IMPORTANCE OF THE BARREN ISLAND VOLCANO (ANDAMAN SEA, NE INDIAN OCEAN) IN THE INDONESIAN VOLCANIC ARC STUDY

Barren Island (12.29°N, 93.85°E, Andaman Sea, NE Indian Ocean) volcano is the northernmost of the active volcanoes of the Indonesian Volcanic Arc (IVA). Barren Island (hereafter BI) volcanics can be classified in two broad groups: (i) Pre-caldera, comprising the volcanics of unknown age (No radiometric aage available, hence referred as Prehistoric) that constitute the caldera wall - the remnant of the primitive cone of BI volcano and (ii) Post-caldera, comprising the rocks that are confined within the caldera and are known to have erupted during historic times and recently from the the coalescent summit craters of the main cinder cone and subsidiary vents of the eccentric spatter cones at its base. BI volcanics show a narrow compositional variation, from low-K basalts to basaltic andesites. Pre-caldera vocanics have less evolved composition and a certain variability of the Sr isotopic ratios (0.70385–0.70400), whereas post-caldera magmas are compositionally more homogeneous and have an almost constant Sr isotopic ratio (0.70399–0.70403). Most of BI pre-caldera magmas have low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd, low Pb isotope values and low Th/Ce and U/Th ratios, which imply little or no contribution from fluids and/or sediments in the source. In the Sunda Arc (a part of the IVA), the tholeiitic rocks at Galunggung are considered near-primitive lavas (MgO = 10.6–12.5%) representing the best estimated composition of unmodified mantle melt entering the crust (Turner and Foden, 2001). The trace element pattern corresponding to the mean Galunggung tholeiitic basalts shows higher abundances of the most incompatible trace elements, except Ba, and almost similar or lower abundances of Nd to Yb elements when compared to the most primitive BI volcanics composition (Mg# = 71, Ni = 218 ppm, Cr = 557 ppm) displaying the lowest Th (0.39 ppm) content in the entire IVA. It endorses the importance of the Barren Island volcano in the IVA study.

Reference:

Turner, S. and Foden, J. (2001) U, Th and Ra disequilibria, Sr, Nd and Pb isotope and trace element variations in Sunda arc lavas: predominance of a subducted sediment component. Contributions to Mineralogy and Petrology 142, 43–57.