Petrogenesis Of The Mesozoic Igneous Rocks In The Ejiu Area,north Tibet And Their Constraints On The Evolution Of Bangong-Nujiang Ocean

The formation of Tibetan Plateau,during which the evolution of the ancient oceans is an inevitable key issue,has consistently been focused research in geology area.Studying on the evolution of the Bangong-Nujiang Ocean(BNO)is of great theorical and pratical significance to the research for Tibetan Plateau.In the middle section of the Bangong-Nujiang Suture Zone(BNSZ),the Ejiu area yields multiple igneous rocks which offer us an ideal opportunity to study the evolution of the BNO in a relatively limited extent of locations.In this article,6 types of igneous rocks from Jiaomuri,Yongqinco,Rigenco and Xiabieco were collected after a careful field observation.A systematical study of the petrology,petrogeochemistry and chronology was carried out,in order to clarify their source and tectonic attribution and discuss the implications for the evolutionary history of the BNO.Combining with regional geological data,the evolutionary history of the BNO was reconstructed.In the early Triassic(-230 Ma),the BNO initiated.In the early Jurassic(?189 Ma),the BNO grew into a mature ocean.In the middle-late Jurassic,the BNO had started subduction,and a back-arc basin was formed in the late Jurassic(?150Ma).In the early Cretaceous(?120 Ma),the BNO finally closed and disappeared.The Jiaomuri diabases(JMRDs)are alkaline sodic gabbro dykes,representing the product of the rift period of BNO.SHRIMP U-Pb dating shows that the dykes formed at 230 Ma.Petrogeochemical results indicate that the magma for JMRDs was derived from an enriched mantle and was generated from 4-9%partial melting of garnet peridotite containing 2-4%garnet.During the formation of JMRDs,the magma experienced remarkable fractional crystallization,but was not affected by crustal contamination.Their formation may be related to the upwelling of a mantle plume.The Yongqinco basalts(YQCBs)geochemically belong to normal mid-ocean-ridge basalt(N-MORB).They are the remnant of BNO floor during its mature ocean period.Chronology research shows that YQCBs formed at 186.8 Ma.Petrogeochemical results show that the magma was derived from a depleted mantle and crustal contamination was not discernible.YQCBs were product of mid-ridge oceanic magmatism.The Rigenco gabbros(RGCGsl)and granodiorites(RGCGs2)constitute a bimodal magmatic rocks serie,which were formed during the subduction of BNO Their ages are 151.7Ma and 150Ma respectively.After detailed petrogeochemistry research,it is suggested that RGCGsl were derived from an enriched mantle modified by metasomatic subduction fluid and was generated from partial melting of spinel peridotite.Their magma experienced obvious fractional crystallization,and was affected by crustal contamination.RGCGs2 belong to metaluminous granitiods.Their magma was generated by partial melting of an ancient continental crust or a juvenile basaltic lower crust evoked by underplating.Both of RGCGs1 and RGCGs2 were formed in a back-arc basin environment.The Xiabieco granites can be divided into normal granites(XBCGls)and high fractional granites(XBCG2s).Both of them were formed during the close of BNO.Geochronology research show that the age of XBCGls is 119.7Ma,but XBCG2s crystalized during a long period of time,form 127.6±0.7Ma to 113.1 ± 1.4Ma.Petrogeochemistry research implies that XBCGls show affinity to metaluminous granitoids.They belong to A1-type granites,whose magma was derived from patial melting of the metagraywacke with a shallow depth.XBCG2s,characterized of peraluminous granitoids,belong to I-type granites.The magma of XBCG2s was derived from the clay rocks area with a relatively deep depth.Both of the granites were formed in a collision-related environment.This work supplements the research achievement in BNZS and Tibet Plateau.