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The Palaeo- to Neoarchaean Bundelkhand Craton (BkC) in central India has received attention due to its felsic crust evolution. There are two linear patches of relict greenstone belts in the BkC, and the central greenstone belt has been the focus of extensive research for the past two decades. In contrast, the southern greenstone belt has received inadequate attention. At the southern BkC, the E-W Madawara Greenstone Belt is represented by patches of remnant mantle serpentinites and metasedimentary rocks that are tectonically juxtaposed with Archaean basement TTG gneisses and Proterozoic Bijawar Group of rocks. This study highlights key findings from fields, geological maps, petrography, and a large number of whole rock chemical analyses of serpentinites and spinel chemistry to unravel the tectonic evolution story. We identified two serpentinites with contrasting petrological and geochemical properties: spinel peridotite (SP) and talc-tremolite schist (TTS). SP is primarily dunite to harzburgite with relatively high MgO (33 wt%), enriched Yb, and shows strong re-fertilization. TTS, on the other hand, is lherzolitic and less refractory (MgO = 25 wt%). Chemically, these two serpentinites were mantle wedge and subducted peridotites, respectively. The unaltered cores of the chromium spinels represent a fore-arc setting. These findings, in conjunction with prior research, paint a compelling picture of a Neoarchaean (~2.5 Ga) subduction system once present in the southern Bundelkhand Craton. The serpentinites represent relict ophiolites – from a northerly dipping trench. It also explains a Mesoarchaean rift basin system between the two cratonic nuclei of southern Bundelkhand, which amalgamated during the Kenorland Supercontinent formation. [ABSTRACT FROM AUTHOR]

Singhbhum Craton (SC), one of the oldest continental crusts, has a nucleus of Singhbhum Granite Complex with volcano-sedimentary belts in peripheral parts. Deciphering the inter-relationship between volcano-sedimentary packages and surrounding granitoids offers considerable challenges. The present study area lies in the northwestern parts of SC. Lithounits of volcano-sedimentary packages drapes around the Paleoarchean Bonai Granite Complex (BGC). An attempt has been made to understand the basin development over BGC based on the mutual relationship between lithopackages of Iron Ore Group (IOG) and their structural disposition. It is proposed that two distinct basins, Koira and Gurundia, developed over BGC during Mesoarchean. While the Eastern Koira basin witnessed continuous development accompanied by subsidence wherein lithopackages of Bonai–Kendujhar Iron-ore Belt were deposited, Western Gurundia Basin is marked by a hiatus after the deposition of Gurundia quartzite and mafic volcanics in a shoreline to shallow marine environment. Darjing Group of rocks deposited as platformal package over Gurundia, Koira groups and BGC. Litho packages of Koira, Gurundia and Darjing groups exhibit structural unity as they co-deformed during Iron ore orogeny. The first deformation phase has led to a series of NE–SW to ENE–WSW trending low plunging overturned folds with southeasterly vergence. The second deformation, near coaxial with the first, has led to low northerly plunging open folds with steep northwesterly dipping axial planes. BGC occupies the core of a major second-generation fold with volcano-sedimentary sequence draping around it. It is proposed to put them under Iron Ore Super Group (IOSG), where IOGs (Koira and Gurundia groups) represent the lower part and Darjing Group as the upper part. Research Highlights: IOSG consists of IOGs (Koira and Gurundia groups) and Darjing Group as per present observation. Volcano sedimentary litho packages of IOG were deposited in two distinct basins (Koira and Gurundia basins) developed over BGC in a continental shelf-slope facies. Darjing Group deposited all over Gurundia and Koira groups and BGC after a hiatus which is marked by polymictic basal conglomerate. All the litho members of IOSG were co-deformed during Iron-Ore Orogeny (during the formation of Ur) and exhibit two distinct phases of folding as observed from field and this is reported for the first time with an attempt to bring Darjing group within Iron Ore Supergroup (IOSG). [ABSTRACT FROM AUTHOR]

Due to the inherently fluid‐mobile nature of W, the 182W record of the early Earth may have been obscured by fluid‐induced mobilization of W. To investigate W mobilization in Archean greenstone sequences, we analyzed 182W isotope systematics and major and trace element concentrations in samples from the 3.53 Ga old Onverwacht Group of the Kaapvaal Craton (South Africa) and the >3.51 Ga old Badampahar Group of the Singhbhum Craton (India). Our results for mafic and ultramafic metavolcanic rocks show W/Th ratios significantly higher than primary magmatic values, which suggests fluid‐induced W enrichment. Samples least affected by secondary W enrichment (W/Th < 0.26) show no resolvable W isotope anomalies from modern mantle values in both cratons. Samples from the Kaapvaal Craton with elevated W/Th exhibit deficits in 182W as low as −8.1 ± 4.3 ppm compared to the modern mantle. Covariations of μ182W with W/Th, and Ce/Pb suggest that negative isotope signatures were introduced during secondary fluid‐mediated processes. The enrichment of W is most evident in altered ultramafic rocks comprising serpentine, resulting in additional covariations between MgO, LOI, and W/Th. The W isotope composition of serpentinized komatiites reflects the composition of younger intruding granitoids. We therefore infer the latter as a possible source of W‐rich fluids. The Badampahar Group samples exhibit little W isotope variability. A well‐resolved 182W deficit of −6.2 ± 2.9 ppm was determined in a single komatiite sample, which indicates an unknown fluid source, currently not represented in any other unit of the Singhbhum Craton. Plain Language Summary: The tungsten (W) isotope composition of ancient rocks can be used to trace processes that occurred during Earth's early evolution. However, interactions between rocks and fluids may alter the W concentration and therefore influence the interpretation of W isotope data. To identify the source of such fluids and the processes by which they affect the W isotope composition of rocks, we analyzed ancient rock samples from South Africa and India. The isotope composition of rocks with a low W concentration reflects that of the modern Earth. Therefore, they do not trace the processes that occurred during Earth's early evolution. Samples from South Africa with untypically high W concentrations show a different isotopic composition. The variation in the W isotope signature correlates with other chemical indices that are susceptible to modification by fluid‐related processes. This shows that the W within the rocks is derived from an external fluid source and not from their original magmatic source. Samples with the highest W enrichment have a similar isotope composition as spatially associated intrusive rocks. By inference, the latter likely represent the source of W‐rich fluids. The samples from India show similar enrichment in W, indicating similar fluid‐related processes and W sources at both localities. Key Points: The magmatic sources of metavolcanic rocks from the Onverwacht Group and the Badampahar Group do not exhibit W isotope anomaliesNegative W isotope signatures in the Onverwacht Group are likely derived from fluids sourced from younger intrusive granitoidsFelsic intrusive rocks are a major source of W‐rich fluids in Paleoarchean greenstone units [ABSTRACT FROM AUTHOR]

Petrographic, whole‐rock geochemical and geochronological analysis of polymictic conglomerate, associated siltstone and greywacke in the Neoarchean Gadag Greenstone Belt in the Western Dharwar Craton, India, provides new insights into sedimentary provenance and tectonic setting. The polymictic conglomerate contains granitic, gneissic, felsic and mafic volcanic, quartzite and banded ferruginous chert clasts within a quartzo‐feldspathic matrix. Framework grains in the siltstone and greywacke are quartz, feldspar with rock fragments, embedded in a chlorite‐dominant matrix. The chemical index of alteration values (~70) and A‐CN‐K plot for the rocks suggest a low‐to‐moderate degree of weathering. Rare earth element (REE) patterns show moderate fractionation (LaN/YbN ~ 12.31) with a weak negative europium anomaly (0.82). Enrichment in transition elements (Ni, Cr, Co, Sc) and depletion in high‐field‐strength elements, as well as relatively low La/Co (0.62), moderate to high Th/Yb (3.06), La/Th (5.16), La/Yb (17.5), and La/Sc (~2.37) ratios, suggest a mixed felsic‐mafic provenance. U–Pb dating of detrital zircon from the conglomerate and greywacke revealed major age peaks at ca. (3.2, 2.9, 2.8, 2.6, 2.5 Ga) and a maximum age of deposition of 2,508 ± 26 Ma, 2,493 ± 20 Ma, respectively. The above data suggest low to moderately weathered proximal sources for both conglomerate and greywacke, indicating that these two rock types were not deposited simultaneously, but rather separately. The association of these conglomerates and greywacke with continental arc‐related volcanics indicates the closing of an arc at the time of sedimentation. Complex provenance consisting of recycled and magmatic arc during basin closure confirm the notion of modern style plate tectonics operating during Neoarchaean. [ABSTRACT FROM AUTHOR]

The metamorphosed ultramafic-mafic bodies of the Kudada area are located close to the Singhbhum Shear Zone (SSZ) in eastern India, where the major rock types are talc-magnesite schist and serpentinite with accessory chromite and magnetite veins. The ultramafic bodies and associated metavolcanic rocks are part of the northern extension of the Early Archean Gorumahisani greenstone belt and belong to the Iron Ore Group (IOG) supracrustal sequence. This study reveals intense compositional variability in accessory chromites of serpentinite with core composition of chromites are characterized by the variable Cr# [Cr/(Cr+Al)] = 0.53–0.82 and Mg# [Mg/(Mg+Fe2+)] = 0.01–0.17. Compositional variability on the scale of a single chromite grain occurs in the form of multi-stage zoning. To identify the patterns of compositional zoning, chromites of serpentinite are subdivided into four types depending on their grain size, reflectivity of different rims, intensity of fracture, and porosity, and supported by in-situ mineral chemistry. The type-I chromites are less fractured and non-porous variety showing the outermost chrome magnetite rim envelops the inner ferritchromit rim. Porosity is mainly developed in the type-II grains where the inner ferritchromit is formed surrounding the pore spaces. The type-III chromites are small clustered grains having ferritchromit core and chrome magnetite rim while the type-IV grains are completely altered to chrome magnetite. Textural relations and mineral chemistry indicate that metamorphism and activities of H2O and CO2-rich hydrothermal fluids during tectonic evolution of the Singhbhum Shear Zone (SSZ) might have caused these zoning patterns and compositional variabilities in accessory chromites of the Kudada area. Cation exchange between chromite and silicate minerals along with intra-grain cation diffusion within different Cr-spinel zones further intensified these processes. [ABSTRACT FROM AUTHOR]

The carbonates of the Dodguni area of the Chitradurga greenstone belt, Dharwar Craton, India were studied to understand the Archean diagenetic and palaeo‐depositional conditions using carbon and oxygen isotopes along with whole‐rock geochemistry. These carbonates show significant variation in δ13C and δ18O (−1.73‰ Vienna Peedee Belemnite [VPDB] to −0.45‰ VPDB and δ18O vary from −13.51‰ VPDB to −8.46‰ VPDB, respectively). The rare earth element (REE) + Y (rare earth elements and yttrium) of the studied carbonate rocks exhibit flat and uniform patterns (PrSN/YbSN = 0.4–1), slight positive La, Eu and Gd anomalies (CeSN/CeSN* = 0.8–0.9; EuSN/EuSN* = 0.7–1.4 and GdSN/GdSN* = 2–2.3, respectively) with slight positive Ce anomalies (PrSN/PrSN* = 0.84–0.86) and superchondritic Y/Ho ratios (Y/Ho = 29–45; chondritic Y/Ho = 28). Their negative δ13C (−1.75‰ to −0.45‰) values are attributed to the influence of higher bio‐productivity during their deposition and oxidation of organic carbon. The geochemical and isotopic systematics suggest that the Dodguni limestones are not affected by the diagenetic and metamorphic processes and record the REE geochemistry of the ancient seawater in which they were precipitated. The comprehensive geochemical including the isotopic studies of the Archean carbonate rocks of the Dharwar Craton, indicate their deposition in shallow marine conditions, precipitation in high temperatures with significant hydrothermal input under the influence of high bio‐productivity, thereby providing valuable constraints on the palaeoceanography and geochemical evolution of the Archean sea. [ABSTRACT FROM AUTHOR]

