Petrophysical studies on rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt, Central China: implications for the composition and delamination of the lower crust

Abstract

The Dabie–Sulu ultrahigh-pressure (UHP) metamorphic belt of Central China represents a zone in which upper and lower continental crust has been subducted and then rapidly exhumed from the mantle back into the crust. About 30 UHP rock samples representing major lithologies were collected from surface exposures and their physical properties were measured in the laboratory at pressures up to 600 MPa (room temperature) and temperatures up to 600°C (600 MPa confining pressure). The experimentally determined data include compressional ( V p) and shear wave velocities ( V s), velocity anisotropy (shear wave splitting), density, and intrinsic pressure and temperature derivatives of V p and V s. Using a regional geotherm, velocity–depth profiles were calculated for the different lithologies spanning compositions from felsic through intermediate to mafic, and metamorphic grades up to granulite and eclogite facies. The velocity data, in particular the Poisson ratio ( V p/ V s ratio), were used to interpret lithologically the seismic models evaluated from seismic refraction data. The seismic velocity profiles reveal a four-layer structure (upper, middle, upper-lower and lowermost crust) with an average thickness of 34 km. From our results, we infer that a mixture of about 90% felsic gneiss with variable amounts of high V p amphibolite/gabbro constitute the middle crust. Intermediate granulite and mafic granulite fit both the V p and Poisson ratio of the upper-lower and lowermost crust, respectively. From the combined V p and Poisson ratio ( V p/ V s) data, we conclude that eclogite is not a major constituent of the present Dabie deep crust. Combining the laboratory-derived in-situ seismic data ( V p and Poisson ratio) with refraction seismic data, along with evidence from geological and geochemical investigations, suggests that lower crustal delamination played an important role in the modification of the East China crust.