samples suggests that, unless the BIF was either initially contaminated during formation with or infiltrated from the surrounding rocks by Mg-, Al-, and Ca-bearing saline fluids, these silicate minerals could not and would not have formed from the inherent magnetite-quartz during granulite-facies metamorphism. These results indicate that saline fluids played a key role in progressing the metamorphic reactions during subduction of sediments.
In this study, we use the U-Pb-Hf isotope composition of detrital zircon in metasedimentary rocks from the Western Dharwar Craton to identify older crusts, their nature, source, timing of extraction, and reworking. The samples come from the Holenarsipur region in the central and Sargur region in the southern part of the craton and belong to both the Sargur and the Bababudan Groups. Both detrital and metamorphic domains were identified in the zircon. The metamorphic overgrowths correspond to two age populations at 2536–2516 Ma, and 2478–2460 Ma. Rutile grains provide either concordant 2453 Ma, or discordant younger ages suggestive of thorough re-equilibration during metamorphism. Detrital zircon grains define notable clusters at 3685–3638 Ma, 3573–3524 Ma, 3471–3411 Ma, 3373–3341 Ma, 3277–3249 Ma, 3172–3148 Ma, 3084–2996 Ma, and 2659–2647 Ma which correspond to major episodes of granitoid crust formation in the craton. Based on detrital and metamorphic zircon populations, deposition of both the older Sargur Group and the basal unit of the younger Bababudan Group in the Holenarsipur region were broadly contemporaneous after c. 3160 Ma, while the sediments of the purported Sargur Group in the Sargur region were deposited after c. 2650 Ma. The new age data requires revision of the stratigraphic positions of the Sargur-type and Dharwar-type greenstone belt successions. The detrital zircon suites have both radiogenic and crust-like Hf isotopic compositions. However, the majority have radiogenic compositions that were acquired during major events of juvenile crust extraction between c. 3850 Ma and 3250 Ma. Strongly unradiogenic ɛHf(t) of some Paleoarchean detrital zircon require protoliths extracted from the mantle in the Hadean, but which persisted for a long period and contributed to granitoid formation in the Paleoarchean and the Mesoarchean. Mixing of juvenile magmas with preexisting crustal components also played a significant role in the petrogenesis of the granitoids. A distinct shift in the ɛHf(t) of the detrital zircon from chondritic to positive value at c. 3600 Ma is noted, reflecting a marked increase in the contribution of strongly depleted mantle reservoir to the source of the post-c. 3600 Ma granitoids.
The Southern Granulite Terrane (SGT) in India is composed of Archaean to Proterozoic crustal blocks and intervening shear/suture zones. The Nallamalai shear zone lies along the eastern flank of the Shevaroy Block. This study is focused on the eastern part of this shear zone where the basement rocks are dominantly composed of charnockite, quartzo-feldspathic gneiss and metagabbro, all of which preserve original magmatic texture. Zircon U-Pb LA-ICPMS analysis on the charnockite samples yielded an age of 2680 to 2500 Ma for the protolith emplacement followed by metamorphism at 2520 to 2450 Ma. Magmatic zircon grains in the quartzo-feldspathic gneiss and metagabbro yielded weighted mean ages of ca. 2557 Ma and ca. 2583 Ma with metamorphism at ca. 2518 Ma. Zircon Lu-Hf data suggest that the protolith of metagabbro was sourced mainly from the recycled Mesoarchaean crustal component. The ƐHf(t) of charnockite (−0.4 to 4.1) and quartzo-feldspathic gneiss (0.5 to 6.0) represent a combined source of the mantle as well as recycling crust. All the rocks underwent amphibolite to granulite facies metamorphism and petrological studies and pseudosection modelling shows a similar P-T range of 7–8 kbar and 650–830°C. Results from the present study suggest that crustal evolution along the eastern flank of the Nallamalai Shear Zone involved melting of older crustal components (Mesoarchaean age) with significant juvenile input (ƐHf(t) = ~+6.0) within a continental arc setting.
Zircon age-Hf isotopic data on the Archean Singhbhum and Dharwar cratons and the Archean-Proterozoic Southern Granulite Terrain (SGT) obtained at the CSIR-NGRI and by others elsewhere are in focus here. These data are used to decipher episodes of juvenile crust formation in the protracted (collectively spanning ∼3.7 billion years) geologic history of the three terranes in the light of their regional geology, structure and deep-crustal architecture based on recent geophysical experiments as well as current perspectives on early Earth crust forming processes and geodynamics. Our important observations and inferences include: (1) the Hf-isotopic compositions of the Hadean-Eoarchean aged (ca. 4.2–3.6 Ga) zircon grains from the Singhbhum craton have distinctly unradiogenic Hf-isotopic compositions quite similar to the Jack Hills Hadean-Eoarchean detrital zircons, suggesting derivation from TTG-like melts generated by the internal reworking of a long-lived, geochemically enriched mafic reservoir formed around ca. 4.5 Ga; (2) a shift to strongly radiogenic zircon Hf isotope compositions during the early Paleoarchean around ca. 3.6–3.5 Ga (Singhbhum craton) and ca. 3.5–3.4 Ga (Western Dharwar craton) is conspicuous. This may relate to the time of development of depleted mantle reservoirs, the source of the voluminous Paleo-Mesoarchean juvenile felsic magmatism and crust formation events that extended for ca. 400–300 million years; (3) in the entire Dharwar craton and the northern parts of the SGT there is clear evidence for widespread juvenile magmatic episodes during the Neoarchean, around ca. 2.7 Ga and ca. 2.55 Ga, the latter being predominant and widespread; (4) in the southernmost part of the SGT, prominent juvenile magmatic episodes are also evident during the Paleoproterozoic (ca. 2.0 Ga, Trivandrum block) and early Neoproterozoic (ca. 1.0–0.9 Ga, in parts of the Madurai block); (5) onset of plate tectonic processes in the Singhbhum and Western Dharwar cratons during early Paleoarchean (ca. 3.6–3.5 Ga) cannot be ruled out, but there is clear evidence for the operation of plate tectonics, significant crustal growth and terrane amalgamation only after ∼3.0 Ga in the Dharwar craton and the SGT and (6) regional dome and basin structural pattern of the pre-3.0 Ga crust attests to the role of internal differentiation processes (Rayleigh-Taylor Inversions) and vertical tectonics for the Paleo-Mesoarchean crust of the Singhbhum and Dharwar cratons. Together with other lines of evidence; changes in bulk crustal composition, deep crustal architecture, zircon age-Hf isotope distribution etc., we infer a transition to plate tectonics around 3.0 Ga in the Singhbhum and Dharwar cratons.
We report petrological and geochronological data for metamafic volcanics and metasediments from the Sargur Greenstone Belt, Western Dharwar Craton (WDC), southern India and discuss the end-Archean metamorphic P–T conditions of the Sargur Greenstone Belt, as well as geotectonic implications of the end-Archean metamorphism in the WDC. The LA-ICP MS zircon U-Pb dating yields a metamorphic age of 2500 ± 20 Ma for the garnet-bearing amphibolite. The garnet-staurolite-kyanite mica schist sample has metamorphic zircon and monazite U-Pb ages of 2443 ± 7 Ma and 2468 ± 10 Ma, respectively, consistent with metamorphic zircon U-Pb ages of 2459 ± 17 Ma and 2455 ± 18 Ma yield by the amphibole-garnet schist samples. The peak mineral assemblage of the garnet-bearing amphibolite is garnet + clinopyroxene + amphibole + plagioclase + quartz + rutile + ilmenite, and that of the metapelitic schist is garnet + staurolite + kyanite + biotite + muscovite + quartz + plagioclase. Phase equilibrium modelling, together with geothermobarometers yield P–T conditions of 7.9–8.9 kbar/681–769 °C and 7.5–9.3 kbar/683–694 °C for the garnet-bearing amphibolite and garnet-staurolite-kyanite mica schist, respectively, corresponding to a medium geothermal gradient of 20–25 °C/km. These data indicate that the Sargur Greenstone Belt has experienced a typical Barrovian-type (amphibolite-facies) metamorphism at ca. 2.50–2.44 Ga. Combined with the previous studies, it is suggested that the Sargur Greenstone Belt was most likely to be formed during 3.35–3.12 Ga and was reworked through four episodes of magmatic-metamorphic events occurred at ca. 3.1 Ga, ca. 3.0 Ga, ca. 2.8–2.6 Ga and ca. 2.5–2.4 Ga. The prominent ca. 2.5–2.4 Ga regional metamorphism is the last stage of the Neoarchean tectono-thermal event in the WDC and thus most possibly indicate accretion-collision of different Archean blocks at ca. 2.5–2.4 Ga and lead to the final cratonization (stabilization) of the whole Dharwar Craton.
