Geochronology And Geochemistry Of Magmatic Rocks From Nuba Area In Southern Lhasa Terrane,Tibet

As part of the East Gondwana continent,the Lhasa Terrane has experienced complex evolutionary history at different stages since the Paleozoic,which reflects the geological evolution of the Neo-Tethys subduction.Especially the late Mesozoic magmatisms,which are widely developed in the southern Lhasa terrane,can help us accurately characterize and deeply understand the deep process of the conversion of oceanic subduction to continental collision.A large number of late Cretaceous magmatic rocks developed in the southern Lhasa terrane have recorded important information of the process since ocean subduction to continental collision.In this paper,a systematic studies of petrology,zircon U-Pb geochronology,Hf isotopes,whole rock elements and Sr-Nd isotopic geochemical research has been carried out,attempting to reveal the deep dynamic mechanism by analyzing the nature of magma source region and petrogenesis,so as to provide more evidence for the controversial tectono-magmatic evolution at the end of Mesozoic in southern Lhasa Terrane.There have been granodiorite,host granites and dioritic enclaves exposed in the Nuba area of southern Lhasa Terrane.LA-ICP-MS zircon U-Pb dating results show that the Nuba pluton was intruded in the Late Cretaceous,the age of granodiorite is90.3±0.8 Ma,the age of the host granite are 97.3±0.6?93±0.8 Ma,and the age of the dioritic enclavesis 93.90±1.0?90.5±1.1 Ma?The host granite has high SiO₂?68.9?69.18%?,K₂O?3.13?3.16%?content,high Sr?503?641 ppm?,Sr/Y ratio?60.00?80.62?and low Yb?0.85?0.89 ppm?,Y?7.76?8.39 ppm?content,The right-inclined rare earth element distribution form,with weak positive anomaly of Eu,enrichment of K?La?Pb?Hf and other elements,loss of Ba?Nb?Ta and other elements,belongs to quasi-aluminous high potassium calcium alkaline granite.The dark diorite inclusions are rich in large ion philophile elements and deficient in high field strength elements,which has relatively flatter rare earth element distribution pattern,belonging to the calcareous-alkaline series of metaaluminous rocks.The major elements of granodiorites are similar to the host granites which have relatively flatter rare earth element distribution pattern,with negative of Eu.Isotope studies have shown that the magmatic rocks in the Nuba area have the characteristics of a depleted source region.The rock origin can be explained by the intrusion of the mantle-derived magma underplated the lower crust,causing partially melting of the lower crust rocks and magmatic mixing.In this paper,it believes that the formation of magmatic rocks in the Nuba area is controlled by the subduction of the Neo-Tethys mid-ridge.