Definition of Neoproterozoic bimodal volcanic rocks in the south of Hangzhou and its tectonic significance

Abstract：Volcanic rocks of the Neoproterozoic Qingbaikouan Shangshu Formation in southern Hangzhou are composed of basic and acid rocks, while rocks of intermediate and intermediate-acid compositions with 57%-68% SiO2 are lacking. They form a bimodal volcanic rock association. There are evident differences in trace elements and rare earth elements (REE) between the basic and acid volcanic rocks, which indicates that the acid volcanic rocks were not produced by fractional crystallization of basic magma but formed by melting of different sources. The basic volcanic rocks originated from the less-depleted mantle or from magma of the depleted mantle contaminated with crustal material. The acid volcanic rocks mainly formed by different degrees of partial fusion of upper crust in an intracontinental to continental-margin extensional environment. The formation of the bimodal volcanic rocks in the Shangshu Formation is a petrological record of the breakup event of the supercontinent Rodinia in South China, which reveals that the breakup of the supercontinent Rodinia in South China started in the Qingbaikouan Period.