Petrogenesis And Tectonic Setting Of The Late Cretaceous Magmatic Rocks In Songdo Area, Tibet

There has been much controversy about the subduction mechanism of the Neo-Tethyan ocean in Late Cretaceous period(88~68Ma)and whether the Neo-Tethyan ocean were closed at the end of Late Cretaceous(Ca.68Ma).In this paper,two Cretaceous magmatic rocks were sampled in the Songdo area of southern Tibet,including granodiorite,monzogranite,granite porphyry and diorite porphyrite.According to their petrology,LA-ICP-MS zircon U-Pb dating,the whole rock geochemical analysis and Hf isotope research,we investigated their petrogenesis and tectonic evolution history of Neo-Tethyan ocean during the Late Cretaceous.The zircon U-Pb dating results show that the granodiorite age is about 88 Ma at the early phase of Late Cretaceous,and the monzogranite,granite porphyry and diorite porphyrite are around 68 Ma at the last phase of Late Cretaceous.The granodiorite in early phase of Late Cretaceous the can be subdivided into normal granodiorite and giant porphyritic diorite based on the presence or absence of giant phenocrysts.Both of them have high Sr,low Y,Yb and high Sr/Y and La/Yb ratios,which are typical characteristics of the adakites.The results show that the adakitic granodiorite in Songdo area are formed by partial melting of ocean crust,and later different degree of ancient lower crust material were added,which resulted in the formation of normal granodiorite and giant porphyry granodiorite.The monzogranite and granite porphyry in the last phase of Late Cretaceous can be identified as I-type granites based on geochemical characteristics.Their magma source are all basaltic lower crust,and the granite porphyry is closer to the ancient lower crust than the monzogranite.The diorite porphyrite in the last phase of Late Cretaceous have same characteristics to high Mg andesite(i.e.,high MgO contents and Mg# ratios).In addition,according to the geochemical characteristics,it is further confirmed that it is sanukitoid in high-magnesian andesite and formed in direct partial melting of mantle peridotite induced by subducted sediment.Based on the study of two periods of igneous rocks in the Songdo area,and combined with previous regional research,we draw a conclusion that the Neo-Tethyan ocean began to mid-ocean ridge subduct and formed a large mass of adakites and charnockite in the southern Gandese,and the mid-ocean ridge subduction continued until the last phase of Late Cretaceous(Ca.68Ma)ended with the formation of sanukitoid and normal arc igneous rocks.There may be flat suduction during the mid-ocean ridge subduction.At the same time,the study of adakites and sanukitoid rocks in the Late Cretaceous in Songdo area provided important evidence for the growth of the crust in Lhasa.