Central Indian Shield Research Articles

Geochemical Constraints on Granulites of the Central Indian Shield: Key Insights into Crustal Evolution

Geochemical Constraints on Granulites of the Central Indian Shield: Key Insights into Crustal Evolution

Geochemistry, Petrogenesis and Tectonic Setting of the Mafic Dykes from the Amgaon and Khairagarh Regions, Bastar Craton, Central Indian Shield: Constraints on the Precambrian Crustal Evolution

The presence of dominantly dykes, dyke swarms and sills along with the basement rocks indicate that the study area has undergone deep erosion exposing the basement rocks and these magmatic bodies (dykes, dyke swarms and sills) actually represent the plumbing system for the mantle derived melts, which contributed significantly to the Precambrian crustal evolution processes in the Central Indian shield

Geochronological and geochemical constraints on the petrogenesis of alkali granites from the Makrohar Granulite Belt: Evidence for Mesoproterozoic extensional regime in the eastern Central Indian Shield

The Chotanagpur Gneissic Complex (CGC) is characterized by the Makrohar Granulite Belt (MGB) at its north‐western margin parallel to Son‐Narmada lineaments. The MGB exhibits a typical association of mantle‐derived mafic tholeiitic and alkaline rocks along with the felsic units. The zircon U–Pb data on the intrusive granite gneiss from this association define a crystallization age of 1,498 ± 38 Ma, representing a widespread Mesoproterozoic within‐plate magmatic event in CGC synchronous with the breakup of the Columbia Supercontinent. These granite gneisses have an alkaline affinity and are metaluminous in character. They exhibit many of the geochemical characteristics of anorogenic (A‐type) magmas, including enrichment of incompatible trace elements, like Zr, Hf, Nb, Y, Ga, and rare earth elements (REE), but low in Ba and Sr as well as high FeOt/(FeOt + MgO) (~0.98) values and Ga/Al (~3.92) ratios. They show less fractionated REE patterns, characterized by relatively enriched LREE and flat HREE with negative Eu anomaly, and have high Zr‐saturation temperatures (~955°C). Considering their field characteristics and all the geochemical and isotopic features, we conclude that they were derived from melting the lower crust metasomatized by mantle‐derived alkali‐rich fluids due to asthenospheric upwelling.

Magneto-thermometric modeling of Central India: Implications for the thermal lithosphere

We estimated the Curie depth using magnetic anomalies in the tectonically complex central Indian shield. The modified centroid method for the scaling distribution of sources is applied to aeromagnetic data and satellite magnetic data (EMAG2) to estimate the Curie depths. We selected 80% overlapping with a window length of 300 × 300 km2 for Curie depth estimation. The estimated Curie depths are 22–46 km being shallower in the Bastar craton, northern end of the Aravalli-Delhi fold belt, and eastern part of Central Indian tectonic zone in the Deccan Volcanic Province. Deeper Curie depths are found in the eastern parts of the Aravalli-Delhi fold belt, Deccan Volcanic Province to the south of Tapti basin, parts of the Ganga and Vindhyan basins. The heat flow values estimated from the Curie depths vary from 30 to 90 mW/m2 being higher are in the northern part covering the Aravalli-Delhi fold belt and the Ganga basin and low heat flow values in the remaining study region. The computed thermal lithospheric thickness ranges 62–120 km, being 62 km thin in the Vindhyan basin, 68 km in the Aravalli-Delhi fold belt, and 73 km in the Bastar craton. We noticed a relatively thick thermal lithospheric thickness of 99 km beneath the foreland Ganga basin and 100 km in the Bundelkhand craton. The thermal lithosphere beneath the Deccan Volcanic Province is found to have a thickness ~ 120 km , which is interpreted as due to the rapid cooling of the lithosphere after Reúnion plume activity. Probably, the plume activities in the past could have modified the thermal lithospheric structure beneath central India. The upper mantle in the Deccan Volcanic Province to the south of the Tapti basin and the Aravalli-Delhi fold belt show magnetic character.

