Petrogenesis of Middle Triassic bimodal volcanic rocks in the Weixi area, Yunnan Province and geological implication for the formation and evolution of the Jinshajiang arc-basin

The magmatic activities developed in the Jinshajiang suture zone and its west side record the process of ocean subduction and collision, which is the key to the inversion of ocean evolution.The Middle Triassic volcanic rocks (Cuiyibi Formation) constitute a unique bimodal volcanic assemblage in space and time in the southern Jomda-Weixi continental arc, which is mainly characterized by the interaction of basic and acidic volcanic rocks.The SHRIMP zircon U-Pb age of 244±1.3 Ma of rhyolite indicates that the Cuiyibi Formation volcanic rocks were formed in the Late Middle Triassic.The basalts are highly depleted in high field strength elements such as Nb, Ta and Ti, with low Zr content, Zr/Y and (Th/Nb)_N values, which is similar to the geochemical characteristics of arc magma.The rhyolite are relatively rich in silicon, poor in TiO₂ and MgO, and characterized by low Al₂O₃.The rhyolite are highly depleted in high field strength elements such as Nb.Sr and Ti show weak negative anomaly.They all have characteristics of intra-plate and arc volcanic rocks.Combined with previous studies, it is suggested that the bimodal volcanic rocks of the Cuiyibi Formation is the product of magmatic activity derived from plate fragmentation under the background of regional extension caused by the westward subduction of the Jinshajiang oceanic crust.