Geochemical characteristics and chronological significance of Middle Ordovician granite in Halahu area of southern Qilian Mountain

The Ordovician granite in the Hala Lake area is characterized by relatively rich sodium (Na₂O>K₂O) and medium potassium-high potassium calc-alkaline series.Rock types include gabbroic diorite and granodiorite.The whole-rock rare earth and trace element characteristics indicate that the material source was derived from mantle material, and the tectonic environment was most likely the island arc granite formed during the ocean subduction.The zircon U-Pb isotope ages were determined by LA-ICP-MS.These zircons yielded a ²⁰⁶Pb/²³⁸U weighted mean age of 466±4 Ma, suggesting that the granite was formed in Middle Ordovician.In the two-way expansion and subduction stage of the North Qilian Ocean, the southern Zhongqilian block was subjected to subduction and extrusion, which caused the mantle material to migrate upward.At the same time, the continental crust was partially melted and the intrusions were formed by the mixture of the two melts.The age of the intrusion provides new evidence for multi-stage subduction of the Qilian Ocean.