In South India, the WNW-striking ~ 130-km-long Achankovil Shear Zone (AKSZ) separating the Trivandrum Block and the Madurai Block is speculated to continue within southern Madagascar in the East Gondwanaland. Two aspects continue to be debated, whether the AKSZ is a terrain boundary shear zone, and if so, when the accretion occurred. We present, for the first time, a detailed field-based analysis of contrasting mesoscale structures and deformation kinematics in the granulite facies rocks across the AKSZ. The mineralogical segregation layering (S2T) in the Trivandrum Block is sub-horizontal to gently inclined and describes F3T regional-scale open asymmetric WNW-trending sub-horizontal folds. By contrast, the S2M granulite facies layering in the Madurai Block describes regional-scale steeply plunging F3M folds. Both blocks share the NNW/NNE-trending sub-parallel sinistral and dextral S3 shears of the AKSZ. Deformation microstructures and garnet-bearing melt-hosted syn-S3 extensional shears attest to the persistence of high temperature during D2–D3 deformations. Monazite chemical dates and existing Pb–Pb zircon dates are overwhelmingly Pan-African in and across the AKSZ, but the Paleoproterozoic (2.1–1.8 Ga) dates in the Trivandrum Block, equivalent to those reported in Anosyan and Androyan domains of southern Madagascar, are lacking in the Madurai Block. By contrast, the Mid Neoproterozoic (800–750 Ma) Pb–Pb zircon dates in the Madurai Block, equivalent to the Antananarivo domain (Central Madagascar), are lacking in the Trivandrum Block. The structural–chronological contrasts between the two blocks suggest the AKSZ to be a Pan-African terrain boundary shear zone system that is continuous with the Ranotsara shear zone in Madagascar. [248]
Mantle-derived magmas at the base of the lower crust exerted a key control on late Archean cratonization in many continents. Since well-preserved, complete lower crustal section is rarely exposed, direct studies on the genetic link between mantle-derived magmas and cratonic lower crustal stabilization are inadequate. Cratonic lower crustal section is well-preserved in the southern margin of the Dharwar Craton (southern India), with a number of late Archean anorthositic-gabbroic complexes. Among these complexes, the Sittampundi anorthosite complex (SAC) consists of white- and dark-anorthosite (>60 vol.%), gabbros, and ultramafic rocks. In this study, SIMS zircon U-Pb dating of the anorthosite revealed a minimum emplacement age of 2522 ± 12 Ma, similar to the chromite Os model ages (2528–2563 Ma) of the anorthosite-hosted chromitite. In-situ plagioclase (⁸⁷Sr/⁸⁶Sr)i ratios (0.70079–0.70100) of the dark anorthosite and the chromite γOs (T) values (-0.2 to -0.4) of the chromitite suggest that the SAC was derived from a depleted mantle source. From the dark to white anorthosite, the (⁸⁷Sr/⁸⁶Sr)i ratios increase while the An contents decrease, suggesting crustal assimilation occurred during fractionation. Similarly, the mantle-like zircon δ¹⁸O values and relatively-wide εHf(T) (-2.1 to +8.4) range of the SAC anorthosite suggest that the parental magma had assimilated the ancient mafic lower crust. Emplacement ages of the SAC and published ages of the mafic/felsic granulites and charnockites altogether indicate that the anorthosites were formed during the Dharwar cratonization, and that the mantle-derived magma underplating may have led to extensive lower crustal melting. We argued that during underplating, high-density olivine-pyroxene cumulates (from fractionation of the mantle-derived magma) and partial-melting residues (in the overlying lower crust) mostly sank back to the underlying mantle. In contrast, the lower-density plagioclase and minor amphibole remained in the lower crust to form anorthositic-gabbroic sills. The magmas underplating and subsequent lower-crustal melting have likely made the cratonic lower crust more refractory and buoyant, which facilitated cratonization.
The Precambrian Peninsular India is comprised of a few ancient cratonic nuclei that were formed during prolonged geological history during Archean to Paleoproterozoic and are classified into two blocks: The North Indian Block (NIB) and the South Indian block (SIB) (Naqvi and Rogers 1987). The former comprises of the Bundelkhand and Aravalli Cratons and the latter is made up of the Dharwar, Bastar and Singhbhum cratons; these are all surrounded by younger Proterozoic Fold Belts. A prominent ENE–WSW trending Central India Tectonic Zone (CITZ) separates these blocks whose fabric extends eastward through Chhotanagpur Plateau, while the isolated Meghalaya makes the sixth craton (Sharma 2009).
We document kinematics and rheological behaviour of brittle shears (~50 cm wide) postdating solid-state tectonic fabric in the Salem–Attur (SASZ) and Bhavani (BSZ) shear zone that constitute a Paleoproterozoic (~2500 Ma) suture juxtaposing disparate granulite blocks in south India. We constrain brittle deformation mechanisms from established relationship between changing orientation of deflected strain marker (quartz vein) and foliation within the shear band with respect to their orientation outside the shear band. Quartz c-axis orientation in charnockite (host lithology) and phyllonite (reworked charnockite) from the SASZ show presence of mixed basal 〈a〉 (low-T) and prism 〈a〉 (high-T) slip, and single basal 〈a〉 slip mechanism, respectively. This suggests considerable cooling of the granulite block prior to the onset of brittle shearing. Distribution of strain parameters – effective shear strain (Γ), shear strain (γ), stretch K2 along intermediate strain axis Y – from margin to the centre of the shear band, show peaked distribution with a single maximum at the shear zone centre. This implies rheological-weakening/strain-softening induced localizing shear zone character. Kinematically heterogeneous strain distribution during brittle shearing varies from transpression dominated for the BSZ to transpression-to-transtension switchover for the SASZ. Demonstrably, contrasting cooling-exhumation, hitherto unexplored, characterizes post-accretionary tectonics along the paleo-suture zone.
Gondwana correlation studies had rationally positioned the western continental margin of India (WCMI) against the eastern continental margin of Madagascar (ECMM), and the eastern continental margin of India (ECMI) against the eastern Antarctica continental margin (EACM). This contribution computes the effective elastic thickness (Te) of the lithospheres of these once-conjugated continental margins using the multitaper Bouguer coherence method. The results reveal significantly low strength values (Te∼2 km) in the central segment of the WCMI that correlate with consistently low Te values (2-3 km) obtained throughout the entire marginal length of the ECMM. This result is consistent with the previous Te estimates of these margins, and confirms the idea that the low-Te segments in the central part of the WCMI and along the ECMM represents paleo-rift inception points of the lithospheric margins that was thermally and mechanically weakened by the combined action of the Marion hotspot and lithospheric extension during the rifting. The uniformly low-Te value (∼2 km) along the EACM indicates a mechanically weak lithospheric margin, probably due to considerable stretching of the lithosphere, considering the fact that this margin remained almost stationary throughout its rift history. In contrast, the ECMI has comparatively high-Te variations (5-11 km) that lack any correlation with the regional tectonic setting. Using gravity forward and inversion applications, we find a leading order of influence of sediment load on the flexural properties of this marginal lithosphere. The study concludes that the thick pile of the Bengal Fan sediments in the ECMI masks and has erased the signal of the original load-induced topography, and its gravity effect has biased the long-wavelength part of the observed gravity signal. The hence uncorrelated flat topography and deep lithospheric flexure together contribute a bias in the flexure modeling, which likely accounts a relatively high Te estimate.
The Nilgiri Block is one of the major Archean crustal blocks that define the tectonic framework of southern India. Here we report geologic, petrologic, geochemical, and zircon U-Pb,-REE, and -Lu-Hf data of a highly metamorphosed and disrupted sequence of amphibolite, meta-gabbro, websterite, volcanic tuff, meta-sediment, and banded iron formation (BIF) from the northern fringe of the Nilgiri Block. Geochemically, the amphibolite shows altered ocean floor basalt signature, whereas the meta-gabbro and the websterite samples form part of a volcanic arc. The metamorphosed volcanic tuff shows subalkaline rhyolitic signature. U-Pb isotope analysis of zircon grains from the volcanic tuff and meta-gabbro shows Pb-207/Pb-206 ages of 2490 +/- 12 Ma and 2448 +/- 16 Ma, respectively. Zircons from the meta-sediments show an age range of 2563 +/- 33 Ma to 2447 +/- 34 Ma. The dominantly positive epsilon Hf (t) values of the zircons in the analyzed rock suite suggest that the magmas from which the zircons crystallized evolved from a Neoarchean depleted mantle source. The Hf model ages (T-DM) of volcanic tuff, meta-sediment and meta-gabbro samples are ranging between 2908-2706 Ma, 2849-2682 Ma, and 2743-2607 Ma, respectively. The ca. 2500 Ma ages for the arc-related magmatic rock suite identified along the northern periphery of Nilgiri Block suggest prominent Neoarchean arc magmatism and early Paleoproterozoic convergent margin processes contributing to the early Precambrian crustal growth in Peninsular India.
Accretion along continental or island arcs at cratonic margins was responsible for most Paleoproterozoic crustal growth. For the development of the Krishna orogen, India, at the southeastern margin of the Eastern Dharwar craton (EDC), two contrasting models, one by long-lived accretion between ~ 1.85 Ga and 1.33 Ga terminating in continental collision with the Napier Complex and the other involving continental collision with the Napier Complex at ~ 1.6 Ga have been proposed. Here we report the geology and geochemistry of the granitoid rocks grouping them into the Vinjamuru arc terrane. These comprise biotite ± hornblende high-silica granite which are mostly calc-alkaline, weakly metaluminous to peraluminous with normalized trace and rare earth element plots resembling those derived from arc sources as seen by negative Nb, Ti, Zr anomalies, enriched LREE and moderate Eu anomalies. On (La/Yb)CN vs YbCN and Sr/Y vs Y discrimination diagrams these rocks plot in the field of liquids from mantle-derived melts resembling Cordilleran type granitoids. Petrography, major oxide and trace element concentrations suggest formation in an arc tectonic setting during convergent tectonics at the active continental margin of the EDC with evidence for crustal assimilation. To generate the observed high-silica granite, using selected trace and REE, we modeled 10% aggregate continuous melting of a lower crustal hydrous, high K2O-bearing gabbro yielding a granodiorite magma that underwent fractional crystallization at mid-to lower crust followed by mixing with country rock tonalite and minor assimilation with metasedimentary crustal rocks resulting in the observed heterogeneity in trace elements from the granite. We interpret Paleoproterozoic paleopostions of component Indian cratons leading to their Mesoproterozoic assembly and in that context relate the crustal growth along the southeastern margin of the EDC. In contrast to the existing two models, we propose an alternative Cordilleran-style accretion involving development of an intra-oceanic arc due to ocean-ward migration following the earlier choking of the subduction zone at an active continental margin, caused probably by the North China crustal ribbon that had by ~ 1.78 Ga accreted to the EDC margin. The formation of the outboard intra-oceanic Ongole arc terrane occurred thereafter and was eventually accreted (and metamorphosed) to the Vinjamuru arc terrane between ~ 1.63 and 1.55 Ga to form the two arc terranes of the Krishna orogen; we discount any continental collision at this stage as tectonics along this margin, post 1.5 Ga, switched to an extensional regime.
