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

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 plagioclase (18–25 vol.%), olivine (1–4%), clinopyroxene (0.5–2.5 vol.%) and few iron oxides.Olivine of the 2005-lava shows a variation in Fo content from core (Fo81–85) to rim (Fo73–75) but those in the 2006-lava show a uniform composition from core to rim (Fo75–78). Feldspar grains of both the 2005 and 2006-eruptions are normally zoned, with a variation from core (An93–80) to rim (An76–55). 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.