The Reconfirmation of Age and Tectonic Setting of the Volcanic Rocks of Yingyangguan Group in the Eastern Guangxi: Constraints on the Structural Pattern of the Southwestern Segment of Qinzhou–Hangzhou Joint Belt

Yingyangguan Group is widely distributed in the junction area of the Hunan, Guangdong and Guangxi Provinces. It consists dominantly of the green schist facies metamorphic spilite,(quartz-) keratophyre and related to volcanic clastic rocks, with small quantities of fine clastic rocks and carbonate rocks. Previously it had been thought that this group formed in a continental rift environment under the background of Rodinia supercontinent breakup during the Neoproterozoic time. The combined petrology, geochemistry and zircon LA-ICP-MS U-Pb dating study reveal that the volcanic rocks of Yingyangguan Group belong primarily to basic-intermediate volcanic(-clastic) rocks, and petrochemically are pronounced enrichment in large-ion incompatible elements(LILEs, e.g., U, Th, Ba, K and Rb) and LREEs and pronounced depletion in high field strength elements(HFSEs, e.g., Nb, Ta, P and Ti) and HREEs. Such signatures display geochemical characteristics of typical subduction-related arc-back arc basin volcanic rocks. A metamorphic keratophyre sample of Yingyangguan Group yielded a zircon LA-ICPMS U-Pb concordia age of(415.1±2.1) Ma(n=13, MSWD=1.8), indicate that it is to be the product of Marine volcanic eruption during Caledonian period. Together with early Paleozoic MORB-type and arc-type metabasic volcanic rocks in the northern margin of Yunkai block, appears to support a development of an early Paleozoic oceanic basin within the southwestern segment of the joint belt between the Yangtze and Cathaysian blocks(so-called Qinzhou-Hangzhou joint belt). Therefore, we conclude that Yingyangguan arc-back arc basin volcanic rocks were likely an important record of the oceanic subduction and subsequent continental collision in the southwestern segment of Qinzhou-Hangzhou joint belt during Early Paleozoic time, and Caledonian structural pattern of the southwestern segment of Qinzhou-Hangzhou joint belt is a subduction-accretion orogenic belt and not an intracontinental orogenic belt.