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Foliated lherzolites show tabular olivines with 120 o triple point junctions and sub-idiomorphic chrome spinels (ol: olivine, crt: chromite) (BxP).
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The mantle beneath the Western Dharwar Craton of the Indian shield comprises a suite of refractory and fertile peridotites and mafic granulites. Detailed petrographic studies coupled with new mineral analysis and geothermobarometric estimations permit to decipher the thermal architecture and get an insight into the evolution of this ancient craton....
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... lherzolites locally show tabular olivines with 120 o triple point junctions (Fig. 3). Orthopyroxene occurs as slightly elongate grains that exhibit stretching and deformation; they sometimes display deformation lamellae and kink bands and may contain inclusions of olivine. Clinopyroxene is generally irregular, interstitial and largely free from deformation. It is at times spatially associated with phlogopite and ...
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Citations
... Another example of modified cratons is the Indian Craton. A recent xenoliths study (Dessai et al., 2021) suggests that the western parts of the Indian Craton lost parts of its lithospheric root. Furthermore, some seismic studies suggest a somewhat thin lithosphere for parts of the Indian Craton (e.g., Maurya et al., 2016). ...
... Since the V p /V s ratio is a better index for constraining the compositional changes (Lee, 2003), we interpret these features as a scenario of a depleted Archean uppermost mantle lithosphere at shallow parts overlying a fertile younger one at deeper parts beneath the Indian Craton. This scenario is in agreement with the recent mantle xenolith suits from the westernmost side of the Indian craton, which suggests a very thin Archean uppermost mantle overlying a newly accreted fertile one beneath the western parts of the Indian Craton (Dessai et al., 2021), although our larger study area might generalize this scenario for the larger parts of the Indian Craton. ...
Cratons formed due to the specific melting regime of the primitive mantle with elevated mantle temperature during Archean. However, each craton has undergone a distinct evolution history, and some have lost their stability. To investigate to what degree cratons in comparison with one another have been modified from their analogous initial form, we employed Sn-Pn differential (PSn) traveltimes to derive Vp/Vs ratio, which is thought to be related to Mg# of the uppermantle. We assessed Pn, Sn, and PSn data using three datasets based on epicentral distance: (1) 2°–12°, (2) 2°–7°, and (3) 7°–12°. The results suggest that most cratons show comparable seismic properties with high velocities and low Vp/Vs ratio, implying a highly depleted uppermost mantle that resembles the original residue from the partial melt extraction of the primitive mantle during the Archean. Conversely, the Eastern North China Craton (ENCC) displays the lowest P- and S-wave velocities, and noticeable high Vp/Vs ratios in all datasets, implying a systematic difference with other cratons. This observation suggests a scenario of total removal of the depleted Archean mantle lithosphere beneath the ENCC. In contrast, the Ordos Block located at the western part of the North China Craton (WNCC) shows velocities and Vp/Vs ratio comparable with those of the typical cratons, suggesting that it has still maintained its Archean mantle lithosphere. The Wyoming Craton has a high Vp/Vs ratio similar to that of the ENCC and a high P-wave velocity comparable to that of the typical cratons. These features suggest that the Archean mantle lithosphere has been significantly modified rather than totally removed and replaced by a younger fertile mantle. The Indian Craton presents a low Vp/Vs ratio and comparatively high velocities at shallow parts of the mantle lithosphere but a high Vp/Vs ratio at deeper parts similar to that of the ENCC, suggesting a partial modification of the Indian Craton at deeper parts.
... Highly fractured and anomalous shaped olivine xenocrysts, occurring as disaggregate components of spinel lherzolite-and dunite-nodules, have been also reported by Kshirsagar et al. (2011). The xenoliths on which significant information is available are those from the Bhujia, Dhrubia, Sayala Devi, Dinodhar Dongar, and Lodai in Kutch regions (Karmalkar et al. 2009;Dessai et al. 2021). These fragments are hosted by the nephelinite/basanite plugs and sill-like intrusives associated with the alkaline rocks. ...
