Geochemistry And Zircon U-Pb Chronology Of Qushenla Group Volcanic Rocks In Gerze,Tibet

The Himalayan-Tibetan orogenic system is most distinctive landform on ourplanet.However,the geological evolution of the Tibetan plateau still hotly debated.TheTibetan Plateau has experienced the subduction of Tethyan oceaniclithosphere(involving the Proto-Tethys,Paleo-Tethys,Neo-Tethys)and the India-Asiacontinental collision,in the processes of which widespread magmatic activitiesoccurred. Studying the Mesozoic volcanic rocks widely exposed in Lhasa block canbetter understand the geological evolution of the Tethys and construction of the wholeTibet Plateau.In the paper,we present field geology,petrography,SHRIMP zircon U-Pbgeochronology,geochemistry compositions for the Gangdese basic-intermediatevolcanic rocks,with the aim of constraining the prtrogenesis and origin of these rocksand the tectonic evolution of the Gangdese belt. Volcanic rocks of Qushenla formationgeographically locate in west of Gerze county,Tibet.They are series ofbasic-intermediate volcanic rocks.Rock associations are basalt、basaltic andesite、andesite.Mineral associations are hornblende、anorthosite、quartz.The results showthat volcanic rocks of Qushenla formation are characterized by SiO2content and theybelong to the cal-alkaline series.Trace elements analyses indicates that thebasic-intermediate volcanic rocks are enriched in LILE,such as Cs、K、Ba，depletedin HFSE,such as Nb、Ta，clearly displaying compositional characteristics of island-arcvolcanic rock,and their rare earth elements alse mainly show a right pattern.Eu takeson weak anomaly. The method of Zircon U-Pb dating reports the age of granodiorite126Ma. Considering field survey data and previous research results, we suggest thatthe Qushenla Formation volcanic rocks were resulted from the southward subductionof Bangong Co-Nujiang ocean, which provide directpetrology evidences ofBangongCo-Nujiang oceans southward subduction in Mesozoic.