DATING OF THE DEEP CRUST BENEATH AN ISLAND ARC: 40AR/39AR GEOCHRONOLOGY OF THE BARREN ISLAND VOLCANO, ANDAMAN SEA

The knowledge of age, structure and composition of the deep crust beneath island arcs is crucial to our understanding of the formation and evolution of continental crust. Due to lack of geophysical data the nature of the deep crust beneath the Andaman Island Arc, a northward extension of the Indonesian magmatic arc, remained unknown. The Andaman Arc is located on the Burma plate, an overriding sliver plate present at the convergent boundary between Indian and Eurasian plates. The arc contains only two subaerial volcanoes: the active Barren Island and the dormant/extinct Narcondam with the former representing the northernmost active center of the arc. While the Barren Island volcano was known to have erupted many times during the past 72 ka and contained rocks as old as ~1.8 Ma in its plumbing system, its age of emergence from the sea surface remained uncertain. Information on the type and age of its basement was also lacking in spite of the fact that its lava flows hosted numerous crustal xenoliths. Incremental-heating ⁴⁰Ar/³⁹Ar dating was carried out on three carefully selected whole-rock samples from lava flows from the subaerial base of the volcano and on a plagioclase megacryst that was geochemically proven to belong to a lava-incorporated crustal xenolith. Our efforts yielded precise, statistically robust and indistinguishable plateau and isochron ages for the lava flows, based on which we infer that at 1.58 ± 0.04 (2σ) Ma – the weighted mean of the plateau ages, the volcano breached the sea surface. The plagioclase xenocryst yielded a robust plateau age of 106 ± 3 (2σ) Ma, which is deemed to be the age of the deep crust. Although isotopically similar, this crust is at least 10 Ma older than the ophiolites of the Andaman forearc – suggesting its existence prior to the current subduction along the Andaman Trench. We speculate that it formed part of the oceanic crust that was attached to the western margin of the Eurasian plate at the initiation of subduction of the Indian plate during the early Cretaceous.