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Vast clay deposits of Chitradurga and Sandur greenstone belts of the Dharwar Craton, southern peninsular India, hosting pockets of high‐grade manganese (Mn) ore, provide remarkable clues related to the ore‐forming processes and palaeo‐environmental conditions. In this study these Fe–Mn‐rich clays were investigated through mineralogical and geochemical characteristics to comprehend their palaeo‐weathering and genetic constraints and their role in the supergene accumulation of Mn ore. Their mineralogy is characterized by predominant kaolinite, muscovite, birnessite, haematite, goethite, halloysite along with quartz and traces of ilmenite. Chemical indices of alteration and weathering indicate intense weathering conditions under high palaeo‐precipitation rates in a humid, tropical climate under shallow burial conditions and K‐metasomatism. The presence of authigenic kaolinite and absence of illite reflect on diagenesis up to mesodiagenetic stage. Positive to negative Ce anomalies (Ce/Ce* = 0.67–5.93) of these clays suggest varying oxic‐anoxic conditions during their genesis, whereas conspicuous negative Eu anomalies (Eu/Eu* = 0.61–0.99) are attributed to precursor sediment signatures and hydrothermal imprints of the Mn ore. These kaolinite‐rich clays are suggested to have been formed by in situ chemical alteration, whereas kaolinitization of the Fe–Mn‐rich arenaceous and argillaceous sediments was derived from predominant tonalite and felsic provenance in a shallow marine passive margin setting. Based on the morphologies of birnessite, haematite and goethite in the investigated clays, we propose that these clays have played a key role in the oxidation and deposition of Mn (II) from the migrating aqueous solutions and acted as organo‐polymerization templates for the proliferation of ancient microbial life on the planet Earth. [ABSTRACT FROM AUTHOR]

The NW–NNW trending Paleoproterozoic (2.5–1.6 Ga) mafic dykes from Pakhanjore, Muramgaon and Gariaband–Chhura areas of central Bastar craton are medium-grained, consisting of orthopyroxene, clinopyroxene and plagioclase with ophitic texture. They represent basalt to basaltic trachyandesite composition in TAS (total alkali silica) and tholeiitic magma series in AFM diagram. On element ratio plots, the dykes exhibit fractional crystallisation of plagioclase, clinopyroxene and olivine. High Th concentrations, deviation from MORB-OIB array observed in Nb/Yb vs. Th/Yb plot, and positive Zr–Hf anomalies in multi-element spidergram support crustal contamination of the dykes. Geochemical characteristics such as affinity towards subduction-modified lithospheric mantle (SZLM) field and diagonal trend towards plume array in TiO2/Yb vs. Th/Nb diagram, support the generation of the magma from different degrees of interactions between plume and SZLM. Non-modal REE modelling implies derivation of the magma from different degrees of partial melting of spinel-garnet containing lherzolitic source. Reynolds numbers in order of 103–104 obtained from quantitative calculations of the dyke widths support turbulent ascent of the magma. Our calculations suggest while magma ascending from lower crust to upper crust, parameters such as magma flow rate (Q), heat flow (H), wall-rock melting rate (U) values change depending upon the nature of magma flow and dyke thickness. [ABSTRACT FROM AUTHOR]

Singhbhum craton in eastern India is one of the oldest and perhaps geologically and geophysically most complex geological terrains on the surface of the Earth, containing a number of intra-cratonic Archean volcanic suites. In the present study, we investigate Mesoarchean volcanic rocks of the Simlipal complex, a tiger reserve in the Mayurbhanj district of Odisha (India). Our results provide a new understanding of the regional geodynamic scenario of the Eoarchean Singhbhum craton, including the nature of plume–lithosphere interaction based on the detailed analysis of geological, geochemical and geophysical data. Whole-rock geochemical studies exhibit large variation in SiO2 (39.30–60.57 wt%), TiO2 (0.20–1.55 wt%), Al2O3 (4.45–15.63 wt%), MgO (5.25–37.00 wt%) and CaO (3.46–11.26 wt%) with low to moderately high Cr (50–1503 ppm) and Ni (36–550 ppm) contents. These rocks are cumulates to porphyritic and petrographically and geochemically, can be classified as ultramafic (lherzolite, pyroxenite), picrite, basalt, basaltic andesite, andesite and boninite. Their trace elements and HFSE/REE patterns indicate that they belong to the same parental magma. Rare earth elements and trace element patterns exhibit moderate fractionation with a coherent pattern of (La/Yb)n: 1.38–8.50, (Gd/Yb)n: 0.92–2.59 and (La/Sm)n: 1.11–4.40, consistent with polybaric melting. The present study suggests that these rocks were generated by decompressive melting of a mantle plume head at a depth of garnet to spinel lherzolite field together with fractional crystallization and crustal contamination in the subcontinental lithospheric mantle (SCLM), which is corroborated by the highly uplifted nature of this terrain, associated with positive gravity anomalies. This plume had a mantle potential temperature between 1400 and 1700 °C and pressure reaching 4.4 GPa. Massive upwarping of the lower crust to an extremely shallow depth of about 4 km from the surface, 21 km thick magmatic underplating above the Moho and the lithosphere asthenosphere boundary at 81 km, would support plume-induced active crust–mantle thermal interaction and extremely warm and deformed nature of the lithosphere beneath the Simlipal volcanic complex. Based on the findings of millerite (NiS), quench/spinifex texture and shock metamorphic features in these volcanic rocks, we infer that there could be a possibility that the suggested mantle plume in the study area may have been triggered by an asteroidal impact. [ABSTRACT FROM AUTHOR]

