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Multi-stage crustal growth and Neoarchean geodynamics in the Eastern Dharwar Craton, Southern India
Abstract
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 Archean-Proterozoic transition witnessed some of the significant felsic magmatic events (as in Table-1) broadly in a subduction zone tectonic regime in Central India. An age of early Paleoproterozoic has been ascribed to the emplacement of Malanjkhand, Dongargarh and Kanker granitoids (Narayana et al. 2000;Panigrahi et al. 2004;Pandit and Panigrahi, 2012), while Meso to Neoproterozoic granite magmatism has been widely recorded in Mul area of Bastar craton (BC) and in the Sausar metasedimentary Belt of Central India (Chattopadhyay, 2017;Dora et al. 2019;Santosh et al. 2020;Dora et al. 2021). Felsic magmatism in the Proterozoic Sausar Belt depicts late Mesoproterozoic to early Neoproterozoic tectono-thermal events (Roy et al., 2006, Bhowmik et al., 2012, Chattopadhyay et al., 2015. ...
... In stage-1, 'phase 1 granitoids' or 'transitional TTG' formed during shift of tectonic style from 'subduction' to 'collision' (see Dey et al., 2017;Jayananda et al., 2020;Mohan et al., 2019;Brown, 2010;Kemp et al., 2009). In stage 2, sanukitoids, hybrid granites and biotite granites formed, after the collision between the eastern and western cratonic blocks in the following ways: Type 1: 'Sanukitoids' are formed in two steps: After the subduction, a) the slab slided into the mantle (Fig.6a), asthenosphere upwelled and broken the slab; and, b) mantle melted and got mixed with the slab and formed sanukitoid melts. ...
... The Archean-Proterozoic boundary in the Dharwar Craton is characterized by the occurrence of Neoarchean granitic activity which is a significant event in the process of crustal melting and differentiation (Jayananda et al., 2020. Granites often display a wide variety of geochemistry which is influenced by factors such as their origin, extent of melting and history of crystallization (DePaolo, 1981;Ray et al., 2011). ...
The segment of the Eastern Ghats Granulite Belt (EGGB) situated between the Godavari and Gundlakamma rivers is a mosaic of litho-assemblages welded together along approximately NNE-SSW planes. These assemblages are (a) enderbite and charno-enderbite with bands of pyroxene granulites and garnetiferous gneiss; (b) garnetiferous gneiss (+ sillimanite) with bands of enderbite, charno-enderbite and pyroxene granulites. Garnetiferous gneisses show compositions of peraluminous granite, granodiorite and tonalite. Pyroxene granulites of komatiitic-basalt, Fe-tholeiite and calc-alkaline basalt compositions are co-banded with enderbite, charno-enderbite and garnetiferous gneiss. Occasionally, veins of A-type granite intrude enderbite, charno-enderbite and garnetiferous gneiss. Field relations and geochemistry indicate (i) intra-crustal recycling of successive lithounits, (ii) possible mafic magma underplating between the incipient granodioritic crust and the mantle, and (iii) repeated accretions to the granulitic kernel from a fertile lower crust during the evolution of the EGGB segment. (Page 120-131)
... The Archean-Proterozoic transition witnessed some of the significant felsic magmatic events (as in Table-1) broadly in a subduction zone tectonic regime in Central India. An age of early Paleoproterozoic has been ascribed to the emplacement of Malanjkhand, Dongargarh and Kanker granitoids (Narayana et al. 2000;Panigrahi et al. 2004;Pandit and Panigrahi, 2012), while Meso to Neoproterozoic granite magmatism has been widely recorded in Mul area of Bastar craton (BC) and in the Sausar metasedimentary Belt of Central India (Chattopadhyay, 2017;Dora et al. 2019;Santosh et al. 2020;Dora et al. 2021). Felsic magmatism in the Proterozoic Sausar Belt depicts late Mesoproterozoic to early Neoproterozoic tectono-thermal events (Roy et al., 2006, Bhowmik et al., 2012, Chattopadhyay et al., 2015. ...
... In stage-1, 'phase 1 granitoids' or 'transitional TTG' formed during shift of tectonic style from 'subduction' to 'collision' (see Dey et al., 2017;Jayananda et al., 2020;Mohan et al., 2019;Brown, 2010;Kemp et al., 2009). In stage 2, sanukitoids, hybrid granites and biotite granites formed, after the collision between the eastern and western cratonic blocks in the following ways: Type 1: 'Sanukitoids' are formed in two steps: After the subduction, a) the slab slided into the mantle (Fig.6a), asthenosphere upwelled and broken the slab; and, b) mantle melted and got mixed with the slab and formed sanukitoid melts. ...
... The Archean-Proterozoic boundary in the Dharwar Craton is characterized by the occurrence of Neoarchean granitic activity which is a significant event in the process of crustal melting and differentiation (Jayananda et al., 2020. Granites often display a wide variety of geochemistry which is influenced by factors such as their origin, extent of melting and history of crystallization (DePaolo, 1981;Ray et al., 2011). ...
The terrain situated to the south of the Dharwar craton along the (i) periphery of the Nilagiri Hills, (ii) Bhavani-Erode-Salem-Attur area, (iii) Mettur-Dharmapuri-Palar corridor, (iv) Bhavani-Sankari-Sittampundi tract, (v) Salem-Attur segment and (vi) Hogenakal-Pikkili zone in southern peninsular India show several traits of Marginal Zone. These terrain segments are mapped as tectonised parts of a marginal zone that has tectonically developed between a granulitic mobile belt and a stable craton. The marginal zone is identified by remote-sensing using IRS imagery followed by validation in the field, marked in a map and presented in this paper.
... The Archean-Proterozoic transition witnessed some of the significant felsic magmatic events (as in Table-1) broadly in a subduction zone tectonic regime in Central India. An age of early Paleoproterozoic has been ascribed to the emplacement of Malanjkhand, Dongargarh and Kanker granitoids (Narayana et al. 2000;Panigrahi et al. 2004;Pandit and Panigrahi, 2012), while Meso to Neoproterozoic granite magmatism has been widely recorded in Mul area of Bastar craton (BC) and in the Sausar metasedimentary Belt of Central India (Chattopadhyay, 2017;Dora et al. 2019;Santosh et al. 2020;Dora et al. 2021). Felsic magmatism in the Proterozoic Sausar Belt depicts late Mesoproterozoic to early Neoproterozoic tectono-thermal events (Roy et al., 2006, Bhowmik et al., 2012, Chattopadhyay et al., 2015. ...
... In stage-1, 'phase 1 granitoids' or 'transitional TTG' formed during shift of tectonic style from 'subduction' to 'collision' (see Dey et al., 2017;Jayananda et al., 2020;Mohan et al., 2019;Brown, 2010;Kemp et al., 2009). In stage 2, sanukitoids, hybrid granites and biotite granites formed, after the collision between the eastern and western cratonic blocks in the following ways: Type 1: 'Sanukitoids' are formed in two steps: After the subduction, a) the slab slided into the mantle (Fig.6a), asthenosphere upwelled and broken the slab; and, b) mantle melted and got mixed with the slab and formed sanukitoid melts. ...
... The Archean-Proterozoic boundary in the Dharwar Craton is characterized by the occurrence of Neoarchean granitic activity which is a significant event in the process of crustal melting and differentiation (Jayananda et al., 2020. Granites often display a wide variety of geochemistry which is influenced by factors such as their origin, extent of melting and history of crystallization (DePaolo, 1981;Ray et al., 2011). ...
The Kannegiri Granulite Massif (KGM) comprises an E-W chain of hills rising amidst a pediplain in the area south of Kothagudem and Godavari coalfields. The granulites of KGM are located in a marginal zone (MZ) made up of reworked gneisses, granulites and amphibolites; the MZ occurs between Dharwar craton and Eastern Ghats Granulite Belt (EGGB). KGM has no direct spatial link with the EGGB. To the west and southwest, KGM is flanked by Khammam schist belt (KSB); the Chimalpad anorthosite lies to the northwest of KGM. The Godavari graben that contains sediments of Gondwana Group is situated to the north, northeast and east of KGM. Besides, the Tallada-Kallur outlier (TKO) of Gondwana Group occurs to the south of KGM. The spatial limits of KGM and TKO are controlled by several faults. Along one of the intersections of faults, (Kannegiri) carbonatite laden with ultramafic xenoliths of garnetiferous hornblendite is emplaced. Along the foot-hill of KGM, the granulites exhibit slickensides, fault breccia, intense mylonitisation and profuse injection of aplite veins. The geomorphology, structural fabrics and lithological contrasts in the MZ suggest that the KGM has been uplifted as a horst along a set of faults, complementary to the Godavari graben during a protracted extensional tectonism that commenced from Archaean.
... The Archean-Proterozoic transition witnessed some of the significant felsic magmatic events (as in Table-1) broadly in a subduction zone tectonic regime in Central India. An age of early Paleoproterozoic has been ascribed to the emplacement of Malanjkhand, Dongargarh and Kanker granitoids (Narayana et al. 2000;Panigrahi et al. 2004;Pandit and Panigrahi, 2012), while Meso to Neoproterozoic granite magmatism has been widely recorded in Mul area of Bastar craton (BC) and in the Sausar metasedimentary Belt of Central India (Chattopadhyay, 2017;Dora et al. 2019;Santosh et al. 2020;Dora et al. 2021). Felsic magmatism in the Proterozoic Sausar Belt depicts late Mesoproterozoic to early Neoproterozoic tectono-thermal events (Roy et al., 2006, Bhowmik et al., 2012, Chattopadhyay et al., 2015. ...
... In stage-1, 'phase 1 granitoids' or 'transitional TTG' formed during shift of tectonic style from 'subduction' to 'collision' (see Dey et al., 2017;Jayananda et al., 2020;Mohan et al., 2019;Brown, 2010;Kemp et al., 2009). In stage 2, sanukitoids, hybrid granites and biotite granites formed, after the collision between the eastern and western cratonic blocks in the following ways: Type 1: 'Sanukitoids' are formed in two steps: After the subduction, a) the slab slided into the mantle (Fig.6a), asthenosphere upwelled and broken the slab; and, b) mantle melted and got mixed with the slab and formed sanukitoid melts. ...
... The Archean-Proterozoic boundary in the Dharwar Craton is characterized by the occurrence of Neoarchean granitic activity which is a significant event in the process of crustal melting and differentiation (Jayananda et al., 2020. Granites often display a wide variety of geochemistry which is influenced by factors such as their origin, extent of melting and history of crystallization (DePaolo, 1981;Ray et al., 2011). ...
This conference was focused to enable the field geologists to liberally express their field findings and petrography. Twenty-one abstracts were received and fifteen presentations were made in the conference. The twelve papers appearing in this Proceedings Volume highlight the Field Geology and Petrology, with geochemistry data in a few of them. A few papers disseminate the field geology as base-line data.
... The Neoarchean crust of the Eastern Dharwar Craton (EDC) consisted primarily of granitoids with diverse mineralogical, geochemical, and source characteristics (Dey et al. 2017;Jayananda et al. 2018Jayananda et al. , 2020Mohan et al. 2019;Bhaskar Rao et al. 2020). In the northern section of EDC, researchers have given significance to studying the origin and evolution of granitoids from field, petrography, geochemistry and geochronological aspects (Dey et al. 2009(Dey et al. , 2017Jayananda et al. 2018Jayananda et al. , 2019Jayananda et al. , 2020Mohan et al. 2019;Bhaskar Rao et al. 2020;Ajay Kumar et al. 2022;Nagamma et al. 2023). ...
... The Neoarchean crust of the Eastern Dharwar Craton (EDC) consisted primarily of granitoids with diverse mineralogical, geochemical, and source characteristics (Dey et al. 2017;Jayananda et al. 2018Jayananda et al. , 2020Mohan et al. 2019;Bhaskar Rao et al. 2020). In the northern section of EDC, researchers have given significance to studying the origin and evolution of granitoids from field, petrography, geochemistry and geochronological aspects (Dey et al. 2009(Dey et al. , 2017Jayananda et al. 2018Jayananda et al. , 2019Jayananda et al. , 2020Mohan et al. 2019;Bhaskar Rao et al. 2020;Ajay Kumar et al. 2022;Nagamma et al. 2023). However, Mineralogical and Redox conditions studies are limited in the northern section of EDC (Ashok et al. 2022a;Raju et al. 2022). ...
... Due to differing scholarly opinions, the origin of these granitoids remains a topic of debate. These granitoid magmas are mostly linked with collisional or subduction rifting, intraplate, and oceanic settings (Manikyamba and Kerrich 2012;Moyen et al. 2017;Jayananda et al. 2018Jayananda et al. , 2020. ...
The mineralogical and geochemical characteristics of the K-rich granites from the Armoor granitic rocks in the northeastern portion of the Eastern Dharwar Craton (EDC) are presented. In order to understand its physicochemical conditions, the petrogenesis of the granitoid was explained from biotite chemistry and geochemical systematics. Studies of mineral chemistry expose that compositionally, K-feldspar and plagioclase in Armoor granite rocks range from An0, Ab3−5.9, Or94−96.9 and An5−29, Ab71.9−94.9, Or0−1.5, respectively. The mineral chemistry of biotite crystals exhibits composition that varies from primary to re-equilibrated primary biotites. Although biotites from the Armoor granites generally exhibit an I-type trend, with calc-alkaline parental magma in a subduction setting. Biotite chemistry of granites displays magnetite (oxidized) series nature, which has oxygen fugacity (fO2) = − 15.1 to − 16.7(log10 bar), under high oxidizing conditions. Temperature and pressure estimates for the crystallization of Armoor granites based on biotite composition are T = 612–716 °C and 1.0−0.4 kbar, respectively. Geochemically, these rocks are metaluminous to slightly peraluminous and magnesian, with calc-alkaline potassium-rich granite. On the chondrite normalized REE diagram, the granites have positive europium anomalies; rich Sr/Y, (Dy/Yb)N ratios and reduced Mg#, Rb/Sr, Rb, Sr indicate that the melting of earlier rocks, crystal accumulation and residual garnet source formed at high pressures. The examined granites show that they are produced from the melting of crustal sources. Thus, the extensive analyses of the described Armoor granite suggest that they were produced by crust sources and developed under oxidizing conditions in subduction setting.
... The DC has mainly three major tectonic domains (1. > 3.2 Ga cratonic nuclei with Late Archaean younger granite plutons, 2. Proterozoic mobile belt, 3. Proterozoic basins). Recently the DC is divided into three cratonic blocks (i.e., east, central and west blocks afterPeucat et al. 2013;Jayananda et al. 2018Jayananda et al. , 2020Dey 2013;Li et al. 2018a, b) on the basis of age relationships of rocks, thermal records and interpreted accretionary histories. However, most of the literature suggests about two major components viz., East Dharwar Craton (EDC) and West Dharwar Craton (WDC) on the basis of contrasts in lithounits, metamorphic grade and crustal thickness (Fig. 1). ...
... Geology of the Dharwar Craton is well known and there are extensive numbers of literature available (e.g., Swami Nath and Ramakrishnan 1981; Radhakrishna and Naqvi 1986;Naqvi and Rogers 1987;Chadwick et al. 2000Chadwick et al. , 2007Chardon et al. 2008Chardon et al. , 2011Jayananda et al. 2006Jayananda et al. , 2015Jayananda et al. , 2018Jayananda et al. , 2020. The DC covering ~ 350,000 km 2 area is one of the major constituents cratonic nucleus in Peninsular India (Rogers 1986). ...
... This craton is bounded to the south by Proterozoic Southern Granulite Terrain (SGT), to the north by the Cretaceous Deccan Traps and the Bastar Craton, to the northeast by the 2.6 Ga Karimnagar Granulite belt and Godavari graben, to the east by the Eastern Ghats Mobile Belt and to the west by Arabian Sea. The EDC comprises mainly of greenschist to amphibolite facies greenstone belts of ~ 2.7 Ga age, along with felsic calc-alkaline plutonic, and volcanic rocks of ~ 2.7-~ 2.5 Ga age (e.g., Jayananda et al. 2000Jayananda et al. , 2020Chardon et al. 2002Chardon et al. , 2011Dey et al. 2012Dey et al. , 2016Manikyamba and Kerrich 2012;Manikyamba et al. 2017). Older (3.3-3.0 ...
Structural fabrics of schist belt, gneiss, migmatites and younger granitoids are different. Various morphology of migmatites is recognized for the first time in the southern part of EDC along with gneiss. The younger granite plutons are examined from northern through central to southern part of EDC. The paleosome-dominated portions of migmatite were possibly formed from low degree partial melting of older rocks and identified as metatexites. Whereas, the neosome dominated parts were developed by complete melting and marked as diatexites. There is a link between diatexite and granite in the southern part, which is chracterized as catazone segment. The four sub-types of metatexites are observed (viz, patch, dilatant, net and stromatic). The diatexites are dominated by two major structures (viz, schollen or raft and schlieren). Gneisses show lit-per-lit as well as fold and leucocratic material flow features. The litho-structural variation from north to south connotes systematic changes from epizone through mesozone to catazone. The qualitative study of mineral content and textural aspects from rocks at different localities suggest preserved portions of different parts of a pressure–temperature path. However, exact quantification should be possible after thermo-barometry and pseudo section study. The present contribution focuses on documentation of classic representative outcrops and petrographic features of gneissic and migmatitic rocks of EDC. It is observed that the six (6) major ductile deformation stages of Kenoran orogeny implies ductile signature before granite plutonism. Subsequently post granite emplacement deformation phenomenon indicates a transition from ductile to brittle regime and entrance into Hudsonian orogeny.
