Age and origin of the Bulangshan and Mengsong granitoids and their significance for post-collisional tectonics in the Changning–Menglian Paleo-Tethys Orogen

• The Bulangshan and Mengsong granitoids were emplaced at 216–228 Ma.
• The granitoids formed during the post-collision.
• The transition from syn-collisional to post-collisional occurred around 235 Ma.

The subduction and collision tectonics in the Sanjiang region provide important insights into the evolution of the Paleo-Tethyan Orogen. Here we present LA-ICP-MS zircon U–Pb ages, Sm–Nd and Lu–Hf isotopic data, whole-rock major and trace element compositions of the Late Triassic magmatic rocks from the Mengsong and Bulangshan granitoids in this region. Zircon U–Pb dating of the Bulangshan and Mengsong granitoids yields crystallization age of 216–228 Ma, with zircon εHf(t) values ranging from −15.2 to −1.5. These granitoids are high-K calc-alkaline and muscovite-bearing. The rocks show peraluminous nature, and are enriched in large ion lithophile elements (Rb, K, U, and Th) and depleted in high field strength elements (Nb and Ti). They also display enrichment in light rare earth elements with strong negative Eu anomalies (δEu = 0.16). The geochemical and isotopic features suggest that the primary magma of the Bulangshan and Mengsong granitoids most likely originated from partial melting of the underlying Proterozoic metasedimentary rocks. The granitoid magmatism is correlated with post-collisional tectonics, and provides important constraints on the timing of closure of the Paleo-Tethyan Ocean.

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