Zircon LA-ICP-MS Dating and Geochemical Characteristics of Rhyolites from the Qushi Area, Tengchong Terrane, Yunnan Province

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The Qushi rhyolites, situated in the eastern sector of the Tengchong terrane, are critical to understanding the Early Cretaceous tectono-magmatic evolution of the Eastern Tethyan Tectonic Domain. Zircon LA-ICP-MS U-Pb geochronology indicates crystallization ages of 118.3–120.5 Ma, with Ti-in-zircon temperatures of 641–816 °C (mean = 716 °C), representing the Early Cretaceous magmatic activity in the Tengchong terrane. Inherited zircons within the rhyolites yield a zircon age of ca. 198.5 Ma, with corresponding Ti-in-zircon temperatures of 615–699 °C (mean = 657 °C), implying the potential presence of an Early Jurassic igneous basement beneath the Qushi region. Geochemically, the rhyolites are classified as calc-alkaline and weakly to moderately peraluminous (A/CNK = 1.07–2.86). These rocks display signatures typical of acidic magmas, marked by significant enrichments in light rare earth elements (LREE: La and Ce) and large ion lithophile elements (LILE: Rb, K, Th and U) while simultaneously exhibiting depletions in high-field-strength elements (HFSE: Nb, Ta, Ti, and P) and heavy rare earth elements (HREE). Trace element signatures further reveal marked depletions in Sr (12.4–244.7 ppm) and Ba while displaying enrichments in Zr and Hf. These geochemical features, including the huge range of the Sr content and A/CNK ratios, suggest both I-type and S-type granite affinities. The Early Cretaceous volcanism of the Qushi rhyolites is likely attributed to the combined effects of subduction and the closure of the Meso-Tethyan Ocean (MTO). This volcanic activity is interpreted to result from subduction-related processes associated with the MTO, potentially involving slab rollback, slab break-off, and subsequent asthenospheric upwelling. The formation of these rhyolites may also be linked to the final closure of the MTO, characterized by the Late Cretaceous collision and amalgamation of the Burma and Tengchong terranes.