... 3400-3200 TTG-greenstone volcanic assemblages also preserved in the regions adjoining to Holenarsipur greenstone belt (see Fig. 2) which include Nuggihalli, Kalyadi, Jayachamaraja Pura (J.C. Pura), Banasandra, Nagamangala, southern margin of the Bababudan basin and western margin of the Chitradurga greenstone belt (Nutman et al., 1992;Ramakrishnan et al., 1994;Hokada et al., 2013;Lancaster et al., 2015;Wang et al., 2020;Jayananda et al., 2008Jayananda et al., , 2016Jayananda et al., , 2020Ravindran et al., 2020). ...
... The Holenarsipur greenstone belt and adjacent granitoids (TTG-type Peninsular gneisses) have long been considered as early Precambrian crustal nuclei (Radhakrishna and Naqvi, 1986). Komatiite-dominated volcanic sequences stratigraphically equivalent to the Holenarsipur greenstone belt can be found in ca.3400-3200 Ma Nuggihalli, Kalyadi, J.C. Pura, Ghattihosahalli, Banasandra and Nagamangala greenstone belts (Chadwick et al., 1981;Swami Nath and Ramakrishnan, 1981;Venkata Dasu et al., 1991;Jayananda et al., 2008Jayananda et al., , 2016Jayananda et al., , 2020. However, spatial link between greenstone volcanism and adjoining granitoids formation, mantle evolution and continental growth, evolving tectonics and craton building processes are still largely conjectural. ...
... On the cratonic scale, tectonic fabrics mapping, and kinematic analysis reveal dome and basin patterns in the western block with widely spaced shear zones Chardon et al., 2008;Tarun Kumar et al., 2022), whereas the central and eastern blocks have spectacular flat fabrics linked to lateral constrictional flow of hot orogenic crust with closely spaced shear zone network (Chardon et al., 2011). Recent strain fabric data in combination with metamorphic facies, age zonation patterns, Sm-Nd isotopes, and strong seismic reflectivity pointed towards Neoarchean craton formation through assembly of the three crustal blocks through westward convergence of oceanic lithosphere along the eastern margin of the Chitradurga greenstone belt (Chadwick et al., 2000;Chardon et al., 2011;Vijaya Rao et al., 2015;Jayananda et al., 2013bJayananda et al., , 2018Jayananda et al., , 2020. ...
... The younger Dharwar Supergroup comprises Meso to Neoarchean greenstone successions that unconformably overlies the basement gneisses (Chadwick et al. 1981;Srinivasan and Ojakangas 1986;Kumar et al. 1996;Jayananda et al. 2013). The WDC has undergone multiple crustal reworking events at 3.0 and 2.60 Ga, resulting in younger granitic magmatism (Jayananda et al. 2006(Jayananda et al. , 2020. ...
... The Eastern Dharwar Craton is dominated by voluminous granitoids and linear greenstone belts of the Neoarchean age (Manikyamba et al. 2017;Jayananda et al. 2018Jayananda et al. , 2020Mohan et al. 2020). Compared to the WDC, the greenstone successions of the EDC are volcanic-dominated and reveal a bimodal age distribution at ca. 2.75-2.67 ...
... Ga episode of TTG magmatism and a major 2.57-2.52 Ga episode of calc-alkaline granitic emplacement (Jayananda et al. 2020;Mohan et al. 2020). After the stabilization of the DC, the Archean basement was intruded by numerous maBc dyke swarms of Paleoproterozoic to Mesoproterozoic age Yadav et al. 2020). ...
We present field, petrographic and mineral compositions of biotite, amphibole, and feldspars from Neoarchean Nizamabad granites from the northeastern part of the Eastern Dharwar Craton, southern India. These granites are classified as hornblende biotite granite (HBG), biotite granite (BTG), monzogranite (MG), and microgranular enclaves (ME) hosted in HBG and BTG. The temperature estimates using amphibole and biotite thermometry exhibit similar results, with higher temperatures for HBG (818 to 859 ±22°C) and ME (800 to 855 ±22°C), and slightly lower temperatures for BTG and MG (829 to 830 ±12°C and 820 to 829 ±12°C). Based on barometry, HBG amphiboles crystallized at pressures between 363 and 448 ±60 MPa (avg. PHBG = 398 MPa), whereas the MEs crystallized at pressures between 313 and 438 ±60 MPa (avg. PME = 386 MPa). The estimated pressures suggest that these granites crystallized at depths of 14–15 km, corresponding to the upper to mid-continental crust. The amphibole compositions reveal that these granites crystallized from a water-rich magma, with >5 wt.% H2O and evolved under high oxidizing conditions NNO + 2 (Nickel–Nickel–Oxide), corresponding to magnetite (oxidized) series granites. The amphibole and biotite compositions suggest a crust-mantle mixed source for HBG, ME, and BTG, while the MG is purely crustal derived. The water-rich and highly oxidizing conditions of the parental magmas rule out a lower crustal granulitic source for the Nizamabad granites. The amphibole and biotite compositions suggest their crystallization from calc-alkaline parental magma in a subduction setting at high oxygen fugacity (fO2) conditions. This study infers the role of convergent margin tectonics in the emplacement of these granites, and their compositional variability is attributed to crust–mantle interactions in this domain of the Eastern Dharwar Craton. The granitoids from the NE part of the Eastern Dharwar Craton are characterized as hornblende biotite granites (HBG), biotite granites (BTG), monzogranite (MG), and mafic enclave (ME).Estimated pressures suggest that these granites crystallized at depths of 14 to 15 km, corresponding to the upper to mid-continental crust.The water-rich (> 5 wt.% H2O content) and high oxidizing conditions (NNO+2) of HBG, BTG, and ME corresponds to magnetite (oxidized) series granites.The amphibole and biotite compositions from the Nizamabad granites suggest the crust-mantle mixed source for HBG, ME, and BTG, while the MG are purely crustal derived.The compositions of these granites suggest their crystallization from calc-alkaline parental magma in subduction settings at high oxygen fugacity (fO2) conditions.The study infers the role of convergent margin tectonics in the emplacement of compositionally variable granitoids in the NE part of the Eastern Dharwar Craton. The granitoids from the NE part of the Eastern Dharwar Craton are characterized as hornblende biotite granites (HBG), biotite granites (BTG), monzogranite (MG), and mafic enclave (ME). Estimated pressures suggest that these granites crystallized at depths of 14 to 15 km, corresponding to the upper to mid-continental crust. The water-rich (> 5 wt.% H2O content) and high oxidizing conditions (NNO+2) of HBG, BTG, and ME corresponds to magnetite (oxidized) series granites. The amphibole and biotite compositions from the Nizamabad granites suggest the crust-mantle mixed source for HBG, ME, and BTG, while the MG are purely crustal derived. The compositions of these granites suggest their crystallization from calc-alkaline parental magma in subduction settings at high oxygen fugacity (fO2) conditions. The study infers the role of convergent margin tectonics in the emplacement of compositionally variable granitoids in the NE part of the Eastern Dharwar Craton.
... While the WDC is predominantly made of the 3.4-3.0 Ga tonalite-trondhjemite-granodiorite (TTG) gneisses commonly referred to as Peninsular Gneisses (e.g., Jayananda et al., 2018;Ranjan et al., 2020), the basement gneisses of the EDC are of 2.7-2.5 Ga age (e.g., Jayananda et al., 2018Jayananda et al., , 2020. The greenstone belts of the WDC are sediment-dominated, whereas those in the EDC are sparse and dominated by volcanic rocks (e.g., Jayananda et al., 2018Jayananda et al., , 2020Ranjan et al., 2020;Swami Nath and Ramakrishnan, 1981). ...
... Ga tonalite-trondhjemite-granodiorite (TTG) gneisses commonly referred to as Peninsular Gneisses (e.g., Jayananda et al., 2018;Ranjan et al., 2020), the basement gneisses of the EDC are of 2.7-2.5 Ga age (e.g., Jayananda et al., 2018Jayananda et al., , 2020. The greenstone belts of the WDC are sediment-dominated, whereas those in the EDC are sparse and dominated by volcanic rocks (e.g., Jayananda et al., 2018Jayananda et al., , 2020Ranjan et al., 2020;Swami Nath and Ramakrishnan, 1981). The rocks of the WDC have been metamorphosed from lower greenschist to upper amphibolite facies, and metamorphic grade in the EDC ranges from lower amphibolite to upper amphibolite facies (e.g., Jayananda et al., 2018Jayananda et al., , 2020Ranjan et al., 2020;Swami Nath and Ramakrishnan, 1981). ...
... The greenstone belts of the WDC are sediment-dominated, whereas those in the EDC are sparse and dominated by volcanic rocks (e.g., Jayananda et al., 2018Jayananda et al., , 2020Ranjan et al., 2020;Swami Nath and Ramakrishnan, 1981). The rocks of the WDC have been metamorphosed from lower greenschist to upper amphibolite facies, and metamorphic grade in the EDC ranges from lower amphibolite to upper amphibolite facies (e.g., Jayananda et al., 2018Jayananda et al., , 2020Ranjan et al., 2020;Swami Nath and Ramakrishnan, 1981). The two blocks were likely amalgamated by ca. ...
The Indian Shield is comprising of the Dharwar, Bastar, Singhbhum, Bundelkhand, and Aravalli Cratons, intruded by distinct mafic dyke swarms of different generations (ca. 2.8–0.8 Ga). Most of these dykes are tholeiitic basalt to basaltic-andesite, including boninite. Some other subordinate dyke rocks are of komatiitic, picritic, and andesitic compositions. The vast areal extent of these dykes indicates that they are remnants of Precambrian Large Igneous Provinces (LIPs). The present study reviews the existing geochronological and geochemical data of various Precambrian mafic dyke swarms intruding in all Indian cratons, to track temporal changes in composition of the underlying subcontinental lithospheric mantle (SCLM). Most of these dykes have crust-like abundances of incompatible trace-element. Even the primitive dykes (Mg# = 82–64) exhibit crust-like incompatible element patterns. However, some also have depleted mantle-like abundances of incompatible elements. Most of the dykes also have high concentrations of compatible trace elements (e.g., Cr and Ni). The age-corrected radiogenic Nd isotope compositions (ɛNd(i)) of these dykes vary between the upper continental crust and depleted mantle ɛNd growth arrays. The elemental composition and ɛNd(i) of these dykes suggest their derivation from a heterogeneous SCLM comprising enriched (metasomatized) and depleted mantle components. The negative as well as positive ɛNd(i) values exhibited by the ca. 2.8 Ga (oldest) dykes suggest the presence of enriched mantle materials distributed within a depleted-SCLM already by ca. 2.8 Ga. The enrichment must have occurred before ca. 2.8 Ga due to subduction-released fluids. Later, the SCLM beneath Indian cratons evolved with a composition consisting of an enriched/metasomatized mantle component and a depleted mantle component.
... Recent studies on the geology of the Dharwar Craton have provided a comprehensive overview of the geochronological framework of both the Basement Gneiss-granite and volcanosedimentary rocks of the Western Dharwar Craton (Jayananda et al., 2018(Jayananda et al., , 2020Krapež et al., 2020;). The age of the craton is determined by U-Pb geochronological analysis on the TTGs and detrital zircons in metasediments (Ao et al., 2021;Guitreau et al., 2017;Jayananda et al., 2018;, Maibam et al., 2011Ranjan et al., 2022). ...
... Polarity of the subduction also debated in the craton-scale evolutionary models. Westward subduction-associated collision models (Chadwick et al., 2000;Jayananda et al., 2018Jayananda et al., , 2020Krapež et al., 2020;Manikyamba et al., 2015) and eastward subduction and associated collision models (Mohan et al., 2013;Ratheesh-Kumarar et al., 2020). Detailed studies of the key formations in the Chitradurga Schist Belt aim to clarify the interlink between stratigraphy, deformation, and the age of volcanism, and magmatism, which can help resolve the mismatches in the different evolutionary models proposed for the region. ...
... The Dharwar Craton is a low-grade granite-greenstone terrain. Many earlier studies (e.g., Naqvi & Rogers, 1987;Li et al. 2018;Jayananda et al. 2018Jayananda et al. , 2020; and references therein) have explored its intricate geology. Broadly, the craton has an Archean tonalitetrondhjemite-granodiorite gneissic basement (ca. ...
... 2.9-2.6 Ga; Taylor et al. 1984;Kumar et al. 1996). Further, the craton is home to ca. 2.6-2.5 Ga Potashrich granites, which are well documented as the last magmatic episode before its stabilization (Jayananda et al. 2020). Based on the difference in thermal and accretionary history, the craton is classically divided into two sub-blocks, i.e., the Eastern and the Western Dharwar Craton (EDC and WDC), along the Chitradurga Shear Zone (Chadwick et al. 2000), which is a prominent shear zone running continuously along the NNW-trending Chitradurga Greenstone belt (Fig. 1a). ...
Mafic dykes are typically emplaced through primary hydraulic fracturing of undeformed crust or may make use of pre-existing crustal inhomogeneities, representing the plumbing systems of a large igneous province. The Eastern Dharwar Craton has dense exposures of several generations of Paleoproterozoic mafic dyke swarms ranging from ca. 2.37 Ga to ca. 1.79 Ga. Herein, using anisotropy of magnetic susceptibility fabric data of mafic dykes and associated host granites, the emplacement systematics of the NW- to W-trending ca. 2.21 Ga Anantapur–Kunigal dyke swarm, displaying a radiating geometry, have been studied to understand magma flow dynamics. A low-angle relationship between the silicate and opaque fabrics and good correlation with magnetic lineation, identified via petrographic studies and shape preferred orientation analyses of multiple oriented thin sections, suggest a primary flow-related magnetic anisotropy for the studied dyke samples. The classic subparallel relationship between the trend of the dyke planes and magnetic fabric of the associated host granites suggests that the radiating geometry of the ca. 2.21 Ga dyke swarm was supported by a favourable pre-existing structural grain of the country rock. We interpret the magma for the studied dyke swarm was fed laterally from a distant plume. It was emplaced as laterally propagating primary dyke fractures as well as injected into the pre-existing subparallel crustal inhomogeneities. Corroborating all these inferences, a detailed emplacement model for ca. 2.21 Ga Anantapur–Kunigal dyke swarm is also proposed.
... Ga calc-alkaline to potassic (Closepet granites) plutonic bodies (Nutman et al., 1996;Jayananda et al., 2000). The EDC is subdivided into two domains based on thermal events that occurred earlier than 2.5 Ga (Jayananda et al., , 2020Gao and Santosh, 2020). The Archean suture is characterized by a steep mylonitic shear zone between the Chitradurga schist belt and the Kushtagi-Hungund greenstone belts in the northern part of the Dharwar craton (Jayananda et al., 2011(Jayananda et al., , 2020. ...
... The EDC is subdivided into two domains based on thermal events that occurred earlier than 2.5 Ga (Jayananda et al., , 2020Gao and Santosh, 2020). The Archean suture is characterized by a steep mylonitic shear zone between the Chitradurga schist belt and the Kushtagi-Hungund greenstone belts in the northern part of the Dharwar craton (Jayananda et al., 2011(Jayananda et al., , 2020. ...
The disintegration of the Columbia supercontinent during the late Paleoproterozoic generated major rift basins in the constituent continental fragments. The Kaladgi basin, located between the southern part of the Deccan volcanic province (DVP) and the northern part of the Dharwar craton, is a Columbia rift-related basin in southwestern India that preserves a complex history from initial fault-controlled mechanical subsidence during rifting, thermal subsidence along a collision zone, crustal thinning due to stretching and erosion associated with doming. The Paleoproterozoic basins worldwide show higher uranium concentration and many deposits are also established in the Purana basins of India. In the present study, the lithotectonic architecture of this basin using broadband magnetotelluric (∼320 Hz–3000 s) soundings in the western segment of the Kaladgi rift basin along two profiles. Two-dimensional (2-D) inversion of data using a 2-D nonlinear conjugate gradient algorithm along both profiles provides insights into the deeper structure of the basin. Our results reveal a thin sheet of Deccan volcanic, sedimentary successions belonging to the Badami and Bagalkot groups, and Proterozoic sediments from top to bottom beneath this basin. The crustal structure is highly heterogeneous and associated with deep-seated faults, and its thickness increases from the eastern Dharwar craton (∼30 km) to the western Dharwar craton (∼45 km). The crustal conductors are interpreted as mafic intrusions derived from the underplated basalts. The moderate conductive features may correspond to carbonate fluids trapped within the faults/fractures zone during basin initiation. The conductive features in the lower crust and the Moho are interpreted as fluids derived from underplated intrusions through plume impact. The NNW trending Chitradurga Suture Zone (CSZ) signature and the Bababudan-Nallur Shear (BNS) in the crust and upper mantle depth are imaged along both MT profiles. This study provides insights into the lithology and tectonic architecture of a long-lived rift basin involved in multiple tectonic events from the late Paleoproterozoic to the late Cretaceous.
... There are significant contrasts in lithounits, metamorphic grade and outcrop pattern between EDC and WDC ( [16,20,21]; Chardon et al., 2008). Generalized regional geology of DC is well known and explained in several literature [18,[22][23][24][25][26][27][28][29][30][31]. Different rock types of this Archaean Craton, varying from metavolcano-sedimentary rocks of greenstone belts and gneiss-migmatites to younger Neoarchaean granitoids, show variation in structural and metamorphic attributes. ...
... But, structural deformation may cause pressure release and thus fall in melting point, i.e., decompression melting associated with isothermal decompression. In the convergent setting, fluid is added to the system and induce partial melting with increasing pressure and thus calcalkaline magmatism was possible for the composite plutons of Late Archaean [28,[92][93][94]. Further, in the later stage of plate collision initiation of rifting started with alkaline magmatism and opening of rift basins [95]. ...
... The Eastern Dharwar Craton (EDC) consists of Neoarchean greenstone and granite belts cover most of the craton, leaving a scant relic of 3.0 Ga TTG. The crust formation, its reworking, and tectono-magmatic processes of EDC are explained through the plume-arc accretion and cratonization (Smithies et al. 2009;Manikyamba and Kerrich 2012;Barnes and Van Kranendonk 2014;Jayananda et al. 2020). ...
