Petrogenesis of the Early Cretaceous Shoshonitic Rocks in Myitkina of Myanmar and its Tectonic Implications

Myanmar is located in the east of the Himalaya orogenic zone and proximal to the Eastern Syntaxis. Voluminous late Mesozoic-Cenozoic magmatic rocks developed in Myanmar, which are the products of the Himalaya orogeny. The origin and tectonic regime of these igneous rocks are critical for the understanding of the geodynamic evolution of the Neo-Tethys and the tectonic correlation of the different terranes in between Myanmar and Tibetan Plateau. However, little attention has been paid to these igneous rocks, particularly for those in Myitkyina of Myanmar. In this study, a suite of shoshonitic rocks were first recognized, which are characterized by high SiO2 (58.92%-63.73%), total alkali (Na2O+K2O=7.94%-10.14%), K2O (3.88%-6.14%), and K2O/Na2O ratios (>1) together with the strong enrichment of LILE and LREE and depletion of Nb, Ta and Ti. LA-ICP-MS zircon U-Pb dating results show that they were emplaced at ~130 Ma. The above signatures coupled with their depleted Sr-Nd isotopic compositions, low Sr/Y and La/Yb ratios, and high Y and Yb contents indicate that the Myitkyina shoshonitic rocks were derived from partial melting of the enriched, phlogopite-bearing lithospheric mantle metasomatized by the subduction-related fluids/melts. Combined with other geological data, we propose that the Myitkyna early Cretaceous shoshonitic rocks were generated in an oceanic arc in response to the slab rollback or breakoff. It is thus noted that the Myitkyina Tethyan Ocean was not closed before ~130 Ma, and the southern continuation of the Bangonghu-Nujiang Meso-Tethys was an important component of the Neo-Tethys. © 2021, Science Press. All right reserved.