Geochemistry of volcanic rocks from the Andaman Sea: Insights into the nature of back-arc crust.

The forearc and back-arc region of the Andaman-Nicobar-Sumatra Subduction Zone are mainly submarine and poorly understood. To gain new insights sixty representative volcanic rocks were collected from the Andaman Sea region. Rock types range from basalt, basaltic andesite, andesite, dacite to rhyolite reflecting calc-alkaline, intermediate to acidic magma compositions. The rocks exhibit pronounced LREE enrichment with significant negative Nb, Ta and Ti anomalies and relative HREE depletion indicative of subduction zone related volcanism. The calc-alkaline rocks were generated by fractional crystallization along with crustal assimilation of older crustal rocks. A lower level of contamination is recorded in Barren Island samples, and mafic rocks from the Central Andaman Trough and Southern arc. Miocene zircon ²³⁸U/²⁰⁶Pb ages (10.8 ± 0.3 Ma, 19.8 ± 0.3 Ma and 19.5 ± 0.2 Ma) are recorded by West Sewell Ridge samples. Previous suggestions that hyperextended Malayan crust (Sundaland) existed below much of the Northern arc, Southern arc, Alcock and Sewell Rises and were later covered by volcanic crustal material is supported by the presence of inherited zircon cores and xenocrysts with ages spanning the Phanerozoic to Neoproterozoic (250, 320, 460–690, 800–1100, and 1400 Ma). These fit with the Malay-Sundaland as the main source of crustal contamination.