Pn tomography and anisotropic study of the Indian shield and the adjacent regions

High-resolution P-wave velocity and anisotropy structure of the hitherto elusive uppermost mantle beneath the Indian shield and its surrounding regions are presented to unravel the tectonic imprints in the lithosphere. We inverted high quality 19,500 regional Pn phases from 172 seismological stations for 4780 earthquakes at a distance range of 2° to 15° with a mean apparent Pn velocity of 8.22 km/s. The results suggest that the Pn velocity anomalies with fast anisotropic directions are consistent with the collision environments in the Himalaya, Tibetan Plateau, Tarim Basin, and Burmese arc regions. The higher Pn anomalies along the Himalayan arc explicate the subducting cold Indian lithosphere. The cratonic upper mantle of the Indian shield is characterized by Pn velocity of 8.12–8.42 km/s, while the large part of the central Indian shield has higher mantle-lid velocity of ~8.42 km/s with dominant anisotropic value of 0.2–0.3 km/s (~7.5%) suggesting the presence of mafic ‘lava pillow’ related to the Deccan volcanism. The impressions of the rifts and the mobile belts are conspicuous in the velocity anomaly image indicating their deep seated origin. The Pn anisotropy in the Indian shield exhibits a complex pattern and deviates from the absolute plate motion directions derived from the SKS study, demonstrating the presence of frozen anisotropy in the Indian lithospheric uppermost mantle, due to the large scale tectonic deformation after its breakup from the Gondwanaland. Whereas, Pn and SKS anisotropic observations are well consistent in Tarim basin, Tibetan regions, eastern Himalayan syntaxis and the Burmese arc. The modeled anisotropic Pn clearly manifests a lower velocity anomaly bounded by 85°E and 90°E ridges in the southern Bay of Bengal. Further, 85°E ridge spatially separates the BoB lithosphere into faster and slower regions consistent with the body wave tomography and free-air gravity observation.

Mineral chemistry of high-Al chromian spinel from ultramafic rocks of the Babina–Prithvipur transect, Bundelkhand Craton, central India: Implication for petrogenesis and tectonic setting

Bundelkhand Craton is an important Archaean cratonic nuclei of the Central Indian Shield and comprises two greenstone complexes, i.e., the Babina–Mauranipur Greenstone Belt and the Girar–Madawara Greenstone Belt. The E–W trending Babina–Mauranipur Greenstone Belt in the central part of the craton, encloses several isolated lensoid shaped ultramafic bodies which has suffered various degrees of alteration and metamorphism. As per modal mineral analysis, the ultramafic rocks of the Babina–Prithvipur section belong to harzburgite and also contain high-Al chromian spinel along with olivine, ortho-pyroxene and amphibole. Mineral chemistry reveals that the spinels are of high Al and Cr poor variety, where Al2O3 and Cr2O3 contents range from 40.06 to 54.34 wt.% and 9.05 to 14.89 wt.%, respectively. The TiO2 content is extremely low (average ≈0.07 wt.%). The Cr# value of the spinel is <0.2, whereas Mg# ranges from 0.495 to 0.633. The forsterite contents of the olivine ranges from 86.088 to 88.105 wt.%. Average CaO and NiO contents of the olivine stand 0.03 and 0.24 wt.%, respectively. Composition of the analyzed ortho-pyroxene belongs to En84.20–87.75Wo0.15–0.39 with low CaO content of 0.080 to 0.207 wt.%. As per mineral chemistry, these harzburgite rocks of the Babina–Prithvipur section belong to mantle peridotite. Melt calculation for the spinel also suggests a least differentiated magmatic product, which is also supported by the olivine spinel mantle array diagram as all the samples are plotted within the mantle array field very close to the fertile mantle source. Low TiO2 and high Al contents of spinel also reflect the MORB type peridotite characteristics for these ultramafic rocks which probably originated from least differentiated plagioclase free mantle derived harzburgite/lherzolitic magma in a rift related spreading centre. As a whole these ultramafic rocks appear to be the remnant of the early crust that existed during the Archaean time.

