THE GEOCHRONOLOGY AND ITS TECTONIC SIGNIFICANCE OF QINYANG-JIUHUASHAN COMPLEX GRANITE, ANHUI PROVINCE, CHINA

Jiuhuashan is located in the northern margin of the Yangtze Block. Qingyang-Jiuhuashan complex granite is a multi-stage complex batholith, and could be divided into two stages, Qingyang granite and Jiuhuashang granite, respectively. Qingyang granite is granodiorite and monzogranite. Jiuhuashan granite is K-feldspar granite with miarolitic structure. Jiuhuashan granite intruded into the Qingyang granite.

Eighteen spots were analysed for granodiorite sample. The ²⁰⁶Pb/²³⁸U ages of 18 plots range from 140 to 147 Ma, with a weighted mean age of 142.3±1.0 Ma. Twenty seven plots were analysed for monzogranite with the ages of 139~149 Ma. Its weighted mean age is 143.6±1.5 Ma. It indicated that the Qingyang granite intruded in the early stage of Early Cretaceous.

Jiuhuashan K-feldspar granite analysed eighteen spots. The ²⁰⁶Pb/²³⁸U ages of 18 plots range from 123 to 147 Ma, with a weighted mean age of 126.6±1.6 Ma. It means that Jiuhuashan K-feldspar granite intruded in the late stage of Early Cretaceous.

The Qingyang granodiorite had the lower SiO₂ and Zr contents, and the ACNK values were less than 1.0. It has the characteristics of I-type granite. Monzogranite had higher SiO₂ contents, DI values and (K₂O+Na₂O)/CaO ratios. It indicates that monzogranite had higher differentiation evolution degree, and belongs to high fractionated granite. Jiuhuashan granite had the high HSFE composition and 10000×Ga/Al values, and it was also depleted in Sr and Ba. The geochemical characteristics were similar as A-type granites.

The Cretaceous granites in South Anhui could be divided into two age groups, 136~145 Ma and 120~136 Ma, respectively. Two stages magmatic often formed a series of multi-stage complex batholith. The zircon LA-ICP-MS U-Pb dating indicated that the Qingyang granite was formed in the early stage, and the Jiuhuashan granite was formed in the late stage. During the early stage of Early Cretaceous, a series of I-type granites formed, and indicated the compressional tectonic environment. Subsequently, South Anhui evolved into the extensional tectonic environment. It was also be indicated by the contemporaneous A-type granites. TheQingyang-Jiuhuashanmulti-stage complex granites in South Anhui indicated a transformation of tectonic setting from compressional to extensional during Early Cretaceous.