Volcanological and petrological evolution of Barren Island (Andaman Sea, Indian Ocean).

The Island of Barren (Andaman Sea, Indian Ocean) consists of a horseshoe-shaped caldera opened to the west that was formed by either a giant, lateral landslide of the original volcanic cone or a vertical collapse of a surficial magma chamber due to a paroxysmal eruption. The reconstruction of the pre-and post-calderic activity indicates that the subaerial volcanic history of Barren Island was characterized by effusive activities accompanied by subordinate explosive (scoria fall) eruptions. The collapse of the central portion of the edifice terminated this mainly effusive phase. The inner flanks and the rim of the depression are covered by a 20 m thick sequence of breccias and tuffs. Beds are structurally massive with steep fronts, likely due to secondary mass flowage. The lack of caldera-forming eruption together with the absence of chemically evolved magmas could suggest a lateral landslide as the most probable process responsible for the caldera formation. The active polygenic tuff cone, presently occupying the inner portion of the caldera, consists of at least five coalescent summit craters and four eccentric spatter cones at the base. These features are attributed to the historic activities that are dominated by the emplacement of lava flows and pyroclastic deposits (e.g. 1803, 1991, 1995 and, possibly, 2000 eruptions). Lava flows moved westward from lateral spatter cones and occasionally reached the sea. A few months after the 2004 tsunami and Sumatra earthquake, Barren Island volcano violently resumed its eruptive activity that lasted for almost two years. The volcanic products ejected from May 2005 eruption have covered the historic lava flows and pyroclastic deposits. Petrologically, the lava flows show a narrow compositional variation, from low-K basalts to basaltic andesites. Pre-caldera magmas display a general less evolved composition and a certain variability of the Sr isotopic ratios (0.70385–0.70400); post-caldera magmas are compositionally more homogeneous and have an almost constant Sr isotopic ratio (0.70399–0.70403). Petrographic and geochemical characteristics indicate an important role played by a crystal–liquid fractionation process. However, the presence of large phenocrysts of xenocrystic origin, the small but significant variations of Sr isotopic ratios in the pre-caldera magmas, the relationships between 87Sr/86Sr and trace elements and the ratios of incompatible trace elements in the basaltic rocks point to the occurrence of additional evolutionary processes and/or to a role of a heterogeneous magma source. The most primitive magma (Mg# 71, Ni 218 ppm, Cr 557 ppm), displaying the lowest Th content (0.39 ppm), found in the Barren Island is also the most primitive in the Burma-Andaman-Java Arc, thus representing an important reference composition for the region.