The Chronological And Geochemical Characteristics Of The Dark Enclave Of The Guposhan Granite In Guangxi And Its Geological Significances

Dark enclaves are widely developed in granitic rocks.They contain abundant information of crust-mantle interaction.Systematic geological and geochemical research on dark enclaves and their host surrounding rocks can effectively decipher the information of deep magmatism and reveal the origin,evolution and genetic mechanism of granite.Therefore,dark enclaves have become a key topic in the study of granitic rocks.Guposhan composite granite is distributed in the northeastern part of Guangxi,mainly in the Zhongshan,Fuchuan,and Hezhou regions.It is an important part of the Shiwandashan-Hangzhou Granite Belt(Shi-Hang Belt)in South China.Due to the large scale of the rock mass and its close relationship with non-ferrous and rare metal minerals,it occupies a very important position in the Nanling area.In order to explore the Mesozoic tectonic evolution of the northeastern Guangxi,this paper has been conducted detailed petrological,chronological and geochemical studies on the Guposhan composite granite and its dark enclaves.The main progresses are as follows:1.The chronological results show that the formation age of the dioritic dark enclaves in the Guposhan composite granite is 142±1 Ma,the host granites formed at 141±2 Ma,142±2 Ma and 141±2 Ma,for the first time to system identify Early Cretaceous dark enclaves in this area.2.Elemental geochemical analysis results show that the contents of SiO₂,MgO,and Fe₂O₃^T of the dioritic dark enclaves are 55.66%?56.72%,2.26%?2.30%and 10.43%?10.77%,respectively;the contents of SiO₂,MgO,and Fe₂O₃^T of the host granite sample are68.33%?68.67%,0.60%?0.64%and 3.73%?3.89%,respectively.The granites are characterized by high 10000*Ga/Al ratios and Zr+Nb+Ce+Y contents,similar with those of A-type granite.Both the host rock and the dark enclave are enriched in large ion lithophile elements and high field strength elements.3.The?_Nd(t)of the dioritic dark enclaves are-1.5?+1.4,with second-stage Nd model ages(T_DM2)of 0.82 Ga?1.06 Ga.The?_Nd(t)of the host granite is-2.9,with T_DM2 of 1.17 Ga.4.Based on the above data,it is indicated that the dioritic dark enclaves in the Guposhan granite are produced by magmatic mixing,and their formation is related to extension and thinning of the lithosphere in the Late Mesozoic.Together with regional geological data,it is suggested that the extension of the lithosphere in South China during the Mesozoic period were occurred by stages,which led to the development of multi-stage magmatism and mineralization in South China.