It is well established that the major and minor element contents of chromites are subject to change during greenschist to amphibolite facies metamorphism. During upper amphibolite facies metamorphism, chromite can be completely converted to chrome magnetite. However, not all elements are affected to the same degree, the concentrations of +2 ions (e.g. Zn, Co, Mn) being particularly vulnerable to modification. The degree to which trace elements, particularly the platinum-group elements (PGE), are affected has not been closely examined. The compositions and textures of chromites from komatiites of the Gorumahishani greenstone belt of the Singhbhum Craton (India) have experienced a range of metamorphic conditions from greenschist to amphibolite facies, providing the opportunity to study the changes of trace and platinum-group element composition with metamorphic grade. Five types of altered chromites are identified from the komatiitic suite of rocks in the ~120-km-long greenstone belt. The type-I chromites are non-porous and characterized by the least modified cores. These chromites are mostly present in the northern Maharajgunj-Tua Dungri section where rocks show metamorphism from greenschist to greenschist-amphibolite transition facies. The type-II and type-III chromites are porous and mostly found in the southern Kapili section of the greenstone belt where rocks show metamorphism up to the mid-amphibolite facies. Type-IV and type-V chromites are completely modified to ferritchromit and chrome magnetite, respectively, and are present in the komatiitic rocks from the entire greenstone belt. The central cores of the type-I and type-II grains have relatively higher concentrations of mobile trace elements (e.g. Zn, Co, and Mn) with higher Mg# [Mg/(Mg + Fe2+)], lower Cr# [Cr/(Cr + Al)], and lower Fe3+/R3+ (R3+ = Fe3+ + Cr3+ + Al3+) ratios than their respective rims. Significantly higher concentrations of the immobile trace elements (e.g. Ti and V) in the cores of the type-II grains relative to their chrome magnetite rims from the Kapili section and to the type-I varieties from other sections might be due to the metamorphism of the komatiitic rocks under higher-grade conditions (amphibolite facies). In situ LA-ICPMS analysis for PGE reveals a relatively higher concentration of Ru and Rh in the rims of the type-I chromites than in the cores which is due to the diffusion of these elements from the normal spinel structure of the cores towards the bivalent octahedral sites of the inverse spinel structure of the chrome magnetite rims during metamorphic processes. The lower concentrations of Os, Ir, Ru, and Rh in the cores of the type-II chromites from the Kapili section might be related to the metamorphism of the rocks under higher-grade conditions that facilitated the diffusion of these elements to associated sulphide or platinum-group mineral or alloy phases. The calculated partition coefficients of Sc, Ti, V, Mn, Ni, Ga, Os, Ir, Ru, and Rh from the least altered chromite cores assuming equilibrium with the parental komatiitic melt also suggest the variable effects of metamorphism when compared with global experimental and empirical values of the natural samples. [ABSTRACT FROM AUTHOR]

The Hole Narsipur Greenstone Belt (HGB), in the Western Dharwar Craton, South India, preserves evidence for three cycles of metamorphism spanning the Archaean–Proterozoic transition. Changes in metamorphic style across the transition are investigated using integrated petrographic, mineral chemical, metamorphic evolutionary history, conventional geothermobarometry, and phase equilibria modeling studies of rocks of diverse bulk rock composition from the belt. These rocks record signatures of three overprinting metamorphic cycles (M2 → M3 → M4 in sequence), postdating a Mesoarchaean, M1 event in the craton. The M2 cycle represents characteristic LP–HT metamorphism along a counterclockwise (CCW) metamorphic P–T path, peak metamorphism at 4–5 kbar, 515–595°C, and high thermobaric ratios (T/P ~1050–1525°C/GPa) at the M2 peak. The intermediate M3 metamorphic cycle records a prograde burial of partially cooled M2 rocks to middle-lower crustal depths, peak metamorphism at P ~7.5–10.2 kbar, T ~630–655°C, and corresponding intermediate T/P ratios (T/P ~620–875°C/GPa) and a retrograde stage, marked by combined exhumation and cooling to P ~5.3–6.4 kbar, T ~530–575°C. The resultant P–T paths of evolution show variability of clockwise (CW) paths from classical hairpin type to two-stage prograde heating segments. During the M4 metamorphism (T Max ~630–640°C at ~8.9–10.3 kbar and with lower T/P ratios at ~610–720°C/GPa), the partially exhumed M3 crust is re-buried to lower crustal depths along an intermediate thermal gradient as during the M3 metamorphic cycle. We interpret the thermal transition, tectonic thickening along a cooler thermal gradient, repeated burial-exhumation cycles in the orogenic wedge, and tectonic mixing of the three metamorphic cycles in terms of a three-stage tectonic evolution of the HGB. These in a sequence are (a) the development of a hot and thinned crust (cf. M2 cycle), (b) the transition to a thickened crustal zone tectonic domain (cf. M3 cycle), with both the domains being part of the lithospheric peel-back driven convergence setting and (c) finally continental collision tectonics (cf. M4 cycle). The pulsating nature of orogenesis at the Archaean–Proterozoic boundary deduced here distinguishes the Western Dharwar Craton from other cratonic blocks globally at the same time frame. [ABSTRACT FROM AUTHOR]

Clast morphometry and sedimentary facies analysis of polymictic conglomerate of the Hiriyur Formation in the Gadag greenstone Belt, Western Dharwar Craton, India, was conducted to constrain the palaeohydraulic condition, depositional environment, and type of sedimentary basin. The conglomerate grades from clast‐supported to matrix‐supported with siltstone‐sandstone intercalations. Eight lithofacies along with five major lithofacies associations are recognized in the Majjur and Attikatti Domains of the Gadag greenstone Belt. The lithofacies architecture suggests sedimentation commenced with rapid deposition of slope‐derived debris in a proximal high‐energy alluvial fan setting followed by a braided river environment. Clast morphometry and palaeohydraulic data indicate a low palaeoslope (0.000024 m/m), and stream discharge value (1.913 m3/s) calculated from the channel fill conglomerates. Greywacke overlying the conglomerate has been interpreted as turbidites from a continental slope. The accompanying 40‐m‐thick banded iron formation and carbonate rocks also support a marine context. The lack of transitional deposits between river and deep marine settings suggests that the greywacke turbidites sequence is separated from the conglomerate deposits by substantial unconformities. This change in depositional setting could be due to a change in basin subsidence rate, tectonic rejuvenation, or major sea‐level changes. [ABSTRACT FROM AUTHOR]

A comparative study of the Precambrian Sonakhan (SGB) and Mahakoshal (MGB) greenstones belts of Central India has been undertaken to decipher their provenance, paleoweathering, paleogeography, and tectonics. As compared to the Upper Continental Crust (UCC), the MGB samples are enriched while the SGB samples are depleted in mafic elements indicating the presence of mafic rocks in the source of the MGB. This is complemented by the Ni–Cr diagram. The REE concentrations, LREE fractionated patterns and negative Eu anomalies of the MGB and SGB samples indicate derivation of sediments from a highly fractionated granitic source. Since MGB samples also contain the geochemical signature of mafic rocks, it is, therefore proposed that the MGB clastic load were derived from two sources (mafic + felsic) with arc character. This is attested by Cr and Zr relationships, and LILE enrichment, and HFSE depletion. These features suggest that the SGB developed as autochthonous while the MGB developed as an allochthonous belt. The chemical alteration indices such as chemical index of alteration (CIA), plagioclase index of alteration (PIA), and index of compositional variability for MGB samples indicate that they were dominantly derived as the first cycle (with minor recycled) sediments from bimodal sources (dominantly continental arcs) by intense chemical weathering as compared to the SGB samples, which were derived from felsic sources (dominantly cratonic rocks), and partly by recycling through a low chemical weathering. The CIA and PIA values of the samples reveal a change in the climatic conditions from Late Archean to Late Paleoproterozoic. Such change is interpreted in terms of migration of the Indian plate from high latitudes in the Late Archean to lower latitudes during the Late Paleoproterozoic. This is consistent with the paleomagnetic data that placed India in the configuration of 2.45 Ga Ur and 1.78 Ga Columbia supercontinents. [ABSTRACT FROM AUTHOR]

This study presents a comprehensive account of the petrogenetic and geodynamic evolution of the Bellara Trap volcanic rocks from the Ingaldhal Formation, Chitradurga Group, western Dharwar Craton (WDC). Geochemical attributes of these rocks are consistent with two groups with distinct evolutionary trends: one comprising tholeiitic, MORB (mid-ocean ridge basalt) type basalts (BTB) and the other corresponding to calc-alkaline andesites (BTA). Basalts are essentially composed of clinopyroxene and plagioclase whereas the andesites are porphyritic with phenocrysts of plagioclase, clinopyroxene and polycrystalline quartz embedded in a groundmass of K-feldspar, quartz and opaques. Primary igneous mineralogy is overprinted by greenschist facies metamorphism resulting in chlorite-actinolite-plagioclase assemblage. The BTB samples reflect nearly flat REE patterns with weak LREE enrichment in contrast to pronounced LREE enhancement over HREE discernible for BTA. Tectonically, the BTB samples correspond to an active mid-oceanic ridge-rift setting with a MORB composition, whereas a back-arc basin (BAB) regime is corroborated for the BTA samples fractionating from back-arc basin basalts. Geochemical imprints of subduction input are more pronounced in BTA compared to BTB as mirrored by their elevated abundances of incompatible fluid mobile elements like Ba, Th, U and LREE. The BTB is endowed with an N- to E-MORB signature attributable to minor contributions from subduction-related components at the inception of a back-arc basin in the vicinity of an active subduction system. The BTA derived through differentiation of a basaltic magma with BABB (back-arc basin basalt) affinity compositionally akin to a heterogeneous source mantle carrying depleted MORB-type and enriched arc-type components inducted with progressive subduction. The BABB-type andesites and MORB-type basalts from Bellara Traps record a compositional heterogeneity of mantle in an intraoceanic arc-back arc system. Mantle processes invoke a BABB-MORB spectrum with a MORB-like endmember and an arc-like endmember associated with a juvenile back-arc basin. This study infers a Neoarchean analogue of Mariana-type back-arc rift setting proximal to the arc with a gradual transition from anhydrous to hydrous melting processes synchronized with MORB-mantle and arc-mantle interaction during initiation of a nascent back arc adjacent to the arc. The MORB-BABB compositional spectrum for the Bellara Traps conforms to a Neoarchean back-arc basin that evolved under an extensional tectonic regime associated with incipient stages of back-arc rifting and incorporation of subduction-derived components in the mantle output. This study complies with Neoarchean intraoceanic accretionary cycle plate tectonics in WDC. [ABSTRACT FROM AUTHOR]

The Sonakhan greenstone belt in Central India is under-explored with respect to gold in spite of its similarity to auriferous greenstone belts in general, which prompted a prospectivity analysis. The workflow involved adoption of a conceptual mineral systems model, recognizing indicative spatial proxies, processing exploration datasets, generating evidence maps and integrating into GIS-based mineral prospectivity mapping. Available geological information such as key lithologic units and their contacts was combined with geochemical anomalies of selected pathfinder elements, geophysical data (aeromagnetic anomaly and K/Th ratio map) and satellite digital image data (ASTER and Landsat 7 ETM +), leading to generation of 17 evidential layers. The lack of a significant number of known mineral occurrences in the study area precludes the use of data-driven prospectivity modeling techniques. Therefore, knowledge-driven approaches such as binary and multiclass index overlay, fuzzy logic and fuzzy AHP (analytic hierarchy process) were adopted to integrate the evidential layers resulting in four prospectivity maps. The variation of cumulative prospectivity with respect to cumulative area in each model was used to determine threshold to produce binary prospectivity maps separating high and low prospectivity zones. An approach based on the unique conditions of the binary prospectivity maps was used to illustrate the combined results of different models. In order to quantify the intuitive uncertainty in exploration targeting that arose due to different model outputs, a modulated predictive model was generated taking the mean prospectivity values at each pixel. The pixels having mean values above 95th percentile were grouped and the area delineated as potential exploration targets for gold that comprises merely 5% of the study area. The estimated uncertainty and confidence values for each pixel were used in the risk analysis that returned 1.95% and 3.05% of the study area as low- and high-risk exploration targets, respectively. [ABSTRACT FROM AUTHOR]

