Browsing by Author "Bikramaditya R.K."

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    Plagioclase ultraphyric basalts of the Abor magmatic complex: Implications for a plumbing system at the eastern Himalaya
    (Elsevier B.V., 2024) Oinam G.; Singh A.K.; Santosh M.; Joshi M.; Dutt A.; Khogenkumar S.; Das B.P.; Bikramaditya R.K.
    Plagioclase ultraphyric basalts (PUBs) are an important unit of the Abor magmatic complex (AMC) of the eastern Himalaya, containing ?35 vol% plagioclase phenocrysts. Apart from the eastern region, PUBs have not been reported in any other part of the Himalayas. However, very little information is available about their origin and significance in the evolution of the eastern margin of the Indian plate and the Himalayan orogeny. In this contribution, we present the first zircon U[sbnd]Pb age data of the PUBs along with whole-rock geochemistry, Sr[sbnd]Nd isotopic ratios, mineral chemistry, and quantitative textural analysis, to understand the evolutionary history of the AMC and subsurface magma chamber activities. The PUBs formed from highly evolved magma (<6 MgO wt%), having high Fe2O3 (9.06�12.29 wt%) and Ti/Y ratios (>500). Their ?Nd (t) values (?0.02 to +2.66) suggest plume magma source. A small difference in anorthite contents (<5 mol%) is observed from the thick core (An47�58) with lower anorthite contents towards the rim (An34�47) of the plagioclase phenocrysts, which is an indication of weakly zoned characteristics. Crystal size distribution shows a non-linear and concave upward trend with a relatively gentler slope towards the coarser plagioclase populations, which can be attributed to the hybrid crystallization of plagioclase-bearing magma and its subsequent differentiation with cumulates of plagioclase inside the magma chamber. The zircon U[sbnd]Pb age of these PUBs records two magmatic events - Early Paleozoic (505�473 Ma) at the core and Early Cretaceous (134�126 Ma) at the rim that are consistent with the previously proposed magmatic events of AMC with Gondwana assembly and break-up. Encounter of such dual ages in zircons does not support the usual condition of PUBs formation through crystal floatation in a slow cooling process of a single magma chamber. Therefore, considering the evidences observed in crystal size distribution, core-rim anorthite variation, geochemistry, and age data, we propose that the PUBs of AMC, eastern Himalaya were formed due to injection of a hot and young magma during the Early Cretaceous into an old and cold mush zone containing pre-existing plagioclase phenocrysts formed during the Early Paleozoic. The results further support that the newly injected magma formed the rim of the plagioclase phenocrysts and the groundmass of the PUBs. � 2024
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    The Kerguelen mantle plume activity in Sylhet Trap mafic rocks of Southern Shillong Plateau, NE India: Implications for its role in magmatism of eastern India
    (John Wiley and Sons Ltd, 2024) Kumar M.; Singh A.K.; Bikramaditya R.K.; Singh N.S.; Imtisunep S.
    Mantle plumes related to Large Igneous Provinces have been linked to continental break-up and validated by the outpouring of mafic-ultramafic magmas that range from continental flood basalt magmatism to submarine plateau volcanism. This study presents a new set of geochemical and mineralogical data on mafic magmatic rocks from the Sylhet Trap of the Shillong Plateau, northeast India. The investigated mafic rocks (basalt and dolerite) are predominantly sub-alkaline-tholeiitic, composed of bytownite+labradorite and diopside+augite, with ophitic to sub-ophitic and glomeroporphyritic textures, the dark interstitial region of much finer grains consisting of opaque minerals and devitrified glass. The mafic rocks of Sylhet Trap show light rare earth elements enrichment with (La/Yb)N ratio (1.92�2.86) and (La/Sm)N ratio (1.11�1.40), an almost flat pattern of heavy rare earth elements along with mild europium anomalies (Eu/Eu*= 0.94�1.11). Trace element characteristics suggest their affinity towards enriched mid-oceanic ridge basalt and generated from low degree of partial melting of spinel source with minor involvement of crustal contamination. The similarity in geochemical characteristics of the investigated mafic rocks with the magmatism of Rajmahal Traps, eastern Peninsular India, Abor Volcanics, eastern Himalaya, along with Bunbury Basalt of western Australia and Cona Mafic exposed in southeastern Tibet, suggests their genetic linkage with mantle plume activities. Thus, we argue that the magmatic event of the Sylhet Trap is related to the Kerguelen mantle plume activity that played a significant role in the fragmentation of eastern Gondwana during the Lower Cretaceous period, giving rise to Greater India, Antarctica and northwest Australia. � 2024 John Wiley & Sons Ltd.