SHRIMP Zircon U-Pb Age of the Dashantounan Granodioritein from Beishan Mountains in Gansu Province and its Signification

Using the SHRIMP zircon U-Pb dating methods, the diagenesisage of the Dashantounangranodiorite has been obtained for the first time. The obtained ²⁰⁶Pb/²⁰⁸Pb weighted average age is (386±6)Ma (MSWD=1.06), standing for the Dashantounan granodiorite was formed in middle Devonian. At the same time, the inherited zircon age has been obtained for the first time, and the yielded age is (885±20)Ma (MSWD=1.18), whichserves as a reflection of the Neoproterozoic tectonic-thermal event. The diagenetic age between the age of Dashantourock mass (397±4)Ma and the ones of Dashantounan rock mass (374±3)Ma. After analyzing the previous research achievements, it's believed that themagmatic intrusionin the Middle-lateDevoniannot only includeintermediate-acidic magmas,but also include basic-ultrabasicmagma in the studying area. Theintermediate-acidic and basic-ultrabasicmagma might be formed in the same geological background, that is, theymight beformed in a post-orogenic extensional setting or a late-stage of syn-orogenic setting.