The Late Jurassic extensional event in the central part of the South China Block - evidence from the Laoshan'ao shear zone and Xiangdong Tungsten deposit (Hunan, SE China)

摘要

The Late Mesozoic tectonics strongly reworked the tectonic framework of East Asia. In the South China Block (SCB), the major Late Mesozoic phenomena are featured by numerous magmatic activities, (half-) graben basins opening, and abundant ore deposits formation. The intrinsic relationships of these phenomena with the tectonic regime are still hotly debated, partly due to the lack of structural data. To advance the understanding of these issues, we conducted a detailed structural analysis including field and microscopic observations as well as an Anisotropy of Magnetic Susceptibility (AMS) investigation in the Laoshan'ao shear zone (LSA), which is the northern branch of the major Chaling-Chenzhou-Linwu Fault (CCLF) in the SCB. The new data enable us to reach the following conclusions: the evolution of the LSA can be divided into three deformation phases. D-1 is a NW-SE-stretching event featured by the SE-dipping normal fault with a top-to-the-SE shear sense, coeval with the emplacement of the 154Ma Batuan syn-tectonic pluton that leads to the development of the Xiangdong tungsten deposit and the opening of the Cretaceous Chaling basin; D-2 is a NE-SW-striking dextral strike-slip event that dislocated the ore veins formed during the D-1 event; D-3 is a NW-directed thrusting event that cross-cut the previous gneissic foliation. D-1 is the major event of the LSA shear zone, interpreted as the reactivation of the CCLF in response to the Late Mesozoic tectonics in the SCB, and indicates a NW-SE extensional regime since the Late Jurassic in the study area. This study also provides an example for an ore-forming process controlled by both the hydrothermal fluid coming from a syn-tectonic granite and the accommodation of tension gashes opened by the regional extensional event.