Yanshanian thermo-upwelling extension and palaeo-hydrothermal system in southeast Guangdong: A case study of Enping-Xinfeng detachment fault

NE-SW trending Yanshanian detachment faults are widely spread in southeast Guangdong. The authors carried out geological survey and physical and chemical test of Enping-Xinfeng detachment fault, with emphasis placed on the mylonite in the ductile shear zone below this detachment fault and the silicified rocks related to the high angle normal fault on the hanging wall of the detachment fault. The authors believe that the initiation age of the upwelling-extension structure was in early Yanshanian, the deformation temperatures of the mylonite which had the kinematic property of ductile rheology in the ductile shear zone were about 350℃, and the mylonite was brought onto the surface during later tectonic uplift. The microthermometric study of the fluid inclusions in the silicified rocks indicates that the homogeneous temperatures of the fluid inclusions are between 155℃ and 326℃. The salinities are between 2.74% and 21.61% NaCleqv, with an average of 11.17% NaCleqv, the temperatures of the silicon-contained hot liquid are about 241℃, and the silicon-contained hot liquid is believed to be NaCl-H₂O solution with rich silicon and carbon dioxide according to Laser-Raman microspectroscopy. Compositions of H-O stable isotope imply that the paleo-hydrothermal fluids were derived from the mixing of magmatic water and rain water in the study area. The silicified rocks distributed along the strike of the normal fault possibly are the significant indications of the paleo-hydrothermal system in Yanshanian. The thermo-upwelling extension system and the deep and shallow integrated palaeo-hydrothermal system in the late Yanshanian are significant for the understanding of the type, the mechanism and the distribution regularities of the modern geothermal system in southeast Guangdong.