Zircon SHRIMP U-Pb Dating of the Dashantou Basic Complex and Its Geological Significance in Beishan Area, Gansu Province

Dashantou rock is a large basic complex of Heishan Cu-Ni sulfide metallogenic belt in Beishan area,of which the geochemical characteristics,petrogenic age,nature of magma source,diagenesis setting are the basic scientific problems that must be solved in studying magma Cu-Ni sulfide deposit rock-forming and ore-forming processes.A SHRIMP zircon U-Pb age of (359.3±5.7)Ma has been first obtained in this paper,which determining diagenesis stages of Heishan metallogenic belt is in the Late Devonian.The rock geochemistry shows that primary magma of Dashantou complex may be high magnesian basaltic magma,of which the magma source belongs to the enriched mantle and has been strongly metasomasised by fluid formed in the subduction process of oceanic crust and contaminated by crustal material.The study on tectonic setting of Dashantou area suggests that the subduction of Hongliuhe-Niujuanzi-Xichangjing paleo-suture zone in the early Paleozoic continued to the end of the Devonian possibly.In about (359.3±5.7)Ma the oceanic crust subduction in Dashantou-Heishan still affected magmatism activity in initial rift of continental margin.