Discovery of Late Triassic (Rhaetian) quartz diorite in the Caojian area of western Yunnan and its geological significance

The Chongshan complex belt in western Yunnan is located in the western margin of the southern belt of the Sanjiang orogenic system, specifically in the "peak waist" of the Sanjiang orogenic belt, with strong tectonic compression metamorphism and intense magmatic activity. In the course of geological investigation, the author and the research team newly discovered a set of quartz diorite in the Chongshan complex belt, which underwent low-grade metamorphism. The zircon U-Pb age of 208.2 Ma indicates that it belongs to the Late Triassic Rhaetian age. Geochemical analysis of the rocks shows that it is potassium to high-potassium peraluminous, with high alkali content but low water content, enriched in elements such as Th, U, and Rb, poor in elements such as Sr, Eu, Ba, Ti and P, and it formed in a low oxygen environment. It is concluded that the quartz diorite belongs to the post-orogenic type, and it formed in the post-orogenic extensional stage following the closure and collision orogeny of the Paleo-Tethys Ocean (Changning-Menglian Ocean) of the Indochinese period. The Chongshan complex belt is a complex of the southern section of Chongshan-Lincang block (Lincang magmatic arc) and the Lancang accretionary complex, which is an accretionary complex.