Emplacement and deformation of Shigujian syntectonic granite in central part of the Dabie orogen: Implications for tectonic regime transformation

Abstract

The Shigujian pluton is a gneissic quartz monzonite located in Tiantangzhai area in central part of the Dabie orogen. Anisotropy of magnetic susceptibility (AMS) data show that most magnetic foliations dip steeply to southeast. About 85% of sampling points dip from 40° to 90°. Magnetic foliations are generally parallel to the foliations measured in the field. The pluton has NWW-SEE trending lineations in the southeast and NE-SW trending lineations in central part and north, but the lineations plunge to SW in central part and to NE in the north. All plunges are moderate. The anisotropy degree (P) is between 1.065 and 1.532 and the shape parameter (T) is between 0.005 and 0.694. A Flinn diagram of the magnetic fabrics shows that the value of K is less than 1. The analysis of AMS suggests that the pluton was emplaced and deformed under a SE-NW compressional stress regime. The analysis of quartz C-axis fabrics indicates that the pluton was deformed under compressional stress and deformation temperatures range from 400 to 500°C. Microstructures indicate that the pluton is deformed in near solidus conditions and the pluton is a synkinematic intrusion. The emplacement of the Shigujian granite is inferred to have taken place syntectonically. The zircon U-Pb dating of the granite suggests that the pluton was intruded at 141±2.3 Ma. By synthesizing all data, it seems that the Shigujian pluton was emplaced in a compressional environment and the transformation time of the Dabie orogen from compression to extension took place after 141 Ma. The structural evolution of the Dabie orogen was controlled by the Pacific tectonic domain when the Shigujian pluton was emplaced, whereas the adjacent Tiantangzhai complex massif is the result of an extensional environment.