The Newly Discovered Cambrian Gabbro-Diorite In Bange,Tibet And Their Tectonic Implications

The tectonic structure of Bange area,part of north Lhasa ternane,is located in the south of the Bangong-Nujiang suture zone.Cambrian magmatism in the northern Lhasa terrane has not been reported,which limits the understanding of the origin and tectonic evolution of Tibetan Plateau.To determine the Pan-African-Early Paleozoic tectono-thermal events and the tectonic evolution of the Lhasa terrane,detailed field geological survey,zircon U-Pb dating and petrographic and geochemical studies of the gabbro-diorite,which is firstly discovered in the Bange area,have been performed.These newly obtained data are used to explore the age,nature and genesis of the rock.It's of great significance for discussing the evolution of time and space,geodynamic settings of the North Lhasa terrane.The gabbro-diorite with medium SiO₂?51.03%?53.97%?and high K₂O?1.26%?1.46%?,Mg^#?65?69?value,belongs to the high-K calc-alkaline series.They are rich in large ion lithophilic elements?e.g.,Rb and Ba?and light rare earth elements?e.g.,La and Ce?,and are depleted in high-field strength elements?e.g.,Nb,Ta,Zr,Hf,and Ti?,similar to the Andean island arc magmatic rocks.LA-ICP-MS zircon U-Pb dating demonstrates that the gabbro-diorite formed at512±3 Ma,which indicates that it formed during the Cambrian period and provides reliable chronological evidence for the existence of Early Paleozoic magmatic events in the northern Lhasa terrane.Comprehensive analysis shows that the rock is a product of the Andean-type orogeny caused by the Proto-Tethys Ocean subducted southward in the Early Paleozoic,and geochemical characteristics of the rock is interpreted as resulting from partial melting of subduction sediments metasomatized with the overlying mantle wedge.The intrusion is located on the margin of Gondwana.It is confirmed that the northern Lhasa terrane is part of northern margin of the Gondwana land.The results provide a new window for further study of the tectonic evolution of the Tibetan Plateau.In addition,the age data shows that Pan-African magmatic activities exist in the Bange area,south of the Bang-Nu suture zone,which confirms that the Northern Lhasa terrane to the south of Bangong-Nujiang suture zone is the basement rock formed in Pan-African period.