An occurrence of Palaeoarchaean ferropicrite cumulates and ferrobasalts is being reported from the Jojohatu area of the Iron Ore Group (IOG) greenstone belt, which is situated in the northern part of the Singhbhum Craton (eastern India). The ferropicrite cumulates contain serpentine and clinopyroxene as major constituents and spinel, magnetite, and ilmenite as accessories. Their bulk rock chemistry exhibits an extremely high amount of FeOT (19.0–20.8 wt%) and MgO (22.0–23.9 wt%) and quite low Al2O3 (4.5–5.2 wt%) and Na2O + K2O (<1.0 wt%), thus making them as one of the most Fe‐rich ferropicrite cumulates in the world. They are also characterized by high contents of compatible trace elements (Ni: 986–1,470 ppm; Cr: 990–1,256 ppm) and moderate TiO2 (1.5–1.7 wt%), Nb (15–18 ppm), and Zr (82–90 ppm). These cumulates are possibly formed due to accumulation of olivine from the ferrobasaltic melts. In comparison, ferrobasalts consisting of plagioclase, pyroxene, and opaques are alkaline in nature. Some of the plagioclase and pyroxene grains have shown a signature of metasomatic alteration, which resulted into formation of secondary minerals such as epidote, chlorite, and hornblende. They exhibit relatively low contents of FeOT (14.6–18.4 wt%), MgO (8.4–10.8 wt%), Ni (32–122 ppm), and Cr (41–131 ppm) than the ferropicrite cumulates, while TiO2 (2.9–4.1 wt%), Nb (42–61 ppm), and Zr (216–318 ppm) contents are higher. Incompatible elemental patterns and their ratios, as well as Zr/Y versus Nb/Y plot, indicate their ocean island basalts (OIB) affinity and genesis from a hot mantle plume, having mantle potential temperature of about 1,600°C and pressure of 3.5–5 GPa. Considerably low CaO (3.00–7.16 wt%) and FeO/MgO and CaO/MgO systematics indicate that the primary source of the basaltic melt could be pyroxenite. These volcanic occurrences possibly represent remnants of the OIB formed during the Palaeoarchean in the eastern part of the Indian shield. [ABSTRACT FROM AUTHOR]

The Sonakhan greenstone belt (SGB), located in the north‐eastern Bastar craton of the Indian shield, comprises mafic‐ultramafic and volcanic‐intrusive sequences in the lower stratigraphic units. We investigate the Platinum‐group element (PGE) relations of Boradih intrusion of the SGB to evaluate its tectono‐magmatic evolution. The chondrite‐normalized PGE patterns of boninitic cumulate rocks exhibit higher abundance of Palladium group PGEs (∑PPGE = 292–496 ppb) relative to the Iridium group PGEs (∑IPGE = 32–52 ppb) along with variable Au concentrations (51.34–718.05 ppb). The PGE concentrations are attributed to a boninitic parental melt, where the IPGEs in the source possibly partitioned into a monosulphide solid solution. The Cu (22–80 ppm), elevated Pt (22–238 ppb), and Pd (31–377 ppb) concentrations indicate Pt and Pd have partitioned into a semi‐metal rich melt during the later stages of crystallization. The geochemical characteristics of the basalts and ultramafic cumulates of the SGB indicate a supra‐subduction zone tectonic setting for its formation. Similar geochemical and litho‐tectonic correlations are also noticed between the SGB of Bastar craton and greenstone belts of the Eastern Dharwar craton of south India. The SGB (V/Yb =146 ± 25) and greenstone belts of Eastern Dharwar craton (V/Yb = 134 ± 52) record similar oxidation conditions of Phanerozoic subduction zones. Accordingly, we propose magmatic as well as tectonic correlations are possible for the Archean‐Palaeoproterozoic Bastar and Eastern Dharwar cratons. [ABSTRACT FROM AUTHOR]

The southern basement of the Cuddapah Basin comprises the Dharwar Batholith and greenstone belt complex. Granitoids of the batholith exhibit extensive variation in terms of geomorphology, age, mineralogy, and micro/meso scale structures. The eastern part of Dharwar Craton along 13°50′ to 14°8′N latitude and 78°45′ to 79°05′E longitude was studied to enlighten the rheological influence on crustal evolution. Frequent occurrences of migmatites of restricted dimension are observed in the south of 14°10′N latitude. The granite‐migmatite contacts are not sharp in general. Different types of migmatite complex and their relationships with granitoids as well as older country rocks represent an exhumed segment of the crustal catazone. The widespread group of migmatitic rocks are classified in a composite manner on the basis of morphology and structure. Furthermore, genetic implication vis‐a‐vis anatexis history is also evaluated. Static and dynamic modes of migmatites are recognized with reference to geothermal gradient and tectonics. Based on the degree of anatexis, two categories of migmatites are identified in the field, that is, metatexites and diatexites. In addition, metatexites are classified into four sub‐types (viz, patch, dilatant, net, and stromatic) and diatexites are also sub‐divided into two categories (viz, schollen or raft and schlieren). The hybrid nature of migmatitic rocks with both metamorphic and igneous characteristics are used to analyse pre‐ and post‐anatectic events. The preserved evidences of partial melting are marked as leucocratic patches. In situ stagnation of the melt or subsequent separation from the remaining solid provides different morphology of static mode. Importance of dihedral angle at solid–liquid contacts is also considered in the present context to describe the grain boundary penetration by partial melt. Folds, veins, and boudins of different styles and generations played significant role in dynamic mode migmatitization. The syn‐ and post‐metamorphic deformation events and granite melt generation from migmatites are schematically defined. Spatial and temporal relationships of schist‐gneiss‐migmatites of both static as well as dynamic mode reveal initiation of the crustal development by vertical accretion of ultramafic‐mafic lava and TTG. Cyclic partial remelting of the metabasic lava and TTG and underplating led to development of the lithospheric plate. Later upwelling material at convergent plate and associated heat transfer led to generation of granitic magma. The established prograde and retrograde cycle of metamorphism were possibly interrupted by crustal reworking events. This study confirms about the crustal catazone segment (with >15 km depth and >500°C) in which physical processes control generation, segregation, ascent, and emplacement of juvenile granite from migmatites. [ABSTRACT FROM AUTHOR]

The timing of emplacement of the oldest supracrustal rocks represented by Sargur Group is significant in understanding crustal evolution processes in the western Dharwar craton. The age of ultramafic-mafic rocks of the Sargur Group is not well-constrained as the komatiitic and komatiitic basaltic rocks have always given unreliable ages due to larger errors attributed to their alteration. New Sm-Nd isotope data from relatively less-altered layered intrusive ultramafic-mafic rocks of Nuggihalli and Holenarsipur greenstone belts of Sargur Group, western Dharwar craton give a much precise age of 2934 ± 88 Ma. This is the youngest age obtained for the ultramafic-mafic rock complexes of Sargur Group and is indistinguishable from the oldest ages reported for the overlying Dharwar Supergroup rocks. Sargur Group rocks might merely represent older ultramafic equivalents of the Dharwar Supergroup. A clear temporal distinction does not exist to support the stratigraphic classification of these two groups separately. However, the sub-contemporaneous to older ages obtained for the tonalite-trondhjemite-granodiorite gneisses that surround these greenstone belts would imply that the narrow belts tucked within the gneisses are younger, and therefore, the greenstone belts, whose dismembered roots and parts are exposed alongside the gneisses, could represent an Archaean analogue of an ophiolite suite. [ABSTRACT FROM AUTHOR]

The Neoarchean potassic granite from the Hosadurga region adjacent to the Chitradurga greenstone belt (CGB), part of the western Dharwar Craton (WDC), is studied for its petrogenesis. The combined field, petrographic, whole‐rock elemental, and Nd–Sr isotope data of granite and associated Tonalite–Trondhjemite–Granodiorite (TTG) gneiss from this region are presented here. The granites are alkali‐calcic to calc‐alkalic in terms of modified alkali lime index (MALI) and are weakly peraluminous (molar Al2O3/(CaO + Na2O + K2O) = 1.0–1.1). The geochemical data indicate that these are formed by the melting of TTG gneiss at depths above the garnet stability field. Isotopic data (εNd = −2.9 to −7.6 at T = 2,600 Ma, ISr = 0.715–0.729, TDM > 3.0 Ga) also show the involvement of Archean crustal sources. Trace element modelling was carried out using non‐modal equilibrium melting equation, which constraints partial melting of TTG source to a low degree (11%) for the formation of this granite. This model could also produce a major element composition consistent with this granite. The upper bounds on the magmatic temperatures (724–785°C) assessed from zircon saturation thermometry suggest that fluid influx is required. TTG samples show variable and very high positive epsilon Nd values (1.5–8.9), higher than the contemporaneous mantle, suggesting re‐melting of mafic crust. The observed high Nd isotope ratios in TTG indicate involvement of a mafic source that formed as early as ~3.8 Ga, probably as a result of global differentiation of the early Earth. [ABSTRACT FROM AUTHOR]

The potential field (gravity & magnetic) data, along with results of available seismic and magnetotelluric studies over the Southern India Shield comprising the Dharwar craton (DC), Southern Granulite Terrane (SGT) and southern part of the Eastern Granulite Mobile Belt (EGMB), were synthesized. The study attempts to elucidate the geometry and continuity of greenstone belts/shear zones, isostatic condition and deeper crustal architecture. We observed a characteristic elevation-gravity relationship for each of the tectonic domains. The greenstone belts of the DC are characterized by N-S to NW-SE trending positive residual gravity anomalies and continue beneath the Cuddapah sediments. The basement of the Cuddapah basin is characterized by NW-SE trending greenstone belts with low-density intrusive rocks. The gravity anomaly suggests occurrence of a high-density rock unit occurs at mid-crustal depth beneath the 'Closepet granite' and 'Chitradurga schist belt'. The potential field signatures of the DC and SGT are distinctly different. The Cauvery Shear Zone is characterized by positive residual gravity and magnetic anomalies with ∼E-W to ENE-WSW trend that continues further east under sediment covers of the east coast. The residual gravity signatures and horizontal gravity gradient signatures suggest that the orogenic systems encompassing the Cauvery Shear Zone and the eastern part of the EGMB may be a part of a common structural belt or tectonic regime associated with the assembly of the Gondwana supercontinent. The potential field modelling reveals the presence of a thick, high-density material resting over the base of the crust in the entire stretch of the east-coast of India. (a) Gravity anomaly map, (b) magnetic (reduced to pole) anomaly map, (c) residual gravity map, (d) Isostatic anomaly map and (e) gravity-magnetic modelling. The residual gravity and horizontal gravity gradient signatures suggest that the orogenic systems encompassing the Cauvery Shear Zone and eastern part of the Eastern Granulite Mobile Belt may be a part of a common structural belt or tectonic regime associated with the assembly of the Gondwana supercontinent. The study also reveals magmatic under plating along east coast of India. [Display omitted] • Comprehensive potential field analysis of parts of Southern Indian Shield (SIS). • Outlines continuation and disposition of the greenstone belt and shear zones. • Continuation of structural belt from across different tectonic domains of the SIS. • The study reveals magmatic under plating along east coast of India. • Each geo-tectonic unit shows characteristic gravity-elevation relationship. [ABSTRACT FROM AUTHOR]

In comparison to the greenstone belt volcano-sedimentary successions, mature arenite dominated sedimentary deposits are very rarely preserved in Paleo-Mesoarchean rock record. Such arenitic deposits may be important for the recognition of extensive development of continental freeboard and onset of Phanerozoic style plate tectonics. The Keonjhar siliciclastics are ∼715-m thick mature arenite-dominated Mesoarchean succession that unconformably overlies Paleo-Mesoarchean granite-greenstone core of the Singhbhum craton. The succession is mapped on 1:50,000 scale, and is designated as the Keonjhar Quartzite. It is subdivided into conglomerate dominated lower unit as the Asurkhol Member and mature arenite dominated upper unit as the Kamalpur Member. A lensoid body of iron ore clast bearing conglomerate unconformably overlies the Keonjhar Quartzite, and is designated here as the Chamakpur Conglomerate. The succession is repeated by three westerly dipping reverse faults that have upthrown the deeper parts of the basinal transects against shallower parts. The Asurkhol Member represents subaerial part of a coastal alluvial fan. The Kamalpur Member represents shallow-shelf depositional setting. Detailed mapping and recognition of the fault slices led to the reconstruction of the paleogeographic setting. The present study also highlights the importance for the stratigraphic correlation of the Keonjhar Quartzite with underlying metabasics of Malangtaoli-Jagannathpur lava and the overlying western IOG succession. The internal stratigraphic classification proposed here is also significant from economic point of view with regard to the occurrences of uraniferous QPCs in the Asurkhol Member and detrital iron ore deposits in the Chamakpur Conglomerate. [ABSTRACT FROM AUTHOR]

