Determination of Middle Triassic rhyolite in Caojian area, Yunlong County of western Yunnan, and its constraints on the Paleo-Tethys tectonic system transition

In recent years, 1:50 000 regional geological survey reveals that a set of intermediate-acid and acid volcanic rocks, pyroclastic rocks and volcanic-sedimentary rocks of the Middle-Late Triassic are intermittently outcropped between the Lancang River fault in western Yunnan and Baoshan block. They are mainly outcropped in Caojian, Yunlong County, Zhushan, Changning County, Dasijia, Fengqing County, Mengku, Shuangjiang County, Manyan, Menghai County and other places, belonging to the volcanic-sedimentary rock series in the interarc basin. The study focuses on the rhyolite outcropped in Caojian of Yunlong County. Its A/CNK value of 1.05-2.44 indicates a peraluminous high-potassium calc-alkaline-potassic series. The zircon LA-ICP-MS U-Pb ages of 235.8±4.9 Ma and 235.2±5.6 Ma implies Middle Triassic. This set of volcanic rocks is chemically characterized by significant enrichment of high field strength elements such as Nb, Ta, Th, Zr, Hf and Y, depletion of large ionic elements such as Sr and Ba, and high field strength elements such as P and Ti. It is a low-grade partial melt under the condition of low-pressure amphibolite facies of metamorphic sedimentary rocks in the middle-upper crust. It is the result of the combined heating, warming, uplifting, decompression and melting caused by upwelling of deep mantle materials in the process of regional crustal relaxation after collision orogeny. Southward to Fengqing County, Shuangjiang County and other places, this set of volcanic rocks gradually transit to a quasi-aluminum-peraluminous series of high potassium calc-alkaline-calc-alkaline. The cobweb diagram of trace element ratio shows obvious negative anomaly of "T-N-T", which indicates a high degree of partial melting under the conditions of high-pressure amphibolite facies-granulite facies of the middle-lower crust metasomatized by subduction fluid under the conditions of uplift and decompression. Although there are some differences in magma source, partial melting degree and temperature-pressure conditions for magma formation, these Triassic volcanic rocks are all the reflection of the deep process of transition from oceanic tectonic system to continental tectonic system on the surface after the closure of the Paleo-Tethys Ocean.