Abstract

Quaternary to recent volcanic centres located in Central Myanmar basin and in western Yunnan (Tengchong rift) display contrasted geochemical signatures. Mt. Popa lavas range in composition from calc-alkaline to potassic calc-alkaline and derive from partial melting of a subduction-modified mantle. The Monywa Holocene maars are made up of absarokites thought to result from low degrees of melting of a similar source. The Tengchong rift potassic calc-alkaline andesites and dacites also display typical subduction-related imprints. On the contrary, the geochemical signature of the Singu alkaline mafic basaltic trachyandesites which are offset by the Sagaing Fault suggests that they derive from the melting of deep enriched intraplate-type mantle, similar to the source of common Plio-Quaternary alkali basalts and related rocks from the Sundaland.

The origin of this uncommon spatial and temporal magmatic association is connected to an unusual tectonic setting of hyper-oblique convergence. The collision between India and the Sundaland is marked by shear partitioning within Myanmar, a significant part of the right lateral motion being accommodated by the Sagaing Fault. Rapid uprise of high temperature alkaline magmas derived from the melting of enriched Sundaland-type mantle and channelled by Neogene fault planes parallel to the Sagaing Fault led to the emplacement of the Singu lavas. Active subduction of the Gulf of Bengal oceanic lithosphere resulted in the development of a Wadati-Benioff plane located 100 to 140 km beneath Mt. Popa and Monywa. The dehydration of this slab led to metasomatism and subsequent melting of the Central Myanmar mantle wedge, and finally to the emplacement of calc-alkaline and shoshonitic magmas in those volcanoes. The origin of the subduction-related signature of the Tengchong lavas has to be related to an older subduction event.

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