In this article, we present the textural, mineralogical, and geochemical studies of a suite of volcanic rocks and their economic mineral potential from the Kadiri Volcanic Province (KVP) – a greenstone belt in the Eastern Dharwar Craton (EDC) of southern peninsular India. The volcanic rocks are represented by primary pyroclastic deposits dominated by tuff breccia, minor lapilli tuff, and pyroclastic flows together with lava flows, namely, metabasalts, andesites, dacites, and rhyolites. Metabasalts and high‐Mg andesites in the KVP are associated with sporadic occurrences of disseminated‐type gold. The ancient works on gold are confined to the ductile‐brittle shear zones in en‐echelon pattern. The source of gold is likely to be from deep fractures (>200 km) at mantle depths that paved the magma to move upper crustal levels due to partial melting of peridotites which tapped the siderophile elements (Au–Ag) from deeper source. Some andesites and dacites contain atomic minerals [uranium, thorium, and rare earth element (REE)], which requires attention in future studies. Monazite occurring in such dacites is a major source of REE such as cerium (Ce), lanthanum (La), neodymium (Nd), and Yttrium (Y), contributing 50–60% of REEs. Presence of thorite (ThSiO4) in dacite, a metamict mineral, strongly radioactive containing upto 10% uranium (U). This study confirms multi‐metal mineralization in KVP, for example, gold, silver, atomic (U and Th), rare‐earths along with base metals. Emplacements of rhyolite and alkali granite appears to be synchronous with the major thermal event at 2.51 Ga of the Dharwar Craton. The diverse volcanic associations are formed in a subduction–accretion orogeny at ~2.7 Ga, coeval with the global accretion event and gold mineralization. Au–U–Th–REE abundances in the composite arc‐back arc system of Kolar‐Hutti‐Kadiri‐Jonnagiri greenstone belts of EDC are primarily attributed to devolatilization and melting of subducted oceanic slab, fluid‐fluxed metasomatism of mantle wedge and elemental cycling associated with different stages of subduction. [ABSTRACT FROM AUTHOR]

The Bailadila Group of the Bastar Craton, India, is host to a 200-m-thick banded iron formation (BIF). We document the lithostratigraphic context for the BIF, informally referred to as the Bose iron formation, and provide radiometric constraints for its depositional age. Field evidence illustrates that the BIF was deposited on an inner-shelf succession with a quartz arenite that grades upward into the BIF through storm-dominated offshore shelf deposits. The quartz arenite to BIF transition records a relative sea level rise from transgressive to highstand systems tract when the BIFs were deposited in a starved outer continental shelf. U-Pb SHRIMP analyses of zircons from the basement of the Bailadila Group yielded mostly highly discordant U-Pb SHRIMP ages. However, the ages fall on well-defined discordia lines from which concordia intercept ages could be determined. These ages, in combination with the ages of a few zircons that are less than 6% discordant, indicate that the granitoid basement crystallized at 3500–3550 Ma. The maximum depositional age of the Bailadila Group is constrained from the weighted mean 207Pb/206Pb SHRIMP age of 2725 ± 57 Ma from detrital zircons from the basal arenites. A well-constrained weighted mean 207Pb/206Pb SHRIMP age of 2733 ± 53 Ma for zircons from a unit that unconformably overlies the Bailadila Group is within error of that age. Stratigraphic relationships suggest that the Bailadila succession is unconformably overlain by the ~2.5 Ga Kotri and Dongargarh Supergroups. The depositional age of the Bailadila Group is well constrained between ~2.7 and 2.5 Ga. In contrast to most other Archean Algoma-type iron formations of peninsular India, which are closely related to volcanic rocks in greenstone belts, the Bose iron formation is associated with siliciclastic shelf succession. It thus is considered a Superior-type iron formation that represents the oldest known one of its kind in India. [ABSTRACT FROM AUTHOR]

Most of the upper crustal fluid flows are strongly influenced by the pre-existing fractures/foliations in the rocks under a certain state of tectonic stress and fluid pressure condition. In the present study, we analyzed a wide range of crosscutting fractures that are filled with quartz veins of variable orientations and thicknesses, from the gold-bearing massive metabasalts (supracrustals) of the Chitradurga Schist Belt adjacent to the Chitradurga Shear Zone (CSZ), Western Dharwar Craton, southern India. The study involves the following steps: (1) analyzing the internal magnetic fabric, using anisotropy of magnetic susceptibility (AMS) studies, and determining strength of the host metabasalts, (2) quantifying the fluid pressure condition through lower hemisphere equal area projection of pole to veins by determining the driving pressure ratio (R′), stress ratio (ϕ), and susceptibility to fracturing, and (3) deciphering the paleostress condition using fault-slip analysis. We interpret the NNW–SSE to NW–SE (mean 337/69 ∘ NE) oriented magnetic fabric in the rocks of the region as having developed during regional D1/D2 deformation on account of NE–SW shortening. However, D3 deformation manifested by NW–SE to E–W shortening led to the sinistral movement along CSZ. As a consequence of this sinistral shearing, fractures with prominent orientations formed riedel shear components, with CSZ as the shear boundary. Subsequently, all the pre-existing fabrics along with the riedel shear components were reactivated and vein emplacement took place through episodic fluid pressure fluctuation from high to low Pf at shallow depth (∼ 2.4 km). However, NNW–SSE orientations were prone to reactivate under both high- and low- Pf conditions, thereby attaining maximum vein thickness along these orientations. The deduced paleostress from fault-slip analysis along with the kinematics of the fractures and veins are in good agreement with previously estimated regional tectonics. Thus, integrating multiple domains of studies helps in the logical interpretation of fluid flow conditions and vein emplacement mechanisms in the study area that has not been ventured before. [ABSTRACT FROM AUTHOR]

Low saline aqueous carbonic fluids are considered to be the ore forming solutions for orogenic lode gold deposits. Phase separation/fluid immiscibility of the ore fluid is quite common and is one of the major reasons for deposition of gold in these deposits. Abundant carbonic fluid inclusions have been observed in quartz grains of Hira-Buddnini Gold Deposit. Theoretical estimation indicates that more volume of H2O compared to CO2 is likely to be trapped in inclusions at different P-T conditions. Preferential loss of H2O from fluid inclusions during ductile deformation of quartz grains have been attributed as the suitable reason for abundance of carbonic fluid inclusions. [ABSTRACT FROM AUTHOR]

The Eastern Dharwar Craton (EDC) in southern India comprises several thin, linear N-S and NW-SE trending greenstone belts that hold rich economic mineral deposits, surrounded by gneisses and granites of different ages. Very long, linear, high-frequency, high-amplitude magnetic anomalies trending along the E-W, NE-SW, ENE-WSW, NW-SE and N-S directions associated with several sets of dike swarms, fault shears and fractures forming a rhombic pattern over the granite-gneissic terrain in the western part of EDC hindered the recognition of these thin, linear belts. The N-S strike of the belts in the low-magnetic latitudes posed another limitation to demarcate these zones. A comprehensive relook into the regional aero-magnetic data using the prevailing interpretation techniques through diverse approaches helped identify the feeble magnetic signatures due to known narrow zones and enabled us to map hitherto unknown belts, namely Kurnool and Lattavaram-Atmakuru. 2.5D GM-SYS modelling has been carried out to delineate the vertical disposition of known belts that reaffirmed the existence of newly inferred belts. Isolated magnetic anomalies are also modelled for source-body parameters. [ABSTRACT FROM AUTHOR]

The present study exploits high-resolution hyperspectral imagery acquired by the Airborne Visible/Infrared Imaging Spectrometer-Next Generation (AVIRIS-NG) sensor from the Hutti-Maski gold deposit area, India, to map hydrothermal alteration minerals. The study area is a volcanic-dominated late Archean greenstone belt that hosts major gold mineralization in the Eastern Dharwar Craton of southern India. The study encompasses pre-processing, spectral and spatial image reduction using Minimum Noise Fraction (MNF) and Fast Pixel Purity Index (FPPI), followed by endmember extraction using n-dimensional visualizer and the United States Geological Survey (USGS) mineral spectral library. Image derived endmembers such as goethite, chlorite, chlorite at the mine site (chlorite mixed with mined materials), kaolinite, and muscovite were subsequently used in spectral mapping methods such as Spectral Angle Mapper (SAM), Spectral Information Divergence (SID) and its hybrid, i.e. SIDSAMtan. Spectral similarity matrix of the target and non-target-based method has been proposed to find the possible optimum threshold needed to obtain mineral map using spectral mapping methods. Relative Spectral Discrimination Power (RSDPW) and Confusion Matrix (CM) have been used to evaluate the performance of SAM, SID, and SIDSAMtan. The RSDPW and CM illustrate that the SIDSAMtan benefits from the unique characteristics of SAM and SID to achieve better discrimination capability. The Overall Accuracy (OA) and kappa coefficient (ҡ) of SAM, SID, and SIDSAMtan were computed using 900 random validation points and obtained 90% (OA) and 0.88 (ҡ), 91.4% and 0.90, and 94.4% and 0.93, respectively. Obtained mineral map demonstrates that the northern portion of the area mainly consists of muscovite whereas the southern part is marked by chlorite, goethite, muscovite and kaolinite, indicating the propylitic alteration. Most of these minerals are associated with altered metavolcanic rocks and migmatite. [ABSTRACT FROM AUTHOR]

The Neoarchean Sonakhan greenstone belt (SGB) in the north‐eastern Bastar Craton is mainly composed of an association of pillowed and massive tholeiitic basalts emplaced within an intraoceanic environment. Two types of ultrabasic rocks were identified and are designated as TH‐1 and TH‐2 on the basis of geochemical parameters. Both sample suites exhibit depletion of HFSE with reference to LILE and LREE, LREE/HFSE ratios, and negative Nb–Ta–Ti anomalies in the primitive mantle‐normalized multi‐element diagrams. The geochemical characters of TH‐1 and TH‐2 are consistent with Island arc tholeiites (IAT) and boninite‐like rocks of both Archean and Phanerozoic terranes. Mineral chemistry of clinopyroxenes from both volcanic suites indicate an oceanic arc affinity. The chromite chemistry indicate its derivation from a boninite‐like magma in a suprasubduction zone (SSZ) environment. Trace element modelling depicts that source depletion with significant influence of fluids derived from the subducting oceanic slab collectively controlled the trace element inventory of the mantle wedge, which result to the higher abundance of LREE and LILE compared to HREE and HFSE. The occurrence of lower basalt sequence followed by IAT and boninite‐like rocks in the SGB define a magmatic evolution which is comparable to the Phanerozoic ophiolite suites that exhibit subduction initiation prior to forearc rifting and suprasubduction zone magmatism. The subduction initiation in the intraoceanic lithosphere and the initial decompression melting followed by the upwelling of the MORB‐related magmas were attributed to the formation of the lower pillow basalts. Dehydration of the subducting plate and the melt extraction processes in the hydrous lherzolite mantle wedge account for the formation of TH‐1. Subsequent depletion and significant modification of a more refractory harzburgite source by the slab‐derived fluids generated the TH‐2 rocks. The compositionally diverse magmatic imprints with IAT and boninite affinity in the SGB indicate the significance of episodic melt extraction processes, dehydration of the subducting plate, and hydrous fluxing in SSZ tectonic setting. [ABSTRACT FROM AUTHOR]