We present a comprehensive review on the alkaline rocks from the Deccan Large Igneous Province (DLIP) and discuss their (i) temporal and spatial association with the Deccan Traps, (ii) petrography, mineral and whole rock- geochemistry (including radiogenic and stable isotopes) and geophysical aspects, and (iii) P-T data available on their entrained xenoliths. The alkaline rocks occur in seven sub-provinces, viz., (i) the Kachchh, (ii) the Saurashtra, (iii) the Gujarat Central and Chhotaudepur, (iv) the Mumbai-Trombay, (v) the Central Deccan, (vi) the Aravalli, and (vii) the Tethyan Himalayan, with the first five occurring in association with the Deccan Traps. A diverse variety of silica under-saturated to over-saturated alkaline rocks nature with varied mineralogical and geochemical compositions, have been reported from these sub-provinces. These include alkali basalt, basanite, carbonatite, ijolite, lamprophyre, leucite melteigite, mugearite, nephelinite, nepheline syenite, orangeites, alkali pyroxenite, phonolite, tinguaite etc. Available geochronological data on the Deccan alkaline rocks reveals a wide duration of the related magmatic activity (124 to 55 My), and suggest the presence of pre-, syn- and post- emplacement ages of the DLIP units. Alkaline rocks of the DLIP are hosted by discrete aged lithotypes in a variety of stratigraphic horizons, such as the Deccan Traps, Cretaceous Bagh beds, Jurassic sandstones, Triassic Shrinab sediments, Proterozoic Godhra Granite and unclassified gneisses. In majority of the sub-provinces, intrusions of alkaline rocks are controlled by fractures, rift or lineaments systems such as the Kutch rift, the Son-Narmada Tapti rift etc. Their major mineralogy is dominated by pyroxene, feldspar, amphibole, mica, olivine, nepheline, leucite, sodalite and carbonate minerals whereas accessory and minor minerals, include sphene, apatite, spinel, rutile, pyrite, chalcopyrite, epidote, zircon, pyrochlore, garnet, perovskite and other REE bearing phases. Geochemical studies reveal their sodic to potassic nature, with distinct shoshonitic character for some alkaline rocks. Isotopic studies highlight the role of mixed mantle sources ranging from spinel to garnet stability depths and involvement of the lower degrees of partial melting. Source modification by subduction and crustal contamination are evaluated. Geodynamic implications for the orogenic and anorogenic signatures found in various occurrences, depth of the lithosphere-asthenosphere boundary, and economic resources are also examined and future research directions identified.
... 65 Ma). Based on the xenolith studies (Griffin et al, 2009;Dessai et al., 2020) it has been suggested that lithospheric thickness beneath WDC varied from 185-125 km during the Proterozoic due to the magmatic and metasomatic processes associated with the intrusion of Proterozoic mafic dyke swarms (ca. 2.36 Ga). ...
... With increasing mantle melt flux, shallowing lithosphere-asthenosphere boundary (Dessai et al., 2020;N. Kumar et al., 2013;Maurya et al., 2016;H. ...
... In this study, we use a set of idealized models (Section 2) to constrain the conditions required for CFB eruptive episodes. However to self-consistently calculate how the eruptive tempo varies throughout a CFB event, a full thermo-chemo-physical model is required both for the crust and the plume associated lithospheric thermochemical evolution (Black & Gibson, 2019;Black et al., 2021;Dessai et al., 2020). Our results illustrate that such a model needs to have a few key features. ...
Continental flood basalts intruded and erupted millions of km³ of magma over ∼1–5 Ma. Previous work proposed the presence of large (>105 $ > {\mathrm{10}}^{5}$–10⁶ km³) crustal magma reservoirs to feed these eruptions. However, in Paper I, we illustrated that this model is inconsistent with observations, by combining eruptive rate constraints with geochemical and geophysical observations from the Deccan Traps and other Continental flood basalt provinces (CFBs). Here, we use a new mechanical magma reservoir model to calculate the variation of eruptive fluxes (km³/year) and volumes for different magmatic architectures. We find that a single magma reservoir cannot explain the eruptive rate and duration constraints for CFBs. Using a 1D thermal model and characteristic timescales for magma reservoirs, we conclude that CFB eruptions were likely fed by a number of interconnected small‐medium (∼10²–10³ km³) magma reservoirs. It is unlikely that each individual magma reservoir participated in every eruption, thus permitting the occasional formation of large xenocrysts (e.g., megacrystic plagioclase). This magmatic architecture permits (a) large volume eruptive episodes with 10–100s of years duration, and (b) relatively short time‐periods separating eruptive episodes (1000s of years) since multiple mechanisms can trigger eruptions (via magma recharge or volatile exsolution, as opposed to long term (10⁵–10⁶ year) accumulation of buoyancy overpressure), and (c) lack of large upper‐crustal intrusive bodies in various geophysical datasets. Our new proposed magmatic architecture has significant implications for the tempo of CFB volatile release (CO2 and SO2), potentially helping explain the pre‐K‐Pg warming associated with Deccan Traps.