Neotectonic fault movement and intraplate seismicity in the central Indian shield: A review and reappraisal

Neotectonic fault movement and intraplate seismicity in the central Indian shield: A review and reappraisal

Geochemistry of ultramafic–mafic rocks of the Madawara Ultramafic Complex in the southern part of the Bundelkhand Craton, Central Indian Shield: Implications for mantle sources and geodynamic setting

Detailed field, petrography and whole‐rock geochemical study was carried out in order to constrain the mantle sources and geodynamic setting of the Madawara Ultramafic Complex (MUC) of the Bundelkhand Craton. Studies reveal that there are two types of ultramafic rocks: (a) high‐Mg ultrabasic/basic rocks and (b) undeformed ultramafic–mafic plutonic rocks. The high‐Mg ultrabasic/basic rocks have undergone severe low‐grade (greenschist) metamorphism and are characterized by stringer and veinlet structures of talc–tremolite–actinolite schists with alternate layers of serpentinites showing comparatively higher SiO2 (46.1–49.4 wt%), lower MgO (24.6–26.2 wt%), and higher Al2O3 (4.58–7.06 wt%) and CaO (2.72–6.77 wt%) compared to the undeformed ultramafic rocks. The undeformed ultramafic rocks (mainly harzburgite, lherzolite, and olivine websterite) are characterized by globular structures and have lower SiO2 (40–44.1 wt%), higher MgO (30.4–38 wt%) and lower Al2O3 (1.84–4.03 wt%) and CaO (0.16–3.14 wt%). The undeformed mafic rocks (mainly gabbro) occur as small pockets within the undeformed ultramafic rocks as well as independent outcrops. Limited variation in Nb/Th against Nb/Yb along with negative Nb‐Ti anomalies of all the rock types in the multi‐element diagram reveals the significant role of the metasomatized mantle in their genesis. All the rocks show enrichment in light rare earth element and large‐ion lithophile elements compared to heavy rare earth elements and high‐field strength elements. The geochemical characteristics coupled with Ce/Yb versus Ce variation of the rocks of MUC point towards two different sources for their genesis. The high‐Mg ultrabasic/basic rocks are derived from partial melting of metasomatized mantle at shallow depth, while the undeformed ultramafic rocks were formed as a result of asthenospheric upwelling from a greater depth that induced the melting in the overlying lithosphere. Gabbro rocks represent the last and most evolved phase of the complex. Geochemical signatures suggest that the rocks of MUC were formed in a continental arc setting.

Petrogenesis of mafic magmatic enclaves of the Bundelkhand granitoids near Orchha, Central Indian shield: evidence for rapid crystallization

Abstract An attempt has been made to understand the origin and emplacement of the widespread mafic magmatic enclaves (MMEs) in the Neoarchean–Palaeoproterozoic Bundelkhand granitoids in the central Indian shield. These MMEs are very fine grained in texture, elliptical and ovoidal in shape and have a very sharp contact with the host granitoids. The MMEs exhibit sub-ophitic texture, acicular apatite and overgrowth of orthopyroxene over olivine crystals pointing towards a rapid crystallization of the MMEs magma in the granitoid magma. The host granitoids are calc-alkaline while the MMEs are tholeiitic, indicating contrasting geochemical composition. Low concentration of Rb, Sr, Ba and K in MMEs points away from the magma mixing with the granitoid magma. Both MMEs and host granitoids are metaluminous and are formed in a subduction zone environment. Although both MMEs and the granitoids were formed at the same time as the mafic magma was injected into the granitoid magma which was still crystallizing (semimolten stage), negligible to no mixing took place between the two contrasting magmas. We propose that the MMEs in the Bundelkhand granitoids are the result of rapid crystallization of the mafic magma in the cooler felsic one.