The Late Archaean regional amphibolite facies to granulite facies progression in southern Karnataka contains two textural varieties of charnockite (orthopyroxene-bearing quartzofeldspathic rocks). Banded charnockites typified by the banded pyroxene gneisses in the vicinity of Halaguru are thought to be older than the "incipient" coarse-grained charnockilic alteration of amphibolite facies gneisses, as at Kabbaldurga, and similar alteration of the banded charnockites in many places south of there. Whole-rock Rb-Sr isochrons were obtained for banded charnockites from the Chillapura quarry near Halaguru, and for incipient charnockite in amphibolite facies gneiss from a quarry near Honganuru, just east of Chamarajnagar. Large multi-layered samples and individual layers (light versus dark layers) were analysed separately in an attempt to discriminate older and younger metamorphic ages. For both quarries, all data yielded good isochrons with ages near 2.5 Ga, or terminal Archaean (2.50± 0.05 Ga with initial 87Sr/86Sr = 0.70500+ 0.00009 for Chillapura and 2.54± 0.17 Ga, 0.7088± 0.0016 for Honganuru). More than one age of charnockitic metamorphism based on isotope systematics of samples collected from single quarries is not evident. The high initial Sr ratios for both localities suggest, however, that the country rocks in southern Karnataka had a protracted crustal history prior to 2.5 Ga ago. Moreover, when the mean isotopic data of both quarries are plotted on the same diagram, "a regional isochron" close to 3.0 Ga with low initial Sr ratio results. This plausibly corresponds to a crustal accretion age, in accord with the interpretaiton of previous whole-rock isotope studies of this region based on combining data from widely-separated quarries. A widespread high-grade metamorphic event coinciding with, or following shortly after, crustal accretion remains an open possibility.
The high-grade gneiss complex of the Satnur-Halagur-Sivasamudram area in southern Karnataka forms a part of amphibolite-granulite facies transition zone of southern India. The major lithologies are metasediments, amphibolite facies gneisses, foliated charnockites, mafic granulites, granite sheets. All these lithologies show a prominent N-S fabric which appears to have been produced during late Archaean shear deformation. Rb-Sr whole rock isochron and U-Pb zircon and monazite ages suggest that much of the crust accreted during 2.96 Ga magmatic event followed by the 2.5 Ga granulite facies metamorphism; the occurrence of a previous high-grade metamorphism close to 2.9 Ga remains debatable. -from Authors
Hf isotopic compositions of four standard zircons, i.e., GJ-1, Temora, 91500 and Mud Tank, were analyzed using MCICPMS( Neptune) coupled with excimer laser ablation system (New Wave 193nm FX). The 176Hf/177Hf ratios obtained are 0.282006± 24(n=159, 2SD) for GJ-1, 0.282684±46(n=20, 2SD) for Temora, 0.282305±32(n=20, 2SD) for 91500, and 282509±25(n=48, 2SD) for Mud Tank respectively. The results are in excellent agreement with the previously reported data.
Understanding the origin of the continental crust is one of the key objectives of earth sciences because as a land species we owe our existence to continents. In addition, change in the volume of the continental crust and distribution of continents on Earth's surface have profound effects on major
The recognition that Earth history has been punctuated by supercontinents, the assembly and breakup of which have profoundly influenced the evolution of the geosphere, hydrosphere, atmosphere and biosphere, is arguably the most important development in Earth Science since the advent of plate tectonics. But whereas the widespread recognition of the importance of supercontinents is quite recent, the concept of a supercon-tinent cycle is not new and advocacy of episodicity in tectonic processes predates plate tectonics. In order to give current deliberations on the supercontinent cycle some historical perspective, we trace the development of ideas concerning long-term episodicity in tectonic processes from early views on episodic orogeny and continental crust formation, such as those embodied in the chelogenic cycle, through the first realization that such episodicity was the manifestation of the cyclic assembly and breakup of supercontinents, to the surge in interest in supercontinent reconstructions. We then chronicle some of the key contributions that led to the cycle's widespread recognition and the rapidly expanding developments of the past ten years.
Large charnockite massifs occur in the high-grade Southern Granulite Terrain (SGT) and Eastern Ghats Belt (EGB) crustal provinces of Peninsular India. Available geochronological data indicate that the magmatism is episodic, associated with distinct orogenic cycles in the different crustal domains. The geochemical data also indicate a change in composition from trondhjemitic at ∼3.0–2.9 Ga to dominantly tonalitic at ∼2.6–2.5 Ga to tonalitic-granodiorite-granitic at ∼2.0–1.9 Ga to dominantly tonalitic at 1.7–1.6 Ga to quartz monzonitic or tonalitic at ∼1.0–0.9 Ga to granodiorite-granitic at ∼0.8–0.7 Ga. The trondhjemitic and tonalitic end members are metaluminous, magnesian and calcic to calc-alkalic, characteristic of magnesian group charnockites. The granodioritic to granitic end members are metaluminous to slightly peraluminous, ferroan and calc-alkalic to alkali-calcic, characteristic of ferroan group charnockites. The quartz monzonitic end members are metaluminous to peraluminous, magnesian to ferroan and calcic to calc-alkalic, neither characteristic of the magnesian group nor of the ferroan group of charnockites. Based on the occurrence and difference in composition of the charnockite massifs, it is suggested that the charnockite magmatism registers the crustal growth of the Indian plate on its southern (SGT) and eastern (EGB) sides, along active continental margins by accretion of arcs.
Granites form when silica-rich magma intrudes into the crust and slowly crystallizes. Such intrusions are discontinuous in space and episodic in time, resulting in a mosaic of ages for granites from a given region. Although the episodicity of granitoid ages is well known [Condie, 1998; Kemp et al., 2006], how representative samples are of age distributions in the preserved continental crust is unclear. This is because sampling strategies are generally dictated by mineral exploration, accessibility of outcrops, or availability of geologic maps, none of which necessarily leads to an unbiased representation of age distributions in the continental crust. For instance, large age peaks at 2.7 and 1.9 billion years ago in the Canadian and Baltic shields may reflect extensive sampling of these shields [Condie et al., 2009a].
Low-pressure charnockites, their tonalitic precursors, and syn-metamorphic granites in the granulite facies transition zone south of Krishnagiri, Tamil Nadu, India, define single Rb/Sr and Sm/Nd isochrons at, respectively, 2463±65 and 2455±121Ma. Rb depletion occurred at this time, while Sm/Nd ratios were not significatively changed. Common lead ratios are identical for the granitic and tonalitic gneisses as well as the low-pressure charnockites; they are very unradiogenic (206Pb/204Pb: 14.5 to 15.5), indicating U depletion around 2500Ma ago. These data suggest 1) that granulites derive from a 2.5Ga-out crust and 2) the time elapsed between the crust-forming event and the metamorphism was short. The geochronological relationships observed in Tamil Nadu suggest that tonalitic precursors and charnockites may have formed simultaneously in the same geodynamic context (probably involving subduction) during the storage of tonalitic magmas. This category of syn-accretion granulite, often observed in the Archean and Lower Proterozoic, would be genetically different from a second type (possibly illustrated by the Biligirirangan rocks) where the time elapsed between crustal formation and metamorphism is very long. The latter would be explained better by collisional or hot-spot models. -from Authors
This study addresses the tectonic and mechanical significance of crustal-scale strain and shear zone patterns in the South Indian Precambrian lithosphere. It is based on a tectonic map of the Dharwar craton derived from LANDSAT imagery, existing documentation, and our own field observations. We document the contrasted responses of the two main parts of the craton to late Archean (2.56-2.51 Ga) shortening. The old (>3 Ga) lithosphere of the Western Dharwar craton stabilized at 2.61 Ga underwent moderate shortening and strain localization along spaced shear zones. The Eastern Dharwar craton, rejuvenated by late Archean juvenile magmatic accretion, responded to shortening by flowing laterally against the Western Dharwar craton. Shortening operated without significant thickening because of the high buoyancy of the juvenile crust and the very low strength of its mantle lithosphere. The late Archean crustal-scale shear zone network did not accommodate large displacements and only contributed to smoothing out of strain heterogeneities during the latest stages of flow. The shear zone pattern only reflects the symmetry of the regional finite strain field of a wide hot orogen. It does not result from terrain amalgamation but accompanies crustal flow and does not compare with those of modern active margins, which accommodate unidirectional transpression. Our analysis further suggests that the shear zones bounding the craton did not record large horizontal displacements during final assembly of Gondwana but rather transverse shortening and vertical extrusion.