... As a result, it is hypothesized that the Madugulapalli granites originated through subduction accompanied by collisional orogeny in the EDC. Most of the researchers have recommended the general geodynamic two-stage growth model (Fig. 11) for Neoarchean granites (Laurent et al. 2014;Dey et al. 2017;Mohan et al. 2019;Jayananda et al. 2020), which is also applicable to the research region. The first magmatic stage is controlled by subduction (of the hydrous basaltic oceanic crust); the fluids metasomatized the underlying mantle wedge, resulting in many pulses of juvenile TTG magmatism over a prolonged period (derived due to the melting of the basaltic source or terrigenous sediments). ...
Magma produced by melting of continental crust and mantle at the Archean-Proterozoic boundary are compositionally variable and chemical compositions provide evidence for the mixing of two sources. Understanding the composition of hybrid magma is essential for determining the comparative influence of crust and mantle sources during orogenesis. The hybrid granites are less documented in Indian cratons, especially less in Dharwar Craton. Here we present petrographic and whole-rock geochemical data of Madgulapalli granitic rocks situated in the NE part of the Eastern Dharwar Craton (EDC), to elucidate their petrogenesis and role in crust formation. The Madugulapalli granites (MPG) are composed chiefly of plagioclase, quartz, and alkali feldspar with associated biotite showing alteration and inter-granular textures. Geochemically, they are metaluminous to peraluminous in nature with calc-alkaline hybrid granite. The hybrid granites exhibit both negative and positive europium anomalies; the lower Rb/Sr, Rb, Sr, and higher Sr/Y, (Dy/Yb)N ratios suggest that the interaction of older rocks with residual garnet source melted at high pressures. We hypothesize that hybrid granites are formed by interaction (e.g., metasomatism, mingling, or mixing) between parental magmas and pre-existing rocks with the influence of sanukitoid melts (heat source) in a subduction environment. The genesis of the hybrid granites demonstrates the mixing coupled with differentiation in the petrogeny’s residue system in a syn-collision setting followed by continental crust stability in EDC during the Neoarchean period.
... The CDC mainly exposes the 2.52 Ga old Closepet Granite intruded within the older (~2.7 Ga) TTGs (Zachariah et al., 1995;Nutman et al.,1996;Balakrishnan et al., 1999;Naqvi et al., 2002 a, b;Peucat et al., 2013;Dey, 2013;Jayananda et al., 2013aJayananda et al., , 2020Vasanthi and Santosh, 2021). The >3.0 Ga ages for the 'basement' TTGs however, are only encountered in the southern extremes of the CDC (Peucat et al., 2013), south of the orthopyroxene isograd that demarcates the boundary between the amphibolite-facies terrane of the DC and the granulite-facies terrane of the DC, the latter is usually grouped with the Southern Granulite Terrane (SGT). ...
... The Kolar-type greenstone belts are composed of low grade metamorphosed volcano-sedimentary associations including mafic and felsic volcanic rocks and minor amounts of ultramafic rocks along with subordinate amounts of greywackes, argillites, quartzites and BIFs (Rajamani et al., 1985;Balakrishnan et al., 1991;Manikyamba andKerrich, 2011, 2012;Jayananda et al., 2013a, b;Dey et al., 2014Dey et al., , 2017. Despite their distributed occurrence within the EDC and at the boundary of EDC and CDC, lithologically these belts are very similar and multiple studies on these belts have established a subduction origin for these belts (Zachariah et al., 1996;Balakrishnan et al., 1999;Mishra & Rajamani, 2003;Manikyamba et al., 2004a,b;Naqvi et al., 2006;Kerrich & Manikyamba, 2012;Dey, 2013;Khanna, 2013Khanna, , 2017Ram Mohan et al., 2013;Ugarkar et al., 2013;Anand et al., 2014;Rajamanickam et al., 2014;Khanna et al., 2015;Pandey et al., 2018;Jayananda et al., 2020;Manikyamba et al., 2020). All these belts also have been emplaced and metamorphosed at around 2.7 Ga. ...
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.
... The Neoarchean period in the Earth's history is characterized by rapid crustal growth, with voluminous ultramaBc to maBc volcanism, eruption of bimodal arc lavas, emplacement of voluminous tonalite-trondhjemite-granodiorite (TTG) and K-rich granitic suites, eruption of boninites and deposition of thick greywacke turbidite sequences in most of the Archean cratons Kerrich 2000, 2001;Polat 2009;Jayananda et al. 2013Jayananda et al. , 2020. The excellent preservation of these diverse lithologies in the Neoarchean greenstone belts over the globe provides an opportunity to study the geodynamic processes responsible for this enhanced crustal growth. ...
... Pb-Pb age obtained from this study falls much closer to the 2.7 Ga Sm-Nd age reported by Kumar et al. (1996). However, the Pb-Pb age 2627 ± 82 Ma, within the error, seems to overlap with the thermal event associated with the 2.62 Ga felsic volcanism in the Chitradurga greenstone belt (Jayananda et al. 2020). ...
The well-preserved exposures of pillow basalts from the late Archean (2.7 Ga) Chitradurga greenstone belt (Western Dharwar Craton) have been studied in detail using petrological, bulk-rock geochemical and isotope data. The pillows are compositionally basalts to basaltic andesites, and constituted of actinolite and plagioclase. The pillows show slight to moderately depleted LREE (La/SmN = 0.6−1.03), and nearly flat HREE patterns (Gd/YbN = 1.03−1.17) that are comparable with MORBs, but have an overall depletion in REEs relative to the latter. The trace-element patterns of the pillows on an N-MORB normalized multi-element diagram, however, are broadly comparable with that of Island Arc Basalts. The pillows were perhaps generated during the initiation of an intra-oceanic subduction zone where a depleted upper mantle (MORB-source) was metasomatized with slab-derived aqueous fluids. The pillows show a whole-rock Sm–Nd errorchron age of 2433 ± 400 Ma, which is within error of the previously reported Sm–Nd age of 2747 ± 15 Ma. The Pb–Pb isochron age (2627 ± 82 Ma) of the pillows is much closer to the previously reported Sm–Nd age, and it may also indicate the 2.62 Ga thermal event associated with felsic magmatism in the Western Dharwar Craton.
... (b) Age distribution of various granitic events in the Dharwar, Bundelkhand, and North China craton (modified and updated from [10,55]). Additional data sources: Bundelkhand craton [9,43,44,52,[54][55][56][57][58], South India (Dharwar craton and Coorg Block) [80,81,104,[106][107][108][109][110], and North China craton [98,99,111,112]. 22 Lithosphere inheritance from older sources. ...
... εNd(t) vs. time (zircon ages) evolution diagram for TTG, LSHM (sanukitoid and hybrid), and HSLM granitoids from the Bundelkhand, Aravalli, and Dharwar cratons. Data are from this study and previous studies[43,44,77,78,80,83,104,105]. (a) Multicationic classification diagram for Bundelkhand granitoids (after[73]). ...
The Bundelkhand craton in central India consists mainly of abundant high-K granitoids formed at the Archaean-Proterozoic boundary and several enclosed rafts of TTGs (tonalite-trondhjemite-granodiorites) up to 3.5 Ga. Therefore, the Bundelkhand craton is a key locality for studies on Archaean crustal growth and the emergence of multisource granitoid batholiths that stabilised a supercontinent at 2.5 Ga. Based on their geochemical characteristics, the high-K granitoids are divided into low silica – high Mg (sanukitoids and hybrids) and high silica – low Mg (anatectic) groups. We aim to provide new insights into the role of juvenile versus crustal sources in the evolution of the TTG, sanukitoid, hybrid and anatectic granitoids of the Bundelkhand craton by comparing their key geochemical signatures with new Nd isotope evidence on crustal contributions and residence times. The ages, geochemical signatures as well as εNd(t) values and Nd model ages of TTGs point towards partial melting of a juvenile or short-lived mafic crust at different depths. Paleoarchaean TTGs show short crustal residence times and contributions from the newly formed crust, whereas Neoarchaean TTGs have long crustal residence times and contributions from the Paleoarchaean crust. This may reflect the transition from melting in a primitive oceanic plateau (3.4-3.2 Ga) in plume settings, resulting in a Paleoarchaean protocontinent, to 2.7 Ga subduction and island arc accretion along the protocontinent. The 2.5 Ga high-K granitoids formed at convergent subduction settings by partial melting of the mantle wedge and pre-existing crust. Sanukitoids and hybrid granitoids originated in the mantle, the latter showing stronger crustal contributions, whereas abundant anatectic granitoids were products of pure crustal melting. Our Nd data and geochemical signatures support a change from early mafic sources to strong crust-mantle interactions towards the A-P boundary, probably reflecting the onset of supercontinent cycles.
... The transfer zone comprises K-feldspar porphyritic monzogranite to granodiorite (which are calc-alkaline granitoids and are referred to as sanukitoids). In addition, mafic magmatic enclaves (MMEs) and fragmented synplutonic dykes commonly occur in this zone (Moyen et al., 2002;Jayananda et al., 2020). A magmatic gap couples the central and northern zones and features porphyritic monzogranite that transitions into grayish homogeneous granite to the north. ...
... In this paper, an attempt is taken to interpret the mode of occurrence, structural constraints and timing of uranium mineralization in KKF. Therefore, the following scientific issues are addressed in this paper: in several literature [e.g., [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. The present study is mainly confined to around the southern basement of the CB where the NNE-SSW fracture zone exhibits uranium mineralization over a narrow deformation zone. ...
... Ga,~3-2.96 Ga,~2.7-2.6 Ga and~2.56-2.5 Ga), similar to the other cratons of the globe (Jayananda et al. 2020). ...
Deciphering the evolution and dynamics of the upper mantle is obscured due to the scarcity of the exhumed ultra-high-pressure (UHP) rocks from a depth of more than 100 km. These UHP rocks, especially the subduction-related pre-Mesoproterozoic eclogites, are very rarely exhumed back to the surface from the upper mantle. Due to such rarity of the exhumed UHP rocks, our understanding regarding the evolution and recycling of subducted crust within the upper mantle during the pre-Mesoproterozoic era is limited. However, the subduction-related eclogite xenoliths hosted in Mesoproterozoic or pre-Mesoproterozoic kimberlites, lamproites, lamprophyres, or alkali basaltic dykes are the ideal candidates to unravel pre-Mesoproterozoic evolution of the subducted crust within the upper mantle. Here, we report an eclogite xenolith hosted in a Mesoproterozoic (~1.1 Ga) kimberlite from the Kalyandurg kimberlite cluster of Eastern Dharwar craton, India, which contains a plethora of UHP minerals. These UHP minerals were identified by the characteristic in situ XRD and laser Raman spectra and EPMA analysis. The presence of coesite points to the subduction origin of the eclogite. The geothermobarometric estimations involving garnet-omphacite-kyanite-coesite revealed that such eclogitic assemblage equilibrated at ~5–8 GPa pressure during ultra-deep subduction up to ~175–280 km in the pre-Mesoproterozoic era. Subsequently, the subducted crust sank deeper within the upper mantle as evident from the development of majoritic garnet on omphacite. The textural relationship between omphacite and majoritic garnet combined with the EPMA and laser Raman spectroscopical data obtained from the majoritic garnet demonstrated that the majoritic garnet formed at ~8–19 GPa pressure between ~280–660 km due to the disassociation of omphacite during the increment of pressure. Thus, the mineralogical and geothermobarometric data suggest that the subducted crust traveled down to the base of the mantle transition zone before it was entrained in a Mesoproterozoic kimberlite as an eclogite xenolith. Hence, the discovery of this sample not only suggests that this is the deepest rock ever found in India, but also opens a new window to study the dynamics of the pre-Mesoproterozoic upper mantle.
... The shear zone to the east of Chitradurga Greenstone belt is considered the boundary between these two blocks (Jayananda et al., 2006). Recent studies divide the craton into three blocks (Western, Central and Eastern blocks) based on the age, thermal and accretionary histories, separated by major shear zones (Peucat et al., 2013;Jayananda et al., 2018;Jayananda et al., 2020). Similar to other Archean cratons of the world, Dharwar craton is predominantly composed of tonalitetrondhjemite-granodiorite (TTG) type gneisses, volcanosedimentary greenstone belts and calc-alkaline to potassic granitoid rocks (Peucat et al., 2013;Jayananda et al., 2018;Swami Nath and Ramakrishnan, 1981 and the references therein). ...
Introduction: Metadolerite dykes in the Western Dharwar Craton represent the oldest generation of mafic dyke swarms in the craton. The emplacement of these dykes after a period of crust building activity and komatiite volcanism, helps to understand the evolution of Subcontinental Lithospheric Mantle (SCLM) and Archean dynamics.
Methods: We report whole rock major, trace element geochemistry and Sr-Nd isotope characteristics for this weakly metamorphosed suite of dykes. Remnant igneous textures and mineralogy are well preserved.
Results: The trace and rare earth element concentrations and an overall flat pattern suggests depleted mantle source for these dykes. Three groups are primarily identified: Group one with initial ⁸⁷Sr/⁸⁶Sr ratios varying between 0.70041 and 0.70102, Group two dykes and Group three dykes with initial ratios 0.70045–0.70154, and 0.70041–0.70153 respectively. Group one dykes show a good Rb-Sr isochron relationship and an errorchron age of ca. 3,003 ± 102 Ma is obtained. The initial ¹⁴³Nd/¹⁴⁴Nd ratios varies from 0.508,245 to 0.509,172. The epsilon Nd values are mostly negative, ranging between −12 and +5. Group one and two show an epsilon Nd value ranging between −1 and +5 and 0.1 to +5 respectively and group three varies between −0.5 and −12.
Discussion: The geochemical characteristics suggest that the group one dykes are derived from a homogenous depleted SCLM source, group two formed by a lower degree of partial melting of a source mantle with enriched components. Group three may have formed from a progressively enriched group one source. All these dykes can be considered as exposed remnants of feeders for the greenstone volcanism in the Western Dharwar Craton.
... The Dharwar Craton is a classical granite-greenstone terrane representing a gently northward-tilted crustal section of the Archean-Proterozoic crust. Broadly, it is divided into Eastern and Western Dharwar Craton subprovinces based on lithological characteristics, grade of metamorphism, crustal thickness, degree of partial melting, and attributes of greenstone belts (e.g., Swami Nath et al., 1976;Jayananda et al., 2018Jayananda et al., , 2020 and references therein) (see Fig. 1). The Chitradurga shear zone marks the boundary between the EDC and Western Dharwar Craton (WDC). ...
Understanding emplacement mechanism and depth of origin of continental mafic dyke swarms is pivotal in resolving outstanding questions regarding magma flow dynamics, plumbing system architecture, and amount of crustal dilation related to large igneous provinces. Paleoproterozoic mafic dyke swarms are often deemed as suitable candidates to establish the crustal evolution of cratonic regions. Here, we present the geometric and statistical
analyses of five distinct Paleoproterozoic mafic dyke swarms and associated fracture systems in the Eastern Dharwar Craton (EDC) to constrain their magmatic overpressures and magma chamber depths. The geometric analysis is further validated with field evidence and measurements. It is suggested that mafic dykes of different swarms are formed primarily due to vertical magma injection from deep-seated magma reservoirs, followed by
lateral magma flow at shallower depths. This study suggests a direct derivation of magma from deep-seated magma reservoirs, extending from lower crustal depths (~17 km) to the crust-mantle boundary (~36 km). It is also suggested that the emplacement of mafic dykes at ca. 2.37–2.36 Ga, ca. 2.26–2.25 Ga, ca. 2.22 Ga, and ca. 2.21 Ga can be traced back to the development of fracture system formed during and subsequent to the
Neoarchean accretion event of Eastern and Western Dharwar Cratons (< 2.52 Ga). Therefore, the emplacement systems of the studied mafic dyke swarms have been largely controlled by the regional stress field and large-scale structural architecture of the Dharwar Craton.
... Ga,~3-2.96 Ga,~2.7-2.6 Ga and~2.56-2.5 Ga), similar to the other cratons of the globe (Jayananda et al. 2020). ...
Subduction-related kimberlite-borne eclogite xenoliths of the Precambrian age may provide significant information about the evolution and recycling of a subducting crust as exhumed/orogenic
eclogites of the pre-Mesoproterozoic time-frame are globally rare. In this paper, we report a kimberlite-borne eclogite xenolith from the diamondiferous Kalyandurg kimberlite cluster of the Eastern Dharwar Craton, India, which contains a plethora of ultra-high-pressure minerals such as coesite, majoritic garnet, and supersilicic K-rich omphacite. The presence of these ultra-high-pressure minerals is confirmed by in-situ X-ray diffractometry, laser Raman spectra and electron probe microanalysis. The presence of coesite undisputedly pinpoints a subduction origin for the eclogite at ~2.8 GPa pressure, which corresponds to ~100 km depth. The geothermobarometric estimations involving garnet–omphacite–kyanite–coesite reveal that such an eclogitic assemblage equilibrated at ~5–8 GPa (~175–280 km) pressure during ultra-deep subduction. The textural relationship between omphacite, coarse-grained garnet and majoritic garnet coupled with the laser Raman spectra and geobarometric estimations obtained from the majoritic garnet demonstrate that the majoritic garnet formed at ~8–19 GPa (~280–660 km) owing to disassociation of omphacite and coarse-grained garnet to majoritic garnet during increment of pressure up to the mantle transition zone. Thus, the mineralogical and geothermobarometric data suggest that the studied eclogite possibly travelled down to the mantle transition zone before it was rapidly carried up by a pre-Mesoproterozoic mantle plume, and subsequently entrained as a xenolith by the Mesoproterozoic (~1.1 Ga) kimberlite.