In this study, we utilize high-alumina pelitic schist samples from the Holenarsipur Greenstone Belt, a component of the ca. 3.30–3.15 Ga Sargur Supergroup in the Western Dharwar Craton, South India to establish a Mesoarchaean metamorphic history in the terrane. On the basis of integrated textural, mineral compositional, metamorphic reaction history and P-T pseudosection modelling studies, garnet-monazite trace element thermometry and coupled monazite electron probe micro-analyzer and sensitive high-resolution ion microprobe U-Pb dating results, we constrain for the first time a Mesoarchaean metamorphism along a hairpin clockwise metamorphic P-T path in the Dharwar Craton. The path reveals a two-stage prograde segment with an initial phase of heating accompanying burial (dP/dT gradient ~ 30 bar/°C) and a later phase of steep burial (∆P ~ 1.1 kbar, dP/dT gradient ~110 bar/°C), peak metamorphism at 7.5 kbar, 640 °C and a post-peak, steep decompression stage (P~6.3 kbar at 635 °C). Chemically zoned monazite grains shielded within garnet or occurring in low-strain matrix domains were formed in two stages during the Mesoarchaean metamorphism: (a) Monazite cores with relatively lower Y concentrations, higher Th/U ratios, a strong negative Eu anomaly; their REE partitioning relationships with the garnet inner rim reveal their crystallization was synchronous with the growth of garnet. (b) Monazite rims with higher Y and uraniferous compositions and lower Th/U ratios which, combined with Y and HREE enrichments in the outer rim of garnet, indicate their formation during the post-peak decompression stage associated with coupled decompositions of garnet and zircon. We correlate U-Pb monazite core and rim ages at 3144 ± 7 Ma and 3105 ± 10 Ma respectively with the timings of peak and retrograde metamorphism. Our findings, when evaluated with the available geological, isotopic, geochronological data of Meso- to Palaeoarchaean extant and extinct felsic crust as well as thermo-mechanical models, predict the ca.3.14–3.11 Ga metamorphism in the Holenarsipur Greenstone Belt as part of a widespread tectono-thermal event in the Western Dharwar Craton. The event marks late stages of sagduction tectonics in the craton as it evolves from a predominantly vertical to a horizontal shortening mode. • We have studied a Mesoarchaean metamorphism in the western Dharwar craton, S India. • High-alumina pelitic schists from the Holenarsipur Greenstone Belt record a hairpin CW P-T loop. • Peak metamorphism (T Max ~ 640 °C at ~7.5 kbar) is dated at ca. 3.14 Ga. • The Mesoarchaean metamorphism is part of a widespread tectonothermal event in the WDC. • The event marks a shift in tectonic style from initial sagduction to later horizontal plate motions. [ABSTRACT FROM AUTHOR]

Archean ultramafic–mafic magmatism associated with dynamic mantle melting processes represents an integral part of crustal evolution and continental growth. Archean mantle plumes and their high temperature derivatives in komatiitic magmas provide a cornerstone in understanding the thermal and chemical evolution of the early Earth and its mantle. In this study, we evaluate the geochemical characteristics of the temporally distinct Sargur Group and Dharwar Supergroup greenstone belts of Dharwar Craton in southern India. The komatiites associated with these greenstone belts are characterized by Al‐depleted and Al‐undepleted compositions. Their MgO, Ni, Co, and Cr concentrations reflect primitive, undifferentiated nature of precursor melts, whereas the HFSE and REE chemistry corroborates high‐degree (30–50%) partial melting of mantle with prominent role of majorite garnet and perovskite in the generation of komatiitic melts. The Al‐depleted komatiites were formed at a greater depth (~13 GPa) by equilibrium melting leaving a garnet‐bearing residue. The resultant komatiitic melt retained buoyancy under extreme pressure and high temperature conditions, segregated and accumulated in the ambient peridotite mantle source and subsequently escaped with progressive decrease in pressure in the ascending plume. The Al‐undepleted and relatively rare Al‐enriched komatiites were derived by fractional melting at intermediate to shallow depths after the discharge of a large proportion of melt ensued by the collapse and entrance of residual garnet into the melt phase. The Mesoarchean–Neoarchean Dharwar komatiites provide evidence for heterogeneous, hydrated Archean upper mantle trapped by ascending mantle plumes. The non‐plume, arc‐related signatures invoke contributions from (a) sub‐cratonic lithospheric mantle roots influenced by ancient subduction events and (b) hydrated Archean upper mantle. The Sargur Group and Dharwar Supergroup komatiites record a distinct temporal transition and geochemical heterogeneity in the Archean mantle beneath the Dharwar Craton and can be attributed to Archean upper mantle hydration by flat slab subduction of >3.3 Ga oceanic crust at shallow level with vestiges of primordial oceanic crust forming deep cratonic mantle roots beneath the Dharwar Craton. The temporal transition towards mantle hydration beneath Dharwar Craton during the Mesoarchean can be correlated with the onset of subduction‐driven plate tectonics and ocean–crust–mantle interactions. [ABSTRACT FROM AUTHOR]

Shales are significant sedimentary units in the Archean greenstone belts and Proterozoic Cuddapah sequence of Dharwar Craton. We present the geochemical studies of the Archean shales from the Chitradurga greenstone belt and the Proterozoic Vempalle Formation of the Cuddapah Basin to evaluate their chemical composition, weathering, provenance, and depositional conditions. The Archean shales are depleted in transition metals (Ni and Co) but enriched in V, Cr, and Sc relative to upper continental crust, while the Proterozoic shales are depleted in Cr, Co, Ni, Sc, and V compared with Post‐Archean Australian shale reflecting on negligible mafic source during their deposition. The REE patterns of these shales are uniform with moderately flat LREE and negative to positive Ce anomalies reflecting on marine conditions of their deposition. The presence of both positive and negative Ce anomalies indicate fluctuating oxic and anoxic conditions. The Archean and Proterozoic shales show negative and positive Eu anomalies, respectively, along with slightly enriched HREE. Compositional variation is observed in the Archean shales, whereas the Proterozoic counterparts were affected by sediment recycling as displayed by the Th/Sc and Zr/Sc relationship. Th/U and Th along with (Gd/Yb)n ratios display significant heavy mineral enrichment in these shales. The chemical index of alteration (CIA) and chemical index of weathering (CIW) of these shales suggest moderate to intense chemical weathering during the Archean and low to moderate weathering during Proterozoic times. The overall geochemical signatures collectively indicate granitic and tonalitic provenance for the Archean and Proterozoic shales, which were deposited in an active and passive continental margins. [ABSTRACT FROM AUTHOR]

• Cherty stromatolitic carbonates of Dharwar Craton exhibit limited morphologies. • REE characteristics reflect marine water conditions with heterogenous hydrothermal input. • Negative to positive δ13C indicate oxic-anoxic conditions of Archean oceans. • Negative δ18O endorse local variations in the Archean sea water temperature. The well-preserved stromatolitic carbonates from Neoarchean Sandur, Chitradurga and Shimoga greenstone belts of Dharwar Craton were studied to constrain the depositional conditions, geochemical, isotopic characteristics and the Archean seawater temperatures. These rocks display limited diversity in their morphological features such as columnar (occasionally branched), pseudo-columnar, domal, crinkly laminated, and oval/elliptical shapes. The stromatolites are associated with Fe-Mn formations along with arenites, argillites, limestones and carbonaceous shales. These stromatolites are cherty dolomites having variable SiO 2 , MgO and CaO contents. The stromatolitic carbonates of the three greenstone belts possess low total Rare earth elements plus Ytterium (∑REY) along with positive La, Eu, Gd, Y anomalies and Y/Ho ratios reflecting their deposition in marine conditions with variable hydrothermal and terrigenous inputs. These stromatolites display predominantly negative δ13C values ranging from −1.61‰ to 0.01‰ VPDB of Sandur, −2.53‰ to 1.35‰ VPDB of Chitradurga and positive δ13C of Shimoga varying from −0.30‰ to 1.29‰ VPDB combined with the negative δ18O values (−20.95‰ to −7.72‰ VPDB). Our studies indicate fluctuating oxic to anoxic; acidic to alkaline environment responsible for the deposition of these stromatolites with locally varying Archean ocean temperatures from 25 to 75 °C. These conditions favoured the deposition of cherty dolomitic stromatolites in a shallow marine environment ranging from supra to subtidal zones with the minor influence of hydrothermal activity which is evident for the existence of Archean smaller plate tectonics. [ABSTRACT FROM AUTHOR]

The present work combines Concentration-Area (C-A) fractal model with the Fuzzy Analytical Hierarchy Process (FAHP) on a GIS platform for mapping gold potential in the Sonakhan Greenstone Belt, India. A set of stream sediment geochemical data obtained over a part of the study area was used to delineate potential regions of gold mineralisation. In order to develop a suitable predictive model, the gold exploration target was taken as target (dependent variable) and concentration of pathfinder elements of gold such as Au, As, Ag, Hg, Sb and Se in the stream sediments were used as predictors (independent variables). The C-A fractal model was applied to the geochemical data of each element to decompose anomaly and background components of the spatial dispersion of element concentration in the study area. AHP combined with fuzzy set theory (FAHP) was used to determine the priority or, the weight of each evidential geochemical anomaly map. The Gold exploration targets are delineated by multiplying the weights with the respective fuzzy normalised geochemical anomaly map of the gold pathfinder elements and integrating the weighted geochemical evidential layers using fuzzy Gamma operator. The reliability of the outcome was assured by the coincidence of known gold mineralisation with the potential regions delineated in the final gold potential map. The areas with high potential are good targets for detailed exploration of gold in the region. The results highlight that (a) application C-A fractal model can effectively separate geochemical anomalies as it considers the spatial variation of the data unlike the conventional statistical methods which account only the frequency of the data, (b) the FAHP allows more flexibility in judgements of multiple decision makers in a group and reduces the inconsistency of the result which is essentially required in the knowledge-driven predictive mapping of mineral prospectivity. • Maiden attempt in knowledge-driven computerized mineral potential mapping in India. • Effective use of existing database for mineral exploration in India. • Novel combination of the multifractal and fuzzy based AHP methodology for mineral exploration. [ABSTRACT FROM AUTHOR]

• The MS presents 18 U-Pb baddeleyite Paleoproterozoic ages on seven different dyke swarms across the Dharwar craton, India. • This is a major step forward in our understanding of the LIP record of this important craton (which is thought to be a principal constituent of Superia, Sclavia, Nuna/Columbia and Rodinia supercontinents), with implications for Paleoproterozoic reconstructions. • We also infer a 30° counter clock-wise rotation of the northern Dharwar block relative to the southern block from changes in trends of various swarms and basement geology. 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. [ABSTRACT FROM AUTHOR]

• A new U-Pb ID-TIMS age 2256 ± 6 Ma is obtained for NE to ENE trending mafic dyke swarm. • Seven distinct Precambrian mafic dyke swarms are identified in the Singhbhum craton. • Two of them i.e. ~2.25 Ga and ~1.77 Ga have close match with the Dharwar craton. • This also implies that both swarms should be present in intervening Bastar craton. • 2.25–2.26 Ga and 1.77–1.79 Ga LIPs are analogous with different crustal blocks of the globe. Based on trend, cross-cutting relationships and U-Pb dating, Precambrian mafic dykes in the Singhbhum craton, earlier collectively identified as 'Newer Dolerite Swarm', have been separated into seven distinct swarms, which are thought to be the plumbing systems for Large Igneous Provinces (LIPs). These Singhbhum swarms range in age from ∼2.80 Ga to ∼1.76 Ga, and include the ∼2.80 Ga NE-SW trending Keshargaria swarm, ∼2.75–2.76 Ga NNE-SSW to NE-SW trending Ghatgaon swarm, the ∼2.26 Ga NE-SW to ENE-WSW trending Kaptipada swarm (based on a new U-Pb ID-TIMS age 2256 ± 6 Ma), the ∼1.77 Ga WNW-ESE trending Pipilia swarm, the early-Paleoproterozoic E-W to ENE-WSW trending Keonjhar swarm, the middle-Paleoproterozoic NW-SE to NNW-SSE trending Bhagamunda swarm, and the late-Paleoproterozoic N-S to NNE-SSW trending Barigaon swarm. Two of the Singhbhum swarms, the ∼2.26 Ga Kaptipada and ∼1.77 Ga Pipilia, are closely matched with the ∼2.26–2.25 Ga Ippaguda-Dhiburahalli and ∼1.79 Ga Pebbair swarms, respectively, of the eastern Dharwar craton. The correlations suggest that the Singhbhum and Dharwar cratons were close enough at these times to share two reconstructed LIPs, a 2.26–2.25 Ga Kaptipada– Ippaguda-Dhiburahalli LIP and a 1.79–1.77 Ga Pipilia-Pebbair LIP, and if so, both swarms must be present in the intervening Bastar craton (candidates are proposed). Also, the 2.76–2.75 Ga Ghatgaon swarm of the Singhbhum craton can be provisionally correlated with ∼2.7 Ga Keshkal swarm of the Bastar craton. The 2.26–2.25 Ga Kaptipada–Ippaguda-Dhiburahalli LIP of the Singhbhum-Bastar-Dharwar reconstruction has age matches in the Vestfold Hills of Antarctica (∼2.24 Ga dykes), the Kaapvaal craton (the ∼2.25–2.23 Ga Hekpoort lavas) and perhaps the Zimbabwe craton (2.26 Ga Chimbadzi troctolite intrusions). The 1.76–1.79 Ga Pipilia-Pebbair LIP of the Singhbhum-Bastar-Dharwar reconstruction has age matches in the North China, Australian Shield, Amazonian, Rio de Plata and Sarmatia cratons. The relevance of these matches for reconstructions will require future testing using paleomagnetic studies. While there are ∼2.7–2.8 Ga LIP-type greenstone belts in many crustal blocks, there are no precise matches with the 2.76–2.75 Ga Ghatgaon swarm of the Singhbhum craton. However, the 2.80 Ga Keshargaria swarm can be potentially linked with units in the Pilbara and Yilgarn cratons. [ABSTRACT FROM AUTHOR]