The continental crust is the archive of the geological history of the Earth. Only 7% of the crust is older than 2.5 Ga, and yet significantly more crust was generated before 2.5 Ga than subsequently. Zircons offer robust records of the magmatic and crust-forming events preserved in the continental crust. They yield marked peaks of ages of crystallization and of crust formation. The latter might reflect periods of high rates of crust generation, and as such be due to magmatism associated with deep-seated mantle plumes. Alternatively the peaks are artefacts of preservation, they mark the times of supercontinent formation, and magmas generated in some tectonic settings may be preferentially preserved. There is increasing evidence that depletion of the upper mantle was in response to early planetary differentiation events. Arguments in favour of large volumes of continental crust before the end of the Archaean, and the thickness of felsic and mafic crust, therefore rely on thermal models for the progressively cooling Earth. They are consistent with recent estimates that the rates of crust generation and destruction along modern subduction zones are strikingly similar. The implication is that the present volume of continental crust was established 2-3 Ga ago.
The present day cumulative distribution of areas of continental basement age provinces follows an exponential function. A similar distribution is displayed by the reserves of their associated economic deposits and by the thickness and areal distribution data for the overlying sediments. This exponential pattern, similar to isotope decay systematics, is a result of recycling (s.l.). Each theoretically possible growth model must be complemented by an appropriate average recycling constat in order to generate the observed present day distribution. Computer simulation of the system has been carried out and the results are presented here. -from Authors
Summarizes recent geophysical, geological, geochemical, and remote sensing data that enable a more detailed tectonic interpretation to be made and linked to crustal evolution of the Precambrian shield of S India and Sri Lanka.-after Authors
The occurrence of high-pressure mafic–ultramafic bodies within major shear zones is one of the indicators of paleo-subduction. In mafic granulites of the Andriamena complex (north-eastern Madagascar) we document unusual textures including garnet–clinopyroxene–quartz coronas that formed after the breakdown of orthopyroxene–plagioclase–ilmenite. Textural evidence and isochemical phase diagram calculations in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 system indicate a pressure–temperature (P–T) evolution from an isothermal (780 °C) pressure up to c. 24 kbar to decompression and cooling. Such a P–T trajectory is typically attained in a subduction zone setting where a gabbroic/ultramafic complex is subducted and later exhumed to the present crustal level during oceanic closure and final continental collision. The present results suggest that the presence of such deeply subducted rocks of the Andriamena complex is related to formation of the Betsimisaraka suture. LA-ICPMS U–Pb zircon dating of pelitic gneisses from the Betsimisaraka suture yields low Th/U ratios and protolith ages ranging from 2535 to 2625 Ma. A granitic gneiss from the Alaotra complex yields a zircon crystallization age of ca. 818 Ma and Th/U ratios vary from 1.08 to 2.09. K–Ar dating of muscovite and biotite from biotite–kyanite–sillimanite gneiss and garnet–biotite gneiss yields age of 486 ± 9 Ma and 459 ± 9 Ma respectively. We have estimated regional crustal thicknesses in NE Madagascar using a flexural inversion technique, which indicates the presence of an anomalously thick crust (c. 43 km) beneath the Antananarivo block. This result is consistent with the present concept that subduction beneath the Antananarivo block resulted in a more competent and thicker crust. The textural data, thermodynamic model, and geophysical evidence together provide a new insight to the subduction history, crustal thickening and evolution of the high-pressure Andriamena complex and its link to the terminal formation of the Betsimisaraka suture in north-eastern Madagascar.
The granulites of the Biligiri Rangan (BR) and Niligiri Hill ranges are characterised by distinct field, geochemical and isotopic signatures. The B.R. Hill granulites contain a significant metasedimentary component (pelite-BIF-Mn-horizons), whilst metasediments are rare in the Niligiri granulite terrain. Lithologic and structure continuity between the two terrains cannot be demonstrated as they are separated by E-W running Moyar shear zone. Isotopic data suggest that B.R. Hills and Niligiri granulites have had different pre-crustal histories. Sm-Nd model TDM ages and U-Pb zircon concordia upper intercept age suggest that the protoliths of B.R. Hill quartzo-feldspathic charnockites accreted at 3.4 Ga. On the other hand, Sm-Nd model TDM ages indicate that the magmatic protoliths of Niligiri granulites differentiated from mantle at 2.6 Ga. -from Authors
The Nilgiri Block, southern India is an exhumed lower crust formed through arc magmatic processes in the Neoarchean. The main lithologies in this terrane include charnockites, gneisses, volcanic tuff, metasediments, banded iron formation and mafic–ultramafic bodies. Mafic–ultramafic rocks are present towards the northern and central part of the Nilgiri Block. We examine the evolution of these mafic granulites/metagabbros by phase diagram modeling and U–Pb sensitive high resolution ion microprobe (SHRIMP) dating. They consist of a garnet–clinopyroxene–plagioclase–hornblende–ilmenite ± orthopyroxene ± rutile assemblage. Garnet and clinopyroxene form major constituents with labradorite and orthopyroxene as the main mineral inclusions. Labradorite, identified using Raman analysis, shows typical peaks at 508 cm− 1, 479 cm− 1, 287 cm− 1 and 177 cm− 1. It is stable along with orthopyroxene towards the low-pressure high-temperature region of the granulite facies (M1 stage). Subsequently, orthopyroxene reacted with plagioclase to form the peak garnet + clinopyroxene + rutile assemblage (M2 stage). The final stage is represented by amphibolite facies–hornblende and plagioclase–rim around the garnet–clinopyroxene assemblage (M3 stage). Phase diagram modeling shows that these mafic granulites followed an anticlockwise P–T–t path during their evolution. The initial high-temperature metamorphism (M1 stage) was at 850–900 °C and ~ 9 kbar followed by high-pressure granulite facies metamorphism (M2 stage) at 850–900 °C and 14–15 kbar. U–Pb isotope studies of zircons using SHRIMP revealed late Neoarchean to early paleoproterozoic ages of crystallization and metamorphism respectively. The age data shows that these mafic granulites have undergone arc magmatism at ca. 2539.2 ± 3 Ma and high-temperature, high-pressure metamorphism at ca. 2458.9 ± 8.6 Ma. Thus our results suggests a late Neoarchean arc magmatism followed by early paleoproterozoic high-temperature, high-pressure granulite facies metamorphism due to the crustal thickening and suturing of the Nilgiri Block onto the Dharwar Craton.
The thermodynamics of order-disorder in mineral solid solutions are handled with symmetric formalism, whereby the mineral is treated as a solid solution between an independent set of end-members with which the range of composition and states of ordering of the phase can be represented. An n-component mineral, requiring s independent order parameters to represent the state of order in the mineral, involves an independent set of n + s end-members. Symmetric formalism involves ideal mixing-on-sites with regular-solution activity coefficients. It is applied Io omphacite, orthopyroxene, ferromagnesian clinoamphibole, and alkali feldspar. The model for omphacite, with a single order parameter, successfully produces the topology of paired irascibility gaps with tricritical points at their apices and with a critical curve connecting them. Ferromagnesian orthopyroxene is shown to behave effectively as an ideal solution at all geologically relevant temperatures. Cummingtonite-grunerite solid solutions are slightly positively nonideal in either a two-site or a three-site model. Na-K alkali feldspars with order-parameter coupling involving tetrahedral site occupancies can show the essential topologic relationships in this system, with only one independent binary interaction energy. The power of symmetric formalism comes from the simplicity of its representation of the thermodynamics of minerals and its flexibility with few adjustable parameters.
New limits have been set on the age of the provenance and the depositional period of the oldest known Archaean supracrustal rocks (Sargur Group) in southern India. Detrital zircon grains from a pelitic schist and a quartzite within major tracts of supracrustal rocks older than their host regional grey orthogneisses (Peninsular Gneiss, c.3000-2900 Ma) have yielded U-Pb ages in the range 3580-2960 Ma. The data indicate that granitoid rocks in the age range 3580-3130 Ma were a significant component of the provenance of the sedimentary protoliths. Exhumation of the granitoid provenance, deposition of the sedimentary protoliths, intrusion of major gabbroic and peridotitic complexes and possible basaltic volcanism took place in the period 3130-2960 Ma. This age range is at variance with previous suggestions that the Sargur Group represents early Archaean or primitive crust. -from Authors
The Archaean Earth is considered to have been characterized by microcontinents that formed, dominantly, through the accretion of oceanic arcs and plateaus. The North China Craton (NCC) provides a typical case where at least six ancient microcontinental nuclei with distinct lithological features and independent tectonic histories were amalgamated into the cratonic framework at the end of the Archaean. Here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks, the Jiaoliao Block that forms part of the composite Eastern Block of the NCC. We present petrological, geochemical and zircon U-Pb geochronological data from the Taipingzhai charnockite suite, and associated amphibolites, metagabbros and orthogneisses from the Qianxi Complex. Geochemically the rocks show a wide range of SiO2 (charnockite suite: 52.57-75.50 wt.%; metagabbro: 43.71wt.%; amphibolite: 50.24 wt.%; garnet-bearing biotite: 63.73 wt.%), and MgO (charnockite suite: 0.89-5.01 wt.%; metagabbro: 3.99 wt.%; amphibolite: 6.23 wt.%; garnet-bearing biotite: 2.08 wt.%). The composition of the felsic units straddle from diorite through syeno-diorite to granite with both alkalic and subalkalic affinity, with dominantly magnesian composition and arc-related features. Their immobile trace element relationships suggest calc-alkaline affinity. They show positive Pb, Ba, La, Nd, and Gd and negative Nb, Ta, Sr, Th and Ti anomalies with slightly negative anomalies of Ce and Y, attesting to arc-related features. In tectonic classification diagrams, the rocks plot in the VAG+syn-COLG field or the VAG area suggesting subduction-related origin.
