Plagioclase ultraphyric basalts of the Abor magmatic complex: Implications for a plumbing system at the eastern Himalaya

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 UPb age data of the PUBs along with whole-rock geochemistry, SrNd 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 epsilon 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 (An(47-58)) with lower anorthite contents towards the rim (An(34-47)) 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 UPb 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.