Geochemical Characteristics and Chronology Significance of Ordovician Quartz Diorite from Suli Area in Middle Qilian Mountains

In Suli area, the Ordovician quartz diorite has poor aluminum and high potassium (K₂O >NaO), which belongs to calc-alkaline series, with the characteristics of partial aluminium shoshonite series and obvious Ⅰ-type granite.Rare earth and trace elements characteristics show that a mantle material was involved in the material source of quartz diorite. The tectonic environment discrimination suggests that the quartz diorite belongs to the island arc granite formed before the plate collision.The zircon U-Pb isotopic age has been tested by LA-ICP-MS method, the obtained zircon ²⁰⁶Pb/²³⁸U weighted average age is (474±2.9)Ma (MSWD=0.12), which limit the formation age of the quartz diorite era to early Ordovician, suggesting that the north Qilian Ocean experienced two-way expansion and subduction stage, while the southern Qilian continental subduction extrusion cause the mantle material upwelling andthe partial melting of continental crust. And then, the rock mass of quartz diorite was formed. The age determination of this rock mass provides a new evidence for the subduction era of the north Qilian Ocean.