Late Cenozoic volcanism in central Myanmar: Geochemical characteristics and geodynamic significance

Abstract

Late Cenozoic volcanism occurred in the central Myanmar basin, a region that is marked by the existence of the dextral Sagaing fault linking the eastern Himalayan Syntaxis to the north and the Andaman Sea to the south. Here we report new geochemical data of volcanic rocks from Monywa, Mt. Popa and Singu areas erupting in two distinct stages, i.e., mid-Miocene and Quaternary, respectively. While calc-alkaline rocks showing arc-like geochemical features are abundant, an apparent change in magma composition is observed between these two stages. The mid-Miocene rocks are mainly dominated by intermediate compositions (SiO2=53-61wt.%) and typical of high-K calc-alkaline nature. They exhibit uniform Sr and Nd isotopic ratios (ISr≈~0.705; εNd(T)=+2.7 to +1.6), suggesting a juvenile mantle origin related to the subduction of Indian oceanic lithosphere beneath this part of Asia. The Quaternary rocks consist mainly of basalts that, however, show variations in geochemical features. Three suites in this stage are observed: (1) calc-alkaline suite (basalts and basaltic andesites from Monywa and Mt. Popa: SiO2=48–56wt.%; ISr=0.704 to 0.705; εNd(T)=+3.1 to +2.1), originating from partial melting of a “remnant” juvenile mantle wedge, (2) alkali basalt suite I (basanites from Monywa: SiO2≈45wt.%; MgO=10–12wt.%; K2O≈2.0wt.%; ISr=0.704 to 0.705; εNd(T)≈+3.6), interpreted as the product of small-degree melting from an amphibole lherzolite in the lithospheric mantle, and (3) alkali basalt suite II (trachybasalts from Singu: SiO2≈51wt.%; K2O=2.6–3.5wt.%; ISr≈0.706; εNd(T)=+1.6 to +0.9), interpreted as the product of partial melting from the asthenosphere. All processes of magma generation were related to the India–Asia collision that caused regional plate reorganization, a transition from oblique subduction to dextral movement in the Miocene, and subsequent “rollback” of the subducted Indian oceanic lithosphere in the Quaternary. Therefore, the arc volcanism ceased at ca. 15Ma and, after a >10-m.y. magmatic gap in the region, volcanism revived in the Quaternary with heterogeneous compositions owing to rise of the geotherm resulting from asthenospheric upwelling that caused small-degree melting of different domains in pre-Miocene subduction-enriched lithospheric mantle and the asthenosphere beneath central Myanmar.