Petrogenesis of plagioclase ultraphyric basalt (PUB) of Abor volcanics, Eastern Himalaya, northeast India

Plagioclase ultraphyric basalts (PUBs) represent magma storage period of a magmatic system. PUB dykes of Abor volcanics, Eastern Arunachal Himalayas have been studied for their morphological, compositional, textural variation and crystallization history in the magma chamber. The centimetre-sized plagioclase phenocrysts constitute >40% of the total volume of PUB. The difference in anorthite contents of the plagioclase phenocrysts and plagioclase in groundmass varies from 3% to 10%. The plagioclase phenocrysts are resorbed, show embayed boundary, at places sieve texture and zoning. The diffusion penetrates through zoning. Some plagioclase phenocrysts have a very thick core with a 3% variation of anorthite content and a thin outer rim indicates its development in an equilibrium condition with an insignificant amount of melt supply and rapid ascent, respectively. Further, plagioclase grains occur as aggregate with clinopyroxene. This textural evidence shows plagioclase and clinopyroxene macrocrysts crystallized from batches of plagioclase and plagioclase-clinopyroxene rich mush formed in the sub-volcanic magma chamber by slow cooling and the parental melts evolve predominantly by clinopyroxene-plagioclase crystallization. The plagioclase phenocrysts crystallized and accumulated in different zones within a single or multi-chambered mush system and later remobilized by a volatile-rich conduit system passing through these chambers and emplaced as PUBs. The occurrences of PUBs support a long period of magma storage in the sub-volcanic chamber at a different level with a variable amount of magma supply and accumulated in origin.