ORIGIN OF THE MIOCENE BUGASI GROUP VOLCANIC ROCKS IN THE CUOQIN COUNTY, WESTERN TIBETAN PLATEAU

Abstract

It is generally believed that Cenozoic potassic and ultrapotassic volcanic rocks in Tibetan plateau are derived from a mantle source, but recent studies showed that some potassic rocks may also come from a lower crust. The Miocene Bugasi Group volcanic rocks are a potassic to ultrapotassic volcanic suite distributed in the Cuoqin County, western Tibetan plateau. They are divided into two subgroups: the intermediate-acid group and the intermediate-basic group on the base of their geochemical composition. The both belong to shoshonitic series. Volcanic rocks of the intermediate-acid group have some compositional features similar to an adakitic melt, hinting that they were probably generated by partial melting of the relatively K-rich mafic lower crust below the Lhasa block. However, volcanic rocks of the intermediate-basic group were probably derived from a mantle that likely contains phlogopite, or generated via fractionation of a basalt-magma from this mantle. The plateau age (15.5Ma) of a intermediate-acid rock is determined by the Ar-Ar isotope dating, confirming that Bugasi Group volcanic rocks erupted in the late Miocene. Base on age, chemical composition, regional tectonic and magmatic association, we suggest that formation of Bugasi Group volcanic rocks was correlation with the N-S direction's grabens produced by tectonic extension in the southern Tibetan plateau about 15~20Ma.