The Metamorphic Volcanic Geological Characteristics And Tectonic Attribute Research At Qinghai Xunhua Daowei Area

The material composition and crustal structure are very complex in the Xunhua area Qinghai.The Structural evolution has a long history.There are many fault trend NWW or NW-NNW.The fault become the boundary between the Hualong micro plot in Qilian orogenic belt and the north of western Qiling orogenic belt,which named Jinkujie-Duozhiba Fault.This article selects different metamorphic grade volcanic rocks as the research object in the Xunhua area Qinghai.We obtain the following results and achievement through geology,petrology,mineralogy,geochemistry and chronology study.(1)The Hualong Group is mainly composed of amphibolite and little granulite in the Xunhua Qinghai.Through the hornblende – plagioclase geothermobarometer,the temperature is about 691 ~ 691 ? and metamorphic pressure is about 0.39 ~ 0.64 GPa.So the metamorphic grade of Hualong Group is low amphibolite facies.The protolith of amphibolites is basalt and granulite is andesite from geochemical data of Hualong Group.These metamorphic volcanic rocks is formed in back-arc basin environment.The zircon U-Pb age is(1852±18)Ma from amphibolites of Hualong Group,indicating the protolith formation in paleoproterozoic.(2)The Shengou Formation is mainly composed of metamorphic andesite and a little metamorphic basalt.This metamorphic basalt is mainly formed by enrichment mantle fractional crystallization.And metamorphic andesite may be formed by partial melting of crust.This metamorphic volcanic rocks are formed in rift environment.The zircon U-Pb age is(503±6.6)Ma from metamorphic basalt of Shengou Formation,indicating the Shengou Formation formed in the middle Cambrian.These facts indicating Lajishan structural belt was in the rift of early stage in the middle Cambrian.(3)The Liudaogou Formation is mainly composed of metamorphic basalt and a little metamorphic andesite.This metamorphic volcanic rocks have the similarity characteristics of E-MORB and formed in rift environment.The metamorphic basalt have the pillow structure in the field and the petrogenesis is fractional crystallization from enrich mantle.These evidences suggest we that the Lajishan structural belt was in the rift of late stage in the late Cambrian.(4)With the southern Qilian Ocean northern subducting and plate of revolution,the centre Qilian block is fractured by ashospheric mantle upwelling.So that the Lajishan structural belt area is formed by intermediate-basic of rift since early-middle Cambrian.Under the sustained effect of subduction,parts of oceanic crust block and oceanic island or seamounts block of South Qilian ocean northward collaged with Mid-Qilian massif piecemeal,Lajishan continental rift closed owing to sustained stress squeeze from subduction.