Japan Geoscience Union Meeting 2024

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[J] Poster

S (Solid Earth Sciences ) » S-VC Volcanology

[S-VC31] Mechanism of volcanic eruptions

Wed. May 29, 2024 5:15 PM - 6:45 PM Poster Hall (Exhibition Hall 6, Makuhari Messe)

convener:Mayumi Mujin(Tohoku University), Ryo Tanaka(Hokkaido University,Institute of Seismology and Volcanology), Takafumi Maruishi(National Research Institute for Earth Science and Disaster Resilience ), Dan Muramatsu(Earthquake Reserch Institute, The University of Tokyo)

5:15 PM - 6:45 PM

[SVC31-P06] Plagioclase ultraphyric basalts of the Abor magmatic complex: implications for a plumbing system at the eastern Himalaya

*Govind Oinam1 (1.Wadia Institute of Himalayan Geology, 33 GMS road Dehradun, India)

Keywords:Plagioclase ultraphyric basalts, Zircon U-Pb ages, Geochemistry, Eastern Himalaya

This contribution presents the first successful zircon U-Pb age determination of the plagioclase ultraphyric basalts (PUBs) of the Abor magmatic complex (AMC) of eastern Himalaya along with whole-rock geochemistry, mineral chemistry, and quantitative textural analysis. The PUBs formed from highly evolved magma (<6 MgO wt.%), having high Fe2O3 (9.06-12.29 wt.%) and Ti/Y ratios (>500). 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-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, I 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.