Trachytic dacitic tuffs in Caijiachong, Qujing basin, Yunnan Province: Evidence for a strong activity of the Qujing deep fault within the Yangtze Craton

As a symbol of the influence extent of the deep faults within the Yangtze Craton by the southeastward escape of the Tibetan Plateau, the volcanic rocks in the Qujing basin are of great research importance. By field observing and sampling, the Cenozoic volcanic rocks were found for the first time in this area. it is confirmed under the microscope that the volcanic rocks are trachytic (ultrapotassic) dacitic tuffs with phenocrystals of orthoclase and biotite, and K and Na of the volcanic rocks may be partly leached by hydrolysis, leading to the falling of volcanic rocks in the region of granodiorite on the volcanic rock classification diagram, as also shown by major and trace elements data. K/Ar dating result shows that the volcanic rock activity lasted for about 3.5Ma, i.e., from 44.93±0.8Ma to 48.39±0.9Ma, suggesting the products of the Cenozoic Middle Eocene (Lutetian, 48.6±0.2~40.4±0.2Ma) volcanic activities. The volcanic rocks in Qujing basin show that the pull-apart of the Qujing deep fault not only formed a series of basins such as Qujing, Luliang, Mile and Kaiyuan, but also brought about the extrusion of the Cenozoic Eocene Lutetian Caijiachong Formation (E2c) volcanic rocks, which implies a strong activity of the Qujing deep fault; like the Xiaojiang deep fault which was responsible for the intrusion in Boka, Dongchuan, the Qujing deep fault was an utracrustal fault during Cenozoic. The volcanic rocks are significantly rich in such elements as Cd, Pb and Zn, indicating that the potentials of Au, Pb-Zn deposits related to the volcanic rocks are worthy of attention. In combination with the finding of the homochromous alkaline intrusion and gold deposits in Boka of the Dongchuan deep fault, the Qujing deep fault, which is closely related to the seismic activities, hot spring resources, agricultural and ecological environment, oil-gas resources, and solid minerals, deserves thorough investigation.