The Archaean Earth is considered to have been characterized by microcontinents that formed, dominantly,
through the accretion of oceanic arcs and plateaus. The North China Craton (NCC) provides a typical case
where at least seven ancient microcontinental nuclei with distinct lithological features and independent tectonic
histories were amalgamated into the cratonic framework at the end of the Archaean. Herewe investigate a suite
ofmagmatic rocks developed at the periphery of one of these microblocks, the Jiaoliao Block that forms part of the
composite Eastern Block of the NCC. We present petrological, geochemical and zircon U–Pb geochronological
data from the Taipingzhai charnockite suite, and associated amphibolites, metagabbros and orthogneisses from
the Qianxi Complex. Geochemically the rocks show a wide range of SiO2 (charnockite suite: 52.57–75.50 wt.%;
metagabbro: 43.71 wt.%; amphibolite: 50.24 wt.%; garnet-bearing biotite: 63.73 wt.%), and MgO (charnockite
suite: 0.89–5.01 wt.%; metagabbro: 3.99 wt.%; amphibolite: 6.23 wt.%; garnet-bearing biotite: 2.08 wt.%). The
composition of the felsic units straddle from diorite through syeno-diorite to granite with both alkalic and
subalkalic affinity, with dominantly magnesian composition and arc-related features. Their immobile trace element
relationships suggest calc-alkaline affinity. They show positive Pb, Ba, La, Nd, and Gd and negative Nb, Ta,
Sr, Th and Ti anomalies with slightly negative anomalies of Ce and Y, attesting to arc-related features. In tectonic
classification diagrams, the rocks plot in the VAG + syn-COLG field or the VAG area suggesting subductionrelated
origin.
The dominant population of zircons in all these rocks displays magmatic crystallization features including high
Th/U values with core-rims textures indicating subsequent thermal events. The zircon U–Pb data yield upper
intercept ages of 2587 ± 10 Ma to 2543 ± 17 Ma and 207Pb/206Pb mean ages of 2578 ± 7.3 Ma to 2536 ±
8 Ma for the charnockite suite, marking the timing of emplacement of the arc magmas. The overgrowth rims
as well as discrete neoformed grains are interpreted as dating subsequent metamorphism and yield
207Pb/206Pb ages between 2533 Ma to 2490 Ma. Zircons in the metagabbro preserve upper intercept ages of
2556± 20 Ma representing the crystallization age of this rock. The younger ages of 2449 ±58 Ma (upper intercept
age) and 1845 ± 25 Ma (207Pb/206Pb spot age) are interpreted to represent subsequent multiple thermal
events in this area. Zircons in the amphibolite preserve the 207Pb/206Pb mean age of 2539 ± 9 Ma, representing
the crystallization age of this rock. The garnet-bearing biotite gneiss shows an upper intercept age of 2562 ±
10 Ma (MSWD = 0.66; N = 36) and the 207Pb/206Pb mean age of 2561 ± 9 Ma (MSWD = 0.63; N = 33)
which is taken to represent the crystallization age of this rock. Some inherited zircons are also identified with
207Pb/206Pb ages of 2664 ± 26 Ma and 2628 ± 26 Ma. Zircon Lu–Hf data show dominantly positive
εHf(t) values and combinedwith crustal residence ages, the results suggestMesoarchean to Neoarchean juvenile
crust formation in the NCC.We interpret the data presented here to represent a phase ofmajor late Neoarchaean
arc magmatism along the western margin of the Jiaoliao Block related to the birth of microcontinental nuclei
within the NCC. Our data suggest that the Western and Eastern Blocksmight not have existed as discrete crustal blocks, and that the construction of the NCC is a result of the assembly of several microblocks or terranes at the end of Archaean. Similar Archean cratonic nuclei in other regions of the world might have formed part of a primitive supercontinent in the early Earth.
The International Mineralogical Association's approved amphibole nomenclature has been revised in order to simplify it, make it more consistent with divisions generally at 50%, define prefixes and modifiers more precisely and include new amphibole species discovered and named since 1978, when the previous scheme was approved. The same reference axes form the basis of the new scheme and most names are little changed but compound species names like tremolitic hornblende (now magnesiohornblende) are abolished and also crossite (now glaucophane or ferroglaucophane or magnesioriebeckite or riebeckite), tirodite (now manganocummingtonite) and dannemorite (now manganogrunerite). The 50% rule has been broken only to retain tremolite and actinolite as in the 1978 scheme so the sodic calcic amphibole range has therefore been expanded. Alkali amphiboles are now sodic amphiboles. The use of hyphens is defined. New amphibole names approved since 1978 include nyböite, leakeite, kornite, ungarettiite, sadanagaite and cannilloite. All abandoned names are listed. The formulae and source of the amphibole end member names are listed and procedures outlined to calculate Fe ³⁺ and Fe ²⁺ when not determined by analysis.
The evolution of continental crust during Hadean and Archean and related geodynamic processes provides important clues to understand the early Earth history. Here we report evidence for Hadean and Eoarchean crust from the fringe of the Coorg Block, one of the oldest crustal blocks in Peninsular India. We present geological, petrological, and geochemical data, together with zircon U-Pb ages and Lu-Hf isotopes from a suite of metaigneous (granitoids, diorite, charnockite, metavolcanics) and metasedimentaty (quartz mica schist, calcareous schist, ferruginous quartzite and BIF) rocks. Petrological and geochemical studies indicate that the igneous suite formed from subduction-related arc magmatism, and that the sedimentary suite represents an imbricated accretionary package of continental shelf sequence and pelagic components. Mineral thermometry suggests metamorphism under temperatures of 710-730 degrees C and pressures up to 8 kbar. Magmatic zircons in the metaigneous suite show oscillatory zoning and high Th/U contents (up to 3.72) and record multiple pulses of magmatism at ca. 3.5, 32, 2.7 and 2.5-2.4 Ga. The metasedimentary rocks accreted along the margins of the Coorg Block show multiple zircon population with mean Pb-207/Pb-206 ages at 3.4, 3.2, 3.1, 2.9, 2.7, 2.6, 2.5, 2.2, 2.0, and 1.3 Ga, and overprinted by younger thermal event at ca. 0.8-0.7 Ga. Zircons in the 3.5 Ga dioritic gneiss show positive epsilon Hf-(t) values ranging from 0.0 to 4.2 and Hf crustal model ages (T-DM
The formation and growth of continental crust in the Archean have been evaluated through mod-els of subduction–accretion and mantle plume. The Nilgiri Block in southern India exposes exhumed Neoarchean lower crust, uplifted to heights of ∼2500 m above sea level along the north western margin of the Peninsula. Major lithologies in this block include charnockite with or without garnet, anorthosite-gabbro suite, pyroxenite, amphibolite and hornblende-biotite gneiss (TTG). All these rock types are closely associated as an arc magmatic suite, with diffuse boundaries and coeval nature. The charnockite and hornblende-biotite gneisses (TTG) show SiO 2 content varying from 64 to 73 wt.%. The hornblende-biotite gneisses (TTG) are high-Al type with Al 2 O 3 >15 wt.% whereas the charnockites show Al 2 O 3 <15 wt.%. The composition of charnockite is mainly magnesian and calcic to calc-alkaline. The mafic-ultramafic rocks show composition close to that of tholeiitic series. The low values of K 2 O (<3 wt.%), (K/Rb)/K 2 O (<500), Zr/Ti, and trace element ratios like (La/Yb) n /(Sr/Y), (Y/Nb), (Y + Nb)/Rb, (Y + Ta)/Rb, Yb/Ta indicate a vol-canic arc signature for these rocks. The geochemical signature is consistent with arc magmatic rocks generated through oceanic plate subduction. The primitive mantle normalized trace element patterns of these rocks display enrichment in large ion lithophile elements (LILE) and comparable high field strength elements (HFSE) in charnockite and hornblende-biotite gneisses (TTG) consistent with subduction-related origin. Primitive mantle normalized REE pattern displays an enrichment in LREE in the charnockite and hornblende-biotite gneisses (TTG) as compared to a flat pattern for the mafic rocks. The chondrite normalized REE patterns of zircons of all the rock types reveal cores with high HREE formed at ca. 2700 Ma and rims with low HREE formed at 2500–2450 Ma. Log-transformed La/Th–Nb/Th–Sm/Th–Yb/Th discrim-ination diagram for the mafic and ultramafic rocks from Nilgiri displays a transition from mid-oceanic ridge basalt (MORB) to island arc basalt (IAB) suggesting a MORB source. The U–Pb zircon data from the charnockites, mafic granulites and hornblende-biotite gneisses (TTG) presented in our study show that the magma generation during subduction and accretion events in this block occurred at 2700–2500 Ma. Together with the recent report on Neoarchean supra-subduction zone ophiolite suite at its southern mar-gin, the Nilgiri Block provides one of the best examples for continental growth through vertical stacking and lateral accretion in a subduction environment during the Neoarchean.