... Compared to the other well-studied Archean Cratons across the Earth (Isua Greenstone Belt, Greenland: [38,39]; Pilbara Craton: [40][41][42], Kaapvaal Craton: [43,44], China [45] Dharwar Craton: [46][47][48]; Singbhum Craton: [49][50][51][52][53]; Bundelkhand Craton: [26,54], among many more), the geological history of the Bastar Craton is loosely constrained. Ramakrishnan and Vaidyanathan [19] compiled geological and geochronological information for Bastar Craton. ...
... The Eastern Dharwar Craton contains 2.7-2.5 Ga TTG gneisses as the major component of the basement while some 3.38-3.0 Ga rocks are exposed close to the boundary between Eastern and Western Dharwar cratons (Dey et al., 2017;Jayananda et al., 2018Jayananda et al., , 2020bWang et al., 2020). Potassic granites within the craton were largely emplaced at 2.61-2.51 ...
River sand detrital zircons from several rivers flowing through the Peninsular Indian cratons were analyzed for U-Pb, Lu-Hf, and O isotopes to characterize the Precambrian crustal evolution of the Indian Shield and to constrain its role in the early supercontinent cycles. Analyzed river sand zircon samples exhibit a prominent age grouping at 2.7-2.4 Ga and additional peaks at 1.8-1.7 Ga, 1.0-0.8 Ga, and 0.6-0.5 Ga. The time-related Hf and O isotope trends of the Indian zircons display a slight offset from the global trends. The age peaks and distinct Hf-O isotopic compositions of the Indian zircons also lack coherence with the global patterns associated with supercontinent cycles, implying a discrete crustal evolutionary history for the Indian Shield. Their contrasting εHf-age trajectories indicate that the Indian Shield was accreted to the Columbia/Nuna supercontinental framework through a collision event that postdated the 2.1-1.8 Ga global-scale orogeny, and was part of a long-lived subduction system along the margin of Rodinia. This implies that the Indian Shield occupied a peripheral paleo-position during the assembly of the two Precambrian supercontinents. The ca. 2.6-2.4 Ga Indian river sand zircons have remarkably low δ¹⁸O values that are distinct from the global zircon O isotopic record. This period coincides with the fragmentation of the Archean supercraton and the flaring-up of a subaerial Large Igneous Province that facilitated the generation of ¹⁸O-depleted magmas. Depending on whether the 2.6-2.4 Ga zircons with low δ¹⁸O values (<4.7‰) are considered or not, approximately 2.77‰ or 1.78‰ rise in the zircon δ¹⁸O values occurred between ca. 2.4 Ga and ca. 1.87 Ga (decoupled from zircon Lu-Hf isotopes), further suggesting that the fine-grained sediments were enriched in ¹⁸O after the Great Oxidation Event. These high δ¹⁸O sediments were subsequently incorporated into the magmatic systems resulting in elevated δ¹⁸O in the zircons crystallizing from such melts. Nevertheless, a ca. 0.9 Ga peak of >10‰ in δ¹⁸O system was followed by a pronounced δ¹⁸O drop at ca. 0.759 Ga, suggesting that δ¹⁸O of recycled sedimentary reservoirs was not the only controlling factor for zircon δ¹⁸O characteristics. Variations in the O-Hf data for Indian and global zircons could be attributed to the initiation of the supercontinent cycle at ca. 2.0 Ga. A significant volume of the juvenile crust was added during the 3.0 - 2.7 Ga mafic magmatism in the Indian Shield, as underlined by the Hf model ages of all the zircon grains and those with mantle-like δ¹⁸O signatures. A gradual addition of continental crust during ca. 3.6 Ga to 3.1 Ga can also be deduced, with the oldest crust being derived from the mantle at ca. 4.4 Ga.
... The two major episodic occurrences are broad volcano-dominated greenstone belts between 2.7 and 2.54 Ga (Manikyamba et al., 2015) and huge North-South trending calc-alkaline plutonic bodies between 2.57 and 2.52 Ga Peucat et al., 2013). A variety of magmatic rocks from the late Archaean period have been reported in EDC, including granodioritic gneisses, calc-alkaline granites, Neoarchean high-Mg diorite, sanukitoids (sanukitoid), and K-rich granite (Peucat et al., 1993;Chadwick et al., 2000;Peucat et al., 2013;Jayananda et al., 2018Jayananda et al., , 2020Bhaskar Rao et al., 2020). Precambrian granitic rocks emplaced in diverse tectonic settings have also been reported in EDC (Sesha Sai, 2013, 2019. ...
Quartz is one of the essential minerals that crystallize at a late-stage in granitic rocks. Understanding the process of crystallization of quartz is thus an important aspect that helps to decipher the late-stage crystal growth phenomenon in felsic magma. These processes are described from crystal shape and studied internal crystal structure of the quartz from the Sircilla granite pluton (SGP). In the present study, the shape and size of quartz crystals and their internal crystal structures were acquired by Cathodoluminescence Scanning Electron Microscopy (CL-SEM). The internal structures of SGP quartz crystals show an oscillatory zonation and partial oscillatory zonation, which may have been formed due to the magmatic processes. Microcracks, heterogeneous CL, dark homogeneous CL, or recrystallized quartz can be seen developed due to local deformation or hydrothermal processes. Magmatic flow conditions forced mineral rotation, which resulted in quartz crystals in subhedral to anhedral forms with fluid-filled fractures. The fluid-filled fractures detected by Energy-Dispersive Spectrometry (EDS) provide a clear distribution of discontinuities filled with elements such as Ca, Na-rich fluid during late magma-fluid interactions. The collective evidences in the present study describe the magmatic to late magmatic stages associated with deformation conditions from the SGP.
... Eastern Dharwar Craton (EDC) in India witnessed extensive granitic activities in a broad time interval between 2.52 and 2.65 Ga (Buhl et al. 1983;Friend and Nutman 1991;Jayananda et al. 1995Jayananda et al. , 2000Jayananda et al. , 2020Nutman et al. 1996;Chadwick et al. 2000;Rogers et al. 2013;Dey et al. 2017). Multiple ore zones in the EDC, such as Kolar Schist Belt, Ramagiri-Penakacherla Schist Belt (RPSB), Hutti Schist Belt surrounded by granitic bodies which are known to be aAected by hydrothermal activities, also evident from abundant quartz and quartzofeldspathic veins (Sarma et al. 2008;Bhattacharya et al. 2014;Prabhakar and Shareef 2015;Roy 2015;Mishra et al. 2018). ...
Orogenic, gold hosting greenstone belts in Eastern Dharwar Craton (EDC), India are surrounded by granitic bodies and affected by hydrothermal activities. In this paper, field, petrographic and geochemical study of granites coupled with LA-ICP-MS analyses of the trace element abundances of zircons from the granites of the Hungund Schist belt, the northwest continuation of Kolar and Ramgiri schist belts in EDC is discussed to report the effects of hydrothermal alteration. The geochemical characteristics such as Nb/Ta ratio (~5), low Zr/Hf ratio, depleted ƩREE from the granites close to the shear zone provide clue towards magmatic-hydrothermal interactions in the source rock, while, granites away from the shear zone show feeble alteration signature. Zircons from the altered granite contain higher concentration of trace elements, LREE enrichment and high ƩREE with weak positive Ce anomalies and strong negative Eu anomalies indicating hydrothermal alteration. While, the zircons from unaltered granites have low to moderate trace element concentration and REE patterns similar to unaltered magmatic zircons. Contrarily, in the ternary plot, the zircons from unaltered granites show minor affinity towards metasomatized hydrothermal field instead of clustering under purely magmatic field. Hence it is considered that these zircons from unaltered granite underwent some minor alteration effect, while the zircons from altered granite are affected completely due to hydrothermal activity. Therefore, it is concluded that movement of hydrothermal solution has taken place along the shear zone in the study area which has affected more on granites close to the shear zone and minor effects are seen in granites away from the shear zone. Hydrothermal alterations often results in concentration of specific economic minerals and hence, can be used as an indicator for mineral exploration.
... The Meso-to Neoarchean Dharwar Craton (DC) with its characteristic association of Tonalite-Trondhjemite-Granite gneisses (TTG)greenstone belt (e.g. Swami Nath and Ramakrishnan, 1981;Ranjan et al., 2020;Jayananda et al., 2020) is bordered to the south by a vast expanse of granulite facies rocks (reviewed in Drury et al., 1984;Braun and Kriegsman, 2003;Peucat et al., 2013;Glorie et al., 2014;Brandt et al., 2014;Santosh, 2020;Fig. 1). ...
Lying to the south of the Dharwar Craton, the expansive granulite massif, known as the Granulite Terrane of South India, preserves protracted history of the earth from ca. 3400–500 Ma. It has been divided into Northern and Southern Granulite Terranes, separated by the Moyar-Bhawani Shear Zone. Unlike the Archean Northern Granulite Terrane, the rocks of the Southern Granulite Terrane bear imprints of magmatism, sedimentation and metamorphism for over 2 billion years from Neoarchean to Cambrian. The Southern Granulite Terrane is subdivided into three geographic divisions, namely Nilgiri-Namakkal Block, Madurai Block and Trivandrum Block, separated from each other by the Palghat-Cauvery Shear Zone and Achankovil Shear Zone. The Nilgiri-Namakkal Block preserves ca. 2900–2550 Ma ensemble of mafic-ultramafic rocks, interpreted as the remnants of Archean oceanic crust formed in a suprasubduction zone. Several phases of felsic magmatism between ca. 2840–2500 Ma and vestigial records of ca. 2600–2530 Ma sedimentation and ultrahigh-temperature metamorphism are found in the Nilgiri-Namakkal and Madurai Blocks. These felsic rocks, showing distinct TTG-type chemistry, rarely record the late Archean ultrahigh-temperature metamorphism, but underwent extensive high-pressure metamorphism (>11 kbar) along 43–65 °C/kbar geothermal gradients during early Siderian (2490–2440 Ma). The late Archean felsic magmatism and the superimposed early Siderian high-pressure metamorphism has been explained by ‘peel-back’ convergence mechanism that takes in account higher mantle temperature during this time. The Siderian crust of the Nilgiri-Namakkal, Madurai and possibly Trivandrum Blocks received extensive multi-phase sedimentation, presumably in a stable continental shelf, spanning from ca. 1800–700 Ma, partially sourced from the Northern Granulite Terrane and Western Dharwar Craton. This together with the unbroken magmatic and metamorphic episodes across the Palghat-Cauvery Shear Zone do not support the view that Palghat-Cauvery Shear Zone represents a Neoproterozoic suture along which the ‘Mozambique Ocean’ was closed. The remarkable similarity of geological events across the Moyar-Bhawani Shear Zone supports that the Northern and Southern Granulite Terranes formed a coherent block at least from ca. 2600 Ma. The late Paleoproterozoic (ca. 1740–1620 Ma) subduction related granitoid magmatism signifies crustal growth during Columbia supercontinent formation. The (Northern + Southern) Granulite Terrane coherent block was extended (without opening of an ocean basin) and received magmatism ranging from alkaline–carbonatite to A-type granitoids during ca. 850–600 Ma, which is tentatively related to the breakdown of Rodinia. Subsequent Ediacaran-Cambrian high- to ultrahigh-temperature metamorphism and associated magmatism, preponderant throughout the Southern Granulite Terrane, marks the last phase of tectonothermal event in the region and is widely correlated to the amalgamation of Gondwana supercontinent.
The geological history of the Northern and Southern Granulite Terranes shows striking resemblance with the Antongil-Masora-Bemarivo and Antananarivo-Ikalamavony–Itremo domains of Madagascar, respectively. Formation of a unified Indo-Madagascar landmass since ca. 2600 Ma or earlier is consistent with the extant geological evidence. Precambrian rocks of Sri Lanka share the Ediacaran-Cambrian metamorphism with the Southern Granulite Terrane and the basements rocks of Madagascar. However, inadequate data from Sri Lanka does not allow us to infer if Sri Lanka was part of the coherent Indo-Madagascar landmass since late Archean.
... Ga poly-phase tonalite-trondhjemite granodiorite (TTG) suites dominate the western Dharwar vraton (WDC) compared to the 2.7-2.5 Ga calc-alkaline granitic rocks of the eastern Dharwar craton (EDC) where some older remnant granites preserved (3.3-3.0 Ga) (Meen et al. 1992;Jayananda et al. 2020). Two major episodic events in the EDC include large volcanicdominated greenstone belts at 2.7 -2.54 Ga (Manikyamba et al. 2009;Manikyamba and Kerrich, 2012) and extensive North-South trending calc-alkaline plutonic bodies at 2.57-2.52 ...
Field, petrography, and crystal size distributions (CSD) of different lithological variants from Sirsilla granitic pluton (SGP), southern India, is described here to understand operative magmatic processes. The SGP contains many mafic microgranular enclaves (MMEs) and syn-plutonic dykes. The contact relationship between MMEs and the host granite is often diffusive or gradational and rarely sharp, implying disaggregation and under-cooling of MMEs. Petrographic features like resorption textures, quartz ocelli, and the poikilitic nature of the large K-feldspar grains enclosed within plagioclase indicate interaction and magma mixing/mingling processes in an open magma chamber. Bladed biotite and acicular apatite grains in MMEs are due to rapid crystallization during the magma mingling process. The CSD curves generated for plagioclase provide an inverse relationship between population density and crystal size. Multiple crystal populations, i.e., a gently sloping line for the core samples and a steeply sloping line for margin samples, are interpreted to be caused by the mafic — felsic magma mixing and mingling processes.
... Wyman, 1999;Mole et al., 2021), India (e.g. Jayananda et al., 2013Jayananda et al., , 2020, Greenland (Dziggel et al., 2019;Nutman et al., 2021) and Southern Africa (e.g. Anhaeusser, 2019;Schneider et al., 2019). ...
Greenstone belts are important windows to the geodynamic evolution of the early Earth. Comprehensive and continuous stratigraphic columns of greenstone belt sections are rarely preserved in South America due to polyphasic deformation, metamorphism and recycling at the cratonic margins and thick regolith layers mainly yielded by heavy weathering. Some recent studies in rocks intersected by drill cores have been pivotal to unveil the stratigraphy of Archean and Paleoproterozoic belts in covered greenstone belts in the São Francisco and Amazonian cratons. This work investigates drill cores from the Pitangui Greenstone belt (PGB), in the Southern São Francisco Craton (SSFC). Rapid localized tectonic transformations are revealed by lithogeochemistry. Two distinct lithochemical associations occur spatially intercalated. The first one is the komatiite-tholeiite association that presents plume signature (low LREE/HREE and no negative HFSE anomalies), with associated low Al2O3/TiO2 and elevated (Gd/Yb)N indicative of a garnet-bearing source, which was also slightly depleted, as suggested by the subchondritic Nb/Ta ratios. In contrast the arc-basalts, andesites and graywackes association displays high LREE/HREE and negative Nb and Ti signatures, which may indicate a subduction-like environment or of crustal contamination. Important part of the subduction-like package are the Nb-enriched basaltic andesites and related volcaniclastic rocks. The presence of this lithotype indicates that metasomatism of the mantle by slab-derived melts was probably an important mechanism in the PGB. It is argued that the assemblages mark a transient stratigraphy from a thick mafic-ultramafic domain generated in an oceanic-plateau to an overlying volcaniclastic-sedimentary dominated package likely generated in an arc-like environment at 2.77 Ga. The intercalation of plume and arc products in the PGB is interpreted as an example of plume-arc interaction, which seems to have been more common in the Archean, as evidenced by numerous occurrences interpreted as such worldwide.
... Several studies point to the reworking of pre-existing continental crust, dominated by TTG, in the generation of granitic rocks in the Archean (e.g., Jayananda et al., 2006Jayananda et al., , 2020Mikkola et al., 2012;Laurent et al., 2014a;Moyen et al., 2021). Partial melting of TTG produces rocks with high SiO 2 (70-75 wt%), K 2 O (2-6 wt%) and Al 2 O 3 /(FeO t + MgO), similar to the analyzed biotite-two-mica granites (BTMG), transitional granitoids group 1 (TRAN.G1) and A-type granites group 1 (AGG1) (Fig. 8; Johnston, 1992, 1993;Patiño Douce, 1997, 2005Patiño Douce and McCarthy, 1998;Castro, 2004;Watkins et al., 2007). ...
The Archean Eon was a time of geodynamic changes. Direct evidence of these transitions come from igneous/metaigneous rocks, which dominate cratonic segments worldwide. New data for granitoids from an Archean basement inlier related to the Southern São Francisco Craton (SSFC), are integrated with geochronological, isotopic and geochemical data on Archean granitoids from the SSFC. The rocks are divided into three main geochemical groups with different ages: (1) TTG (3.02–2.95 Ga); (2) medium- to high-K granitoids (2.79–2.72 Ga); and (3) A-type granites (2.7–2.6 Ga). The juvenile to chondritic (Hf-Nd isotopes) TTG were divided into two sub-groups, TTG 1 (low-HREE) and 2 (high-HREE), derived from partial melting of metamafic rocks similar to those from adjacent greenstone belts. The compositional diversity within the TTG is attributed to different pressures during partial melting, supported by a positive correlation of Dy/Yb and Sr/Zr, and batch melting calculations. The proposed TTG sources are geochemically similar to basaltic rocks from modern island-arcs, indicating the presence of subduction processes concomitant with TTG emplacement. From ∼2.85 Ga to 2.70 Ga, the dominant rocks were K-rich granitoids. These are modeled as crustal melts of TTG, during regional metamorphism indicative of crustal thickening. Their compositional diversity is linked to: (i) differences in source composition; (ii) distinct melt fractions during partial melting; and (iii) different residual mineralogies reflecting varying P–T conditions. Post-collisional (∼2.7–2.6 Ga) A-type granites reflect rifting in that they were at least partially coeval with extension-related dyke swarms, and they are interpreted as differentiation or partial melting products of magmas derived from subduction-modified mantle. The sequence of granitoid emplacement indicates subduction-related magmatism was followed by crustal thickening, regional metamorphism and crustal melting, and post-collisional extension, similar to that seen in younger Wilson Cycles. It is compelling evidence that plate tectonics was active in this segment of Brazil from ∼ 3 Ga.
