Abstract:
The intrusive rocks in Tongling area can be separated into a high-K calc-alkaline series and a shoshonitic series. The Jiaochong orefield is one of the seven orefields in Tongling area. The problems as to whether there are two series of intrusive rocks in the Jiaochong orefield or not and whether they have the same age and genesis as compared with the other intrusions in Tongling area are not clear. Therefore, this research was focused on the study of petrography, geochemistry, LA-ICPMS zircon U-Pb dating and in situ Hf isotopic analysis. It turns out that there are two series of intrusive rocks in the Jiaochong orefield. The high-K calc-alkaline series mainly contains quartz monzodiorite and granodiorite, and the shoshonitic series consists of pyroxene monzodiorite. The results of LA-ICPMS zircon U-Pb dating show that the age of the high-K calc-alkaline series in the Jiaochong orefield is about 142 Ma, identical with the age of the other orefields in Tongling area; the age of the shoshonitic series is around 136 Ma, which is younger than that of the other intrusive bodies in Tongling area. In general, the two series of intrusions indicate the features of the multiphase emplacement. Furthermore, lots of zircons having old inherited core in the high-K calc-alkaline series reveal the existence of the materials of old crust in the magma-forming process. Combined with the characteristics of the intrusive rocks in Tongling area and the geochemical features of Jiaochong intrusions, it is obvious that the genesis of the two series of intrusions in the Jiaochong orefield is similar to that of other intrusions in Tongling area. In other word, the intrusive rocks of the shoshonitic series were formed by the fractional crystallization of the alkali-rich basic magma that originated from the deep magma chamber near the Moho. In the meanwhile, the high-K calc-alkaline series is the product of the magma mixing between the felsic magma derived from the shallow magma chamber and the magma that suffered from the fractional crystallization and originated from the deep magma chamber.