The present study contributes new constraints on, and definitions of, the reconstructed plate margins of India and Madagascar based on flexural isostasy along the Western Continental Margin of India (WCMI) and the Eastern Continental Margin of Madagascar (ECMM). We have estimated the nature of isostasy and crustal geometry along the two margins, and have examined their possible conjugate structure. Here we utilize elastic thickness (Te) and Moho depth data as the primary basis for the correlation of these passive margins. We employ the flexure inversion technique that operates in spatial domain in order to estimate the spatial variation of effective elastic thickness. Gravity inversion and flexure inversion techniques are used to estimate the configuration of the Moho/Crust-Mantle Interface that reveals regional correlations with the elastic thickness variations. These results correlate well with the continental and oceanic segments of the Indian and African plates. The present study has found a linear zone of anomalously low-Te (1-5. km) along the WCMI (~1680. km), which correlates well with the low-Te patterns obtained all along the ECMM. We suggest that the low-Te zones along the WCMI and ECMM represent paleo-rift inception points of lithosphere thermally and mechanically weakened by the combined effects of the Marion hotspot and lithospheric extension due to rifting. We have produced an India-Madagascar paleo-fit representing the initial phase of separation based on the Te estimates of the rifted conjugate margins, which is confirmed by a close-fit correlation of Moho geometry and bathymetry of the shelf margins. The matching of tectonic lineaments, lithologies and geochronological belts between India and Madagascar provide an additional support for the present plate reconstruction.
The architecture of accretionary orogens is a key to understand continental growth. Here we present an overview of the orogenic components and their amalgamation in the western Central Asian Orogenic Belt (CAOB). The CAOB records the convergence and interactions among various types of orogenic components including the Japan-type, Mariana-type, and Alaska-Aleutian-type arc systems, as well as the active marginal sequences of the Siberia Craton, which incorporated wide accretionary complexes and accreted arcs and terranes. During construction of the CAOB, the Kazakhstan arc chain was characterized by multiple subduction, whereas the northern fringe of the Tarim Craton remained mostly as a passive margin. The multiple convergence and accretions among these various orogenic components generated huge orogenic collages in the late Paleozoic and even in the early Triassic, involving parallel amalgamation, circum-microcontinent amalgamation and oroclinal bending. The preservation of trapped basins played a significant role in orogenesis with some parts of the oceanic plate being subducted and others behaving as rigid units. The orogenesis in the CAOB was long-lived, lasting for more than 800 m.y., involving multiple-subduction and long, continuous accretion, and featuring the complexity of accretionary orogenesis and continent growth.
A new evaluation of the elastic thickness (Te) structure of the Indian Shield, derived from isotropic fan wavelet methodology, documents spatial variations of lithospheric deformation in different tectonic provinces correlated with episodic tectono-thermal events. The Te variations corroborated by shear velocity, crustal thickness, and seismogenic thickness reveal the heterogeneous rheology of the Indian lithosphere. The thinned, attenuated lithosphere beneath Peninsular India is considered to be the reason for its mechanically weak strength (< 30 km), where a decoupled crust-mantle rheology under different surface/subsurface loading structures may explain the prominent low Te patterns. The arcuate Te structure of the Western Dharwar province and a NNE-trending band of low Te anomaly in the Southern Granulite Terrane are intriguing patterns. The average Te values (40–50 km) of the Central Indian Tectonic Zone, the Bastar Craton, and the northern Eastern Ghats Mobile Belt are suggestive of old, stable, Indian lithosphere, which was not affected by any major tectono-thermal events after cratonic stabilization. We propose that the anomalously high Te (60–85 km) and high S-wave velocity zone to the north of the Narmada-Son lineament, mainly in NW Himalaya, and the northern Aravalli and Bundelkhand Cratons, suggest that Archean lithosphere characterized by a high velocity mantle keel supports the orogenic topographic loads in/near the Himalaya. The Te map clearly segments the volcanic provinces of the Indian shield, where the signatures of the Reunion, Marion, and Kerguelen hotspots are indicated by significantly low Te patterns that correlate with plume- and rift-related thermal and mechanical rejuvenation, magmatic underplating, and crustal necking. The correlations between Te variations and the occurrence of seismicity over seismically active zones reveal different causal relationships, which led to the current seismogenic zonation of the Indian Shield.
Peninsular India forms a keystone in Gondwana, linking the East African and Malagasy orogens with Ediacaran–Cambrian orogenic belts in Sri Lanka and the Lützow Holm Bay region of Antarctica with similar aged belts in Mozambique, Malawi and Zambia. Ediacaran–Cambrian metamorphism and deformation in the Southern Granulite Terrane (SGT) reflect the past tectonic setting of this region as the leading vertex of Neoproterozoic India as it collided with Azania, the Congo–Tanzania–Bangweulu Block and Kalahari on one side and the Australia/Mawson continent on the other. The high-grade terranes of southern India are made up of four main tectonic units; from north to south these are a) the Salem Block, b) the Madurai Block, c) the Trivandrum Block, and d) the Nagercoil Block. The Salem Block is essentially the metamorphosed Dharwar craton and is bound to the south by the Palghat-Cauvery shear system — here interpreted as a terrane boundary and the Mozambique Ocean suture. The Madurai Block is interpreted as a continuation of the Antananarivo Block (and overlying Palaeoproterozoic sedimentary sequence — the Itremo Group) of Madagascar and a part of the Neoproterozoic microcontinent Azania. The boundary between this and the Trivandrum Block is the Achankovil Zone, that here is not interpreted as a terrane boundary, but may represent an Ediacaran rift zone reactivated in latest Ediacaran–Cambrian times.
Sandwiched between the Dharwar Craton in the north and the Neoarchean–Proterozoic crustal blocks to the south, the Coorg Block in southern India is composed dominantly of a suite of arc magmatic rocks including charnockites, TTG (tonalite–trondhjemite–granodiorite)-related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block. Coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block. Here we present the petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of all the major lithologies from this block. Com-putation of metamorphic P–T conditions from mineral chemical data shows consistent granulite-facies P–T con-ditions of 820–870 °C and up to 6 kbar. Our geochemical data from major, trace and REE on representative samples of the dominant rock types from the Coorg Block corroborate an arc-related signature, with magma gen-eration in a convergent margin setting. The zircon data yield weighted mean 207 Pb/ 206 Pb ages of 3153.4 ± 9 to 3184.0 ± 5.5 Ma for syenogranites, 3170.3 ± 6.8 Ma for biotite granite, 3275 ± 5.1 Ma for trondhjemite, 3133 ± 12 to 3163.8 ±6.9 Ma for charnockites, 3156 ± 10 to 3158.3 ± 8.2 for mafic enclaves, 3161 ± 16 Ma for diorite and 3173 ± 16 Ma for felsic volcanic tuff. An upper intercept age of 3363 ± 59 Ma and a lower inter-cept age of 2896 ± 130 Ma on zircons from a charnockite, as well as an evaluation of the Th/U values of the zircon domains against respective 207 Pb/ 206 Pb ages suggest that the Mesoarchean magma emplacement which probably ranged from N3.3 to 3.1 Ga was immediately followed by metamorphism at ca. 3.0 to 2.9 Ga. The ages of magmatic zircons from the charnockites and their mafic granulite enclaves, as well as those from the vol-canic tuff and biotite granite, are all remarkably consistent and concordant marking ca. 3.1 Ga as the peak of subduction-related crust building in this block, within the tectonic milieu of an active convergent margin. The majority of zircons from the Coorg rocks show Hf isotope features typical of crystallization from magmas derived from juvenile sources. Their Hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8 Ga. The crustal blocks in the Southern Granulite Terrane in India preserve strong imprints of major tectonothermal events at 2.5 Ga, 2.0 Ga, 0.8 Ga and 0.55 Ga associated with various subduction–accretion–collision or rifting events. However, the Coorg Block is exceptional with our data suggesting that none of the above events affected this block. Importantly, there is also no record in the Coorg Block for the 2.5 Ga pervasive regional metamorphism that affected all the other blocks in this region. The geo-chronological data raise the intriguing possibility that this block is an exotic entity within the dominantly Neoarchean collage in the northern domain of the Southern Granulite Terrane of India. The Mesoarchean arc-related rocks in the Coorg Block suggest that the magma factories and their tectonic architecture in the Early Earth were not markedly different from those associated with the modern-style plate tectonics.
The Palghat-Cauvery Suture Zone (PCSZ) in southern India defines the trace of the collisional suture developed during the assembly of the Gondwana supercontinent through the closure of the Mozambique Ocean in the Late Neoproterozoic–Cambrian. Here we report Secondary Ion Mass Spectrometry (SIMS) U–Pb ages from zircons in plagiogranites and gabbros from a suprasubduction zone ophiolitic complex at Manamedu, located along the southern periphery of the PCSZ. The morphology and internal structures, together with the high Th/U values of the zircons from the plagiogranite suggest a magmatic crystallization history. The dominant population of zircons in the two plagiogranite samples analyzed in this study yield 206Pb/238U ages of 737 ± 23 Ma and 782 ± 24 Ma corresponding to the timing of emplacement of these rocks. The plagiogranite from Manamedu also contains two other zircon populations: the first group shows a discordant population with 207Pb/206Pb ages between 2278 and 2527 Ma with an upper intercept age of 2418 ± 65 Ma. Similar Neoarchean–early Paleoproterozoic ages have been widely reported from the surrounding rocks within the PCSZ and also from the Salem Block to the north. We interpret these older zircons as xenocrysts entrained in the plagiogranite during magma ascent and consolidation. The third group of zircons in the plagiogranite yield 206Pb/238U age of 513 ± 4.6 Ma, comparing well with the Cambrian ages reported in several recent studies from magmatic and metamorphic rocks in the PCSZ and the crustal blocks to the south, correlating with the tectono-thermal events associated with the collision and post-collisional extension associated with the assembly of the Gondwana supercontinent. Zircons in the two gabbro samples from the Manamedu complex analyzed in this study yield well-defined clusters on the concordia and show weighted mean 206Pb/238U ages of 744 ± 11 Ma and 786 ± 7.1 Ma. The internal structure of the zircons from these gabbro samples and their high Th/U values also suggest a magmatic crystallization history. The zircon ages that we obtained from the Manamedu forearc complex also compare with the recently reported zircon age of 825 ± 17 from the gabbro–anorthosite complex and the 766 ± 8 Ma age from oscillatory-zoned euhedral crystals of magmatic zircons in felsic volcanic suite of the Kadavur Dome, to the south of Manamedu. The ages also compare with the 819 ± 26 Ma 206Pb/238U age reported from zircons in arc-related rapakivi granite from an adjacent locality within the PCSZ. All these data suggest a prominent mid Cryogenian subduction system along the southern periphery of the PCSZ prior to the destruction of the Mozambique Ocean lithosphere and the final amalgamation of the Gondwana supercontinent.