Geomorphic evidences and chronology of multiple neotectonic events in a cratonic area: Results from the Gavilgarh Fault Zone, central India

The ENE–WSW trending Gavilgarh Fault Zone (GFZ) is an important tectonic lineament within the Central Indian shield. Geomorphological mapping and spatial analyses of rivers were carried out to elucidate the imprints of active tectonics on the fluvial systems of this region. The sinuosity index, width–depth ratio of river valleys, longitudinal profile, S–L index and hypsometric index of the rivers flowing from north to south across the GFZ lineament suggest that the northern side of GFZ was tectonically uplifted. Luminescence dating of sediments from river terraces and calculation of knickpoint migration rates in the rivers indicate occurrence of multiple neotectonic events in GFZ at ca. 65–80ka, ca. 50ka, ca. 30–40ka, and ca. 14ka. Evidences of neotectonic activity, presence of active geothermal springs, and occurrence of recent earthquakes along GFZ suggest that this lineament is tectonically active and there is a need for proper seismic monitoring of this fault zone.

Crustal Evolution and Metallogeny in India

Crustal Evolution and Metallogeny in India . Sanjib Chandra Sarkar and Anupendu Gupta. 2012. Cambridge University Press, Cambridge. Pp. 840 p. Appendix of 40 glossy plates. ISBN 978-1-107-00715-4. $US140.00. This lengthy volume is a comprehensive contribution to earth science literature that attempts to summarize our present-day basic knowledge of regional geology and ore deposits in India. The lead author has been one of India’s most prominent economic geologists for almost 50 years and the second author is a past Deputy Director of the Geological Survey of India; they are two of the more respected experts on the country’s geology and mineral resources. The volume divides India into seven areas, each represented by a chapter, with one section entitled “Geology and Crustal Evolution” and another, “Metallogeny.” It concludes with a final chapter on India’s geology and mineral deposits within a broader global framework. The book opens with an initial chapter on southern India, which includes the Dharwar and Southern Granulite provinces. It includes only five pages on the Kolar goldfields, India’s historically most important gold producer, and an upto-date section on the presently producing Hutti gold deposit. The BIF-hosted iron ores of Karnataka state, rare earths in beach sands, and bauxite resources are also described. Only one page addresses the Mangampeta bedded barite deposit, which contains almost 25% of the global barite resource. Chapter 2 discusses the Bundelkhand (northern) and Bastar (southern) provinces of the central Indian shield. The region contains important deposits of Mn, bauxite, and Fe-rich BIFs, with the latter covered through 20 pages of the text, which include a description of the …

Geochemical characteristics of calcareous and non-calcareous shales from the Meso-Neoproterozoic basins of the Bastar craton, Central Indian Shield: implications for provenance, tectonic setting, paleoweathering and paleoredox conditions

The Meso-Neoproterozoic shales of the Bastar craton, Central Indian Shield have been analyzed geochemically to study their geochemical characteristics and to evaluate paleoweathering, provenance, tectonic setting and paleoredox conditions. The shale samples have been classified into the calcareous and the non-calcareous shales on the basis of presence of calcite and CaO concentrations. In comparison to the calcareous shales, the non-calcareous shales have higher concentrations of most major elements and trace elements including rare earth elements (REEs). The Upper Continental Crust (UCC) normalized elemental ratios of the calcareous and the non-calcareous shales suggest evolved sources similar to the UCC (Upper Continental Crust). The Th/Co versus La/Sc and La-Th-Sc diagrams, chondrite normalized REE patterns and negative Eu/Eu* values of the calcareous and non-calcareous shale samples further reveal that the sediments have been derived from a felsic source and the source rocks have been identified to be the granites and the gneisses of the Bastar craton. The SiO2 versus K2O/Na2O and the SiO2/Al2O3 versus K2O/Na2O tectonic setting discrimination diagrams indicate sediments were deposited in a passive margin tectonic setting. The CIA (Chemical Index of Alteration), PIA (Plagioclase Index of Alteration), Th/U and K/Rb ratios and the K2O-Fe2O3-Al2O3 relations of both the calcareous and the non-calcareous shales indicate that source area was affected by moderate to intense weathering history. The geochemical parameters like Ce/Ce* and Mn* suggest that the calcareous shales were deposited in suboxic environment compared to the non-calcareous shales.

