Early Carboniferous Magmatic Rocks in Eastern Myanmar: Implications for the Tethyan Evolution

Abstract

Eastern Myanmar is the key position linking between SW Yunnan and northern Thailand for better understanding of Tethyan evolution. However, the actual location and evolution of the Tethyan suture zone are still unclear in eastern Myanmar. The present study focuses on the geochronological, geochemical and zircon Lu-Hf isotopic study on the plutonic rocks, including granite, diorite and gabbroic rocks, from the Tachileik area, eastern Myanmar. These plutonic rocks yielded zircon U-Pb weighted mean ages of ca. 353–355 Ma, suggesting the Early Carboniferous emplacement. The Tachileik granites are high-K calc-alkaline, weakly peraluminous and have low P₂O₅ contents, which are typical features of I-type granites. They have positive zircon ε_Hf(t) values (+4.5 − +7.4) with T_DM2 ages of 981–825 Ma, indicating a juvenile mafic lower crust source. The Tachileik gabbros and diorites show high Al₂O₃ contents and Mg^#, but low TiO₂ and K₂O contents, belonging to tholeiitic and calc-alkaline basalt series. They are characterized by enrichments in large ion lithophile elements (LILEs, e.g., Rb, Ba and Sr), depletions in high field strength elements (HFSEs) and distinctly negative Nb and Ta anomalies, similar to the volcanic arc basalt. The zircon Hf isotopic (+4.7− +7.1) and whole-rock geochemical data imply that the Tachileik gabros and diorites probably resulted from partial melting of lithospheric mantle in the spinel stability field within an arc-related setting. The magmatic rocks can be grouped to the Late Devonian–Early Carboniferous magmatic rock and pyroclastic rock zone from SW Yunnan to northern Thailand based on their age and geochemical characters. The zone was formed in the post-collisional extension-related tectonic setting of the Proto-Tethys. This study provides important evidences for the evolution of the Proto-Tethys in Southwest Yunnan and Southeast Asia.

Keywords

Tethys / Early Carboniferous / plutonic rocks / post-collision / eastern Myanmar / geochemistry / tectonics

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