The Palaeo to Mesoproterozoic Dalma volcano-sedimentary basin (DVSB) is a typical greenstone belt sequence within the North Singhbhum Mobile Belt (NSMB) of Singhbhum Crustal Province (SCP), eastern India. The volcano-sedimentary belt is situated in the central portion of NSMB and separates the Chaibasa and Dhalbhum formation lying at its south from the Chandil Formation at its north. The metasedimentary unit comprises carbonaceous phyllite, carbonaceous cherty quartzite, quartz mica schist and quartzite. In the present work, the Au-rich carbonaceous metasedimentary rocks were studied to understand the source rocks characteristic, paleoweathering history and possible tectonic set-up. Geochemical classification diagrams based on the major oxide data indicates that these units belong to arkose to wacke field and the protolith underwent moderate to high degree of chemical weathering. Based on critical elemental ratios, it is inferred that the sediments were subjected to moderate degree of physical weathering, associated with sediment sorting and recycling. A fractionated REE pattern coupled with prominent negative Eu anomaly and elevated Th/Sc ratios indicate a felsic provenance for these metasedimentary units. The data further suggests that the sediments were deposited in a passive margin tectonic setting, during a prolonged period of upliftment and erosion of the Archean Singhbhum granite complex. [ABSTRACT FROM AUTHOR]

Dipole–dipole resistivity and induced polarisation (IP) surveys were conducted over a conductor identified from an audiomagnetotelluric (AMT) survey run in a greenstone schist belt of the Dhanjori Basin, Eastern Singhbhum, India. The shallow conductors identified in the AMT were not well resolved due to insufficient high-frequency data. The geology of the area suggests that gold mineralisation is associated with quartz pebble conglomerates (QPC) or sulfides. The resistivity and IP surveys were conducted over an area of 1.2 × 0.4 km with an electrode spacing of 10 m and a line spacing of 50 m. This data set was inverted using Gauss–Newton optimisation. Based on the resistivity and IP results, responses were categorised into five distinct geological/geophysical signatures defining local lithology and associated mineralisation. Low resistivity and low to moderate chargeability at depths up to 20 m from the surface indicate the weathered zone (which is not a strong feature in some of the IP models). High resistivity and high chargeability zones are identified as quartz vein/QPC with disseminated sulfide ± auriferous concentrations. Moderate resistivities and chargeabilities are interpreted as being related to the greenstone schists that make up the major part of the geology in the Dhanjori Basin. Zones with high resistivity and low chargeability are interpreted as Proterozoic gabbroic anorthosite ultramafics (PGAU) and/or younger dolerites. PGAU is intrusive to the local greenstone schist/phyllites; we have observed that its resistivity is considerably high and there is no significant gain in its chargeability due to the hindrance in ionic and electronic movement in the formation. Low resistivity and low to moderate chargeability zones below the weathered layer are attributed to mineralisation associated with massive sulfides. The integrated geophysical study indicates the presence of quartz vein/QPC and sulfide hosting auriferous mineralisation over the greenstone schist belt of the Dhanjori Basin, Eastern Singhbhum. [ABSTRACT FROM AUTHOR]

• Metamorphosed komatiites and basalts from the Badampahar greenstone belt. • Geochemistry reveals a genetic connection among the volcanic rocks. • Al-depleted and -undepleted komatiites generated at different depths and melting degrees. • Melt evolved from Al-undepleted komatiite to komatiitic basalt to tholeiitic basalt. • Such evolution ensued by assimilation and fractional crystallization processes. The ultramafic–mafic volcanic rocks of Archean greenstone belts are important archives for lithospheric and asthenospheric processes of the early Earth. Despite decades of research on this context, many issues still remain unsolved. For example, the process of komatiite magma genesis and the genetic relationship among komatiites, komatiitic basalts and tholeiitic basalts in Archean greenstone belts are not clearly understood. The metavolcanic rocks of the Badampahar greenstone belt (BGB), Singhbhum Craton are studied by major–trace element geochemistry to address the said problems and better understand the evolution of melts in Archean lithosphere. Our research suggests that the protoliths of the metavolcanic rocks were komatiites (both Al –depleted and –undepleted), komatiitic basalts and tholeiitic basalts. The Al–heavy rare earth element (HREE) depleted komatiites were formed by moderate degree mantle melting at a higher depth and the Al–HREE undepleted komatiites are products of moderate to high degree mantle melting at a shallower depth. The melting–assimilation–fractional crystallization modelling result shows that komatiitic basalts were generated from Al–undepleted komatiites, and tholeiitic basalts were generated from evolved komatiitic basalts by assimilation and fractional crystallization processes. The older age limit of the BGB is determined to be 3.25 Ga. and the basement of sedimentation and volcanism was composed of plutonic felsic rocks. [ABSTRACT FROM AUTHOR]

The Chakradharpur granite gneiss (CGG) represents an uplifted Archaean basement within the North Singhbhum Orogen (NSO) of Singhbhum‐Orissa Craton in the eastern Indian shield and shares a Mesoarchaean 3.12 Ga age equivalent to the emplacement of Singhbhum granite (SBG) phase III. The CGG hosts numerous enclaves of tonalitic gneisses, metamorphosed ultramafic and mafic volcanics, metagabbros, metasediments, and conglomerates that collectively represent vestiges of an older greenstone belt preserved within it. The mineralogical composition of the ultramafic volcanic enclaves of CGG has been overprinted by greenschist facies metamorphism presenting a talc‐tremolite‐phlogopite‐actinolite assemblage with prominent development of crenulation cleavage, pressure solutions and relict spinifex texture. These are characterised by restricted low SiO2 (39.7–46.8 wt.%) at consistently high MgO (20.7–28 wt.%) contents, and geochemically classified as: (a) subalkaline (Na2O + K2O < 2) komatiite and meimechite and (b) an alkaline (Na2O + K2O > 2) type picrobasalt‐foidite‐basanite assemblage. Depleted LILE and LREE chemistry, Al2O3/TiO2 versus (Gd/Yb)N and HFSE abundances suggest that the studied Al‐undepleted Munro‐type komatiite and meimechite were derived by high degree (30%–50%), shallow level (<90 km) partial melting of a hot, anhydrous mantle plume carrying depleted components. It is suggested that the komatiite‐meimechite melts were deflected by an Archaean cratonic keel and erupted through thinned continental lithosphere at a rifted cratonic margin. Geochemical evidence of crustal contamination preserved in these komatiites are equated with minor input from reworking of older continental materials and interaction between plume‐derived asthenospheric melts and metasomatized subcontinental lithospheric mantle (SCLM). Zr/Hf, Zr/Sm, Nb/Ta, Zr/Nb, and Nb/Th ratios for the picrobasalt, high Mg basalt, foidite and basanite rocks corroborate their generation through variable degrees of partial melting of a heterogeneous lithospheric mantle source carrying depleted plume‐derived melts and enriched components from metasomatized lithosphere. Mantle processes involved assimilation of metasomatic components of SCLM by ascending melts of plume incubating at the base of lithosphere ensued by melt generation at different mantle depths extending from garnet to spinel peridotite domains. Therefore, the geochemical signatures of ultramafic enclaves in CGG invoke distinct magmatic episodes during plume‐craton interaction, melt generation at thinned craton margin to rifted continental interior and melting of metasomatized continental lithosphere that collectively contributed to the evolution of Archaean mantle lithosphere system, volcanism with diverse chemical compositions and greenstone generation in the Singhbhum Craton. [ABSTRACT FROM AUTHOR]

Abstract New geochemical and geochronological data are presented for a suite of metavolcanic rocks from Shimoga greenstone belt to understand their petrogenesis and to constrain the timing of final mafic magmatic activity in western Dharwar craton. We report an Sm-Nd isochron age of 2638 ± 66 Ma for metabasaltic and metaandesitic rocks of the stratigraphically uppermost Medur Formation of Dharwar Supergroup exposed in the belt, which is the youngest age obtained yet for Neoarchean mafic magmatism in western Dharwar craton. The emplacement of the basalt-andesite-dacite-rhyolite suite of rocks took place in a magmatic arc culminating in the deposition of vast greywacke-argillites in a continental margin-type setting. The magmatic precursors of the metavolcanic rocks were generated by partial melting of depleted mantle by slab-derived fluids with variable input from the subducting slab. Assimilation-fractional crystallization processes may also have contributed to the magmas to a significant extent. The younger mafic lithologies of the Dharwar stratigraphy represent accretionary orogens and the date obtained in this study points towards the final stages for the assembly of western Dharwar craton by subduction processes during Neoarchean. The thickness and extent of the greywacke-argillite suite in SGB could be attributed to the protracted deposition during amalgamation of western Dharwar craton with a northern Archean continental crust that might underlie Deccan volcanic rocks. Highlights • 2638 ± 66 Ma Sm-Nd age is the youngest obtained for Dharwar Supergroup mafic rocks. • Medur and Daginakatte formations mark the cessation of greenstone magmatism in WDC. • Younger mafic rocks of Shimoga belt represent Neoarchean accretionary orogens. [ABSTRACT FROM AUTHOR]

Abstract The Paleoarchean geodynamical evolution of the Bundelkhand Craton is discussed based on the geochemistry (major, trace and rare earth elements) and Sm Nd isotope systematic of mafic-ultramafic rocks from volcano-sedimentary succession of the Babina and Mauranipur greenstone belts. The petrography and mineral assemblages of these mafic-ultramafic rocks imply that they were gone through greenschist to amphibolite facies metamorphism. Geochemical analyses indicate that mafic rocks from the Babina are characterized by SiO 2 = 43.9–51.2 wt%, MgO = 5.4–11.0 wt%, TiO 2 = 0.3–1.14 wt% and Mg# = 44–77, whereas the Mauranipur are characterized by higher silica (51.8–55.6 wt%), MgO = 6.9–9.5 wt% and Mg# = 59–70. The ultramafic rocks from the Babina and Mauranipur contain SiO 2 = 46.9–50.3 wt%, MgO = 20.2–21.1 wt%, TiO 2 = 0.33–0.46 wt% and Mg# = 77–82. Six samples of mafic-ultramafic rocks from the Babina greenstone belt yield 147Sm 143Nd whole rock isochron age of ca. 3.4 Ga. Sm Nd isotopic studies of mafic rocks from the Babina and Mauranipur reveal that they were derived from a depleted mantle source. Bivariate diagrams such as Cr vs. Ni and V suggest that these magmas might have evolved through fractionation of clinopyroxene and olivine. The mafic rocks from the Babina displayed almost a flat REE and HFSE profile [(La/Yb) PM = 0.87–1.40] with negative Nb (Nb/Nb* = 0.13–0.77) and positive Pb anomalies that could be attributed to metasomatic agents derived from subducting crustal rocks. The Mauranipur mafic rocks show slightly enriched REE [(La/Yb) PM = 1.1–1.7; (La/Sm) PM = 1.1–2.0] with negative Nb anomalies (Nb/Nb* = 0.13–0.77), which are most likely the effects of crustal contamination. The ultramafic rocks from both areas and ultramafic cumulate rock from the Mauranipur display a progressive enrichment from Th, La to Yb, with negative Nb anomalies (Nb/Nb* = 0.40–0.73) with lack of Zr anomalies (Zr/Zr* = 0.10–0.30) that could be attributed to their derivation from shallower mantle. These data collectively suggest that the mafic-ultramafic rocks have been interpreted to be derived from oceanic crust in a subduction-related setting with depleted mantle composition. Highlights • Mafic-ultramafic nature, ultramafic rocks show cumulate texture. • The Babina greenstone belt yield Sm Nd whole rock isochron age of ca. 3.4 Ga • Mafic-ultramafic magmas derived from a depleted mantle sources. • These rocks are derived from oceanic crust in a subduction-related setting. [ABSTRACT FROM AUTHOR]