Zircon megacrysts represent a late stage in the crystallisation of the magmas that produced the low-Cr megacryst suite (Ol+Opx+Cpx+Gnt+Ilm+Phl+Zir) found in many kimberlites, and may carry information on the sources of the parent magmas and the interaction of these magmas with the cratonic lithosphere. The isotopic composition of Hf has been measured in 124 mantle-derived zircon megacrysts from African, Siberian and Australian kimberlites, using a laser-ablation microprobe (LAM) and a multi-collector (MC) ICPMS. The zircons range in age from 90 Ma to ca 2500 Ma, allowing indirect analysis of mantle-derived Hf over a long time span. Most values of εHf fall between 0 and +10, but zircon suites from several kimberlites range down to εHf = −16. Combined with published Nd data on the silicate members of the low-Cr megacryst suite, these data indicate crystallisation of zircon from magmas lying well below the terrestrial εHf-εNd array. LAM-ICPMS analyses of garnets and clinopyroxenes from mantle-derived peridotite xenoliths suggest that cratonic lithospheric mantle has Hf/Nd (0.3–0.5) greater than estimated Bulk Silicate Earth. The depleted and metasomatised lherzolites and harzburgites that make up much of the Archean lithospheric mantle have Lu/Hf ratios (≤0.15) low enough to account for the lowest εHf observed in the zircons, over time spans of 1–3.5 Ga. We therefore suggest that the magmas from which the kimberlitic zircons crystallised were derived from Depleted Mantle or OIB-type sources, and developed negative εHf through reaction with the subcontinental lithospheric mantle.
We report detailed evidence for a new paleo-suture zone (the Kumta suture) on the western margin of southern India. The c. 15-km-wide, westward dipping suture zone contains garnet-biotite, fuchsite-haematite, chlorite-quartz, quartz-phengite schists, biotite augen gneiss, marble and amphibolite. The isochemical phase diagram estimations and the high-Si phengite composition of quartz-phengite schist suggest a near-peak condition of c. 18 kbar at c. 550 degrees C, followed by near-isothermal decompression. The detrital SHRIMP U-Pb zircon ages from quartz-phengite schist give four age populations ranging from 3280 to 2993 Ma. Phengite from quartz-phengite schist and biotite from garnet-biotite schist have K-Ar metamorphic ages of ca. 1326 and ca. 1385 Ma respectively. Electron microprobe-CHIME ages of in situ zircons in quartz-phengite schist (ca. 3750 Ma and ca. 1697 Ma) are consistent with the above results. The Bondla ultramafic-gabbro complex in the west of the Kumta suture compositionally represents an arc with K-Ar biotite ages from gabbro in the range 1644-1536 Ma. On the eastern side of the suture are weakly deformed and unmetamorphosed shallow westward-dipping sedimentary rocks of the Sirsi shelf, which has the following upward stratigraphy: pebbly quartzite/sandstone, turbidite, magnetite iron formation, and limestone; farther east the lower lying quartzite has an unconformable contact with ca. 2571 Ma quartzo-feldspathic gneisses of the Dharwar block with a ca. 1733 Ma biotite cooling age. To the west of the suture is a c. 60-km-wide Karwar block mainly consisting of tonalite-trondhjemite-granodiorite (TTG) and amphibolite. The TTGs have U-Pb zircon magmatic ages of ca. 3200 Ma with a rare inherited core age of ca. 3601 Ma. The K-Ar biotite cooling age from the TTGs (1746 Ma and 1796 Ma) and amphibolite (ca. 1697 Ma) represents late-stage uplift. Integration of geological, structural and geochronological data from western India and eastern Madagascar suggest diachronous ocean closure during the amalgamation of Rodinia; in the north at around ca. 1380 Ma, and a progression toward the south until ca. 750 Ma. Satellite imagery based regional structural lineaments suggests that the Betsimisaraka suture continues into western India as the Kumta suture and possibly farther south toward a suture in the Coorg area, representing in total a c. 1000 km long Rodinian suture.
Altimeter data from the entire ERS-1 and GEOSAT Geodetic Missions have been used in an evaluation of the Earth gravity field. This map is a new and revised version of the Andersen & Knudsen, [1995] global altimetric gravity map from the ERS-1 geodetic mission altimetry. Focus on the gravity field in the Mediterranean sea will be made in this presentation. Furthermore comparison with sea gravity in the Eastern Mediterranean Sea will be presented.
The high-grade rocks of the Southern Granulite Terrain (SGT) of Peninsular India are bounded to the north by the Archean Dharwar Craton. Another high-grade terrane, the Mesoproterozoic Eastern Ghats, occurs to the northeast of the SGT. The tectonic relationship between these crustal domains is complex. We present new geochronological and structural data that indicate a continuation of the Dharwar Craton into the Southern Granulite Terrain as far south as a newly identified Neoproterozoic shear zone, here named the Karur-Kamban-Painavu-Trichur Shear Zone (KKPTSZ). South of the KKPTSZ, Mesoproterozoic dates of the SGT are similar to those recorded in the Eastern Ghats, and the two domains may have been conterminous. Thirty-three new U/Pb/Th single zircon and monazite dates of samples from six structural transects across the regional shear zones indicate that the SGT has experienced at least seven thermo-tectonic events at 2.5 Ga, ~2.0 Ga, ~1.6 Ga, ~1.0 Ga, ~800 Ma, ~600 Ma, and ~550 Ma, and two distinct episodes of metasomatism/charnockitization between 2.50-2.53 and between 0.55-0.53 Ga. Deformation along a number of major shear zones in the SGT is Neoproterozoic to earliest Paleozoic in age, with an early phase (D2) concentrated between 700-800 Ma, and a later phase (D3) between 550 and 600 Ma. Major charnockitization (530-550 Ma) post dates D3, and is, in turn, overprinted by granitization, retrogression, and uplift between 525 and 480 Ma. The KKPTSZ, active between 560 and 570 Ma, is either a terrane boundary, or a tectonized décollement between cover and Archean basement rocks represented by predominantly paragneisses to the south and orthogneisses to the north, respectively. Other regional Neoproterozoic shear zones do not appear to separate allochthonous terranes as previously suggested on the basis of Nd model ages and Rb/Sr biotite/whole rock dates. The Neoproterozoic-Cambrian tectonothermal history of the SGT and Eastern Ghats is similar to that recorded in parts of Madagascar, East Africa, and Antarctica, and is used to reconstruct parts of central Gondwana, here named the Deccan Continent, with more robust confidence.
The enthalpies of solution of a suite of 19 high-structural state synthetic plagioclases were measured in a Pb 2 B 2 O 5 melt at 970 K. The samples were crystallized from analyzed glasses at 1200°C and 20 kbar pressure in a piston-cylinder apparatus. A number of runs were also made on Amelia albite and Amelia albite synthetically disordered at 1050-1080°C and one bar for one month and at 1200°C and 20 kbar for 10 hr. The component oxides of anorthite, CaO, Al 2 O 3 and SiO 2 , were remeasured. The H of disorder of albite inferred in the present study from albite crystallized from glass is 3.23 kcal, which agrees with the 3.4 found by and (1968). It is not certain whether this value includes the H of a reversible displacive transition to monoclinic symmetry, as suggested by et al . (1978) for the Holm-Kleppa results. The enthalpy of solution value for albite accepted for the solid solution series is based on the heat-treated Amelia albite and is 2.86 kcal less than for untreated Amelia albite. The enthalpy of formation from the oxides at 970 K of synthetic anorthite is -24.06 ± 0.31 kcal, significantly higher than the -23.16 kcal found by et al . (1978), and in good agreement with the value of -23.89 ± 0.82 given by et al . (1979), based on acid calorimetry. The excess enthalpy of mixing in high plagioclase can be represented by the expression, valid at 970 K: H ex (±0.16 kcal ) = 6.7461 X ab X 2 An + 2.0247 X An X 2 Ab where X Ab and X An are, respectively, the mole fractions of NaAlSi 3 O 8 and CaAl 2 Si 2 O 8 . This H ex , together with the mixing entropy of and (1975) Al-avoidance model, reproduces almost perfectly the free energy of mixing found by (1972) in aqueous cation-exchange experiments at 700°C. It is likely that Al-avoidance is the significant stabilizing factor in the high plagioclase series, at least for X An 0.3. At high temperatures the plagioclases have nearly the free energies of ideal one-site solid solutions. The Al-avoidance model leads to the following Gibbs energy of mixing for the high plagioclase series: . The entropy and enthalpy of mixing should be very nearly independent of temperature because of the unlikelihood of excess heat capacity in the albite-anorthite join.
