The Early Cretaceous evolution of SE China: Insights from the Changle–Nan'ao Metamorphic Belt

Abstract

The Mesozoic geology in SE China is characterized by widespread Jurassic–Cretaceous magmatism. There appears, however, an episode of “magmatic quiescence” at ca. 130–110Ma in terms of increasing high-precision isotopic age data. The causes and regional tectonic significance of this magmatic quiescence are unclear. In this paper, we report integrated in situ U–Pb, Hf and O isotope analyses of zircons from four mica schist samples along Changle–Nan'ao Metamorphic Belt in SE China coast, and use these data to discuss the provenances of these metasedimentary rocks, the timing of the regional metamorphism and their implications for the regional tectonic evolution. The majority of zircons from the mica schists are euhedral to anhedral crystals showing clear concentric zoning under cathodoluminescence (CL) images. Metamorphic zircon rims with bright luminescence are overgrown on the some of the euhedral to anhedral zoned zircons. The zoned zircons from all mica schist samples are homogeneous in U–Pb age zircon ages (between 131 and 148Ma, peaking at 141Ma), O isotopes (δ18O=5.6±0.8‰, 2SD, n=106), and, to the less extent, Hf isotopes (εHf(T)=−4.7 to −1.4 for the Dongshan mica schists, and −1.4 to 2.2 for the Putian mica schists). They are most likely detrital zircons sourced from the regional Early Cretaceous felsic volcanic rocks (the Nanyuan Formation) and/or coeval intrusions in Fujian Province. The sedimentary protoliths of these mica schists were likely deposited in fore-arc basins. The metamorphic zircon overgrowths have relatively low Th/U ratios; they are dated at ~130–100Ma, with two peaks at ~125Ma and ~108Ma. The first metamorphic age of ~125Ma is coincident with the timing of regional Early Cretaceous magmatic quiescence, while the second metamorphic age of ~108Ma is consistent with the timing of reinitiation of regional Cretaceous magmatism. We use a model of collision between the West Philippines and SE China blocks to interpret our new results. According to this model, the intensive Early Cretaceous (ca. 140–130Ma) magmatism along the coastal region of SE China was attributed to the subduction of the Paleo-Pacific plate beneath SE China. Collision between the West Philippines and SE China blocks at ~130–120Ma resulted in the regional “magmatic quiescence” and formation of the Changle–Nan'ao Metamorphic Belt. Post-collisional extension triggered the reinitiation of magmatism associated with amphibolite-facies metamorphism at ~110Ma.