Petrogenesis of Middle Triassic andesitic rocks in the Weixi area, Southwest China: Implications for the tectonic evolution of the Paleo-Tethys

The Paleo-Tethyan tectonic evolution of the central Sanjiang orogenic belt remains unclear, particularly regarding the types and tectonic setting of volcanic rocks in the Cuiyibi Formation in the Weixi area, southwest China. These volcanic rocks were previously thought to be bimodal, but our detailed geological mapping revealed that the Cuiyibi Formation volcanic rocks include basalt, andesite, and rhyolite. In this paper, we present new geological, geochronological, and geochemical data for the andesitic rocks in the Cuiyibi Formation. Zircon U-Pb ages of the andesitic rocks indicate they formed at 245-242 Ma. The andesitic rocks are part of the high-K calc-alkaline/shoshonite series, and they exhibit light rare earth and large-ion lithophile element (e.g., K, Rb, Th, and U) enrichment and heavy rare earth and high-field-strength element (e.g., Nb, Ta, and Ti) depletion. High Mg# values and Nb contents, low Sm/Yb and Lu/Yb ratios, and negative zircon epsilon Hf(t) and whole-rock epsilon Nd(t) values suggest these rocks were derived from an enriched mantle source. The Cuiyibi Formation volcanic rocks are geochemically similar to those formed in a continental arc. These rocks formed along a vast continental arc in the North Qiangtang-Lanping-Simao Block related to the subduction of the Paleo-Tethyan oceanic slab.