The Indian Shield, composed of diversified rock types, formed at different ages, emplaced in varied tectonic environments, is a natural laboratory for the geoscientific community. Multi-disciplinary studies involving tools like field geology, petrography, geophysics and geochemistry can better probe the Earth's interior at a higher resolution. The Geochemistry Division, equipped with state-of-the-art analytical facilities such as XRF, SEM-EDS, MC-TIMS and LA-HR-ICP-MS, has made pioneering contributions to better understand the evolutionary history of the Indian Shield. This excerpt provides a glimpse of the research activities carried out by the Geochemistry Group during the last 55 years. Significant contributions include (1) geodynamics and crustal evolution of the Indian Plate in terms of nucleation, accretion and amalgamation of the three Protocontinents: Dharwar, Aravalli and Singhbhum; (2) geological and geochemical studies on the evidence for early life; and (3) deciphering the depositional environment and provenance of clastic sediments of the Dharwar Craton. Besides southern India, focused research studies in Bastar, Singhbhum and Bundelkhand cratons, Eastern Ghat Mobile Belt, Central Indian Tectonic Zone, Andaman Islands and Antarctica have yielded significant results that were published in recognised peer-reviewed National and International Journals. In addition to fundamental research, the Geochemistry Group has actively collaborated with industries like Hutti Gold Mines Limited (Hutti), Hindustan Zinc Limited (Udaipur) and National Mineral Development Corporation (Hyderabad) for the exploration of base metals and gold in different parts of the country. The analytical facilities created and developed by the Group are currently helping scores of researchers and students from a host of R&D organisations/universities across the country and even a few organisations abroad in pursuing world-class geochemical and environmental research. [ABSTRACT FROM AUTHOR]

The lower part of the Proterozoic Cuddapah Basin exposes three quartzite sequences, namely, Gulcheru, Pulivendla, and Gandikota formations. Gulcheru Quartzites show angular to subangular texture, whereas Pulivendla and Gandikota quartzites contain rounded to subrounded detrital grains. Mineralogically, all quartzites are predominantly composed of quartz with subordinate amount of feldspar, mica, and heavy minerals. Clay and silt are present as a matrix while silica and carbonate act as a cementing material. Based on detrital modes and major element geochemistry, these sediments are classified as arkose to subarkose. The petrological and geochemical attributes of the basal formations of the Cuddapah quartzite indicate cyclic weathering under variable humid subtropical climate. The CIA values of these quartzites suggest moderate to intense chemical weathering for the Gulcheru and Gandikota quartzites and low to moderate chemical weathering for Pulivendla Quartzites. The major element ratios (SiO2/Al2O, K2O/Na2O), trace element ratios, chondrite‐normalised REE patterns and UCC normalised multi element diagrams suggest that these quartzites were predominantly derived from the felsic sources, with subordinate contribution from mafic sources. Provenance studies and mixing models, in conjunction with palaeocurrent data suggest that these quartzites of the Lower Cuddapah Supergroup received sediments from various sources including Eastern Dharwar Craton as well as from more juvenile crust. [ABSTRACT FROM AUTHOR]

Abstract: SHRIMP U-Pb zircon and titanite age data for granites adjacent to the Gadag greenstone belt (GGB) in the Western Dharwar Craton (WDC), India, establish the occurrence of voluminous ∼2.56Ga granitic magmatism and indicate the development and evolution of the GGB over a time span of ∼70m.y. These new data show that the GGB has an evolutionary history distinct from the Chitradurga greenstone belt (CGB) to the south, which was previously considered to be linked to that of the GGB, based on the continuity of lithological associations. Whole rock geochemical data indicate granites marginal to the GGB are potassic, were derived from intracrustal melting of an arc crust, without the involvement of mantle, and demonstrates the prevalence of horizontal tectonics in the evolution of Dharwar Craton. Lu-Hf zircon studies indicate that these rocks were derived from a heterogeneous arc-crust with distinct crustal histories on either side of the GGB. The voluminous ∼2.56Ga granitic magmatism surrounding the GGB in the WDC, in addition to the known ∼2.61 and 2.56–2.52Ga felsic magmatism supports a number of short-lived, episodic crustal growth events within the Dharwar Craton during the Neoarchean over a period of ∼100m.y. Short-lived episodic crustal growth during the Archean, typically within the time scale of few million years marks the distinction with long-lived modern-style subduction processes; these variations are often attributed to hotter Archean mantle and prevailing geodynamic regime. [Copyright &y& Elsevier]

