Geochronology and geochemistry of Early Mesozoic acid volcanic rocks from Southwest Guangxi: Constraints on tectonic evolution of the southwestern segment of Qinzhou-Hangzhou joint belt

Early Mesozoic acid volcanic rocks are exposed dominantly in the southwestern segment of Qinzhou-Hangzhou joint belt, and are mainly distributed in both sides of the Shiwandashan Mesozoic-Cenozoic basin. The volcanic rocks of Lower and Middle Triassic Beisi Formation are exposed in northwest side of the basin, which consists of alternating volcanic succession of felsic lavas (basaltic andesite-dacite-rhyolite) interlayered with pyroclastic rocks (dominantly agglomerate lava, breccia lava, tuffolava, breccia ignimbrites, ignimbrites) and a dacite sample yielded a zircon SHRIMP U-Pb concordia age of 246±2Ma. The volcanic rocks of Triassic Banba Formation are exposed in southeast side of the basin, which composed mainly of rhyolite interlayered perlite, tuffolava, agglomerate lava and rhyolite tuff and a rhyolite sample yielded a zircon SHRIMP U-Pb concordia age of 250±2Ma. Major- and trace-elements as well as Sr-Nd isotope geochemistry researches show: The volcanic rock association belongs to a typical peraluminous high-K calc-alkaline series and are characterized by relative enrichment in LILEs (e.g., K, U, Ba, Rb and Th) and LREEs and pronounced depletion in HFSEs (e.g., Nb, Ta, P and Ti) and HREEs. They also have relatively high (87Sr/86Sr)i (0.713929~0.722178) and low eNd(t)(-10.33~-9.02). Such signatures display geochemical characteristics of typical subduction-related arc volcanic rocks. Together with early Paleozoic MORB-type metabasic volcanic rocks and Permian E-MORB type basalts were formed in an back-arc spreading center setting in the region, appears to support a development of a Paleozoic oceanic basin within the southwestern segment of the joint belt between the Yangtze and Cathaysian blocks (so-called Qinzhou-Hangzhou joint belt), which suggests that the oceanic basin had not been closed until the Indosinian orogeny at middle Triassic resulted in the final collision between the Yangtze and Cathaysian blocks.