• New U–Pb and Lu-Hf data from the Daitari Greenstone Belt, Singhbhum Craton, India. • Detailed geochronological and tectonic evolution of the Daitari Greenstone Belt. • Zircon εHf (t) indicate a juvenile source for 3.5 Ga felsic volcanic rocks. • 3.5 Ga marks a significant period of growth of the continental crust. The Singhbhum Craton in India contains well-preserved Palaeoarchaean greenstone belts that rival their Australian and South African counterparts, but their stratigraphic make-up remains to be resolved. Here we provide new SHRIMP- and LA-ICP-MS-based U–Pb and Lu-Hf data on magmatic and detrital zircons from stratigraphically resolved samples of the Daitari Greenstone Belt (DGB) in the south of the Singhbhum Craton. A minimum age for the Kalisagar Formation, the lowermost mafic–ultramafic volcanic suite of the DGB, is provided by a felsic sill dated at 3505 ± 6 Ma. Felsic volcaniclastic rocks from the overlying Talpada Formation provided a weighted mean age of 3507 ± 5 Ma. Zircon εHf (t) indicate a juvenile source for 3.5 Ga felsic volcanic rocks. Erosion of the volcanic edifice of the Talpada Formation acted as a predominant source for the overlying turbidite sequence of the Sindurimundi Formation, having identical U-Pb detrital zircon ages of ∼ 3505 Ma. A maximum age of deposition of 3502 ± 2 Ma is proposed for the Sindurimundi Formation. Magmatic zircon ages obtained from different intrusive felsic rocks in the Daitari belt ranges from c. 3.37 to 3.35 Ga and support the notion of widespread felsic magmatism of the Singhbhum Suite in the southern part of the craton. Xenocrystic zircon crystals dated at c. 3.55 to 3.52 Ga may correspond to an earlier phase of magmatism. 3.5 Ga marks an important period in the Earth's history, when extensive felsic magmatism facilitated continental crustal growth as recorded in the Singhbhum, Kaapvaal and Pilbara cratons. [ABSTRACT FROM AUTHOR]

The Maevatanana greenstone belt in north-central Madagascar contains widespread exposures of tonalite-trondhjemite-granodiorite (TTG) gneisses, and is important for its concentrations of various metal deposits (e.g., chromium, nickle, iron, gold). In this paper we report on the petrography, and major and trace element compositions of the TTG gneisses within the Berere Complex of the Maevatanana area, as well as LA-ICP-MS U-Pb ages and Lu-Hf isotopic compositions of zircons from the gneisses. The gneisses consist mainly of granitoid gneiss and biotite (± hornblende) plagiogneiss, and analysis of thin sections provides evidence of crushing, recrystallization, and metasomatism related to dynamic metamorphism. Samples have large variations in their major and trace element contents, with SiO2 = 55.87-68.06 wt%, Al2O3 = 13.9-17.8 wt%, and Na2O/K2O = 0.97-2.13. Geochemically, the granitoid gneisses and biotite plagiogneisses fall on a low-Al trondhjemite to granodiorite trend, while the biotite-hornblende plagiogneisses represent a high-Al tonalite TTG assemblage. Zircon U-Pb dating shows that the Berere Complex TTG gneisses formed at 2.5-2.4 Ga. Most ∊Hf(t) values of zircons from the biotite (± hornblende) plagiogneisses are positive, while most ∊Hf(t) values from the granitoid gneisses are negative, suggesting a degree of crustal contamination. Two-stage Hf model ages suggest that the age of the protolith of the TTG gneisses was ca. 3.4-2.6 Ga, representing a period of paleocontinent formation in the Mesoarchean. Geothermometries indicate the temperature of metamorphism of the TTG gneisses was 522-612°C. Based on these data, the protolith of the TTG gneisses is inferred to have formed during the development of a Mesoarchean paleocontinent that is now widely exposed as a TTG gneiss belt (mostly lower amphibolite facies) in the Maevatanana area, and which records a geological evolution related to the subduction of an ancient oceanic crust and the collision of microcontinents during the formation of the Rodinia supercontinent. The lithological similarity of Precambrian basement, the close ages of metamorphism within greenstone belts and the comparable distribution of metamorphic grade all show a pronounced Precambrian geology similarity between Madagascar and India, which can provide significative clues in understanding the possible Precambrian Supercontinent tectonics, and also important constraints on the correlation of the two continental fragments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Granitoids of Chitradurga belt were emplaced at ∼ 3.2 Ga, ∼3.0 Ga and ∼ 2.6 Ga. • Two metamorphic events are documented at ∼ 3.0 Ga and ∼ 2.56 Ga. • Nd isotopes infer juvenile crustal addition at 3.2 Ga, reworking at 3.0 Ga and 2.6 Ga. • Role of convergent margin tectonics in the evolution of Chitradurga granitoids. The Chitradurga Greenstone Belt is one of the largest belts in the central part of the Western Dharwar Craton (WDC). We present new SHRIMP U-Pb zircon and titanite ages, whole-rock geochemical and Sm-Nd isotopic data for granitoids that developed at both its eastern and western margins, and for those that are intrusive into the belt. Western margin TTG samples yield emplacement ages ofca. 3.21 Ga and are of the high-Al TTG type characterized by weakly negative ɛNd t values. Eastern margin granite samples yield emplacement ages of ∼ 3.0 Ga and 2.56 Ga with negative ɛNd t values. The granites intrusive into the belt are of Neoarchean age (∼2.61 Ga) with more-evolved Nd isotopic signatures consistent with the involvement of older crustal material. Metamorphic events are documented at ca. 3.0 Ga and ca. 2.5 Ga. The ca. 3.0 Ga event is synchronous with the diapiric emplacement of trondhjemites and high-K granitic plutons in the WDC. The peak metamorphic event at ∼ 2.5 Ga is the major tectono-thermal event evident throughout the Dharwar Craton. Geochemical systematics imply the involvement of convergent margin tectonics in the evolution of studied granitoids. The role of horizontal tectonics in the crustal growth of WDC is evident from at least 3.2 Ga, whereas signatures for the reworking of the older crust are more prominent from ∼ 3.0 Ga. [ABSTRACT FROM AUTHOR]

Two Archaean supracrustal complexes are documented in the Bundelkhand craton viz. 1) the Central Bundelkhand greenstone complex (formed the Babina and Mauranipur belts), and 2) the Southern Bundelkhand metasedimentary (paragneisses, schists) complex (formed the Girar belt). The Central Bundelkhand greenstone complex consists of (i) an early assemblage composed of basic-ultrabasic, felsic volcanics, banded iron formation (BIF), and (ii) a late assemblage which contains felsic volcanics. The U-Pb zircon ages of felsic volcanics from the early assemblage, formed in an arc-type subduction setting, are 2813 Ma (Mesoarchaean). The U-Pb zircon ages of felsic volcanics from the late assemblage, which also occur in subduction settings as an active continental marginal type, are 2542 Ma (Neoarchaean). The Girar belt, located in the southern part of the craton, consists generally of metasedimentary rocks namely quartzites and BIFs. Quartzites are represented by fuchsite- and hematite- bearing quartz arenite and lesser quartz pebble conglomerates that have been subjected to low-grade metamorphism. BIF is represented by thinly-bedded quartz-hematite (±magnetite) rocks. Some lenses of marble and chlorite schist are present at the contact between the two formations. BIF is fairly rich in Cr and Ni, poor in Zr, Hf, Ba, Th, Sr, Yb and Lu, and displays a distinct positive Euanomaly (Eu/Eu* = 1.14-2.46). Zircons from quartzites give two U-Pb ages: 3432±9.7 Ma and 3252±6.4 Ma. The Sm-Nd isotope study of quartzite from the Girar belt shows that the T is 3.29 Ga. This T correlates well with the U-Pb ages of zircon and indicates that the continental crust in the Girar area began to form in the Paleoarchaean (3.4-3.2 Ga). The Central Bundelkhand greenstone complex was thus formed in a subduction-accretion setting in Mesoarchaean (ca 2.8 Ga) and Neoarchaean (2.54 Ga) time, and the Southern Bundelkhand metasedimentary complex originated in a sedimentary basin on the old continental crust apparently at ca ~ 2.7 Ga. [ABSTRACT FROM AUTHOR]

This contribution presents a comprehensive synthesis on the origin of oldest (3600–3200 Ma) cratonic core in the Western Dharwar Craton (WDC), Southern India based our new field, geochronologic, elemental and Nd-Hf isotopes data on the Holenarsipur greenstone belt and adjacent granitoids combined with published record on the stratigraphically equivalent greenstone belts in the same region. Our study shows that the oldest cratonic core in the WDC formed through assembly of different tectonic units close to 3200 Ma. These tectonic units include microcontinental remnants with oceanic plateaus, oceanic island arcs and preserved oceanic crust similar to modern oceanic crustal section close to a spreading center. Isotopic age data for these greenstone sequences (detrital zircons -ca.3600–3230 Ma; volcanics 3384–3200 Ma) and adjoining granitoids (3430–3400 and 3350–3270 Ma with remnants of older 3600–3500 Ma gneisses) indicate that a large part of the old TTG-type granitoids in the WDC is either coeval to or slightly younger than associated greenstone units. The final stage of assembly of these tectonic elements into cratonic framework through horizontal motion of intervening oceanic crust and eventual slab breakoff is marked by the formation of ca. 3200 Ma trondhjemite magmas, emplacement of which into the lower crust caused partial convective overturn of the crust, thereby leading to the development of dome and keel patterns followed by metamorphism and cratonization of the WDC crust at 3100 Ma. Our study strongly suggests that the tectonic environments in which Archean cratons formed require some sort of horizontal motion but not necessarily modern plate tectonics. However, vertical addition of juvenile crust in hotspot environments associated with mantle plumes played a major role in building of early crustal nuclei. The model proposed for the formation of the cratonic core in the western Dharwar craton is compatible with observations made globally in most of the Paleoarchean cratons. [ABSTRACT FROM AUTHOR]

Turbidite hosted orogenic gold mineralization in the Archean Gadag greenstone belt of the Western Dharwar Craton, forms a major auriferous zone (Central Auriferous Zone) extending over a strike length of about 12 km in the Gadag duplex. The turbidite sequence comprises thick inter-bedded, medium to coarse grained lithic graywacke and thin laminated layers of fine grained carbonaceous phyllite. Gold bearing quartz veins impregnate preferentially along the en-echelon shear planes, fractures and schistosity planes. Auriferous quartz veins are enveloped by the altered wall rocks. Mineralogy of the auriferous zone is dominated by gangue minerals like quartz, ankerite, chlorite, sericite and carbonaceous matter, with subordinate plagioclase. Monazite and xenotime are the important accessory minerals. Arsenopyrite and pyrite are the major sulfide minerals, but pyrrhotite, chalcopyrite, sphalerite, galena and scheelite are also present. Gold in native state occurs within quartz, silicates and arsenopyrite. Notable distinctions in mineral assemblage, texture and in chemical compositions of altered wall rocks compared to the precursor host rock in the study area implies that the metasomatism and wall rock alterations are the results of pervasive infiltration and intense interaction between hydrothermal fluids and the surrounding host rocks over a prolonged period. Sulfides, carbonates, carbonaceous matter, K 2 O, MgO, CaO, Cr, Ni, Cu, Pb, Zn, As and higher values of gold (0.98–4.72 ppm) are added into the altered wall rocks, immediately enveloping the auriferous quartz vein bodies. The chondrite normalized REE pattern of altered wall rocks exhibits enriched LREE (La N /Yb N = av. 9.54), with prominent negative Eu anomaly. The observed variation in geochemical characteristics and mineral assemblages in the alteration zones indicates differential response of the host rock and intensity of alteration depending on the composition of host rocks and hydrothermal fluids. The auriferous hydrothermal fluids were of low salinity (2.0 to 6.6 wt.% NaCl), dominated by CO 2 –H 2 O (about 30 mol% CO 2 ) with moderate densities (0.7 to 1.04 g/cm 3 ), and gold deposition occurred over a wide temperature range between 175 °C and 325 °C. Gold deposition was influenced by fluid mixing, phase separation and redox reactions. Mixing between CO 2 –H 2 O fluids and more reduced fluids, which evolved during fluid reaction with adjacent carbonaceous wall rocks, was the key factor causing gold deposition. The formation of the Gadag duplex, deformation, folds and reverse strike slip faults (discontinuities) was caused by the compression associated with subduction related tectonic processes. During the initial period of intrusive magmatism (2,555 ± 6 Ma), regional metamorphism occurred in the entire greenstone belt, while during later period, hydrothermal fluids responsible for gold mineralization probably were derived from metamorphic processes as well as from intrusive granites. Such fluids channeled through the thrust in host turbidite sequence carrying dissolved gold, associated metals and sulfur, ultimately were precipitated in a reducing environment in the splays to the thrust in the Gadag duplex at about 2,522 ± 6 Ma, resulting in retrograde alteration assemblages. [ABSTRACT FROM AUTHOR]

