Petrogenesis of the ca. 820-810 Ma felsic volcanic rocks in the Bikou Group: Implications for the tectonic setting of the western margin of the Yangtze Block

The petrogenesis of Neoproterozoic igneous rocks at the northwestern margin of the Yangtze Block and their tectonic setting are controversial. Here we present whole-rock major and trace elements and Nd-Hf isotopic data for felsic volcanic rocks from the Bikou Group. These ca. 820-810 Ma volcanic rocks consist of dacite and rhyolite with SiO2 ranging from 64.4 to 77.0 wt%. Most rocks show decreasing P2O5 with increasing SiO2 and Th and have I-type granite affinities (metaluminous volcanic rocks). However, four rhyolites from the lower part of the Bikou Group show A-type granite affinities (alkaline rhyolites), characterized by high 10,000*Ga/Al and FeOt/(FeOt + MgO) ratios and high (Zr + Nb + Ce + Y) contents. The felsic volcanic rocks commonly have relatively high Zr saturation temperature (up to 960 degrees C) and most of the felsic volcanic rocks have epsilon(Nd) (t) and epsilon(Hf) (t) values lower than the contemporaneous basalts in the Bikou Group, but higher than the basement rocks in the study area. Among them, four alkaline rhyolites have epsilon(Nd) (t) and epsilon(Hf) (t) values higher than the metaluminous volcanic rocks in the lower Bikou Group. We propose that the metaluminous volcanic rocks were likely produced by partial melting of newly underplated basaltic crust, with involvement of different proportions of basement rocks, whereas a two-stage model is proposed for the formation of the alkaline rhyolites, i.e. partial melting of newly underplated basaltic crust forming early granitoids, followed by low pressure partial melting of these granitoids to generate the alkaline rhyolitic magma. Combined with previous studies, the bimodal occurrence of volcanic rocks within the Bikou Group, and the A-type granite affinity of some rhyolites, as well as the presence of ocean island (OIB)-type basalts in the upper part of Bikou Group, suggest an extensional setting. This is consistent with opening of the Kangdian rift basin to the southwest at this time. We thus propose that the ca. 820-810 Ma Bikou bimodal volcanic rocks formed in response to the breakup of the Rodinia supercontinent.