Ocean Island Rock Assembly And Its Tectonic Significance In Tangga-Sumdo Area,Tibet

The traditional understanding about the tectonic framework of the Paleo Tethys Ocean was broken by the discovery of Sumdo eclogite belt in Tibet,which may represent a new suture zone of the Paleo Tethys Ocean lying on the most southern of the Paleo Tethys Ocean tectonic domain and be significant to understand the time and space evolution of the Paleo Tethys Ocean.However,the establishment of suture zone must be evidenced by the composition of oceanic crust such as ophiolite and ocean island.Although the predecessors have reported some fragments of oceanic crust in Sumdo and its neighboring area,including eclogite,ophiolite,ocean island,the study level of ocean island is still lower currently.The rock assembly and formation time of the ocean island are known poorly,which restrain to more understanding of the Paleo Tethys Ocean basin evolution.Based on previous studies,this paper completed detailed field geological surveys on the typical outcrops of Pairigang Ocean Island and Wenmulang Ocean Island in Tangga-Sumdo area.And the whole rock geochemistry and zircon U-Pb isotopic geochronology of the magmatic rocks in these outcrops have been carried out.The field geological survey shows that the Pairigang Ocean Island has the characteristics of"dual structure"of typical Ocean Island,which means the basalt basement of the lower layer,the limestone(marble)cover of the upper layer and the conglomerate of the slope facies of Ocean Island.There is interbedding phenomenon of marble and metamorphic basalt on Pagaduo Ocean Island outcrop.The Wenmulang Ocean Island has been seriously reconstructed,which missed the upper sedimentation cover of the Ocean Island,and the rock assembly has undergone metamorphism to varying degrees,including metamorphic gabbro and metamorphic basalt.Zircon U-Pb isotopic geochronology of magmatic rocks shows that the main body of Pairigang Ocean Island was formed in late period of early carboniferous to late carboniferous(330?307 Ma),and the main body of Wenmulang Ocean Island was formed in Middle Permian(269?261 Ma).Whole rock geochemical tests show that the basic rocks in Pairigang and Wenmulang all have high Ti O₂,P₂O₅ and(Na₂O+K₂O)contents,REE partition curve shows obvious enrichment of light REE,and trace element spider diagram shows obvious enrichment of Nb and Ta.However,the analysis shows that the source magma of these basic rocks is different.The source magma of the Paerigang basalt and diabase porphyrite may be from the spinel-garnet lherzolite mantle,while the source magma of the Wenmulang metamorphic gabbro and basalt may be from the garnet lherzolite mantle.Based on the field geological characteristics,geochemical characteristics,geochronological data and previous research results,this paper suggests that the Pairigang Ocean Island is similar to the“Cape Verde type”ocean island,which may have been formed on a thinner oceanic crust in late period of early carboniferous to the late carboniferous,and the Wenmulang Ocean Island is similar to the“Hawaiian type”ocean island,which may have been formed on a thicker oceanic crust in the Middle Permian.The study of the Ocean Island rock assembly in the Tangga-Songdo Paleo-Tethys suture zone shows that the Sumdo Paleo-Tethys ocean had developed mature ocean basin at least in the late period of early carboniferous,which should be opened before this time.In the early stage of the evolution,the Sumdo Paleo-Tethys Ocean expanded at a(ultra)slow speed,which resulted in a large age span of the formation of Pairigang Ocean Island.In the middle stage,the expansion of the Sumdo Paleo-Tethys Ocean speeded up,and the middle Permian Wenmulang Ocean Island was formed.