The Wayand-Nilambur granulite terrain in Kerala (India) is well-known for vein-type gold mineralization within the quartz veins. Based on the previous geological, petro-physical and geophysical characteristics of Wayand-Nilambur gold deposit, the integrated geophysical survey was carried out to delineate the favourable prospecting zones. In the present study, detailed ground geophysical surveys, that is, magnetic, electrical resistivity/chargeability surveys were carried out in an area of 2 km 2 in Kat-tikallu and Kalkulam blocks to delineate the ore deposits in terms of depths and extensions through the structural shear zone, locate the anomalous sources and the geometry of the Au-rich sulphide zone and its depth continuity in the subsurface. These surveys brought out prominent resistivity and chargeability zones over known magnetite-quartz veins that are associated with low-grade sulphide bands at the central part of the Kattikallu block. The prominent potential zone is characterized by strong bipolar magnetic anomalies over the quartz veins. Based on the 2D inversion of resistivity data, the resistivity low zone of order 80-600 Ω m and chargeability of 21-25 mV/V are observed at a depth range of 5-20 m. In Kalkulam block, the magnetic survey has also brought out high intensity anomaly zones over quartz veins, the same quartz veins are mapped by high chargeability of 10-35 mV/V and low resistiv-ity of 185-400 Ω m. The dipole-dipole configuration produces two parameters, that is, resistivity and chargeability, these methods distinguish the anomalies along the two selected profiles in the study area. An attempt was made for combining the resis-tivity and chargeability values to identify the anomalous zone boundaries. The results of inversion indicated that the conductive bodies are located at the subsurface, with depths ranging from 5 to 25 m. Based on this integrated geophysical study, we suggested two borehole sites for further geo-scientific studies in view of mineralization. K E Y W O R D S gold mineralization, magnetic, quartz vein, resistivity
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.
This article highlights the scientific studies published during the last four years from the Paleo-Neoarchean Singhbhum, Dharwar and Bastar cratons. The geochronological and geochemical studies related to TTGs, granitoids, supracrustal sequences and dyke swarms have been documented separately. The Paleoarchean granitoids of Singhbhum Craton are sourced from reworked depleted mantle derived juvenile crust, reflecting episodic magmatic thickening and lower crustal delamination indicative of early crustal maturation in the craton. It is suggested that the continental crust in the Singhbhum Craton experienced a compositional transition from sodic TTG to more potassic transitional TTG and granites at 3.4 Ga. Isostatic modelling suggests that the Singhbhum Craton became subaerial by 3.3–3.2 Ga and favoured terrestrial to shallow marine sedimentation. The circular Simlipal Complex has been suggested as an impact structure and the volcanic rocks are dated at 2.84 Ga, while the sediments are sourced from 3.63 to 2.54 Ga granitoids. In the Dharwar Craton, the oldest Holenarsipur nucleus has been coined as cratonic core representing oceanic plateau, oceanic arc and oceanic crust accreted through horizontal tectonics. Based on the stable isotopes and the U–Pb zircon ages of the sediments, the Archean ocean composition, ocean oxygenation and initiation of biogeochemical processes have been documented. A new tectonic classification of Bastar Craton into western, eastern and central Bastar orogens has been proposed. The co-existence of Dharwar, Singhbhum and Bastar Cratons upto 3.0 Ga has been inferred based on the U–Pb ages of the detrital zircons of the metasediments from the Bastar Craton.
A new cataclastic shear zone, exposed between Rangapur and Shivannaguda, from the East Dharwar Craton, is reported here for the first time. The shear zone is characterized by breccias, cataclasites, mylonites and intense fracturing suggesting that the rocks are subjected to brittle-ductile transitional deformation. Image interpretation and preliminary field observations suggest that the shear zone stands out as a potential field example for demonstration and future geoscientific investigations.
This paper presents the petrology and geochemistry of SE part of Dharmaram granites which are occur near to Karimnagar Granulite belt area, NE of Karimnagar district of Telangana State within Eastern Dharwar Craton (EDC). Geologically, the study area is dominated by granitic rocks of Precambrian age. The lithological variants in the study area mostly covered by pink granites, gneissic grey granites, coarse-grained pegmatite veins, quartz veins and epidote veins are observed as cross-cutting or parallel to the granitoids and are sometimes folded and deformed. The monzogranite is megascopically medium to coarse grained, inequigranular, porphyritic in texture and composed of K-feldspar (microcline perthite; 25-36% by volume), quartz (27-31%) and plagioclase (31-39%) in the order of decreasing abundance. Mafic minerals include biotite (2-8 %) and magnetite, titanite, zircon and apatite are accessory phases. Presence of discrete plagioclase grains in substantial proportions suggests sub-solvus crystallisation of the parental magma. Geochemically monzogranite are having high SiO2 (69-73 wt. %), high K2O (4-6wt. %) and low Na2O (3 wt.%). The chondrite normalised REE patterns of monzogranite display LREE-enriched and HREE-depleted patterns with negative Eu anomaly. The monzogranites show high LREE/HREE ratio that hints at either fractional crystallisation or an LILE-enriched source. The negative Eu anomaly indicates the role of feldspar in the genesis of the monzogranites. And one sample shows positive europium anomaly indicate role plagioclase in equilibrium conditions.
Coupled paleomagnetic and geochronologic data derived from mafic dykes provide valuable records of continental movement. To reconstruct the Proterozoic paleogeographic history of Peninsular India, we report paleomagnetic directions and U-Pb zircon ages from twenty-nine mafic dykes in the Eastern Dharwar Craton near Hyderabad. Paleomagnetic analysis yielded clusters of directional data that correspond to dyke swarms at 2.37 Ga, 2.22 Ga, 2.08 Ga, 1.89–1.86 Ga, 1.79 Ga, and a previously undated dual polarity magnetization. We report new positive baked contact tests for the 2.08 Ga swarm and the 1.89–1.86 Ga swarm(s), and a new inverse baked contact test for the 2.08 Ga swarm. Our results promote the 2.08 Ga Dharwar Craton paleomagnetic pole (43.1° N, 184.5° E; A95 = 4.3°) to a reliability score of R = 7 and suggest a position for the Dharwar Craton at 1.79 Ga based on a virtual geomagnetic pole (VGP) at 33.0° N, 347.5° E (a95 = 16.9°, k = 221, N = 2). The new VGP for the Dharwar Craton provides support for the union of the Dharwar, Singhbhum, and Bastar Cratons in the Southern India Block by at least 1.79 Ga. Combined new and published northeast-southwest moderate-steep dual polarity directions from Dharwar Craton dykes define a new paleomagnetic pole at 20.6° N, 233.1° E (A95 = 9.2°, N = 18; R = 5). Two dykes from this group yielded 1.05–1.01 Ga 207Pb/206Pb zircon ages and this range is taken as the age of the new paleomagnetic pole. A comparison of the previously published poles with our new 1.05–1.01 Ga pole shows India shifting from equatorial to higher (southerly) latitudes from 1.08 Ga to 1.01 Ga as a component of Rodinia.
A spectacular chain of radiating mafic dykes with precise Pb–Pb and U–Pb baddeleyite ages (~2087–2080 Ma) are reported from the Dharwar Craton, southern India. These radiating dykes are spatially disposed over an extent of 70,000 km2 and flank the northern and western flanks of the Cuddapah Basin in the Eastern Dharwar Craton. Here, we present the geochemical results of four precisely dated dykes and 15 similar trending dykes (N134ºW–N28ºE), which gives first insight into the geochemical behaviour, mantle source characteristics, and geodynamic history of the mafic magmatic event at ~2.08 Ga. Field observations, mineralogy and textures classify them as fine to coarse-grained dolerites, exhibiting insignificant post-magmatic alterations. Basaltic komatiite to high-Fe tholeiitic character, variation in Mg-number (Mg#) (50–75) and crystallisation trends suggest their primary to evolved nature. Enriched light rare earth element (LREE) patterns, negative Nb–Ti anomalies and moderate Th/Nb values (0.5–0.7) in few samples suggest crustal input in the genesis. Petrogenetic modelling using batch melting equation and elemental proxies (La/Yb, Dy/Yb) indicate derivation of these dykes from a heterogeneous spinel to spinel-garnet lherzolitic source. The study postulates that a mantle plume activity at ~2.08 Ga, led to mantle upwelling, causing domal uplift beneath the Cuddapah Basin and emplacement of the Cuddapah dyke swarm (~2.08 Ga). This magmatism was followed by crustal extension and thinning, supervened by thermal relaxation and subsidence, that ultimately led to the formation of the Cuddapah Basin. Several geophysical surveys also concur a high-density material peculiar to a mafic volcanic province present beneath the southwest Cuddapah Basin. The Cuddapah dyke swarm is coeval with Fort Frances dykes, Cauchon Lake dykes and Lac Esprit dykes of the Superior Craton, but their paleolatitudes and geochemical behaviour do not represent an obvious match for the two cratons. The study highlights the significant role of ~2.08 Ga mafic dyke swarm in the evolution of the Cuddapah Basin.
The Narmada‐Son lineament is a major tectonic feature in Central India. Here we employ data from geodetic measurements of crustal deformation and upper mantle P‐wave travel time tomography and propose that the Narmada‐Son palaeo‐rift is a present‐day diffuse plate boundary that segments the continental part of the Indian “composite” plate into two component plates, the North and South‐Indian plate. Tomographic images reveal that the collision of an indenter, associated with the relatively faster moving South‐Indian plate, with the Narmada‐Son diffuse plate boundary acted as a pivot point exerting a torque on one of the component plates along with mechanical rotation. This new proposal along with a new identity of the Narmada‐Son diffuse plate boundary is in better agreement with the estimated location of the relative Euler pole across two respective component plates. Our model is also able to better explain the Indian “composite” plate hypothesis, diversity in the topographic architecture along the rift, and contrasting deformation styles on either side of the diffuse plate boundary. This represents the regional seismo‐tectonics across Central India. Schematic inset shows the tectonic setting of the diffuse plate boundary. The spatial location of the relative pole between Component plates A and B must lie close to the diffuse zone and on either side of the pole compression and extension will occur. Regional tectonics across Central India. Faults are marked with a black colour (After GSI report 1995, Acharya et al., 1998, Deshpande and Gupta, 2013), and rivers are marked with a blue colour. Current earthquakes are from United States Geological Survey (USGS) and International Seismological Centre (ISC), and historical earthquakes from Martin and Szeliga (2010) are marked by yellow colour. Significant earthquakes are marked by red stars along with available focal mechanisms. Schematic inset shows the concept of the diffuse plate boundary and its tectonic setting. It can be noted that the spatial location of the relative pole between two plates (Component plate A (N‐India) and B (S‐India)) must lie close to the diffuse zone and on either side of the pole compression and extension will occur. The indenter (or aseismic ridge) collides with the diffuse plate boundary at the pinning point (or location of the plate pair pole), which may produce transform faulting where the diffuse plate boundary microblock ends. Blue squares and vectors represent the Euler rotation pole of the North‐Indian and South‐Indian plates. Note that the South‐Indian plate is moving faster than the North‐Indian plate since the spatial location of the Euler pole for South‐Indian plate is far from the Euler pole of the North‐Indian plate (~10° longitudinal difference).
The Neoarchaean to Palaeoproterozoic Betsiboka Suite of Madagascar mainly consists of granitoid orthogneiss and migmatite variably metamorphosed to amphibolite and granulite facies during the late Neoproterozoic. New U-Pb zircon data yielded emplacement ages of 2512 ± 17 Ma, 2507 ± 17 Ma, 2493 ± 14 Ma, and 2485 ± 16 Ma. Zircon d ¹⁸ O data indicate involvement of the continental crust during magma genesis. Mostly positive ε Hf (t) values plot slightly below the depleted-mantle curve. The isotopic data favour mixing between an older Archaean crustal source and a juvenile depleted-mantle source. Possible local crustal candidates for the older component in the exposed Malagasy basement are Neoarchaean mafic gneisses in the Tsaratanana Complex and the ca. 3200 Ma Nosy Boraha orthogneiss in the Antongil Domain. The new isotopic data support interpretations from previous studies investigating the genesis of Tonian plutonic rocks advocating crustal melting of a Mesoarchaean crustal source in the Antananarivo Domain. In addition, detrital zircon in the Proterozoic Ambatolampy Group and Itremo Group likely had a local source with the same isotopic composition as the Betsiboka Suite. Our study demonstrates the significance of multi-isotopic data from zircon for constraining the sources of detrital zircon grains in sedimentary provenance studies.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.6282672
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.
A widely spaced Neoarchean shear zone network traverses the granite-greenstone terrains of the Western Dharwar craton (WDC). The NNW-SSE trending Balehonnur shear zone traverses the largest part of the preserved tilted Archean crustal ensemble in the Western Dharwar craton (WDC) from the amphibolite-granulite transition in the south to greenschist facies in the north and eventually concealed under Deccan lava flows. Published tectonic fabrics data and kinematic analysis, with our data reveal a sinistral sense of shearing that effectuate greenstone sequences, Tonalite-Trondhjemite-Granodiorite Gneisses (TTG), and Koppa granite as reflected in variable deformation and strain localization. A profound increase of strain towards the core of the shear zone in the ca. 2610 Ma Koppa granite is marked by a transition from weak foliation outside the shear zone through the development of C-S structures and C-prime fabrics, mylonite to ultramylonite. The mineral assemblages in the Koppa granite and adjoining greenstone indicate near peak P-T conditions of 1.2 Gpa, 775-800°C following a slow cooling path of 1.0 GPa and 650°C. Field-based tectonic fabrics data together with U-Pb zircon ages reveal that the Koppa granite emplaced along the contact zone of Shimoga-Bababudan basin ca. 2610 Ma, coinciding with the emplacement of ca. 2600 Ma Arsikere-Banavara, Pandavpura, and Chitradurga granites further east which mark the stabilization of WDC. Significant variation in major element oxide (SiO2 = 56-69 wt.%) together with high content of incompatible elements (REE, Nb, Zr, and Y) and high zircon crystallization temperatures (~1000°C) of Koppa granite suggests derivation by partial melting of composite sources involving enriched uppermost mantle and lower crust. The development of widely spaced shear zones is probably linked to the assembly of eastern and western blocks through westward convergence of hot oceanic lithosphere against already cratonized thick colder western block leading to the development of strain heterogeneities between greenstone and TTGs due to their different mineral assemblages leading to rheological contrast in the cratonic lithologies.
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.
The crust-mantle dynamic system of the formation and evolution of the early Earth crust is a frontier field in current earth science. This paper mainly introduces the popular dynamic models, comprising the stagnant-lid, mantle plume, sagduction, dripping, hot plate subduction and flat plate subduction models. Geological signatures and thermomechanical experiments suggest that the above-mentioned first four dynamic models most likely occurred in the early Archean under high mantle potential temperature and high thermal state conditions although they may have survived in small scales to the late Archean, while the hot- and flat-plate subductions chiefly operated during the late Archean as the mantle potential temperature decreasing although they could have started since the Paleoarchean. The late Mesoarchean is a crucial transition period of the crust-mantle dynamic regimes, the diversified geodynamic regimes can coexist, for example, mantle plume and hot plate subduction, and pre-subduction buckling, sagduction and dripping mechanism. These regimes possibly emerged alternately, affected each other or operated in combination. Therefore, we believe that the Archean geodynamic regime has obvious variations as the mantle potential temperature decreasing from early to late Archean, of which the transition may be an interrelated and gradual evolved process rather than a sudden event.
Resolving the modes of mantle convection through Earth history, i.e. when plate tectonics started and what kind of mantle dynamics reigned before, is essential to the understanding of the evolution of the whole Earth system, because plate tectonics influences almost all aspects of modern geological processes. This is a challenging problem because plate tectonics continuously rejuvenates Earth's surface on a time scale of about 100 Myr, destroying evidence for its past operation. It thus becomes essential to exploit indirect evidence preserved in the buoyant continental crust, part of which has survived over billions of years. This contribution starts with an in-depth review of existing models for continental growth. Growth models proposed so far can be categorized into three types: crust-based, mantle-based and other less direct inferences, and the first two types are particularly important as their difference reflects the extent of crustal recycling, which can be related to subduction. Then, a theoretical basis for a change in the mode of mantle convection in the Precambrian is reviewed, along with a critical appraisal of some popular notions for early Earth dynamics. By combining available geological and geochemical observations with geodynamical considerations, a tentative hypothesis is presented for the evolution of mantle dynamics and its relation to surface environment; the early onset of plate tectonics and gradual mantle hydration are responsible not only for the formation of continental crust but also for its preservation as well as its emergence above sea level. Our current understanding of various material properties and elementary processes is still too premature to build a testable, quantitative model for this hypothesis, but such modelling efforts could potentially transform the nature of the data-starved early Earth research by quantifying the extent of preservation bias.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.