... These data provide an insight into the chemical composition and petrochemical variation as well as the antiquity of the mantle chemical stratigraphy with depth. Above all they serve to record the post-Proterozoic thermal modification to the lithosphere and petrological evidence for the decratonization of the Indian shield primarily by "bottom up" mantle-plume and extensional tectonics (Dessai et al., 2021), as against the subduction tectonics envisaged to account for the heterogeneity of the crustal rocks of the Indian shield. ...
... The existence of fertile olivines in the xenolith suite is indicative of the existence of Proterozoic and Phanerozoic fertile mantle which is clearly distinct from the ancient refractory mantle (Boyd, 1989;Griffin et al., 1999). Assuming that subcontinental lithospheric mantle was coupled with the formation of the overlying crust (O'Reilly ), the SCLM beneath western India should have exhibited refractory composition Dessai, 2021;Dessai et al., 1990, 2021, Dessai and Viegas, 2010Karmalkar et al., 2000Griffin et al., 2009;Paton et al., 2007;Paton et al., 2009;Chalapathi Rao et al., 2010;Dongre et al., 2017;Khan et al., 2018;Pattanaik et al., 2020;Shaikh et al. 2020 (CI: contamination Index;Clement, 1982); macro: macrocryst,(2-10 mm) micro: microphenocryst (0.5 mm), major element: wt%, trace elements: ppm,); ProK: Proterozoic kimberlite; CretK: Cretaceous kimberlite. a All modal ranges are tentative due to sample rarity, poor state of preservation, secondary alteration and contamination. ...
... The existence of fertile olivines in the xenolith suite is indicative of the existence of Proterozoic and Phanerozoic fertile mantle which is clearly distinct from the ancient refractory mantle (Boyd, 1989;Griffin et al., 1999). Assuming that subcontinental lithospheric mantle was coupled with the formation of the overlying crust (O'Reilly ), the SCLM beneath western India should have exhibited refractory composition Dessai, 2021;Dessai et al., 1990, 2021, Dessai and Viegas, 2010Karmalkar et al., 2000Griffin et al., 2009;Paton et al., 2007;Paton et al., 2009;Chalapathi Rao et al., 2010;Dongre et al., 2017;Khan et al., 2018;Pattanaik et al., 2020;Shaikh et al. 2020 (CI: contamination Index;Clement, 1982); macro: macrocryst,(2-10 mm) micro: microphenocryst (0.5 mm), major element: wt%, trace elements: ppm,); ProK: Proterozoic kimberlite; CretK: Cretaceous kimberlite. a All modal ranges are tentative due to sample rarity, poor state of preservation, secondary alteration and contamination. ...
A 150–200 km thick, cold (35–45 mWm⁻²), melt-depleted lithospheric keel characterised the eastern cratons of the Indian shield at the end of the Precambrian. Differing chemical- and isotopic-characteristics, and ages of the crust and mantle rocks reveal the decoupling of the crust and mantle beneath the cratons, beginning at 2.45 Ga, in the Bastar craton. The Pan-African event was more pervasive and brought about widespread reworking in most of the cratons of the shield. Major-, trace- and rare-earth elements combined with Sr, Nd and Hf isotope data suggest a heterogenous SCLM beneath southern India. The trace element signatures of xenoliths and the presence of majoritic garnet inclusions in diamond suggest that some kimberlites were derived from the mantle transition zone. Mesoproterozoic (1.2–1.4 Ga) modal and cryptic refertilisation by asthenosphere-derived, low-degree carbonated melts led to the generation of the fluids responsible for the metasomatic transformation of the source rocks.
The western craton of the shield has witnessed more severe reactivation than the eastern due to the frequent interaction of the Indian plate with mantle plumes. One plume caused major igneous activity during the late Cretaceous, synchronous with crustal attenuation, rifting and the ridge-jump at 66 Ma, in the Indian Ocean. By the end of the Palaeocene the geotherm of the western craton had risen from 50 to 55 mWm⁻² in the Proterozoic to a peak 80–90 mWm⁻². This increase in heat flow not only modulated the mantle thermal regime, but led to a net loss of more than 100 km of lithosphere and to destabilisation of the craton. After this thermal event, the lithosphere preserves a thickness of barely 60–80 km, and a thin crust (10–21 km) beneath the continental margin in the west. These changes decratonized the western part of the shield and the transitional region further west in the Indian Ocean where the continental ridges are almost devoid of crustal sections and the lithosphere is ~60 km thick. The waning of the Deccan Traps (65 Ma) magmatism was marked by alkaline intrusive activity along the western margin of the shield, probably derived from the SCLM in response to the rise of the mantle plume. Low degree (2–3%) partial melting of a modally and cryptically metasomatized source may have been involved in the generation of alkaline magmas from a depleted mantle source variously contaminated by an enriched endmember.
