Petrogenesis of Dacun and Gulong plutons in southeast Guangxi: Constraints from geochemistry, zircon U-Pb ages and Hf isotope

The Dacun and Gulong plutons were found in the southwest orogenic belt of the Yangtze block and the Cathaysia block, and the pluton has important geological significance in analyzing the regional tectonic evolution of South China. In this paper, two quartz diorite samples were collected from Dacun pluton and Gulong pluton for LA-ICP-MS zircon U-Pb dating, respectively, which yielded U-Pb ages of 438±1Ma and 435±2Ma, indicating that the Dacun pluton and Gulong pluton were both formed in Caledonian orogeny. The data show that the Dacun pluton and Gulong pluton belong to I-type granite which has calc-alkaline and aluminous-peraluminous features. The Hf isotopic compositions of zircons show that the rocks have ε_Hf(t) values from 0 to +4, and T_DM2(Hf) model ages are mainly concentrated in the range of 1.15~1.45Ga, implying that the magma was derived from the basic lower crust material, and the lower crust was formed in the Mesoproterozoic. According to geochemical features of the Dacun and Gulong plutons, Hf isotopic composition, and the dark particles-bearing pluton, in combination with regional geological conditions, the authors hold that, after the intracontinental collision orogenic period, the Dacun and Gulong plutons were formed along with the local lithosphere stretching-thinning, the upwelling of high temperature mantle materials from the asthenosphere, and the partial melting of the basic lower crust in the Mesoproterozoic. In the process of melting, there existed mixture of the upwelling mantle-derived magma formed in the source area at different levels, thus leading to the generation of parental magma. And then the magma experienced a certain degree of differentiation and evolution, finally forming rocks through solidification.