Abstract
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 (TP 1330–1340 °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# ~ 63, Sr/Y ~ 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# ~ 45, Sr/Y ~ 50) were produced by assimilation-fractional crystallization processes at mid-crustal depths.
Highlights
Myanmar lies at the junction of the Alpine-Himalayan orogenic belt and Indonesian Island arc system and is geologically divided into three major areas: Western Indo-Myanmar Ranges, Central Lowlands, and Eastern Highlands (Fig. 1)
We integrate petrological and geochemical evidence to underpin a model for generating magma with adakitic affinity in an old subduction setting without slab melting, which is tested against recently published tomographic models for the region
We further examined the behavior of trace elements during fractional crystallization by using Rayleigh fractionation modeling (Table S6); the mafic adakitic rock can be produced by subtraction of the phenocryst phases and garnet (Figs. 2, 4A)
Summary
Myanmar lies at the junction of the Alpine-Himalayan orogenic belt and Indonesian Island arc system and is geologically divided into three major areas: Western Indo-Myanmar Ranges, Central Lowlands, and Eastern Highlands (Fig. 1). Three Quaternary volcanoes are present in the Central Lowlands; Popa, Monywa, and Singu (e.g., ref.[1]). They do not form a clear volcanic front, though Quaternary volcanoes can be traced southwards to volcanic islands in the Andaman S ea[2]. The slab beneath central Myanmar is composed of Cretaceous Indian and/or Neo-Tethyan oceanic crust and the geothermal conditions do not support a slab melting origin for the magmas (e.g., ref.[15]). We integrate petrological and geochemical evidence to underpin a model for generating magma with adakitic affinity in an old subduction setting without slab melting, which is tested against recently published tomographic models for the region
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