• New Pb-Pb ages confirm the craton-wide presence of 2.36 Ga and 1.88 Ga mafic events. • Sr-Nd isotopic data suggest their sources were derived from distinct mantle sources. • The 2.36 Ga mafic dykes were derived from a shallower mantle source. • The 1.88 Ga mafic dykes were derived from relatively deeper mantle source. • The Bastar craton rotated counterclockwise during Paleoproterozoic. This research focuses on the geochronology and geochemistry of WNW-NW-NNW trending Paleoproterozoic mafic dykes from the Bastar craton in India. The weighted mean 207Pb/206Pb ages of baddeleyite grains obtained from the Tuskal, Pakhanjore, and Gariaband regions were 2365.25 ± 0.54 Ma, 2365.70 ± 0.48 Ma, and 2363.85 ± 0.79 Ma, respectively. Baddeleyite grains from a mafic dyke near Narayanpur-Kondagaon yielded a weighted mean 207Pb/206Pb age of 1886.5 ± 1.8 Ma. In the TAS (Total Alkali Silica) diagram, the 2.36 Ga and 1.88 Ga mafic dykes have basalt to basaltic andesite composition. Dykes from both age groups are represented by the tholeiitic magma series in the AFM diagram. In the spidergrams, the dykes show LREE and LILE enrichment, with the 2.36 Ga dykes showing flat HREE and the 1.88 Ga dykes showing slightly inclined HREE patterns. The REE non-modal batch melting model indicates derivation of the 2.36 Ga mafic dykes from a shallower spinel lherzolitic source, whereas the 1.88 Ga mafic dykes were derived from a deeper spinel-garnet lherzolitic mantle source. The 2.36 Ga dykes have near chondritic to slight super chondritic initial Nd isotopic values (εNd (2.36 Ga) = −0.95 to 2.36), whereas the 1.88 Ga dykes have sub chondritic initial Nd isotopic values (εNd (1.88 Ga) = −2.37 to −9.67). Present-day orientations and similar precise ages of the mafic dykes reported from the Bastar and Dharwar cratons indicate the counter-clockwise rotation of the Bastar craton during the Paleoproterozoic. [ABSTRACT FROM AUTHOR]

Abstract: The Neoarchean eastern Dharwar Craton (EDC) is distinct from the Mesoarchean western Dharwar Craton (WDC) in many aspects of its geology. The important distinguishing features of the EDC are the predominance of younger granitoids, abundance of gold mineralization and the exposure at lower crustal depths. No consensus exists on evolutionary models for EDC; mutually exclusive plume and subduction-derived tectonic models have been proposed. Geochemical and radiogenic isotopic studies on the granitoids and volcanic rocks of three greenstone belts along a cross-section in the northern part of EDC are presented herein. The evolved Nd isotopic signatures, radiogenic Pb isotopic ratios and “arc-like” geochemical signatures are suggestive of a subduction regime and the involvement of recycled older crust in the derivation of these rocks. The proposed petrogenetic mechanism involves multi stage processes in a supra subduction regime involving slab dehydration, formation of hydrous basaltic melts, and re-melting and interaction with sub-arc basaltic crust at low pressures where amphibole±plagioclase is the dominant residual phase. There is a notable systematic decrease in the extent of older crustal involvement from west to east in the EDC. This is in concurrence with the younging of Dharwar Craton from west to east and eastward subduction. The proposed petrogenetic model can efficiently explain the variations in older crustal involvement, which is very common in other Archean cratons. [Copyright &y& Elsevier]

Detailed geological, petrological and geochemical studies have been carried out on an arenite–quartzwacke suite of rocks constituting a part of the Late Archaean Sandur schist belt in Dharwar craton, southern India for understanding the nature of provenance for these sedimentary rocks. The arenite–quartzwacke consists of rounded to sub-rounded and angular fragments of monocrystalline–polycrystalline quartz, quartzite and chert embedded in a fine-grained matrix of quartz and sericite. While arenites are more siliceous (SiO2, 80–92 wt.%), the quartzwacke have relatively lower silica content (ca. 69–78 wt.%). The arenites and quartzwackes have CIA values ranging from 76 to 96 which suggest intense chemical weathering. This is further corroborated by the positive correlation between Al2O3 and TiO2 in both these rock types. The A&z.sbnd;CN&z.sbnd;K modeling of arenites and quartzwackes show evidence for addition of K2O during later metasomatic alteration. In the A&z.sbnd;CNK&z.sbnd;FM ternary diagram all the samples plot along a mixing line between chlorite and sericite indicating alteration during K-metasomatism and the presence of mafic rocks in the source. The high concentration of HFSE such as Zr, Hf, Nb and Ta and the trace element ratios Th/Sc, La/Sc, Th/U and Ce/Th in the arenite–quartzwacke indicate a mixed provenance. The rare earth element modeling of quartzwackes considering tonalite, granite and amphibolite end members in the provenance suggests equal proportions of mafic and felsic end members. A composition comprising of 25% tonalite+25% granite+50% amphibolite in the provenance appears to match with the observed range of REE patterns of quartzwackes. The presence of higher proportions of granite in the provenance is evidenced by the large negative Eu anomalies in these sediments. Field evidence and structural discordance suggest that the arenite–quartzwacke suite is an allochthonous part of the Sandur schist belt. [Copyright &y& Elsevier]