The 2005-2006-eruption of the Barren Volcano, Andaman Sea: Evolution of basaltic magmatism in island arc setting of Andaman-Java subduction complex

摘要

The Barren Volcano located in the volcanic chain of Burma-Java subduction complex became active during 2005-2006 initially as Strombolian and later on as sub-Plinian type ejecting out ash laden grey smoke and coarse pyroclasts. The pyroclasts are represented by vesiculated (20-30 vol.% vesicle) porphyritic basaltic rock where the phenocrysts constituting 20-25 vol.% of the total bulk are represented by plagio-clase (18-25 vol.%), olivine (1-4%), clinopyroxene (0.5-2.5 vol.%) and few iron oxides.rnOlivine of the 2005-lava shows a variation in Fo content from core (Fo_(81-85)) to rim (Fo_(73-75)) but those in the 2006-lava show a uniform composition from core to rim (Fo_(75-78)). Feldspar grains of both the 2005 and 2006-eruptions are normally zoned, with a variation from core (An_(93-80)) to rim (An_(76-55)). The major oxides show basaltic composition for the 2005-lava and basaltic to basaltic andesite for the 2006-lava. Abundances of plagioclase phenocrysts, poorly hydrous glass inclusions in the phenocrysts, rarity of the mafic phase and high Zr content (69-75 ppm) are suggestive of the very poorly hydrous nature of magma. The calculated fractional crystallisation trend show the differences in the 2005-lava and 2006-lava where the former show accumulated nature and the latter indicates evolved path. The major oxides and trace elements patterns indicate that both the lava had a different parental composition and the 2006-lava was subjected to fractional crystallisation. The magmatic evolution is explained by the mantle diapir model where the almost anhydrous basaltic magma of the 2005-eruption was produced from hot rind of the diapir and feebly hydrous basaltic andesite magma of the 2006-eruption was formed from cool and wet core of the diapir.