A Mesoarchean greenstone belt (3.5–3.0 Ga) in the western part of the East Indian Shield comprising the Iron Ore Group of the Noamundi basin contains economic resources of both iron and manganese ores in the NNE plunging regional synclinorium. Manganese mineralization in the central and eastern parts of this synclinorium, particularly in Joda–Noamundi sector, has taken place in multiple cycles starting from syngenetic sedimentary and exhalative type through mobilization and remobilization in different stages of tectonism, deformation and hydrothermal activities to latest lateritic or supergene type. A relatively high temperature metamorphic jacobsite–hausmannite–bixbyite–braunite assemblage, low temperature hydrothermal pyrolusite–psilomelane–hollandite assemblage and supergene pyrolusite–manganomelane–groutite–polianite assemblage are present and were formed by recycling of manganese in different stages of mineralization. A detailed structural study of the manganese ore bodies as well as their ore petrographic and mineralogical characteristics with mineral chemistry has revealed systematic mineralization and their relation to deformational phases. Such recycling of manganese and its structural control of mineralization in different phases is unique of its kind in comparison with other Archean manganese deposits in the world. [ABSTRACT FROM AUTHOR]

The Archean Hutti-Maski greenstone belt (HMGB) and the south Kolar greenstone belt (SKGB) are the two richest gold provinces in India. Here, we present results of texturally constrained U-Th-Pbtotal/CHIME dating of monazites and PT pseudosection analyses to understand the geochronological and metamorphic evolution of the two greenstone terrains. The ages of felsic volcanism are constrained at ca. 2669 ± 22 Ma in the HMGB and 2661 ± 32 Ma in the SKGB. The HMGB rocks underwent midamphibolite facies metamorphism at ca. 2±64 ± 12 Ma. The inferred PT path indicates a contemporaneous PT increase of up to ~6 kbar and ~6207C followed by postpeak near-isothermal decompression for the HMGB rocks. The SKGB rocks underwent a lower amphibolite facies metamorphism at ca. 2±46 ± 12 Ma and are characterized by a synchronous PT increase of up to ~4.6 kbar and ~6007C, followed by decompressional cooling. These results indicate a mixed arc/plume setting for the evolution of both the greenstone belts, as suggested by several earlier works. The HMGB rocks record a pervasive postpeak metamorphic K-rich fluid alteration event at ca. 2414 ± 18 Ma that is discernible by monazite growth proximal to K-feldspar veins and garnet breakdown domains. Similar, although less prevalent, monazite age record of ca. 2414 ± 26 Ma from the SKGB rocks is manifested by postmetamorphic shear-induced hydrothermal activity. Considering the worldwide scenario, the HMGB and SKGB are ~100 Ma younger and witnessed a higher grade of metamorphism than the greenstone belts in the Abitibi province and the Yilgarn Craton. [ABSTRACT FROM AUTHOR]

Analysis of 3.3 Ga tonalite–trondhjemite–granodiorite (TTG) series granitoids and greenstone belt assemblages from the Bundelkhand craton in central India reveal that it is a typical Archaean craton. At least two greenstone complexes can be recognized in the Bundelkhand craton, namely the (i) Central Bundelkhand (Babina, Mauranipur belts) and (ii) Southern Bundelkhand (Girar, Madaura belts). The Central Bundelkhand greenstone complex contains three tectonostratigraphic assemblages: (1) metamorphosed basic or metabasic, high-Mg rocks; (2) banded iron formations (BIFs); and (3) felsic volcanics. The first two assemblages are regarded as representing an earlier sequence, which is in tectonic contact with the felsic volcanics. However, the contact between the BIFs and mafic volcanics is also evidently tectonic. Metabasic high-Mg rocks are represented by amphibolites and tremolite-actinolite schists in the Babina greenstone belt and are comparable in composition to tholeiitic basalts-basaltic andesites and komatiites. They are very similar to the metabasic high-Mg rocks of the Mauranipur greenstone belt. Felsic volcanics occur as fine-grained schists with phenocrysts of quartz, albite, and microcline. Felsic volcanics are classified as calc-alkaline dacites, less commonly rhyolites. The chondrite-normalized rare earth element distribution pattern is poorly fractionated (LaN/LuN = 11–16) with a small negative Eu anomaly (Eu/Eu* = 0.68–0.85), being characteristic of volcanics formed in a subduction setting. On Rb – Y + Nb, Nb – Y, Rb – Ta + Yb and Ta – Yb discrimination diagrams, the compositions of the volcanics are also consistent with those of felsic rocks formed in subduction settings. SHRIMP-dating of zircon from the felsic volcanics of the Babina belt of the Central Bundelkhand greenstone complex, performed for the first time, has shown that they were erupted in Neoarchaean time (2542 ± 17 Ma). The early sequence of the Babina belt is correlatable with the rocks of the Mauranipur belt, whose age is tentatively estimated as Mesoarchaean. The Central Bundelkhand greenstone complex consists of two (Meso- and Neoarchaean) sequences, which were formed in subduction settings. [ABSTRACT FROM AUTHOR]

The Neoarchean Kadiri greenstone belt (KGB) of eastern Dharwar Craton is dominated by metavolcanic rocks including basalts, andesites, dacites and rhyolites that have experienced lower amphibolite grade of metamorphism. The geochemical investigations of the basalts and basaltic andesites of KGB show 47–56 wt.% SiO 2 and Mg# ranging from 38 to 64. The rocks display sub-alkaline tholeiitic to transitional affinity. Based on REEs, the KGB basalts are classified as two types. Type I basalts show flat to slightly enriched LREE patterns while type II basalts have relatively enriched LREE patterns. Both types of basalts exhibit negative Nb, Zr, Hf and Ti anomalies along with distinct enrichment in Th and U. Type II basalts are characterized by a greater magnitude of negative Nb anomaly (Nb/Nb* = 0.1–0.32) compared to type I (Nb/Nb* = 0.41–0.82) basalts. Prominent island arc signatures are evident in both the types in terms of their LILE, LREE enrichments and relative HFSE depletion. Ba/Th vs. Nb/Th relationships reflect a frontal arc setting for type I basalts whereas type II basalts correspond to a rear arc setting. Th/Nb, La/Nb and Ce/Yb ratios show a gradual increase from type I to type II basalts reflecting melting of sediments from the subducted slab and their increased addition in type II basalts. The Zr/Hf (31–43), Zr/Sm (11–32) and Nb/Th (1–9) ratios suggest a depleted to enriched mantle source with variable influx of subduction-derived fluids and sediments. The flat to slightly enriched LREE patterns of type I basalts indicate a gradual progression from depleted MORB-type mantle melting in a fore-arc to slab-dehydration–mantle wedge metasomatism and fluid-fluxed melting in an arc regime within spinel peridotite compositional domain. Island arc signatures in type II basalts suggest their generation through melting of a subduction-modified MORB-type mantle within spinel to garnet peridotite stability field in a juvenile back-arc rift system that developed proximal to the intraoceanic arc. Variable input of subduction derived fluids and sediments in type I and type II basalts attests to across-arc geochemical variations. The subduction processes operating within arc–back arc system provided favourable pathways for the migration of gold-bearing fluids leading to economic mineralization in the northern (Hutti) and southern (Kolar) parts of this belt. [ABSTRACT FROM AUTHOR]

The ∼2.7 Ga Veligallu greenstone terrane in the eastern Dharwar craton is one of the least studied Neoarchean greenstone belts in India. To our knowledge, there is no published dataset that provides a detailed account of the petrogenetic processes, which culminated into the evolution of this greenstone belt. This paper presents a comprehensive account of field, petrography and geochemistry of the basalt–high-Mg andesite–adakitic volcanic rock association in the Veligallu greenstone belt. The basalts are tholeiitic in nature and are characterized by primitive geochemical compositions with ∼49 wt% SiO 2 , ∼9 wt% MgO, Mg# 54, Cr ∼ 197 ppm, Ni ∼ 122 ppm, Y ∼ 27 ppm, Yb ∼ 2.7 ppm; Zr/Y < 3 and Ce/Yb < 5. They do not plot on a mixing line between Mesoarchean TTG and N-MORB, therefore, inconsistent with any interaction with the upper continental crust. Accordingly, they are interpreted to have erupted in an oceanic setting. On the basis of Nb/Th ratio the Veligallu basalts are divided onto two groups. The basalts with Nb/Th > 8 display systematically flat to light-REE depleted chondrite normalized REE patterns (La N /Yb N = 0.76–1.0), in contrast to the samples with Nb/Th < 8, which display slight enrichments in light-REE (La N /Yb N = 1.0–1.6). The depleted basalts are characterized by primitive mantle normalized Nb concentrations lower than La but equal to Th, in contrast to the enriched basalts, which are characterized by slight depletions of Nb relative to both La and Th. Collectively, as a group, they display Nb/Yb ratio higher than the N-MORB. The enrichment in Nb is attributed to moderate to low degree (∼10%–4%) partial melting of a depleted MORB source. The trace, REE and their interelement ratios in the Veligallu basalts exhibit transitional characteristics between the average N-MORB and the typical Phanerozoic island arc basalts. Overall, the geochemical characteristics of the Veligallu basalts are identical to basalts erupted in the Phanerozoic back-arc basins. The Veligallu high-Mg andesites are orthopyroxene-bearing calc-alkaline andesites and are characterized by moderate SiO 2 (∼56 wt%), high MgO (∼8 wt%), Mg# (61–67), Cr (174–330 ppm) and Ni (173–242 ppm) contents; low Y (∼13 ppm) and Yb (∼1.2 ppm) concentrations, relative to typical island arc andesites. They exhibit light-REE enriched (La N /Sm N ∼ 1.7) and heavy-REE depleted (Gd N /Yb N ∼ 1.9) fractionated REE patterns. On a primitive mantle normalized trace element variation diagram they exhibit negative Nb and Ti, and contrasting positive Zr anomalies relative to the neighboring REE. The La–Yb systematics suggest that these high-Mg andesites were generated by hydrous partial melting of a slab-melt metasomatized mantle wedge in the sub-arc mantle. The felsic volcanic rocks are characterized by high SiO 2 ∼ 73 wt%, Na 2 O ∼ 4.5 wt%, and significantly low concentrations of MgO (∼0.72 wt%), Cr ∼ 19 ppm, Ni ∼ 23 ppm, Y ∼ 5.9, and Yb ∼ 0.44 ppm; strongly fractionated chondrite normalized REE patterns (La N /Yb N ∼ 30) with occasionally minor positive Eu anomaly, and primitive mantle normalized negative Nb and Ti, and contrasting positive Zr anomaly. The geochemical attributes of the Veligallu felsic volcanic rocks are analogous to the Cenozoic high silica-type adakites that are produced by partial melting in the sub-arc mantle, and presumably without any significant interaction of the slab-melts with the overlying mantle wedge. The juxtaposed occurrence of basalts with the banded iron formations, and their close spatial association with the high-Mg andesites, in the Veligallu greenstone belt, indicates that the petrogenesis of these volcanic rocks is consistent with their eruption in an intraoceanic setting. The close proximal association of the back-arc type basalts, an [ABSTRACT FROM AUTHOR]