Petrogenesis of the Xifanping Porphyries in the Sanjiang Orogenic Belt, SW China: Eastward Propagation of the Indo–Asian Collision

Abstract

Large-scale Cenozoic alkali-rich magmatic rocks are exposed at the eastern margin of Tibet due to the interaction between the Indian and Eurasian plates. However, their petrogenesis and associated geodynamic processes remain poorly understood. We analyzed the Xifanping porphyries in the Sanjiang orogenic belt to provide new insights. Our study shows a successive assembly of porphyry intrusions during three magmatic episodes. The magnitude and duration of the magmatic activities diminished sequentially, and mineralization occurred during the intermediate phase. Geochemically, the Xifangping porphyries display an adakitic affinity. According to zircon Hf isotope data, we propose that these porphyries originated from the partial melting of the thickened mafic lower crust beneath the western Yangtze Craton in response to lithospheric extension and asthenospheric upwelling. Analogous to the coeval alkali-rich porphyries in western Yunnan, the petrogenetic model of the Xifanping porphyries indicates regional north–south and east–west fractures caused by the clockwise rotation of the continental lithosphere and the Jinshajiang–Red River strike-slip during the post-collision phase of the India–Eurasia collision. In this context, we argue that the collision in eastern Tibet may have extended eastward to southwestern Sichuan during the post-collision period, persisting until approximately 30 Ma.