AGE AND GEOCHEMICAL CONSTRAINTS on THE PETROGENESIS OF LATE CENOZOIC VOLCANISM IN MYANMAR

Late Cenozoic volcanism occurs in the central Myanmar basin, a region that is marked by the existence of the dextral Sagaing fault linking the eastern Himalayan Syntaxis in north and the Andaman Sea in south. Here we report new ⁴⁰Ar/³⁹Ar age results for the volcanic rocks from Mt. Popa and two volcanoes in Monywa, central Myanmar. The results suggest two distinct stages of eruptions, a mid-Miocene stage from ~16 to 13 Ma and a Quaternary stage <1 Ma, with a >10-m.y. magmatic gap in between. While calc-alkaline rocks showing arc lava geochemistry are abundant, an apparent change in magma composition is observed between the two stages. The mid-Miocene rocks are typical of high-K calc-alkaline nature and dominated by intermediate compositions (SiO₂=53-62 wt.%). By contrast, the Quaternary rocks are mainly basalts but show heterogeneous compositions, including (1) high-Al basalts from Mt. Popa: ε_Nd=+3.0 to +2.1, ε_Hf=+17.4 to +10.7, (2) absarokites and high-Al basalts from Monywa: ε_Nd≈+3.6, ε_Hf≈+12.1 for the former and ε_Nd≈+3.0, ε_Hf≈+11.8 for the latter, and (3) OIB-type alkali basalts from Singu, a locality off the Sagaing fault: ε_Nd≈+0.9, ε_Hf≈+3.7. We interpret the long magmatic gap as a consequence of cessation of the oblique subduction of the Indian oceanic lithosphere beneath this part of Asia that occurred in the mid-Miocene, when the dextral motion of the Sagaing fault system initiated and opening of the Andaman Sea began. All these processes were related to the India-Asia collision that caused plate reorganization in the region and eventually transformed the subduction system from oblique convergence to dextral movement. Under this framework, the Quaternary volcanism renewed owing to small-degree melting of different types of pre-Miocene subduction-enriched mantle domains beneath central Myanmar related to an extensional or transtensional setting created by the stress partitioning along the Sagaing fault system.