The Diagenesis Research Of Guniujiang Composite Pluton In Guichi Region, Anhui Province

There had happened large-scale late Mesozoci magmatism, and W, Mo and Au polymetallic mineralization in southern Anhui Province. It is meaningful that the massive magmatism appeared as a series of zonal composite plution, such as Huangshan-Taiping, Qingyang-Jiuhuashan, Guniujiang composite pluton. The composite plutons are made of Granite and Granodiorite in rock association. Although certain results on the composite pluton have been abtained, its petrogenesis and tectonic setting remain controversial.In this study, petrology, element geochemistry, zircon chronology and Hf isotope of Guniujiang composite plution (Chengan Granodiorite and Dalishan granite) were systematically investigated in the Guichi region, respectively. Combining previous research results, we obtain the main conclusions as follow:1. Magma mixing was likely to effect the formation of Guniujiang composite pluton, on the basis of petrology characteristics of Guniujiang intrusive rocks.2. The Dalishan granite is A-type granite with the characteristics of alkaline and metaluminous. The very low Mg#values (0.14~0.16) and relatively low Cr concentration (Cr=10×10-6), high Yb (7.08×10-6~9.02×10-6), Y (78.7×10-6~90.8×10-6) concentrations and Th/U ratios (5.17~7.79) show that the partial melting of old continental crust materials might have been the main mechanism of the Guniujiang granite. The Guniujiang granite can be classified into A2subtype, which is usually believe to be formed under a post-collision extensional tectonic setting.3. The Chengan Granodiorite is a high calc-alkaline and peraluminous rock, which is similar with the adakitic rocks in geochemistry characteristic. The high K2O/Na2O (0.89~1.55), La/Nb (2.9~3.3), Nb/Ta (11.1~12.3) and Th/U ratios (5.94~7.49), the low Mg#values (0.34~0.36), the low Cr (10×10-6) and Ni concentration (8.67×10-6in average) indicate tha the Chengan adakitic rocks might be derived from the partial melting of the ancient continental crust materials. The Chengan Granodiorite maybe had formed a new Jiangnan volcanic island arc environment.4. The results of high precise zircon U-Pb isotopic ages yielded two groups of ages (c.a.125~130Ma and138~145Ma) for the composite pluton in southern Anhui Province. It coincided with the twice magma activities (120~126Ma and137~145Ma) in eastern China and the middle and lower Yangtze River.5. The in-situ geochemistry and Lu-Hf isotopic characteristics of zircon (εHf(t)= -1.6~+5.1, tDM2=855~1281Ma) further support that the Guniujiang pluton was formed in the partial melting of the ancient continental crust materials.6. The W, Mo and Au polymetallic mineralization related to granodiorite (adakitic rock) with high zircon Ce+4/Ce+3ratios (>100) might result from the subducted oceanic crust rollback.