Nd-isotope analyses from a range of lithologies that comprise the high-grade terrain of South India define contrasting age provinces. North of the Palghat-Cauvery shear zone, the Karnataka Craton and the granulite blocks of Nilgiri and Madras are characterized by model Nd ages from 3.4 to 2.4 Ga with significant crustal growth during the Late Archean. South of the Palghat-Cauvery shear zone a Pan-African granulite-facies event resulted in Sm/Nd fractionation. Model Nd age calculations that allow for Sm/Nd fractionation at 550 Ma yield ages in the range of 2.9 to 1.3 Ga. The Palghat-Cauvery shear zone therefore represents a boundary between southern terranes that are characterized by crustal reworking and high-grade metamorphism during the Pan-African tectonothermal event and an Archean craton to the north where no Pan-African overprint has been detected. -from Authors
We report age data on zircon, monazite, uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern India using an electron microprobe technique. The rocks analysed in this study cover all of the major lithounits in these terrains and include garnet-bearing and garnet-free charnockites, garnet-biotite gneisses, khondalites, calc-silicate rocks, and a suite of orthogneisses (biotite gneiss, biotite-hornblende gneiss). Two pink metagranites representing the magmatic phase were also analysed. Zircons from the Madras Block yield well-defined isochrons at 2.5-2.6 Ga. Core to rim analyses of single zircon grains show age zoning with 2.6-2.9 Ga igneous cores mantled by 2.4-2.5 Ga rims. Detrital zircons show age up to ca. 3.2 Ga. Monazites in this block have cores and rims with 2.5-2.3 Ga. A suite of 19 samples from the Madurai Block brings out the multiple tectonothermal events in this terrain. Zircons from an orthogneiss yield well-defined isochrons at 1.7±0.1 Ga, 0.82±0.05 Ga and 0.58±0.04 Ga from core, inner rim and rim portions, respectively. Zircon grains in other rocks preserve either core or secondary growth ages at 0.8-1.0 Ga. Zircons in a pink metagranite from this block show sharply defined isochrons of 0.68±0.03 Ga for the core and 0.57±0.01 Ga for the secondary portion. A late Pan-African overprint is observed throughout this block with zircon rims, monazite, uraninite and huttonite yielding age values in the range of 0.45-0.60 Ga. Zircons from both the Trivandrum and Nagercoil blocks show a major tectonothermal event at 0.55 Ga with faint indications of previous tectonothermal events during 0.8-1.0 and 1.7-2.0 Ga. Monazite data from both the Trivandrum and Nagercoil blocks are essentially similar to those from the Madurai Block except for presence of relic monazite in the former.
The mapping of the crust-mantle boundary surface is an important geophysical task, which the method of seismic profiling has dealt with profitably. There are, however, areas where the crustal structure is not known up to the present, and where the Moho has as yet not been determined by geophysical sounding. In such areas the isostatic theory may be applied to give a first estimate of the depths of the crust-mantle boundary. However, young orogenic regions are not necessarily in isostatic equilibrium. Therefore the isostatically calculated crustmantle boundary must be corrected. In our method, the long wavelength observed gravity anomalies are inverted in an iterative process to model the crust-mantle boundary, assuming thus that the mass responsible for the observed gravity anomalies is located at the level of the crust-mantle boundary. After illustrating the proposed method in different model situations, it is applied to two N-S-oriented profiles in the NE/Italian Alps, an area only scarcely studied with seismic deep sounding. We obtain a maximum crustal thickening of 60 km in the western profile, beneath the Hohe Tauern, whereas to the east a lower value of 50 km (Niedere Tauern), is retrieved. The eastern profile shows a secondary crustal thickening further to the south, below the Dinarides. The analysis shows that the Moho depth beneath the crest of the Alps is in the order of 10 km in excess compared to what we would expect for the Airy-Heiskanen (AH) isostatic model.
Phanerozoic addition rates to the continental crust are calculated by
using seismic profiles through magmatic arcs to measure the crustal
volumes added during the active lifespans of the arcs. Data for 17 arcs
give addition rates per kilometer of arc in the range 20 to 40 km³
km-1 Ma-1. From these data we deduce a world-wide
addition rate of 1.65 km³ a-1 after adding other
contributions to the formation of the continental crust, e.g., from hot
spot volcanism. We infer a subtraction rate, mainly by subducting
sediments, of 0.6 km³ a-1 and arrive at a net crustal
growth rate of about 1 km³ a-1. Growth of the
continental crust is necessary to maintain approximately constant
freeboard, because the secular decline in the heat production of the
mantle causes the ocean basins to deepen. An equation for the growth of
the continents as a function of the decline in terrestrial heat flow
yields approximately constant growth rate since the Archean of 0.9
km³ a-1, in good agreement with the above estimate. On
the average, Archean growth rates must have been 3 to 4 times the
present rate. Island arc growth rates are inadequate to explain the
formation of the Arabian-Nubian Shield and the Archean
granite-greenstone terrain of the Superior Province, and a captured
island chain in Oregon. We confirm the oceanic island origin of the
Oregon terrain on the basis of the large growth rates of hotspot
islands.
Satellite altimetry from the Geosat and the ERS-1 Geodetic Missions provide altimeter data with very dense spatial coverage. Therefore the gravity field may be recovered in great detail. As neighboring ground tracks are very closely distributed, cross-track variations in the sea surface heights are extremely sensitive to sea surface variability. To avoid errors in the gravity field caused by such effects, sea surface variability needs to be carefully eliminated from the observations. Initially, a careful removal of gross errors and outliers was performed, and the tracks were fitted individually to a geoid model and crossover adjusted using bias and tilt. Subsequently, sea surface heights were gridded using local collocation in which residual ocean variability was considered. The conversion of the heights into gravity anomalies was carried out using the fast Fourier transform (FFT). In this process, filtering was done in the spectral domain to avoid the so-called ``orange skin'' characteristics. Comparison with marine gravity was finally carried out in three different regions of the Earth to evaluate the accuracy of the global marine gravity field from ERS-1 and Geosat.
We present SIMS U–Pb zircon ages and Nd isotope data for the felsic volcanic rocks from seven Neoarchean greenstone belts of the Eastern Dharwar craton (EDC) and from the Chitradurga greenstone belt in the Western Dharwar craton (WDC). Zircon ages show bimodal age distribution of felsic volcanism. The ca. 2.70–2.65 Ga felsic volcanic event is contemporaneous with 2.7 Ga mafic greenstone volcanism and emplacement of juvenile tonalitic to granodioritic crust, while 2.58–2.54 Ga felsic volcanics are coeval and spatially (and probably genetically in some cases) linked to the major episode calc-alkaline magmatic accretion in the EDC. The Chitradurga and Veligallu greenstone belts host felsic volcanics of the first generation, the latter showing inheritance at ca. 2.95 Ga. Four of the five greenstone belts hosting the second generation of felsic volcanics (Chitradurga, Kolar, Kadiri, Hutti) show crustal inheritance at ca. 2.6, 2.7, 2.9, 3.0, 3.1 and 3.3 Ga. ɛNdt indicate derivation of the felsic volcanics from juvenile sources or short-lived crustal sources with minor influence of older crust.The new and existing data are consistent with two-stage growth of the Dharwar craton in the Neoarchean. First-stage accretion led to the growth of a 2.7–2.6 Ga juvenile crustal province of mafic volcanics and felsic plutons along the eastern margin of the WDC. Second-stage accretion (2.58–2.52 Ga) led to the emplacement of TTG and calc-alkaline plutons and felsic volcanics throughout the Eastern Dharwar craton. An active margin context could apply for the two-stage accretion scenario considering a west-dipping subducting slab beneath the craton in the framework of long-lived ultra-hot accretionary orogen. But the great width of influence of magmatic accretion and/or reworking, particularly during the second stage, would suggest large-scale mantle flow reorganization that would have generated large plume(s).
The Neoarchean eastern Dharwar Craton (EDC) is distinct from the Mesoarchean western Dharwar Craton (WDC) in many aspects of its geology. The important distinguishing features of the EDC are the predominance of younger granitoids, abundance of gold mineralization and the exposure at lower crustal depths. No consensus exists on evolutionary models for EDC; mutually exclusive plume and subduction-derived tectonic models have been proposed. Geochemical and radiogenic isotopic studies on the granitoids and volcanic rocks of three greenstone belts along a cross-section in the northern part of EDC are presented herein. The evolved Nd isotopic signatures, radiogenic Pb isotopic ratios and “arc-like” geochemical signatures are suggestive of a subduction regime and the involvement of recycled older crust in the derivation of these rocks. The proposed petrogenetic mechanism involves multi stage processes in a supra subduction regime involving slab dehydration, formation of hydrous basaltic melts, and re-melting and interaction with sub-arc basaltic crust at low pressures where amphibole ± plagioclase is the dominant residual phase. There is a notable systematic decrease in the extent of older crustal involvement from west to east in the EDC. This is in concurrence with the younging of Dharwar Craton from west to east and eastward subduction. The proposed petrogenetic model can efficiently explain the variations in older crustal involvement, which is very common in other Archean cratons.