Geochronology, Geochemistry and Tectonic Significance of the Early Cretaceous Volcanic Rocks in the Bianba Area, Tibet

An important tectonic switch of the Bangong-Nujiang Ocean took place in the Cretaceous, thus, the volcanic rocks of this period are critical to decode the tectonic evolution of the Bangong-Nujiang Ocean. Petrographical, geochronological and geochemical investigations were conducted on the Cretaceous volcanic rocks in the Bianba area, Tibet. From the bottom to the top, the volcanic rocks are mainly composed of basaltic andesite, andesite, dacitic and rhyolite. U-Pb dating of zircon grains from these rocks yielded ages of 116 Ma to 120 Ma. Geochemical results show that the intermediate-basic volcanic rocks have SiO2 contents of 54.07%-62.63% and Mg# values of 48-67, with a linear relationship between SiO2 and Mg#. The acidic volcanic rocks have SiO2 contents of 72.64%-75.41% and Mg# values of 40-50. The overall volcanic rock samples show enrichment of LREE and large-ion lithophile elements such as K, Rb, Th and depletion of HREE as well as some high field strength elements such as Nb, Ta, P, Ti. There are no significant linear relationships between CaO and δEu, and SiO2 and Na2O. Thus, these intermediate-basic and acidic volcanic rocks from the Bianba area may have different magma sources. The intermediate-basic volcanic rocks are likely results of partial melting of the ancient lithospheric mantle that metasomatized by subduction-derived fluid and underwent fractional crystallization of mafic minerals in the early stage of the closure of the Bangong-Nujiang Ocean. In contrast, the acid volcanic rocks were likely derived from partial melting of the curst in the late stage. © 2022, Science Press. All right reserved.