Geologic Features And Tectonic Significance Of Indosinian Magmatite At The Qinghai Xunhua Area In The Northern Margin Of The West Qinling

The Qinghai Xunhua area locate in the northwest margin of Tibatan Plateau,at the intersection of Qilian orogenic belt and Xiqinling tectonic belt,is an important component of Central Orogenic Belt.Take Guyi complex and Dalijiacuo mongnitic granite as study objects,focus on the geological characteristics,forming age,tectonic attribute with comprehensive research of structure,petrology,geophysics and zircon U-Pb geochronology,based on which investigate evolution history of the Palaeo-Tethys in Late Paleozoic.The main achievements are as follows:1?Guyi complex and Dalijiacuo mongnitic granite are Indosinian intrusive rocks exposed in Xunhua area,they intrude into Lower Triassic Longwuhe Formation,southern of Dalijiacuo mongnitic granite partly intrude into Permian Ganjia Fomation.2?Guyi complex is composed of Gabbro,gabbro diorite,diorite,quartz diorite and granodiorite.The geochemical study of Gabbro,gabbro diorite and granodiorite shows they belongs to high-medium K calc-alkaline series,light rare earth elements enrichment,heavy rare earth elements loss.With the evolvement characteristic of the same magma origin from Gabbro to granodiorite total REE rising,Eu slight enrichment converted to a slight loss.The trace elements are characterizd by systematic enrichment of LILEs(Cs?Rb?Th)and depletion of HFSEs(Nb ? Ta ? Ti),which are consistent with geochemical characteristics of subduction-related magmas.The LA-ICP-MS zicon U-Pb age of granodiorite is 246.7±2.1Ma,belongs to the middle Triassic.The granodiorite of Guyi complex have developed enclaves.Most of enclaves are elliptoid,porphyritic texture,and hypidiomorphic granular structure.Some of enclaves are sharely from the host rocks Which is producted by mafic component magma pouring into the magma which is lower temperature during the magma mixing.The LA-ICP-MS zircon U-Pb age of the enclaves is(245.3±2.1)Ma,which is same with the host rocks' age within the limit error,standing for the age of the complex,in Middle Triassic.3 ? Dalijiacuo mongnitic granite is composed of monzogranite,few-porphyritic monzogranite and porphyritic monzogranite,which show the gradual contact relationship.Geochemistry characteristic shows the rock is high-k calc-alkaline series,peraluminous series,which is samiliar with the S-Type granite.REE patterns show that the rocks are enriched in LREEs,deficited in HREEs,and little deficited in Eu;the trace elements are marked by enrichment in LILEs(Cs?Rb?Th?Zr et al.),and characterized by depetion of HFSE(Nb?Ta?Ti,et al).Discrimination diagram results show that the Dalijiacuo mongnitic granite are formed in a post-collisional environment.Geochemcistry characteristics show the protolith of Dalijiacuo mongnitic granite is metapelitic.The LA-ICP-MS zircon U-Pb age of the monzogranite is(221.1±1.0)Ma,which is stand for the age of Dalijiacuo mongnitic granite,in Late Triassic.4?The most of magmatite in the Xiqinling tectonic belt was composed of late Permian to early middle Triassic,and late Triassic to early Jurassic,which is mainly magmatism stage during the indosinian,represent the mainly composed stage of island arcs magmatite and the post-collisional magmatite.Combining with territorial character data,the formation of the Xiqinling tectonic belt during indosinian is considered that the ocean basin subduction during the late Permian to early middle Triassic,collision stage during the late middle Triassic to early late Triassic;post-collision during the late Triassic to early Jurassic.