Geochemical characterization and petrogenesis of Proterozoic Khairagarh volcanics: implication for Precambrian crustal evolution

The Khairagarh volcano‐sedimentary sequence is exposed along the Kotri–Dongargarh Belt towards the north‐eastern part of the Bastar Craton. This sequence is exposed south of the Central Indian Shear (CIS) and east of the Sakoli Group rocks. The Khairagarh volcanic sequence is represented by low‐Ti, intermediate‐Ti and high‐Ti basalt–basaltic andesite series that probably represent varying degrees of partial melting of an enriched mantle source, thus they appear to be consanguineous. These rocks are associated with a sequence of high magnesium andesitic (HMA) rocks that follow a separate evolutionary trend, thus they appear not to be related to the basalt–basaltic andesite sequence. The presence of the two contrasting sequences probably indicates generation in a hot Andean‐type subduction zone for the HMA, and Andean‐type back‐arc rifting for the basalt–basaltic andesite samples. The possibility of a relatively thick crust around 3.6 Ga in the Bastar Craton and the Amgaon Gneissic Complex, basement for the Khairagarh–Sakoli sequence, probably indicates that it was part of the Supercontinent Ur. The mantle extraction age of about 2.9 to 2.5 Ga, based on the Nd‐model ages for the Khairagarh–Sakoli sequence, probably indicate that this part of the Central Indian Shield became part of the Supercontinent Columbia subsequently. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Evaluation of provenance, tectonic setting, and paleoredox conditions of the Mesoproterozoic–Neoproterozoic basins of the Bastar craton, Central Indian Shield: Using petrography of sandstones and geochemistry of shales

We carried out petrographic analyses of the sandstones and geochemical analyses of the shales from the Mesoproterozoic–Neoproterozoic Chhattisgarh and Indravati Basins to determine their tectonic setting, provenance, and paleoredox conditions. Petrographic study shows that the sandstone samples have high amounts of quartz but are depleted in feldspar and lithic fragments. The shales have been classified into the calcareous and noncalcareous shales. The noncalcareous shales have higher concentrations of most of the major elements and trace elements, including the rare earth elements (REEs), in comparison to the calcareous shales. However, this difference in elemental concentrations between the calcareous and noncalcareous shales proved to be significant only for SiO2, TiO2, Al2O3, MnO, CaO, K2O, loss on ignition, Rb, Sr, Nb, Ce, Pr, Sm, Gd, Hf, and Ta using the Student's t-test at better than 95% confidence level. Upper continental crust (UCC)–normalized elemental ratios of the calcareous and noncalcareous shales suggest evolved sources similar to UCC. The sandstone petrology and Ni versus Cr diagram, chondrite-normalized REE patterns, and negative Eu/Eu* values of the calcareous and noncalcareous shale samples reveal that the sediments have been derived from felsic rocks (granites and gneisses) of the Bastar craton. The SiO2 versus K2O/Na2O and SiO2/Al2O3 versus K2O/Na2O tectonic-setting discrimination diagrams of the shales and the petrology of the sandstones indicate a passive-margin tectonic setting for Chhattisgarh and Indravati Basins. Geochemical parameters such as Ce/Ce* and Mn* suggest that the calcareous shales were deposited in a suboxic environment, compared to the oxic environment of the noncalcareous shales.

A new find of boninite dyke from the Palaeoproterozoic Dongargarh Super group: Inference for a fossil subduction zone in the Archaean of the Bastar craton, Central India

