Early Mesozoic magma-induced mid-crustal flow and high-temperature metamorphism in the Yunkai massif, South China

Abstract

Early Mesozoic evolution and geodynamic processes of the South China Block remain controversial and its rheology is not well understood. The granites and migmatites exposed in the Yunkai massif preserve important records of middle crust mechanical behavior during Indosinian orogeny. They show a significant strength decreasing of the crust during this period, further affecting the rheological behavior of the entire lithosphere in South China. In order to better understand the Early Mesozoic evolution and geodynamic processes of the South China Block, we conducted detailed studies on structural geology, multi-minerals geochronology and P-T estimate on Mesozoic migmatites and granitoids in the Yunkai massif. Three tectonic/metamorphic events are identified. The main one corresponds to the development of a regional NE-SW extension. It constitutes a dome-like structure with amphibolite-facies metamorphism during 243–237 Ma (D1), followed by a rapid cooling to ~300 °C within ~20 Ma. Conventional mineral thermobarometers constrain the D1 metamorphism at 665–692 °C and 6.1–6.2 kbar, indicating an upper amphibolite – granulite facies metamorphism. Based on the phase equilibrium modelling, the grossular component (XGrs=0.12–0.13) in the garnet core, Ti content (XTi=0.15–0.17) of biotite and An value (An=0.84–0.86) of plagioclase in the matrix define a peak P–T condition at 780–810 °C and 6.8–7.0 kbar within the stability field of the observed assemblage, indicating a mid-crustal high-temperature metamorphism. Combined with the field structural relationships, geochemical similarity and inherited zircons, the protoliths of the D1 amphibolite-facies metamorphism are of Silurian (<427 Ma) greywacke and magmatic rocks. Due to the strong reworking during the Early Mesozoic and only a few of these previous records were preserved in the metamorphic rocks by the zircon cores and several monazites. Thereafter, the D1-deformed granites and migmatites are crosscut by Late Jurassic (~157 Ma) greenschist-facies NE-SW dextral strike-slip faults (D2). Both of the above rock units are reworked by a later local vertical shearing (D3). Combined with published age data, our results documented a late local metamorphism of 10 Ma with respect to the regional magmatism, suggesting a magma-induced high temperature metamorphism. We propose that this magma-induced mid-crustal flow and high-temperature metamorphism are in response to the Indo-China collision and the westward subduction of the Paleo-Pacific Plate. Our new results provide new insights on partial melting and its implications on mid-crust rheological behavior during orogenic processes.