Early Cretaceous adakitic rocks in the Anqing region, southeastern China: constraints on petrogenesis and metallogenic significance

摘要

The Anqing region in Lower Yangtze River metallogenic belt is one of the important Cu polymetal producers in China. The origin of Cu polymetallic deposits in the region is closely related to Early Cretaceous adakitic intrusions. To constrain the petrogenetic and metallogenic significance of the adakitic rocks, a detailed geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic study was performed. The Anqing adakitic rocks (SiO2=57.4-64.2wt.%) consist mainly of quartz monzodiorite, formed at 138.2 +/- 1.7Ma (Mean Standard Weighted Deviation (MSWD)=0.61). They have high MgO, Al2O3, Sr, and low Rb, Y, Yb contents, together with high Sr/Y (50.5-222) and La/Yb (31.9-46.9) ratios. They also show negative whole-rock epsilon(Nd)(t) (-9.8 to -8.5) and zircon epsilon(Hf)(t) (-10.0 to -5.4), and high oxygen fugacity (mainly -17.0 to -8.01) values and radiogenic Pb isotopic compositions with (Pb-206/Pb-204)(i)=17.692-17.884, (Pb-207/Pb-204)(i)=15.413-15.511, and (Pb-208/Pb-204)(i)=37.611-37.943. Coupled with negative Nb-Ta anomalies, low K2O/Na2O ratios (0.39-0.62), and high Mg# values (0.44-0.71), these data suggest the adakitic rocks and associated large-scale Cu-Au mineralization of the Anqing region resulted from partial melting of the high oxidized subducted oceanic crust. Addition of mantle-derived magmas and assimilation of crustal materials during emplacement are also possible.