The Dongargarh Supergroup (DSG), a bimodal Large igneous province (LIP), is one of the Palaeoproterozoic greenschist facies-metamorphosed volcano-sedimentary belts in the Bastar craton of the Central Indian shield. Two contrasting models are in vogue for the generation of the mafic volcanics from the DSG - a continental rifting model and an arc related model. In this paper, we report the occurrence of a boninite dyke from the Bijli rhyolite Formation, which is the lower volcanic horizon in the Nandgaon Group of the DSG. The boninite dyke is characterised by high magnesium (MgO : 18.32-18.80 wt.%), primitive Mgnumber (Mg# > 80), abundance of silica (SiO 2 : 51.63-51.95 wt.%), high Ni (~369 ppm), Cr (~2703 ppm), extremely low titania (TiO 2 : 0.04 wt.%), enrichment of LREE over MREE and HFSE and pronounced negative anomalies in Nb, Ti and Zr on primitive mantle normalized multi-element plots. The Dongargarh boninite dyke is inferred to have been derived from a primary magma and shares geochemical characteristics of modern- as well as Archaean-boninites. It comes under the high-Ca boninite category and displays distinct geochemical traits compared to the so far reported boninites from the Bastar craton. Its petrogenesis necessitates a two stage-model involving a refractory mantle as well as fluids derived from subducted sediments. Crustal assimilation (contamination) or a direct plume-derived melt cannot account for its observed geochemical characters. Even though we cannot constrain the generation of the mafic volcanics of DSG vis-a-vis rifting vs convergence with the available data, the occurrence and geochemistry of the boninite dyke indeed demonstrates that this domain represents a fossil subduction zone.

Petrochemistry of sandstones from Neoproterozoic basins of the Bastar craton, Central Indian Shield: Implications for paleoweathering, provenance and tectonic history

AbstractPetrographic analysis and chemical analysis of major and trace elements including rare earth elements of the Neoproterozoic sandstones from the Chandarpur Group and the Tiratgarh Formation have been carried out to determine their provenance, tectonic setting and weathering conditions. All sandstone samples are highly enriched in quartz but very poor in feldspar and lithic fragments. Petrographically and geochemically these sandstones are classified as subarkose, sublitharenite and arenite. The Chemical Index of Alteration (CIA mean 68) and Th/U ratios (mean 4.2) for these sandstones suggest their moderate weathering nature. Generally, all sandstone samples are strongly depleted in major elements (except SiO2), trace elements (except Zr) and REE in comparison with Post Archean Australian Shale (PAAS) and Upper Continental Crust (UCC). Their mineralogy and mean of elemental ratios suitable for determination of provenance and tectonic setting, e.g. Al2O3/SiO2 (0.02), K2O/Na2O (10), Eu/Eu* (0.67), (La/Lu)n (10.4), La/Sc (3), Th/Sc (1.2), La/Co (0.22), Th/Co (0.08), and Cr/Th (7.2), support a felsic source and a passive margin tectonic setting for these sandstones. Also these key elemental ratios do not show much variation over a range of SiO2. Thus we attest their significance in determining source rock characteristics of quartz rich sandstones. Chondrite‐normalized REE patterns with LREE enrichment and a strong negative Eu anomaly are also attributed to felsic source rock characteristics for these sandstones. The source rocks identified are granite and gneiss of the Bastar craton. Minor amounts may have been derived from older supracrustals of the Bastar craton. However, the major element data of the Paleoproterozoic Sakoli schists when compared with those of the Neoproterozoic sandstones indicate that the schists were derived from a mafic source and deposited in an active continental margin tectonic setting. There is, however, little difference in CIA values between the Paleoproterozoic Sakoli schists and Neoproterozoic sandstones, indicating prevailing of similar (moderate‐intense) weathering conditions throughout the Proterozoic in the Bastar craton. Our study also suggests a change in the provenance and tectonic setting of deposition of sediments from dominantly a mafic source and an active continental margin in the Paleoproterozoic to dominantly granite and gneiss (felsic source) and a passive continental margin in the Neoproterozoic in the Bastar craton.

Petrogenesis and tectonic setting of bimodal volcanism in the Sakoli Mobile Belt, Central Indian shield

AbstractThe Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO2 shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial 143Nd/144Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNdt (t = 2000 Ma) varies between −0.19 and +2.22 for the basic volcanic rock and between −2.85 and −4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.