On the contemporary Earth, distinct plate tectonic settings are characterized by differences in heat flow that are recorded in metamorphic rocks as differences in apparent thermal gradients. In this study we compile thermal gradients [defined as temperature/pressure (T/P) at the metamorphic peak] and ages of metamorphism (defined as the timing of the metamorphic peak) for 456 localities from the Eoarchean to Cenozoic Eras to test the null hypothesis that thermal gradients of metamorphism through time did not vary outside of the range expected for each of these distinct plate tectonic settings. Based on thermal gradients, metamorphic rocks are classified into three natural groups: high dT/dP [>775 °C/GPa, mean ∼1110 °C/GPa (n = 199) rates], intermediate dT/dP [775-375 °C/GPa, mean ∼575 °C/GPa (n = 127)], and low dT/dP [<375 °C/GPa, mean ∼255 °C/GPa (n = 130)] metamorphism. Plots of T, P, and T/P against age demonstrate the widespread occurrence of two contrasting types of metamorphism -high dT/dP and intermediate dT/dP -in the rock record by the Neoarchean, the widespread occurrence of low dT/dP metamorphism in the rock record by the end of the Neoproterozoic, and a maximum in the thermal gradients for high dT/dP metamorphism during the period 2.3 to 0.85 Ga. These observations falsify the null hypothesis and support the alternative hypothesis that changes in thermal gradients evident in the metamorphic rock record were related to changes in geodynamic regime. Based on the observed secular changes, we postulate that the Earth has evolved through three geodynamic cycles since the Mesoarchean and has just entered a fourth. Cycle I began with the widespread appearance of paired metamorphism in the rock record, which was coeval with the amalgamation of widely dispersed blocks of protocontinental lithosphere into supercratons, and was terminated by the progressive fragmentation of the supercratons into protocontinents during the Siderian-Rhyacian (2.5 to 2.05 Ga). Cycle II commenced with the progressive reamalgamation of these protocontinents into the supercontinent Columbia and extended until the breakup of the supercontinent Rodinia in the Tonian (1.0 to 0.72 Ga). Thermal gradients of high dT/dP metamorphism rose around 2.3 Ga leading to a thermal maximum in the mid-Mesoproterozoic, reflecting insulation of the mantle beneath the quasi-integral continental lithosphere of Columbia, prior to the geographical reorganization of Columbia into Rodinia. This cycle coincides with the age span of most anorogenic magmatism on Earth and a scarcity of passive margins in the geological record. Intriguingly, the volume of preserved continental crust of Mesoproterozoic age is low relative to the Paleoproterozoic and Neoproterozoic Eras. These features are consistent with a relatively stable association of continental lithosphere between the assembly of Columbia and the breakup of Rodinia. The transition to Cycle III during the Tonian is marked by a steep decline in the thermal gradients of high dT/dP metamorphism to their lowest value and the appearance of low dT/dP metamorphism in the rock record. Again, thermal gradients for high dT/dP metamorphism show a rise to a peak at the end of the Variscides during the formation of Pangea, before another steep decline associated with the breakup of Pangea and the start of a fourth cycle at ca. 0.175 Ga. Although the mechanism by which subduction started and plate boundaries evolved remains uncertain, based on the widespread record of paired metamorphism in the Neoarchean we posit that plate tectonics was established globally during the late Mesoarchean. During the Neoproterozoic there was a change to deep subduction and colder thermal gradients, features characteristic of the modern plate tectonic regime.
The Dharwar Craton in southern peninsular India incorporates well developed Meso-Neoarchean greenstone terranes where komatiite-tholeiite and picrite-boninite-Nb-enriched basalt-high Mg-andesite- and adakite are well-preserved. These lithological associations and their geochemical and geochronological features offer important insights on mantle plume and subduction zone processes in the early Earth. The juxtaposition of these lithounits in most of the greenstone belts of western (WDC) and eastern (EDC) sectors of the Dharwar Craton attests to plume-arc accretion, generation of continental lithosphere and related mineral deposits. The iron and manganese deposits of the greenstone belts of WDC endorse the oxygenated protooceans and biogeochemical processes that are evidenced through the stromatolitic carbonates/cherts. The larger greenstone terranes of WDC and their gradual transition into smaller and fragmented belts appear to reflect a gradual change from plume-arc to arc-continent collision and the Archean higher geothermal gradient. The smaller plates and large number of subduction zones provided ideal setting for the generation of major metallic mineral deposits of Cu, Pb, Zn and Au. The shallow to deeper shelf environment became suitable loci for the deposition of iron and manganese formations. Secular cooling of the mantle was accompanied by a transition from stagnant lid tectonics to rapid development of plate tectonics from 3.5–2.0 Ga with a peak of mantle plumes, crustal growth, BIF deposition and gold mineralization in the greenstone terranes of the Dharwar Craton. The transition from mantle plume activity to subduction zone tectonics recorded in the greenstone belt lithologies of WDC and EDC provide insights into the thermal and tectonic transition in our planet during Archean.
Evidence for episodic crustal growth extending back to the Hadean has recently prompted a number of numerically based geodynamic models that incorporate cyclic changes from stagnant lid to mobile lid tectonics. A large part of the geologic record is missing for the times at which several of these cycles are inferred to have taken place. The cratons, however, are likely to retain important clues relating to similar cycles developed in the Mesoarchean and Neoarchean. Widespread acceptance of a form of plate tectonics by ∼ 3.2 Ga is not at odds with the sporadic occurrence of stagnant lid tectonics after this time. The concept of scale as applied to cratons, mantle plumes and Neoarchean volcanic arcs are likely to provide important constraints on future models of Earth’s geodynamic evolution. The Superior Province will provide some of the most concrete evidence in this regard given that its constituent blocks may have been locked into a stagnant lid relatively soon after their formation and then assembled in the next global plate tectonic interval. Perceived complexities associated with inferred mantle plume – volcanic arc associations in the Superior Province and other cratons may be related to an over estimation of plume size. A possible stagnant lid episode between ∼ 2.9 Ga and ∼ 2.8 Ga is identified by previously unexplained lapses in volcanism on cratons, including the Kaapvaal, Yilgarn and Superior Province cratons. If real, then mantle dynamics associated with this episode likely eliminated any contemporaneous mantle plume incubation sites, which has important implications for widespread plumes developed at ∼2.7 Ga and favours a shallow mantle source in the transition zone. The Superior Province provides a uniquely preserved local proxy for this global event and could serve as the basis for detailed numerical models in the future.
Spinifex-textured, magnesian (MgO >25 wt.%) komatiites from Mesoarchean Banasandra greenstone belt of the Sargur Group in the Dharwar craton, India were analysed for major and trace elements and 147,146Sm-143,142Nd systematics to constrain age, petrogenesis and to understand the evolution of Archean mantle. Major and trace element ratios such as CaO/Al2O3, Al2O3/TiO2, Gd/Yb, La/Nb and Nb/Y suggest aluminium undepleted to enriched compositional range for these komatiites. The depth of melting is estimated to be varying from 120 to 240 km and trace-element modelling indicates that the mantle source would have undergone multiple episodes of melting prior to the generation of magmas parental to these komatiites. Ten samples of these komatiites together with the published results of four samples from the same belt yield 147Sm-143Nd isochron age of ca. 3.14 Ga with an initial εNd(t) value of +3.5. High precision measurements of 142Nd/144Nd ratios were carried out for six komatiite samples along with standards AMES and La Jolla. All results are within uncertainties of the terrestrial samples. The absence of 142Nd/144Nd anomaly indicates that the source of these komatiites formed after the extinction of 146Sm, i.e. 4.3 Ga ago. In order to evolve to the high εNd(t) value of + 3.5 by 3.14 Ga the time-integrated ratio of 147Sm/144Nd should be 0.2178 at the minimum. This is higher than the ratios estimated, so far, for mantle during that time. These results indicate at least two events of mantle differentiation starting with the chondritic composition of the mantle. The first event occurred very early at ∼4.53 Ga to create a global early depleted reservoir with superchondritic Sm/Nd ratio. The source of Isua greenstone rocks with positive 142Nd anomaly was depleted during a second differentiation within the life time of 146Sm, i.e. prior to 4.46 Ga. The source mantle of the Bansandra komatiite was a result of a differentiation event that occurred after the extinction of the 146Sm, i.e. at 4.3 Ga and prior to 3.14 Ga. Banasandra komatiites therefore provide evidence for preservation of heterogeneities generated during mantle differentiation at 4.3 Ga.
We present results of field studies for magmatic processes of 2.57-2.52 Ga calc-alkaline plutonic bodies from three corridors in the eastern Dharwar craton (EDC) corresponding to different crustal levels. At deeper levels plutons are bounded by thick zone of migmatites with numerous melt filled shear bands which often overprinted by incipient charnockite. On the other hand in the mid-to-upper crustal levels plutons show relatively sharp contacts and truncates the adjoining basement. The plutons are composite which comprises voluminous intrusive monzodiorite, quartz-monzonite and porphyritic monzogranite in the central part and minor anatectic granites or diatexite at periphery. Numerous xenoliths, Mafic Magmatic Enclaves (MME), disrupted trains of synplutonic mafic dykes are found in both intrusive and anatectic fades. The plutons show magmatic as well as solid-state plastic fabrics defined by magmatic flow banding and C-S fabrics respectively. Crustal scale shear zone network comprising early melt filled NE trending hot ductile dextral shear bands and slightly later colder NW trending sinistral shear bands defined by rotation of mafic boudins, phenocrysts and C-S fabrics. The internal architecture of plutons is attributed to the crustal scale magma chamber processes where voluminous intrusive magmas emplaced into the crust caused reworking of surrounding basement resulting in production of anatectic magmas. Crystallization of voluminous intrusive magmas in the deep crust probably caused development of fractures to mantle depth causing decompression melting of mantle and resultant mafic magmas penetrated the crystallizing host in magma chambers. Field evidences together with published ages and Nd isotope data reveal a spatial link between late Archaean magmatic accretion, reworking and cratonization.
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
Fluid inclusion studies in the Closepet granite reveal the common occurrence of trapped fluids which show melting temperatures and laser excited Raman spectral characteristics close to those for pure CO2. The quartz-bound inclusions in this polyphase intrusive define two major genetic categories: an earlier CO2-rich fluid with only minor traces of water and a late mixed carbonic-aqueous fluid. Our results indicate that this late Archaean granite body which truncates regional metamorphic isograds has probably been a major carrier of CO2-rich volatiles, subsolidus exsolution and channelisation of which along structural locales caused incipient dehydration and the formation of Kabbaldurga-type arrested granulites in many adjacent localities. -from Authors
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
We present field and petrographic data on Mafic Magmatic Enclaves (MME), hybrid enclaves and synplutonic mafic dykes in the calc-alkaline granitoid plutons from the Dharwar craton to characterize coeval felsic and mafic magmas including interaction of mafic and felsic magmas. The composite host granitoids comprise of voluminous juvenile intrusive facies and minor anatectic facies. MME, hybrid enclaves and synplutonic mafic dykes are common but more abundant along the marginal zone of individual plutons. Circular to ellipsoidal MME are fine to medium grained with occasional chilled margins and frequently contain small alkali feldspar xenocrysts incorporated from host. Hybrid magmatic enclaves are intermediate in composition showing sharp to diffused contacts with adjoining host. Spectacular synplutonic mafic dykes commonly occur as fragmented dykes with necking and back veining. Similar magmatic textures of mafic rocks and their felsic host together with cuspate contacts, magmatic flow structures, mixing, mingling and hybridization suggest their coeval nature. Petrographic evidences such as disequilibrium assemblages, resorption, quartz ocelli, rapakivi-like texture and poikilitically enclosed alkali feldspar in amphibole and plagioclase suggest interaction, mixing/mingling of mafic and felsic magmas. Combined field and petrographic evidences reveal convection and divergent flow in the host magma chamber following the introduction of mafic magmas. Mixing occurs when mafic magma is introduced into host felsic magma before initiation of crystallization leading to formation of hybrid magma under the influence of convection. On the other hand when mafic magmas inject into host magma containing 30–40% crystals, the viscosities of the two magmas are sufficiently different to permit mixing but permit only mingling. Finally, if the mafic magmas are injected when felsic host was largely crystallized (~70% or more crystals), they fill early fractures and interact with the last residual liquids locally resulting in fragmented dykes. The latent heat associated with these mafic injections probably cause reversal of crystallization of adjoining host in magma chamber resulting in back veining in synplutonic mafic dykes. Our field data suggest that substantial volume of mafic magmas were injected into host magma chamber during different stages of crystallization. The origin of mafic magmas may be attributed to decompression melting of mantle associated with development of mantle scale fractures as a consequence of crystallization of voluminous felsic magmas in magma chambers at deep crustal levels.
The strong resilience of the mineral zircon and its ability to host a wealth of isotopic information make it the best deep-time archive of Earth's continental crust. Zircon is found in most felsic igneous rocks, can be precisely dated and can fingerprint magmatic sources; thus, it has been widely used to document the formation and evolution of continental crust, from pluton- to global-scale. Here, we present a review of major contributions that zircon studies have made in terms of understanding key questions involving the formation of the continents. These include the conditions of continent formation on early Earth, the onset of plate tectonics and subduction, the rate of crustal growth through time and the governing balance of continental addition v. continental loss, and the role of preservation bias in the zircon record.
Supplementary material
A compilation used in this study of previously published detrital zircon U-Pb-Hf isotope data are available at http://www.geolsoc.org.uk/SUP18791
New in situ U-Pb-Hf analyses of detrital zircons from across the Archaean Dharwar craton indicate significant juvenile crustal extraction events at ~3.3 and 2.7 Ga, and continuous extraction from 3.7-3.3 Ga. Reworking in the older western block at ~3.0 Ga marks the onset of cratonisation, most likely due to 'modern' plate tectonic processes, while reworking in both the western and younger eastern block at 2.55-2.50 Ga indicates accretion of the two terranes and final cratonisation much later than in most other Archaean terranes (~2.7 Ga). Different patterns of disturbance to the zircon U-Pb systematics reflect variations in both the U content of parent rocks and later metamorphic conditions. Tectonic links are observed between the Kaapvaal and western Dharwar cratons, and between the north China and eastern Dharwar cratons, though none of these links necessarily requires a consanguineous origin.
One of the major obstacles to our understanding of the growth of continental crust is that of estimating the balance between extraction rate of continental crust from the mantle and its recycling rate back into the mantle. As a first step it is important to learn more about how and when juvenile crust is preserved in orogens. The most abundant petrotectonic assemblage preserved in orogens (both collisional and accretionary) is the continental arc, whereas oceanic terranes (arcs, crust, mélange, Large Igneous Provinces, etc.) comprise
The formation and evolution of continental crust in the Early Earth are of fundamental importance in understanding the emergence of continents, their assembly into supercontinents and evolution of life and environment. The Dharwar Craton in southern India is among the major Archean cratons of the world, where recent studies have shown that the craton formation involved the assembly of several micro-continents during Meso- to Neoarchean through subduction-accretion-collision processes. Here we report U-Pb-Hf isotope data from detrital zircons in a suite of metasediments (including quartz mica schist, fuchsite quartzite and metapelite) from the southern domain of the Chitradurga suture zone that marks the boundary between the Western and Central Dharwar Craton. Morphology and internal structure of the zircon grains suggest that the dominant population was derived from proximal granitic (felsic) sources. Zircon U-Pb data are grouped into Paleo-Mesoarchean and Neoarchean to Paleoproterozoic with peaks at 3227 Ma and 2575 Ma. The age spectra of detrital zircon grains, in combination with the Lu-Hf isotopic analyses indicate sediment provenance from magmatic sources with model ages in the range of ca. 3.67 to 2.75 Ga. A transition from dominantly juvenile to a mixture of juvenile and recycled crustal components indicate progressive crustal maturity. The results from this study suggest major crustal growth events during ca. 3.2 Ga and 2.6 Ga in Dharwar. Our study provides insights into continental emergence, weathering and detrital input through river drainage systems into the trench during Eoarchean to Mesoarchean.
We present field and petrographical characteristics, zircon U–Pb ages, Nd isotopes, and major and trace element data for the magmatic epidote-bearing granitic plutons in the Bellur–Nagamangala–Pandavpura corridor, and address successive reworking and cratonization events in the western Dharwar Craton (WDC). U–Pb zircon ages reveal three stages of plutonism including: (i) sparse 3.2 Ga granodiorite plutons intruding the TTG (tonalite–trondhjemite–granodiorite) basement away from the western boundary of the Nagamangala greenstone belt; (ii) 3.0 Ga monzogranite to quartz monzonite plutons adjoining the Nagamangala greenstone belt; and (iii) 2.6 Ga monzogranite plutons in the Pandavpura region. Elemental data of the 3.2 Ga granodiorite indicate their origin through the melting of mafic protoliths without any significant residual garnet. Moderate to poorly fractionated REE patterns of 3.0 Ga plutons with negative Eu anomalies and Nd isotope data with ε Nd (T) = 3.0 Ga ranging from −1.7 to +0.5 indicate the involvement of a major crustal source with minor mantle input. Melts derived from those two components interacted through mixing and mingling processes. Poorly fractionated REE patterns with negative Eu anomalies of 2.6 Ga plutons suggest plagioclase in residue. The presence of magmatic epidote in all of the plutons points to their rapid emplacement and crystallization at about 5 kbars. The 3.2 Ga intrusions could correspond to reworking associated with a major juvenile crust-forming episode, whilst 3.0 Ga potassic granites correspond to cratonization linked to melting of the deep crust. The 2.6 Ga Pandavpura granite could represent lower-crustal melting and final cratonization, as 2.5 Ga plutons are absent 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.
Large igneous provinces (LIPs) and especially their dyke swarms are pivotal to reconstruction of ancient supercontinents. The Dharwar craton of southern Peninsular India represents a substantial portion of Archean crust and has been considered to be a principal constituent of Superia, Sclavia, Nuna/Columbia and Rodinia supercontinents. The craton is intruded by numerous regional-scale mafic dyke swarms of which only a few have robustly constrained emplacement ages. Through this study, the LIP record of the Dharwar craton has been improved by U-Pb geochronology of 18 dykes, which together comprise seven generations of Paleoproterozoic dyke swarms with emplacement ages within the 2.37–1.79 Ga age interval. From oldest to youngest, the new ages (integrated with U-Pb ages previously reported for the Hampi swarm) define the following eight swarms with their currently recommended names: NE–SW to ESE–WNW trending ca. 2.37 Ga Bangalore-Karimnagar swarm. N–S to NNE–SSW trending ca. 2.25 Ga Ippaguda-Dhiburahalli swarm. N–S to NNW–SSE trending ca. 2.22 Ga Kandlamadugu swarm. NW–SE to WNW–ESE trending ca. 2.21 Ga Anantapur-Kunigal swarm. NW–SE to WNW–ESE trending ca. 2.18 Ga Mahbubnagar-Dandeli swarm. N–S, NW–SE, and ENE–WSW trending ca. 2.08 Ga Devarabanda swarm. E–W trending 1.88–1.89 Ga Hampi swarm. NW–SE ca. 1.79 Ga Pebbair swarm. Comparison of the arcuate trends of some swarms along with an apparent oroclinal bend of ancient geological features, such as regional Dharwar greenstone belts and the late Archean (ca. 2.5 Ga) Closepet Granite batholith, have led to the hypothesis that the northern Dharwar block has rotated relative to the southern block. By restoring a 30° counter clockwise rotation of the northern Dharwar block relative to the southern block, we show that pre-2.08 Ga arcuate and fanning dyke swarms consistently become approximately linear. Two possible tectonic models for this apparent bending, and concomitant dyke rotations, are discussed. Regardless of which deformation mechanisms applies, these findings reinforce previous suggestions that the radial patterns of the giant ca. 2.37 Ga Bangalore-Karimnagar dyke swarm, and probably also the ca. 2.21 Ga Anantapur-Kunigal swarm, may not be primary features.
