Zircon U-Pb age and geochemistry of Cretaceous metabasic rocks in Sewu area, Yarlung Zangbo River junction belt of Tibet, and their geological significance

In recent years, new Cretaceous metabasic rocks were found in the vicinity of Sewu, Qiuduojiang Township, Qusong County, southern Tibetan Plateau, by regional geological survey.The rocks are extensively distributed in layers in B rock section of Songre Formation in southern Yarlung Zangbo River suture zone.A series of LA-ICP-MS zircon U-Pb samples, geochemical samples and thin section samples were collected for testing on the basis of detailed field work in order to find out the formation age and geochemical characteristics of these metamorphic rocks.All LA-ICP-MS zircon U-Pb samples belong to the Early Cretaceous.The geochemical characteristics of the major elements show that the rocks belong to the low-K (tholeiite) basalt series.The rare earth elements show that the plagioclase in the original magmatic rocks was less or not crystallized, and there existed a relatively high degree of fractionation between light rare earth and heavy rare earth.The high field strength elements are relatively obvious; the large ion lithophile elements are deficient, and the negative anomalies of Sr and Ba are obvious.It is considered that the metabasic rocks might have been derived from the soft fluid mantle.According to the high field intensity element diagram of stable enrichment, magma was contaminated by the crust in the process of emplacement and formation.According to the analysis of tectonic environment, the metabasic rocks have MORB characteristics, which might have been related to the interaction between the hot spots and the mid-ocean ridges on the passive continental margin of the New Tethys Ocean.Due to the strong metamorphism and deformation of the rocks, the composition of the original rocks can not be determined under the microscope.Field observation shows that they were probably similar to the characteristics of the volcanic rocks.If they were formed by volcanic eruption, they would play an important role in promoting the study of the southern part of the Yarlung Zangbo Suture Zone, and therefore the Cretaceous strata could be established.