Petrogenesis of isotopically enriched Quaternary magma with adakitic affinity associated with subduction of old lithosphere beneath central Myanmar

We present a model for the petrogenesis of magma with adakitic affinity in an old subduction zone, which does not involve slab melting and is constrained by new geochronological and geochemical data for Mt. Popa, the largest of three Quaternary volcanoes in central Myanmar (Popa, Monywa and Singu). The edifice is composed of Popa Plateau (0.8-0.6 Ma) with high-K rocks and a stratovolcano (<0.33 Ma) predominantly composed of medium-K rocks with adakitic affinity (Mg# 45-63, Sr/Y > 40). The distinct K contents indicate that the adakitic magmas cannot be derived from Popa high-K rocks, but they share trace-element signatures and Sr-Nd isotope ratios with medium-K basalts from Monywa volcano. Our estimation of water contents in Popa magma reveals that primary magma for medium-K basalts was generated by partial melting of wedge mantle with normal potential temperature (T-P 1330-1340 degrees C) under wet conditions (H2O 0.25-0.54 wt%). Its melting was probably induced by asthenospheric upwelling that is recognized by tomographic images. Mafic adakitic magma (Mg# similar to 63, Sr/Y similar to 64) was derived from the medium-K basaltic magma in fractional crystallization of a garnet-bearing assemblage at high pressure, and felsic adakitic rocks (Mg# similar to 45, Sr/Y similar to 50) were produced by assimilation-fractional crystallization processes at mid-crustal depths.

Automated summary

A study on the largest volcano in central Myanmar, Mt. Popa, reveals that adakitic magmas with high-pressure crystallization are produced through slab melting and fractional crystallization processes. This research provides important insights into the petrogenesis of magma in an old subduction zone.