It is estimated that around three quarters of Earth's first generation continental crust had been produced by the end of the Archaean Eon, 2.5 billion years ago. This ancient continental crust is mostly composed of variably deformed and metamorphosed magmatic rocks of the tonalite–trondhjemite–granodiorite (TTG) suite that formed by partial melting of hydrated mafic rocks. However, the geodynamic regime under which TTG magmas formed is a matter of ongoing debate. Using a filtered global geochemical dataset of 563 samples with ages ranging from the Eoarchaean to Neoarchaean (4.0–2.5 Ga), we interrogate the bulk rock major oxide and trace element composition of TTGs to assess evidence for secular change. Despite a high degree of scatter in the data, the concentrations or ratios of several key major oxides and trace elements show statistically significant trends that indicate maxima, minima and/or transitions in the interval 3.3–3.0 Ga. Importantly, a change point analysis of K2O/Na2O, Sr/Y and LaN/YbN demonstrates a statistically significant (>99% confidence) change during this 300 Ma period. These shifts may be linked to a fundamental change in geodynamic regime around the peak in upper mantle temperatures from one dominated by non-uniformitarian, deformable stagnant lid processes to another dominated by the emergence of global mobile lid or plate tectonic processes by the end of the Archaean. A notable change is also evident at 2.8–2.7 Ga that coincides with a major jump in the rate of survival of metamorphic rocks with contrasting thermal gradients, which may relate to the emergence of more potassic continental arc magmas and an increased preservation potential during collisional orogenesis. In many cases, the chemical composition of TTGs shows an increasing spread through the Archaean, reflecting the irreversible differentiation of the lithosphere.
Lamprophyres from the greenstone belts play a crucial role in deciphering tectonic and geodynamic processes operating during the Archean. This study presents a comprehensive mineralogical and geochemical study of three lamprophyre dykes with calc-alkaline to shoshonitic affinities from the Neoarchean Kadiri schist belt, eastern Dharwar craton, southern India. These rocks display porphyritic-panidiomorphic texture, typical of the lamprophyres with amphibole (magnesio-hornblende) as phenocrysts, biotite as microphenocrysts and feldspar, epidote, titanite and apatite confined to the groundmass. Alteration of biotite to chlorite is observed along with mild deformation in the amphibole phenocrysts. Based on mineralogy and major oxide geochemistry, these rocks are classified as the calc-alkaline lamprophyres. Higher Ba/Nb and low Nb/La points to their derivation from an enriched lithospheric mantle source and higher Th/Yb ratio along with negative TNT (Ti-Nb-Ta) and Zr-Hf anomalies on the primitive mantle (PM) normalized multi-element diagram indicates dehydrated fluids from the foundering slab could be the possible metasomatic agent. Fractionated HREE ratios (GdN/YbN >1.9) and higher SmN/YbN suggests that the source region lies in the garnet stability field. Higher than PM Rb/Sr along with positive correlation between K/La and Rb/La reveals presence of metasomatic phlogopite in the source region. Strong negative initial εNd along with radiogenic 87Sr/86Sr ratios further support an enriched mantle reservoir involved in their genesis. Non-modal batch melting (1–5%) of a mixed source (phlogopite-garnet peridotite) assuming 5% mixing of subducted sediment with ambient mantle wedge (depleted mantle) satisfies the multi-element concentration pattern shown by the Kadiri lamprophyres. The source enrichment can be linked to the accretion-related growth of Dharwar craton and its schist belts during Neoarchean. Our study shows that a majority of lamprophyres associated with the Archean greenstone belts display a shoshonitic character; this highlights the role of subduction-related processes in the growth and evolution of the greenstone belts
The emergence of Earth's continental crust above sea level is debated. To assess whether emergence can be observed at a regional scale, we present zircon U-Pb-Hf-O isotope data from magmatic rocks of the Coorg Block, southern India. A 3.5-Ga granodiorite records the earliest felsic crust in the region. Younger phases of magmatism at 3.37–3.27 and 3.19–3.14 Ga, comprising both reworked crust and juvenile material, record successive crustal maturation. We interpret an elevation in δ¹⁸O through time as an increase in both the amount of sediment recycling and hence, crustal thickening, as well as an increase in the emerged area of continental crust available for weathering. Geochemical signatures do not point to any apparent change in geodynamic regime. We interpret the isotopic evolution of these rocks as solely reflecting regional emergence and thickening of the continental crust, assisted by the increasing strength of the lithosphere.
Investigation of large scale suture zones in old continental interiors offers insights into the evolution of continents. The Dharwar Craton (DC) and the Southern Granulite Terrain(SGT) of the Indian shield represent large segments of Precambrian middle to lower crust and preserve a geological record spanning from Mesoarchean to Cambrian. This study illuminates the deep structure of the Palghat-Cauvery Shear Zone System (PCSS) and the Palghat-Cauvery Suture Zone (PCSZ) that comprise crustal-scale structures related to multiple episodes of orogeny, crust formation and reworking. We utilize here 3202 high quality P-receiver functions computed using new data from a 23 station seismic network operated by us. Results show a thick (>38 km) mafic (Poisson's ratio >0.25) crust beneath the SGT. The change in crustal thickness is gradual, with a shallower Moho towards the south of PCSZ. We found little evidence for drastic changes in crustal thickness across prominent shear zones like the PCSZ and Moyar-Bhavani. Few seismic stations located along these boundaries have shown evidence for dipping reflectors around 8–20 km depth, with strikes matching well with the trends of surface geological sutures. We opine that these suture zones do not show indications of a terrane boundary. However, a drastic change in the crustal thickness is observed around the prograde metamorphic transition zone or broadly, the “Fermor line”, which separates rocks of Chanockitic (Orthopyroxene bearing granitoid) and non-Charnockitic (Orthopyroxene-free granitoid) mineral assemblage, further north beneath the DC. We suggest that thicknening of crust north of Moyar-Attur Shear Zone (MASZ) and around Fermor line is related to subduction processes operative during the Precambrian.
Several profound changes, including those involving formation of the continental
crust, occurred on Earth during the Neoarchaean Era. However, the tectonic settings
associated with Neoarchaean crustal growth are not well understood and vigorously debated. The Neoarchaean Veligallu greenstone belt, eastern Dharwar craton hosts a variety of ultramafic, mafic and felsic volcanic rocks. Whole-rock elemental and Nd isotope data along with zircon U-Pb dating on these rocks provide significant insights into the origin and tectonic setting of Neoarchaean crust formation. The volcanism in the Veligallu belt started with ~2.67 Ga tholeiitic basalts derived from shallow melting of a slightly depleted mantle (εNdt = +0.6 to +1.1). Moderate negative Nb anomalies, slightly elevated Th/Yb and LREE, and an absence of evidence for crustal contamination are consistent with extraction of these basalts from a mantle source weakly metasomatized by subducted slab-derived fluids in an incipient oceanic arc setting. As the arc matured, clastic sediments started forming with concurrent emplacement of komatiites, komatiitic basalts and ferropicrites showing strong signatures of contamination with continental crust (negative Nb and Ti anomalies, LREE enrichment and negative εNdt). In the final stage (~2.58 Ga), a variety of felsic volcanic rocks (sodic trachyandesite, high Mg# andesite, rhyolite, calc-alkaline andesite) formed. The rock association and distinct geochemical signatures (enrichment of LILE, negative Nb and Ti anomalies, Mesoarchaean Nd model ages and inherited older zircons) suggest a continental margin arc environment which contained older crust. The evolutionary history of the
Veligallu belt implies that both the arc- and plume-related processes, and their interplay
contributed significantly to the growth of Neoarchaean crust.
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.
Major part of the Earth's continental crust is evolved during the Archean, however, the mechanism for its formation is controversial. It could have formed either through horizontal accretion similar to the modern plate tectonic processes or by vertical accretion by plume activity. Here, we present the results of a new deep crustal seismic reflection Profiling, the DHARSEIS experiment conducted along a 200 km long Perur-Chikmagalur profile across the Archean Dharwar craton, to understand the crustal evolutionary processes during the Neoarchean. The data were processed using the Common Reflection Surface (CRS) stack method. Seismic images show distinctly different reflectivity patterns in the Mesoarchean Western and Neoarchean Eastern Dharwar Cratons (WDC and EDC), the two crustal blocks of the composite Dharwar craton. The WDC consists of a simple structure with a major part of the crust from 6 to 28 km displaying a gently dipping reflection fabric and a subhorizontal reflection fabric from 28 to 40 km except beneath the Chitradurga schist belt. On the other hand the EDC displays a complex reflectivity pattern, contrary to the simple crustal structure suggested by various other studies. A dipping Moho, oppositely dipping reflection fabric and a thrust fault are the major crustal features in the EDC. The present seismic study imaged a west-dipping reflection fabric extending from 34 to 43 km in the EDC, which is interpreted to represent an upper-mantle subduction zone. During this process the EDC was thrusted obliquely against the pre-existing proto-continent WDC and accreted to it. Oppositely dipping reflection fabrics with a crustal root at the convergence boundary suggest accretion of WDC and EDC during the Neoarchean orogeny. The collisional boundary coincides with the location of ~2.5 Ga Closepet granite. Seismic images suggest the Moho as a detachment boundary. The present study reveals a likely two-stage subduction-accretion process for the evolution of continental crust during the Neoarchean. Plate tectonic processes were responsible for the Neoarchean crustal growth in the region. © The Authors 2017. Published by Oxford University Press on behalf of The Royal Astronomical Society.
Anorthositic rocks have attracted attention in terms of their possible role as the primordial crust of our planet, and also as markers of Archean-Paleoproterozoic plate tectonic regimes. Here we investigate the Konkanhundi gabbro-anorthosite suite from Peninsular India, adjacent to the major collisional suture between the Western and Eastern Dharwar cratonic blocks. The anorthosites display high Al2O3 contents with negative correlation against MgO, high CaO and Sr contents, and positive Eu anomalies indicating plagioclase flotation in the magma chamber. The pyroxenite and gabbroic rocks of the suite show high Ni, Cr, Co contents and geochemical features typical of pyroxene cumulation. LA-ICPMS U-Pb analyses of magmatic zircon grains yield weighted mean ²⁰⁷Pb/²⁰⁶Pb ages of 2601 ± 12 Ma for pyroxenite, 2616 ± 12 Ma for gabbro, 2615 ± 13 Ma and 2627 ± 14 Ma for anorthositic gabbro, 2594 ± 16 Ma for anorthosite, and 2605 ± 27 Ma for microgabbro. The age data from the different rock types in the anorthosite suite are broadly consistent, marking the emplacement time as ca. 2.6 Ga, and providing insights on one of the oldest anorthosite complexes in Peninsular India. Zircon grains in the surrounding TTG gneisses into which the gabbro-anorthosite suite was emplaced show weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 3321 ± 11 Ma suggesting Mesoarchean basement. The zircon grains display high Th/U values and REE patterns typical of magmatic crystallization with enriched HREE, positive Ce and Sm anomalies, and negative Pr, Nd, Eu anomalies. Zircon Lu-Hf analysis yield negative εHf(t) values of −4.9–−0.7 with TDMC (two stage model ages) of 3123–3376 Ma for pyroxenite, −4.5–−1.7 and 3192–3366 Ma for gabbro, −5.3–0.9 and 3034–3410 Ma for anorthositic gabbro, and −3.9–2.4 and 3221–3311 Ma respectively, suggesting the incorporation of Mesoarchean components within the Neoarchean magmatic suite. However, zircon grains in the TTG gneiss possess more ‘juvenile’ εHf(t) values in the range of −0.2–1.3 with TDMC in the range of 3554–3646 Ma. The zircon Hf isotopes and trace element data, together with the whole rock geochemical features suggest that the parent magma of the Konkanhundi gabbro-anorthosite suite was derived from subduction-related depleted mantle source that also incorporated continental crustal components. The time of emplacement of the gabbro-anorthosite complex broadly correlates with widespread arc magmatism and crust production as well recycling in other domains of the Dharwar Craton. We envisage multiple convergence of microblocks during craton assembly at the end of Archean in Dharwar, with the greenstone belts marking zones of paleo-ocean closure.
This work examines the global distribution of Archaean and modern igneous rock's compositions, without relying on preconceptions about the link between rock compositions and tectonic sites (in contrast with “geotectonic” diagrams). Rather, Archaean and modern geochemical patterns are interpreted and compared in terms of source and melting conditions.
Mafic rocks on the modern Earth show a clear chemical separation between arc and non-arc rocks. This points to the first order difference between wet (arc) and dry (mid-ocean ridges and hotspots) mantle melting. Dry melts are further separated in depleted (MORB) and enriched (OIB) sources. This three-fold pattern is a clear image of the ridge/subduction/plume system that dominates modern tectonics. In contrast, Archaean mafic and ultramafic rocks are clustered in an intermediate position, between the three main modern types. This suggests that the Archaean mantle had lesser amounts of clearly depleted or enriched portions; that true subductions were rare; and that the distinction between oceanic plateaus and ridges may have been less significant.
Modern granitic rocks dominantly belong to two groups: arc-related granitoids, petrologically connected to arc basalts; and collision granitoids, related to felsic sources. In contrast, the Archaean record is dominated by the TTG suite that derives from an alkali-rich mafic source (i.e. altered basalt). The geochemical diversity of the TTG suite points to a great range of melting depths, from ca. 5 to > 20 kbar. This reveals the absence of large sedimentary accumulations, again the paucity of modern-like arc situations, and the importance played by reworking of an earlier basaltic shell, in a range of settings (including some proto-subduction mechanisms). Nonetheless, granitoids in each individual region show a progressive transition towards more modern-looking associations of arc-like and peraluminous granites.
Collectively, the geochemical evidence suggests an Archaean Earth with somewhat different tectonic systems. In particular, the familiar distinction between collision, arcs, ridges and hotspots seems to blur in the Archaean. Rather, the large-scale geochemical pattern reveals a long-lived, altered and periodically resurfaced basaltic crust. This protocrust is reworked, through a range of processes occurring at various depths that correspond to a progressive stabilization of burial systems and the establishment of true subductions. A punctuated onset of global plate tectonics is unlikely to have occurred, but rather short-term episodes of proto-subduction in the late Archaean evolved over time into longer-term, more stable style of plate tectonics as mantle temperature decayed.
The Western Dharwar Craton (WDC) is an Archean crustal segment for which the earliest stages of development have remained poorly constrained because the oldest identified lithologies are chronologically indistinguishable despite vastly different compositions and origins (i.e., 3352 ± 110 Ma Sargur-group komatiites and 3342 ± 6 Ma Hassan-Gorur TTG gneiss). Indication for older crust come from ancient detrital zircons (3450–3610 Ma), although their genetic link to the WDC is purely conjectural. In order to bring new understanding to early development of the WDC, we studied orthogneisses around the Holenarsipur Schist Belt (HSB) for their petrography, major-oxide concentrations, zircon U-Pb geochronology, and Lu-Hf isotope systematics. Our results reveal that the WDC igneous record contains crust older than 3350 Ma in the form of a 3410.8 ± 3.6 Ma granitic gneiss and inherited zircons with ages ranging from 3295 ± 18 to 3607 ± 16 Ma that were found within a 3178 ± 10 Ma trondhjemitic gneiss and a biotite-rich enclave found within it. The presence of muscovite and the peraluminous signature of the granitic gneiss, in spite of mildly-depleted Hf isotopic signature (εHf = +2.2 ± 0.6 at 3410.8 Ma), suggest that this sample formed by reworking of a felsic precursor with short crustal residence time, possibly marking the beginning of WDC formation. The oldest inherited zircons display variable εHf ranging from +10.4 at 3414 Ma to −2.3 at 3607 Ma that did not seem to have influenced the Hf isotopic composition of granitoids of the WDC that formed between 3200 and 3410 Ma, except perhaps in the Sargur area. We suggest that the WDC formed remote from continental crust until a crustal block containing >3410 Ma zircons was accreted to it ∼3200 My ago. This event resulted in the stabilization of the WDC which is marked by diapiric granitoids to which the 3178 Ma trondhjemitic gneiss belongs. After 3200 Ma, the crustal block together with granitoids formed between 3410 and 3200 Ma buffered the Hf isotopic signature of newly formed granitoids, hence, indicating that, by then, the WDC already was a stable continental segment.
