Quaternary Volcanism In Myanmar: A Record Of Indian Slab Tearing In A Transition Zone From Oceanic To Continental Subduction

Magmatic processes that occur during the transition from oceanic to continental subduction and collision in orogens are critical and still poorly resolved. Oceanic slab detachment in particular is hypothesized to mark a fundamental change in magmatism and deformation within an orogen. Here, we report on two Quaternary volcanic centers of Myanmar that may help us better understand the process of slab detachment. The Monywa volcanic rocks are composed of low-K tholeiitic, medium-K calk-alkaline, and high-K to shoshonitic basalts with arc signatures, while the Singu volcanic rocks show geochemical characteristics similar to asthenosphere-derived magmas. These volcanic rocks have low Os concentrations but extremely high Os-187/Os-186(i) ratios (0.1498 to 0.3824) due to minor (<4%) crustal contamination. The Monywa arc-like rocks were generated by small degrees of partial melting of subduction-modified asthenospheric mantle at variable depths from the spinel to garnet stability fields. Distinct from the Monywa arc-like rocks (Sr-87/Sr-86(i) = 0.7043 to 0.7047; epsilon Nd-i = +2.3 to +4.7), the Singu OIB-like rocks exhibit higher Sr-87/Sr-86(i) (0.7056 to 0.7064) and lower epsilon Nd-i (+0.8 to +1.6) values. These isotopic characteristics indicate a large contribution of an isotopically enriched asthenosphere layer beneath the Burmese microplate, which possibly flowed from SE Tibet. We interpret that this short-lived, small-scale, and low-degree melting Quaternary volcanism in Myanmar was triggered by its position above a slab window resulting from the tearing of the oceanic lithosphere from buoyant continental lithosphere of the Indian plate.