2021 年 55 巻 1 号 p. 11-25
The Ghansura Rhyolite Dome (GRD) of Bathani volcano-sedimentary sequence in Eastern India provides direct evidence for mafic-felsic magma interaction during its evolution. The interaction led to the formation of different varieties of hybrid rocks that are now preserved at the outcrop scale. One of the hybrid rocks occupying a significant portion of the rhyolite dome is a porphyritic andesite. Here we try to investigate the magmatic processes that led to the formation of the porphyritic andesite in GRD. From field and petrographical observations and mineral chemical data interpretations we infer that the porphyritic andesite crystallized from a hybrid magma that was produced due to the interaction of an invading crystal-rich basaltic magma and melt-dominated portion of the rhyolite magma chamber. When basaltic magma carrying crystals of plagioclase mixed with the rhyolite melt, the plagioclase crystals present in the mafic endmember were incorporated into the resultant hybrid magma. The melt-dominated nature of the hybrid magma allowed the plagioclase crystals to drift and accumulate as synneutic aggregates in the magma. Eventually, the plagioclase crystals in the synneutic aggregates underwent mutual attachment to form large phenocrysts of plagioclase giving the rock its characteristic porphyritic texture. The compositions of plagioclases from the basalts and those occurring as synneutic aggregates in the porphyritic andesite are almost similar. This compositional similarity suggests that the plagioclase crystals joined by synneusis to produce phenocrysts in the porphyritic andesite were probably derived from the basaltic magma.
