Geochemical characteristics of submarine rhyolitic pumice from Andaman subduction zone: Inferences on magmatism and tectonics of north‐eastern Indian Ocean

Abstract

This paper reports new geochemical data for submarine volcanic pumice sampled from two volcanoes (SM‐1 and SV‐1) which are part of the Andaman subduction system. SM‐1 volcano is located closer to the Andaman back‐arc spreading axis while SV‐1 volcano is part of a linear chain of arc volcanoes in the southern Andaman Basin. The pumice are products of phreatomagmatic eruptions and are highly vesicular in nature; however, immediately after eruption of these lava froth, sea water entering into the vesicles made them sink to the sea floor. A sheet of dull grey to white‐coloured pumice observed on top of the SM‐1 volcano indicates a nonexplosive eruption referred to as Tangoroan type. Pumice from these two spatially apart volcanoes share uniform physical properties but display minor variations in mineralogical compositions and distinct characteristics of selected major and trace elements. Pumice from SM‐1 comprised plagioclase, hornblende, and spinel while those from SV‐1 are composed of plagioclase and quartz as main crystalline phases. Geochemically, the pumice samples from SM‐1 volcano are classified as medium‐K calc‐alkaline rhyolites, while those from SV‐1 volcano show low‐K tholeiitic compositions. Trace and REE compositions for these rhyolitic pumice collectively reflect LILE‐LREE‐enriched, HFSE‐depleted signatures of subduction zone magmatism and implicate an oceanic arc tectonic affinity. Selective enrichment in LILE and LREE with relative HFSE depletion is attributed to infiltration of fluid‐mobile LILE and LREE into the mantle via fluids released from slab dehydration with retention of fluid‐immobile HFSE in the subducted slab. Elevated abundances of LILE/HFSE, LREE/HFSE, and LREE/HREE for the medium‐K calc‐alkaline rhyolitic pumice relative to their low‐K tholeiitic counterparts invoke increased fluid flux into the mantle with progressive maturation of oceanic slab subduction. The precursor magmas parental to the SM‐1 and SV‐1 volcanic pumice were derived by partial melting of a mantle wedge metasomatized by variable slab–mantle interactions, and influx of slab‐dehydrated fluids and sediments. Pronounced REE fractionation trends suggest evolution of resultant melts in a cold, hydrous, oxidizing regime of intraoceanic arc setting that eventually reheated, remelted, remobilized, and assimilated lower oceanic arc crust and erupted as rhyolites. The volcanic pumice from Andaman are geochemically and tectonically analogous to those from Mariana arc and Okinawa Trough of Pacific Ocean.