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Granitoids and orthogneisses (~ 1.7 Ga) from the northern part of the Banded Gneissic Complex (BGC-II domain), Aravalli Craton (NW India) are geochemically analyzed to understand the geodynamic condition and crustal evolution processes. The samples are metaluminous to peraluminous (molar A/CNK: 0.74–2.12), and characterized by Eu-anomaly ranging from 0.17 to 1.06, Fe2O3T/(Fe2O3T + MgO) from 0.8 to 0.9 and high magmatic zircon saturation temperature (> 830 °C) which are the features suggestive of A-type granite affinity. Tectonic discrimination diagrams classify the samples as post-collisional extensional A2-type granites. Geochemical characteristics along with trace element ratios [(Y/Nb)N = 0.15 to 4.33 (avg. 0.76), (Th/Nb)N = 4.63 to 255.47 (avg. 63.13), (Th/Ta)N = 1.37 to 9.84 (avg. 8.86), (Ce/Pb)N = 0.05 to 3.05 (avg. 1.43)] indicate that the rocks were derived from a plagioclase-rich and garnet-free crustal source under low-pressure conditions. Further, it is also proposed that tonalite-trondhjemite-granodiorite (TTG) rocks which occur dominantly in southern Rajasthan (BGC-I) are precursors and their partial melting led to the generation of the studied A-type granite samples. The studied samples also bear close geochemical similarity with A-type granites of similar age (~1.7 Ga) near the Sakhun-Ladera region of northern BGC-II. The studied A-type granites are believed to be coeval to similar aged A-type granites of the Khetri and Alwar sub-basins of the North Delhi Fold Belt (NDFB). They are comparable in age and magmatic history to recorded A-type magmatism in North America and parts of the Chinese craton. The large geographical extents of synchronous A-type granites are proposed to be related to the Columbia Supercontinent assembly (ca. 1.7 Ga; post-collisional granites). Thus, based on the studied extensional granites, we surmise that BGC-II was part of the Columbia Supercontinent. [ABSTRACT FROM AUTHOR]

The Banded Gneissic Complex (BGC) of the Aravalli Craton (India) comprises Archean BGC‐I (3.3–2.5 Ga) and Proterozoic BGC‐II. The BGC‐II is a mosaic of amphibolite facies namely, (a) Mangalwar Gneissic Complex (MGC), (b) Mangalwar Metasedimentary Complex (MMC), and (c) granulite‐facies Sandmata Metamorphic Complex. Here we present field, petrography and geochemical study of the Proterozoic amphibolites from the MGC and MMC. Based on field and geochemical data, the amphibolites have been characterized into three types related to rift settings (G1, G2 and G3). The G1 type occurs as dykes in the MGC and bears ocean island basalt‐type rare earth element (REE) patterns along with negative Nb and Ti anomalies, negative to positive values of εNd(t) (−0.02 to +3.96) and slightly variable initial 87Sr/86Sr (ISr) ratios. They are derived from deep mantle sources and correspond to the pre‐rift magmatic phase. The G2 type occurs as isolated patches associated with chert and is characterized by light REE (LREE) depleted and almost flat heavy REE (HREE) patterns suggesting that they were emplaced in an oceanic setting and were derived from a shallower mantle bearing positive εNd(t) (+2.87 to +6.27) and ISr = 0.7002–0.7083. This phase corresponds to the opening of the Mangalwar sedimentary basin (MMC). The G3 type occurs intercalated with metasedimentary rocks of the MMC and marked by LREE‐enriched and HREE‐depleted to flat patterns that resemble Upper Continental Crust signature, their εNd(t) mostly negative values and variable ISr also corroborate this explanation. They are believed to be derived from heterogeneous sources and represent syn‐sedimentary volcanic phases. All these signatures indicate that the amphibolites distinctly represent three phases of magmatism that occurred during pre‐rift (1.72 Ga), opening of basin (1.62 Ga) and syn‐sedimentary volcanism (1.6–1.3 Ga) in the rift‐basin and they were formed during the Proterozoic. These rifting events might have been connected with the fragmentation of Columbia. [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]