From the scant Hadean records of the Jack Hills to Cenozoic supervolcanoes, the continental crust provides a synoptic view deep into Earth history. However, the information is fragmented, as large volumes of continental crust have been recycled back into the mantle by a variety of processes. The preserved crustal record is the balance between the volume of crust generated by magmatic processes and the volume destroyed through return to the mantle by tectonic erosion and lower crustal delamination. At present-day, the Earth has reached near-equilibrium between the amount of crust being generated and that being returned to the mantle at subduction zones. However, multiple lines of evidence support secular change in crustal processes through time, including magma compositions, mantle temperatures, and metamorphic gradients. Though a variety of isotopic proxies are used to estimate crustal growth through time, none of those currently utilized are able to quantify the volumes of crust recycled back into the mantle. This implies the estimates of preserved continental crust and growth curves derived therefrom represent only a minimum of total crustal growth. We posit that from the Neoarchean, the probable onset of modern-day style plate tectonics (i.e. steep subduction), there has been no net crustal growth (and perhaps even a net loss) of the continental crust. Deciphering changes from this equilibrium state through geologic time remains a continual pursuit of crustal evolution studies.
The Neoarchaean Era is characterized by large preserved record of continental crust formation. Yet the actual mechanism(s) of Neoarchaean crustal growth remains controversial. In the northwestern part of the eastern Dharwar craton (EDC) granitoid magmatism started at 2.68 Ga with gneissic granodiorites showing intermediate character between sanukitoid and tonalite–trondhjemite–granodiorite (TTG). This was followed by intrusion of transitional (large-ion lithophile element-enriched) TTGs at 2.58 Ga. Finally 2.53–2.52 Ga sanukitoid and Closepet-type magmatism and intrusion of K-rich leucogranites mark the cratonization in the area. These granitoids mostly display initial negative εNd and Mesoarchaean depleted mantle model ages, suggesting presence of older crust in the area. Available data show that most of the Neoarchaean sodic granitoids in the EDC are transitional TTGs demonstrating the importance of reworking of older crust. It is suggested that the various c. 2.7 Ga greenstone mafic–ultramafic volcanic rocks of EDC formed in oceanic arcs and plateaus which accreted to form continental margin environment. Subsequent 2.7–2.51 Ga granitoid magmatism involved juvenile addition of crust as well as reworking of felsic crust forming transitional TTGs, sanukitoids and K-rich leucogranites. Microcratons were possibly the source of older crustal signatures and their accretion appears to be one of the important processes of Neoarchaean crustal growth globally.
Supplementary material: Analytical techniques are available at https://doi.org/10.6084/m9.figshare.c.3470724
The lower plate is an active agent in modern convergent margins characterized by active subduction, as negatively buoyant oceanic lithosphere sinks into the asthenosphere under its own weight. This is a strong plate-driving force because the slab-pull force is transmitted through the stiff sub-oceanic lithospheric mantle. As geological and geochemical data seem inconsistent with the existence of modern-style ridges and arcs in the Archaean, a periodically-destabilized stagnant-lid crust system is proposed instead. Stagnant-lid intervals may correspond to periods of layered mantle convection where efficient cooling was restricted to the upper mantle, perturbing Earth’s heat generation/loss balance, eventually triggering mantle overturns. Archaean basalts were derived from fertile mantle in overturn upwelling zones (OUZOs), which were larger and longer-lived than post-Archaean plumes. Early cratons/continents probably formed above OUZOs as large volumes of basalt and komatiite were delivered for protracted periods, allowing basal crustal cannibalism, garnetiferous crustal restite delamination, and coupled development of continental crust and sub-continental lithospheric mantle. Periodic mixing and rehomogenization during overturns retarded development of isotopically depleted MORB (mid-ocean ridge basalt) mantle. Only after the start of true subduction did sequestration of subducted slabs at the core-mantle boundary lead to the development of the depleted MORB mantle source. During Archaean mantle overturns, pre-existing continents located above OUZOs would be strongly reworked; whereas OUZO-distal continents would drift in response to mantle currents. The leading edge of drifting Archaean continents would be convergent margins characterized by terrane accretion, imbrication, subcretion and anatexis of unsubductable oceanic lithosphere. As Earth cooled and the background oceanic lithosphere became denser and stiffer, there would be an increasing probability that oceanic crustal segments could founder in an organized way, producing a gradual evolution of pre-subduction convergent margins into modern-style active subduction systems around 2.5 Ga. Plate tectonics today is constituted of: (1) a continental drift system that started in the Early Archaean, driven by deep mantle currents pressing against the Archaean-age sub-continental lithospheric mantle keels that underlie Archaean cratons; (2) a subduction-driven system that started near the end of the Archaean.
We report on the bulk-rock geochemistry and Lu-Hf isotope systematics, and infer the petrogenesis of the ultramafic rocks in the Neoarchean Veligallu greenstone terrane, eastern Dharwar craton, India. The ultramafics along with the basalts and adakites yield a bulk-rock Lu-Hf isochron age of 2.696 ± 0.054 Ga, consistent with the available SIMS zircon U-Pb age of 2.697 ± 5 Ma, reported for the felsic volcanics, in the literature. The rocks have positive initial εHf(2.696 Ga) = +3.0 to +6.5, consistent with an origin from a long term depleted mantle source relative to a chondritic reservoir at ∼2.7 Ga. Geochemically, the ultramafic rocks are characterized by high MgO = 24–34 wt.%, Mg# ∼ 89, Cr = 2290–3855 ppm and Ni = 604–966 ppm contents; moderate to low SiO2 = 52–49 wt.%, Al2O3 = 9.2–4.7 wt.%, and TiO2 = 0.30–0.14 wt.%. To first order, these chemical compositions broadly resemble komatiites. The rocks exhibit higher than chondrite Al2O3/TiO2 (21–37) ratio, depletions in their middle-REE relative to the heavy-REE i.e. GdN/YbN < 1, and negative Nb, Ti, and positive Zr (Hf) anomalies in a primitive mantle normalized trace element variation diagram; attributes indicating a “boninite-like affinity”. Hence, the Veligallu ultramafics lack the key geochemical characteristics that would indicate a plume origin typically attributed to komatiites. Post-magmatic low grade metamorphism, hydrothermal alteration and element mobility, and/or contamination by Archean upper continental crust cannot be the cause of these geochemical patterns. Present study suggests that these ultramafic rocks are not melts sensu stricto as they have significantly high Mg-number and MgO contents. Rather, these rocks are most likely to be the products of fractional crystallization of orthopyroxene ± olivine from a basaltic melt, and hence interpreted as cumulates. The spatial and temporal association of these ultramafics having “boninite-like affinity” with the associated arc magmatic sequence comprising of basalts, andesites and adakites in the Veligallu greenstone terrane essentially indicates that the ultramafic rocks originated in a Neoarchean subduction-related intraoceanic setting, therefore they represent arc cumulates.
The Kumta and Mercara suture zones welding together Archean crustal blocks in western peninsular India offer critical insights into Precambrian continental juxtapositions and the crustal evolution of eastern Gondwana. Here we present the results from an integrated study of the structure, geology, petrology, mineral chemistry, metamorphic P-T conditions, zircon U-Pb ages and Lu-Hf isotopes of metasedimentary rocks from the two sutures. The dominant rocks in the Kumta suture are greenschist- to amphibolite-facies quartz-phengite schist, garnet-biotite schist, chlorite schist, fuchsite schist and marble. The textural relations, mineral chemistry and thermodynamic modeling of garnet-biotite schist from the Kumta suture indicate peak metamorphic P-T conditions of c. 11 kbar at 790°C, with detrital SHRIMP U-Pb zircon ages ranging from 3420 to 2547 Ma, εHf (t) values from -9.2 to 5.6, and TDMc model ages from 3747 to 2792 Ma. The K-Ar age of phengite from quartz-phengite schist is ca. 1326 Ma and that of biotite from garnet-biotite schist is ca. 1385 Ma, which are interpreted to broadly constrain the timing of metamorphism related to the suturing event. The Mercara suture contains amphibolite- to granulite-facies mylonitic quartzo-feldspathic gneiss, garnet-kyanite-sillimanite gneiss, garnet-biotite-kyanite-gedrite-cordierite gneiss, garnet-biotite-hornblende gneiss, calc-silicate granulite and metagabbro. The textural relations, mineral chemistry and thermodynamic modeling of garnet-biotite-kyanite-gedrite-cordierite gneiss from the Mercara suture indicates peak metamorphic P-T conditions of c. 13 kbar at 825°C, followed by isothermal decompression and cooling. For pelitic gneisses from the Mercara suture, LA-ICPMS U-Pb zircon ages vary from 3249 to 3045 Ma, εHf (t) values range from -18.9 to 4.2, and TDMc model ages vary from 4094 to 3314 Ma. The lower intercept age of detrital zircons in the pelitic gneisses from the Mercara suture range from 1464 to 1106 Ma, indicating the approximate timing of a major lead-loss event, possibly corresponding to metamorphism, and are broadly coeval with events in the Kumta suture. Synthesis of the above results indicates that the Kumta and Mercara suture zones incorporated sediments from Paleoarchean to Mesoproterozoic sources and underwent high-pressure metamorphism in the late Mesoproterozoic. The protolith sediments were derived from regions containing juvenile Paleoarchean crust, together with detritus from the recycling of older continental crust. Integration of the above results with published data suggests that the Mesoproterozoic (1460-1100 Ma) Kumta and Mercara suture zones separate the Archean (3400-2500 Ma) Karwar-Coorg block and Dharwar Cratons in western peninsular India. Based on regional structural and other geological data we interpret the Kumta and Mercara suture zones as extensions of the Betsimisaraka suture of eastern Madagascar into western India.
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.
The gold mineralized Ramagiri Schist Belt is a volcanic dominated, late Archaean belt in the eastern Dharwar Craton. Based on the lithological association, mode of occurrence of rocks, geochemical characteristics of mantle derived rocks and the metamorphic grade of rocks, the belt is divided into three blocks that are tectonically interleaved with and surrounded by granitic rocks of distinct histories. The eastern block has amphibolite facies rocks, dominantly basic metavolcanics having light rare earth element (LREE) depleted patterns with minor banded ferruginous quartzite (BFQ). The central block includes mafic and felsic volcanics, pyroclastics, gabbroic and felsic dykes, argillites and BFQ. The volcanic rocks as well as the intrusives have LREE enriched patterns. The central block has dominantly greenschist facies rocks. The western block is made up of fine grained metabasalts with well preserved pillow structures at places, and retrogressed chlorite-actinolite and chlorite-carbonate schists in shear zones, serpentinite and BFQ. These rocks have flat to moderately LREE depleted patterns. The associated serpentinite is of residual origin, probably representing obducted pieces of Archaean oceanic lithosphere. Available age information and required tectonic settings of the volcanics necessitate subduction related magmatism and corroborate the idea of crustal genesis by accretionary processes in the eastern Dharwar Craton during late Archaean.
Neoarchaean granites adjacent to the Chitradurga schist belt were emplaced in the inner margin of the foreland in the context of the Neoarchaean oblique convergent setting of the Dharwar craton. Two previously unreported granites, one 50 km and the other 80 km NW of Chitradurga town, and a mylonitised granite in the hanging wall of a duplex in the NW of the schist belt yielded SHRIMP U/Pb zircon emplacement ages of 2648±40 Ma, 2598±19 Ma, and ca. 2600 Ma, respectively, the large errors being due to radiogenic Pb loss during an unidentified Neoproterozoic event. Some discrete zircon grains and xenocrystic cores yielded ≥3000 Ma ages that were derived from older rocks during anatexis or emplacement. The granites NW of Chitradurga town were emplaced as steep sheets trending NW-SE. The Chitradurga granite has a similar form, bifurcating N of Chitradurga town into two separate, steeply dipping, NW-SE sheets. Magmatic-and solid-state fabrics in these granites show that emplacement took place during, but was outlasted by, sinistral and dextral strike-parallel shear. Emplacement of the granite above the hanging wall of the duplex in the NW of the schist belt was outlasted by top-SW displacement. The shapes of the granites and their emplacement in relation to the structure of the Ranibennur and Chitradurga schist belts in the west of the craton are modelled as a mid-crustal part of a craton-wide imbricate fold-thrust belt. The relationships show that whereas some Neoarchaean granites in the craton were emplaced prior to, or during, SW-vergent thrust thickening, most granites and related plutonic suites in the foreland and accretionary complex were emplaced later as multipulse injections in steep NW-SE sheets or wedges during orogen-parallel, sinistral and dextral shear. Steep high-strain zones in the foreland and accretionary complex are interpreted as listric structures that root into an attachment at a depth of ca. 18-20 km in accord with the depth of the boundary between upper and lower crust placed at ca. 23 km from seismic reflection data published in 1979 and in more recent studies. The new structural observations and zircon dating, combined with published isotopic age data, show that the inner margin of the foreland in the west of the craton and the outer margin of the accretionary complex in the east are linked in a diffuse, steeply dipping, orogen-parallel boundary zone at least 200 km wide.
The Dharwar Supergroup and its basement of Peninsular Gneiss and Sargur supracrustal rocks in the areas of Ghatti Hosahalli and SE Bababudan display certain textural, structural and unconformable relations which have important implications for the Archaean chronology of the Karnataka craton. In the first instance these relations show that certain tonalitic-granitic parental rocks of the Peninsular Gneiss basement of the Dharwar supracrustal rocks were formed as a series beginning with polyphase gneisses and ending with discordant plutons such as the Chikmagalur granite s.l. The Sargur rocks were deformed and metamorphosed to medium-high grade during intrusion of the polyphase gneisses. After cooling, uplift and erosion of the Peninsular Gneiss and the tracts and enclaves of Sargur rocks, the Dharwar supracrustal association was deposited unconformably on the medium-high grade basement. The pre-Dharwar metamorphic minerals in the Sargur rocks were partly retrogressed and then overprinted by a second major metamorphism, mainly low grade, whose climax was attained after the main deformation of the belts and basins of the Dharwar supracrustal rocks. This major low grade metamorphism in central Karnataka is correlated with the later Archaean high grade terrane (approx 2600Ma) in S Karnataka and elsewhere in Peninsular India.-Authors
The boundary between these volcanosedimentary rocks and their basement (Peninsular gneiss, approx 3100 m.y.) on the S of the Bababudan basin is an unconformity. Within the basin, the structure is dominated by steep faults and upright open folds with strongly curved hinge lines within steep axial surfaces. The geometrical relations suggest that the volcanosedimentary rocks were deformed in response to constriction by segments of basement rising on all sides of the Bababudan basin. (Preceding abstract)-R.A.H.
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 Dharwar Craton in southern India is composed of Neoarchean greenstone successions in association with banded iron formations (BIFs), felsic volcanics and voluminous calc-alkaline to potassic plutons intruding tonalite-trondhjemite-granodiorite (TTG) basement rocks. In this study, we investigate a suite of BIFs (pelagic sediments), amphibolite (oceanic basalts), quartz mica schist (trench sediments from continental source), and porphyritic granite with mafic magmatic enclave (magma mixing and mingling in continental magmatic arc), together representing an ocean-trench-continent transect within a Neoarchean subduction-accretion-collision zone in the Kolar greenstone belt of the Eastern Dharwar Craton. The morphology, internal structure and high Th/U values of zircon grains from these rocks suggest magmatic crystallization, closely followed by metamorphism. The magmatic zircons in the BIFs show upper intercept ages of 2719±31 Ma to 2698±50 Ma and weighted mean 207Pb/206Pb mean ages of 2718±26 Ma to 2696±35 Ma marking the time of deposition of the pelagic sediments with which the ocean floor basalts were intercalated, and from which the magmatic zircons where incorporated within the pelagic sediments. The ca. 2.5 Ga ages from the younger group of zircons in the BIFs correspond to the timing of metamorphism. Zircons from the amphibolite shows upper intercept age of 2581±31 Ma representing the crystallization age of the protolith basalts. Zircons from the porphyritic granite yield upper intercept age of 2576±10 Ma and weighted mean 207Pb/206Pb mean age of 2572±13 Ma suggesting the timing of emplacement of this rock, which is comparable to the ages of magmatic zircons in the amphibolite. Magmatic zircons in the dioritic enclave show upper intercept age of 2564±13 Ma and weighted mean 207Pb/206Pb age of 2560±10 Ma. The younger population of zircons in this rock show upper intercept age of 2492±33 Ma. Some inherited zircon grains in these rocks show older 207Pb/206Pb ages of 2707±21 Ma and 2666±22 Ma. The zircon εHf(t) values are dominantly positive (up to 5.1), although some grains show negative values. The crustal residence ages suggest Mesoarchean to Neoarchean juvenile and reworked sources consistent with vertical and lateral accretion in a continental arc setting. Our data trace crustal growth during Neoarchean during 2.7 to 2.5 Ga in Eastern Dharwar along an active convergent margin. The multiple events of crustal growth recorded in our study are comparable with similar features in some of the other cratonic nuclei elsewhere on the globe, and suggest that the late Archean marks an important period of continent building.
An outstanding feature of the Archaean Eon is that it was a time of major production and preservation of continental lithosphere. Here I review the geological, geochemical and basic geophysical data that hold key information regarding Archaean crust formation and preservation. This insight is then contrasted with the data for the preceding Hadean and following Palaeoproterozoic, both often portrayed as geological times of apparently much poorer crust preservation.
We report newly obtained U-Pb SHRIMP ages of detrital zircons from metagreywackes in the Hiriyur Formation (Chitradurga Group, Dharwar Supergroup) from the central eastern part of the Chitradurga greenstone belt. U-Pb analyses yield three major Neoarchean age populations ranging from 2.70–2.54 Ga with some minor age population of Mesoarchean. The maximum age of deposition is constrained by the youngest detrital zircon population at 2546 Ma. This is the first report of the occurrence of supracrustal rocks less than 2.58 Ga in the central part of Chitradurga greenstone belt. Close evaluation of detrital ages with the published ages of surrounding igneous rocks suggest that the youngest detrital zircons might be derived from rocks of the Eastern Dharwar craton and the inferred docking of the western and eastern Dharwar cratons happened prior to the deposition of the Hiriyur Formation. The Chitradurga shear zone, dividing the Dharwar craton into western and eastern blocks, probably developed after the deposition. Furthermore, the lower intercept is interpreted as evidence for the Pan-African overprints in the study area.
A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post-Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present-day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond-hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction-related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post-Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.