Western Indian Ocean basin shows one of the most complex signatures of the ocean floor anomalies by juxtaposition of the rapidly evolving, multiple spreading ridges, subduction systems and microcontinental slivers. This study based on ocean floor magnetic anomalies, gravity gradient map, tomographic profiles and geometrical kinematic models reports a significant westward drift of the Central Indian Ridge (CIR) segments. Documented precisely between the latitudes 17°S and 21°S the drift is coincident with the Deccan volcanism at ~ 65 ± 2 Ma and we further explain its bearing on the Indian plate kinematics. The progressive stair-step trend of the ridge segments towards NE is marked by anomalous deflection to NW for a brief distance of ~ 217 km between these latitudes represented by the anomalies C30n-C29n. The observed length of the ridge segments moving NW at 17°S match the calculated NW drift rates of Indian plate (Bhagat et al., 2022). We infer that the NW drift and its restoration towards NE triggered short Plume Induced Subduction Initiation along the Amirante trench. Further a plume induced lithospheric tilt of the Indian plate (Sangode et al 2022) led to restoration of subduction along the Sunda trench at ~ 65 Ma imparting new slab pull force over the Indian subcontinent besides the NE trend for CIR. This episode resulted into anticlockwise rotation of the Indian plate along with accelerated drift rates due to vector addition of the plume push and the slab pull forces from Eurasian as well as Sunda subduction systems after 65 Ma. The Deccan eruption thus resulted in major geodynamic reorganization that altered the kinematics of Indian plate; and the signatures of which are well preserved over the ocean floor.
The Deccan Traps form one of the most spectacular geological features of the Indian subcontinent. Significant amount of work focusses on the volcanism and petrological aspects of the large igneous province leaving several lacunae in understanding its geodynamic impact over the Indian plate, the Indian ocean
basin and its subduction regimes. In this paper we have presented the observations on the ocean floor anomalies to explain the kinematic changes in terms of drift velocities and directions of the Indian plate, and the interaction of the subduction zones and spreading ridges as the plate encountered the Reunion plume. A significant westward drift has been reported here precisely at the Deccan event as an anomaly to the NNE drift. The relicts of this drift are scattered across the
Western Indian Ocean. We investigated the CIR for the same and discovered that the ridge segments between 17-21°S exhibit a segmentation pattern that is distinct from the rest of the ridge. Below 21°S the ridge breaks off at NE bearings and starting 21°S the ridge breaks off at NW bearings and this is again reversed to NE at 17°S. Paleolatitude calculations revealed that the site of eruption of main phase of Deccan Traps falls almost exactly between the above
latitudes. We estimate the length of ridge segments trending towards west equivalent to the length of the drift of Indian plate previously reported by Bhagat et al (2022). Amongst the possible forces that could have brought about this directional change, subduction forms the most plausible option. The initiation of Amirante subduction system and its cessation within a short span of less than ~1.5 Ma exactly at the Deccan event makes it geodynamically the best candidate for this position. Further the activation of the Sunda arc at the Deccan event owing to the lithospheric tilting reinstated the NNE drift of the India. Based on the ocean floor records and the postulations presented here, we indicate that the westward drift of India at the Deccan event reflects the kinematic reorganization of the Indian plate which further demands detailed computational modelling as future studies in this region.
Western Indian Ocean basin shows one of the most complex signatures of the ocean floor anomalies by juxtaposition of the rapidly evolving, multiple spreading ridges, subduction systems and microcontinental slivers. This study based on ocean floor magnetic anomalies, gravity gradient map, tomographic profiles and geometrical kinematic models reports a significant westward drift of the Central Indian Ridge (CIR) segments. Documented precisely between the latitudes 17°S and 21°S the drift is coincident with the Deccan volcanism at ~65±2 Ma and we further explain its bearing on the Indian plate kinematics. The progressive stair-step trend of the ridge segments towards NE is marked by anomalous deflection to NW for a brief distance of ~217 km between these latitudes represented by the anomalies C30n-C29n. The observed length of the ridge segments moving NW at 17°S match the calculated NW drift rates of Indian plate (Bhagat et al., 2022). We infer that the NW drift and its restoration towards NE triggered short Plume Induced Subduction Initiation along the Amirante trench. Further a plume induced lithospheric tilt of the Indian plate (Sangode et al 2022) led to restoration of subduction along the Sunda trench at ~65 Ma imparting new slab pull force over the Indian subcontinent besides the NE trend for CIR. This episode resulted into anticlockwise rotation of the Indian plate along with accelerated drift rates due to vector addition of the plume push and the slab pull forces from Eurasian as well as Sunda subduction systems after 65 Ma. The Deccan eruption thus resulted in major geodynamic reorganization that altered the kinematics of Indian plate; and the signatures of which are well preserved over the ocean floor.
