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

The radiolarian cherts of Late Cretaceous age are reported in the Andaman-Nicobar ophiolites of Bay of Bengal, India. They are of chocolate-coloured (Type-I) and light pink-coloured (Type-II) varieties, juxtaposed and tectonically associated with phyllites, claystones and basalts and are exposed in eastern part of North Andaman Island. These cherts are composed of radiolarian tests in a fine-grained ferruginous matrix. Type-I radiolarian cherts have low SiO2, and high Al2O3, Fe2O3(T), MgO, Na2O and TiO2 and trace elements as compared to Type-II cherts. Al–Fe–(∑REE–Ce) diagram of the studied cherts indicates a mixed terrigenous and volcanogenic source. LaN/CeN ratios (0.76–0.89 for Type-I and 0.71–0.88 for Type-II) and Ce-anomalies (Ce/Ce* = 1.15–1.33 for Type-I and 1.07–1.38 for Type-II) and other elemental ratios in these cherts suggest that they were deposited in continental margin environments. It has been suggested that the studied samples of cherts were deposited at different places, were scrapped off the subducting plate, became tectonically juxtaposed. They were obducted onto the leading edge of the Eurasian continent during the Late Cretaceous prior to the currently active Andaman–Java subduction, that was probably initiated during the Late Miocene. [ABSTRACT FROM AUTHOR]

New geological and geochemical data have been presented in this paper to improve the understanding of the petrogenesis and regional geodynamic evolution of the 2.8–2.5 Ga Dhanjori volcanics. These volcanics exhibit porphyritic texture and classified as picritic basalt. The Dhanjori volcanics are MgO-rich (9.97–15.81 wt%) with moderate SiO2 (51.62–53.82 wt%), FeOT (9.97–12.57 wt%) and TiO2 (0.73–1.21 wt%) but relatively higher Ni (123.56–545.35 ppm) and Cr (241.87–1158.41). The rare earth element (REE) patterns and their ratios indicate their derivation from the garnet to spinel stability field due to decompressive melting of a mantle plume head near the crust-mantle boundary, which is supported by the presence of high positive gravity anomaly over the Dhanjori lavas. The thermobarometric studies indicate that the primary magma had an initial mantle potential temperature between 1350 and 1500°C in a pressure range of 1 to 1.8 GPa, thus conforming with the mantle origin, which is further endorsed by supplementary geophysical studies. [ABSTRACT FROM AUTHOR]