Geochemical characterizations of mafic magmatic rocks of the Central Indian Shield: Implication for Precambrian crustal evolution

Geochemical and petrogenetic studies have been conducted on a set of basaltic rocks from Betul, amphibolites and dolerite dykes which intrude the Tirodi and Amgaon Gneissic Complex from Betul-Chindwara and Amgaon regions, respectively. Geochemically Betul mafic volcanics are classified as basalt to basaltic andesite with tholeiitic lineage in the form of Fe-enrichment trend. All the rocks are distinctly enriched in incompatible trace elements including light rare earth elements (LREE) and large ion lithophile elements (LILE), with continental signature in the form of negative Nb, P and Ti anomalies in their primitive mantle-normalised spidergrams. These chemical characteristics indicate their derivation from enriched mantle sources / sub-continental lithosphere. The dykes traversing the Tirodi Gneissic Complex (TGC) in Chindwara region have high abundances of Fe-oxides and alkali elements. These dykes are sub-alkaline, ranging in composition from basalt to basaltic andesite, with tholeiitic affinity. Doleritic dykes of TGC have enriched LREE-LILE with depleted high field strength elements (HFSE) patterns, which probably indicate their derivation by from enriched mantle sources, while the dykes of Amgaon Gneissic Complex (AGC) have lower LREE-LILE enrichment, which probably indicate higher degrees of partial melting of similarly enriched sources. Negative Sr-anomaly is conspicuous, probably indicating role of plagioclase fractionation in their genesis. Depleted mantle Sm-Nd model ages for the Betul mafic volcanics vary between 1951 Ma and 2320 Ma, indicating this period represents a major crustal evolution event in this region.

Fault reactivation and pseudotachylite generation in the semi-brittle and brittle regimes: examples from the Gavilgarh–Tan Shear Zone, central India

AbstractIn the sheared and foliated granitoids of the Proterozoic Gavilgarh–Tan Shear Zone (GTSZ) in central India, two types of pseudotachylite (Pt-M and Pt-C) are recognized. Pt-M layers are interbanded with mylonite and ultramylonite, show strong internal plastic deformation and buckle folding concurrent with the host rocks, and appear to have formed within the greenschist facies (300–400 °C) in the brittle–plastic transitional (semi-brittle) regime. Pt-C layers show sharp contacts with the host rock, exhibit abundant coeval cataclasis, preserve no evidence of subsequent plastic deformation, and formed at shallower depths, at temperature &lt; 300 °C. Sulphide droplets and embayment of quartz grain margins in the pseudotachylite (Pt-C) matrix indicates a melt origin. Ductile shear sense criteria in the host mylonites are consistently sinistral, while those associated with the deformed pseudotachylite (Pt-M) layers are dextral. It appears therefore that the host mylonite/ultramylonite foliation experienced reactivated slip movement in the ‘semi-brittle’ zone when pseudotachylite was generated and subsequently ductilely deformed. The brittle pseudotachylite (Pt-C) layers were generated later at a shallower level, and at a lower temperature. They are spatially associated with a set of foliation-parallel brittle shears with sinistral-sense displacements. The multiple episodes of frictional melt generation within the Gavilgarh–Tan Shear Zone illustrate that it has a complex history of multiple reactivations. It therefore represents an important new area for the study of seismic behaviour of the upper crust along pre-existing structures and may facilitate a better geological understanding of the present seismic activity in the central Indian Shield.

Geology and geochemistry of giant quartz veins from the Bundelkhand Craton, central India and their implications

Giant quartz veins (GQVs; earlier referred to as ‘quartz reefs’) occurring in the Archean Bundelkhand Craton (29,000 km2) represent a gigantic Precambrian (∼2.15 Ga) silica-rich fluid activity in the central Indian shield. These veins form a striking curvilinear feature with positive relief having a preferred orientation NE-SW to NNE-SSW in the Bundelkhand Craton. Their outcrop widths vary from ≤1 to 70m and pervasively extend over tens of kilometers along the strike over the entire craton. Numerous younger thin quartz veins with somewhat similar orientation cut across the giant quartz veins. They show imprints of strong brittle to ductile-brittle deformation, and in places are associated with base metal and gold incidences, and pyrophyllite-diaspore mineralization. The geochemistry of giant quartz veins were studied. Apart from presenting new data on the geology and geochemistry of these veins, an attempt has been made to resolve the long standing debate on their origin, in favour of an emplacement due to tectonically controlled polyphase hydrothermal fluid activity.