Cratons are the stable portions of the continents for at least the past two billion years. Their longevity is generally attributed to their thick, cold, rheologically strong and chemically depleted subcrustal mantle lithosphere. The lithosphere achieved both buoyancy and strength by relatively low thermal regime and compositional differences due to dehydration, presence of melt-depleted peridotites and low iron content. Low temperature of the mantle lithosphere leads to high viscosity, which prevents deformation and thus preserves the cratonic root. However, significant modifications/erosion of the lithospheric root is observed for some cratons from different parts of the globe. The present paper discusses some aspects of lithospheric modification of the Archean Dharwar Craton, southern India. Southern India is a mosaic of several Archean cratonic blocks, the Western and Eastern Dharwar cratons, and Southern Granulite Terrain. It has experienced several tectonic and magmatic events, including mafic dykes of various periods since Paleoproterozoic. The Dharwar Craton was accreted with the Karwar and Coorg blocks on the west and Eastern Ghats and Nallamalai fold belts on the east during the Proterozoic. The Cuddapah, Bhima and Kaladgi basins were formed during the Proterozoic period. A 1.1 Ga kimberlite magmatism and 550 Ma East-African orogen are other important geodynamic events of the region. The western and eastern continental margins, together with the Deccan Large Igneous Province, one of the world's largest igneous provinces, were formed due to mantle plume activities during 130-65 Ma. 2500 km long Himalayan Mountain chain was formed during ∼55 Ma. Active and passive seismic, magnetotelluric, gravity, xenolith and elevation data complimented with geological data were used to understand the lithospheric structure. Subduction, collision and mantle plume activities, which are main processes for crustal evolution are also found responsible for lithospheric destruction in the region. The detached subducted slabs from various geodynamic activities of the region might have led to rheological weakening of the subcontinental lithospheric mantle and thereby craton modification/erosion in the southern India. Mantle plume activities are responsible for epeirogenic uplift and lithospheric thinning in the southern India.
In comparison to the eastern Dharwar Craton, the mantlederived
xenocrysts/xenoliths are extremely rare or even unreported
from the western Dharwar Craton, southern India. A
Neoproterozoic (ca. 800-900 Ma) lamprophyre cropping out in the
Mysuru area of southern India contains chrome-diopside
xenocrysts (Cr2O3 content varying from 0.2 – 1.23 wt%) which
provide important evidence about the pressure-temperature
conditions and lithospheric thickness beneath the western Dharwar
Craton. Studied chrome-diopsides show compositional zoning
which is lacking in the liquidus phases (amphiboles and feldspars)
of the lamprophyre which additionally favors a non-cognate origin
of the former. Based on the compositional zoning, all the chromediopside
xenocrysts can be divided into three groups: (i) Group Iwhich
are euhedral and show reverse zoning with increasing Crcontent
from core to rim; (ii) Group II- which are characterized
by fractures and resorption textures, show complex reverse zoning
and display up to three distinct compositional layers, and (iii) Group
III- which evidence the reaction of chrome-diopsides with
lamprophyric melt and are marked by alteration phases, such as
actinolite and chlorite, together with relicts of some unaltered
xenocrysts. High Cr2O3, moderate MgO and low Al2O3 content of
all the three varieties of chrome-diopside suggest them to represent
disaggregated xenocrysts of mantle-derived garnet peridotite.
Temperature-pressure estimates for chrome-diopside xenocrysts
ranges from 895 - 1026 ºC (± 30 ºC) and 32 – 38 kbar respectively
and correspond to depth range of 106 – 127 km. The study reveals
that lithospheric thickness during the Neoproterozoic beneath the
western Dharwar craton was at least ~115 km and is similar in
composition to that of the cratonic lithosphere found in the other
cratonic domains.
