Geochronology, geochemistry, and petrogenesis of the late Mesozoic Luoyang volcanics: Implications for the geodynamic evolution of the Zhejiang-Fujian region, SE China

The Zhejiang-Fujian region, situated along the southeastern coast of China and in the eastern part of the Cathaysia Block, is characterized by large volumes of late Mesozoic granitoid volcanics. We present new zircon U-Pb ages, whole-rock geochemistry, and SrNdHf-isotope data for 3 rhyolite porphyries from the Luoyang region (LY-1, LY-2, and LY-3). We identify 2 magmatic events. In Stage 1 (146Ma), rhyolite porphyry LY-1 was produced by the partial melting of Proterozoic Mayuan Group amphibolites and metasediments. In Stage 2 (130-126Ma), rhyolite porphyries LY-2 and LY-3 were generated by the continued melting of amphibolites but with only a minor contribution from metasedimentary rocks. The fractionation of plagioclase, biotite, titanite, and allanite (degree of fractionation 0.7-0.5) from an LY-2 parent magma produced LY-3. Our new data, together with other published data, indicate that minor volcanism, related to the initial northwestward subduction of the Paleo-Pacific Plate beneath SE China, occurred in a continental arc setting in the Zhejiang-Fujian coastal region from 160 to 140Ma but that the major period of volcanism took place from 140 to 120Ma in a back-arc setting and occurred because of rollback of the subducting slab.