Pulses of Late Mesozoic magmatism: Zircon ages and Hf-O isotopic composition of the Qingyang-Jiuhuashan granitic complex, southern Anhui province, eastern China

摘要

Late Mesozoic magmatic rocks are widely distributed in southern Anhui province, and are represented by the Qingyang-Jiuhuashan granitic complex, which consists dominantly of the Qingyang pluton of granodiorite and monzogranite, and the Jiuhuashan pluton of K-feldspar granite. These two plutons have zircon U-Pb ages of 138 Ma-128 Ma. Combining the new data with previously reported age data, the magmatism in southern Anhui province can be subdivided into three stages: early stage of similar to 152-136 Ma, middle stage of similar to 136-130 Ma, and latest stage of similar to 130-120 Ma. Granodiorite (early stage) and monzogranite (middle stage) of the Qingyang pluton have SiO2 contents of 66-68 wt%, total alkali contents of 7.5-8.6 wt%, K2O/Na2O ratios of 1.1-1.3, A/CNK ratios of 0.9-1.1, and low FeOT/(FeOT + MgO) values of 0.71-0.73. These rocks show depletion in Nb, Ta, and Zr and slightly negative Eu anomalies. K-feldspar granite (late stage) of the Jiuhuashan pluton is characterized by high SiO2 contents ( 75 wt%), K2O/Na2O ratios of 1.1-1.3, and A/CNK ratios of 0.9-1.2, and high FeOT/(FeOT + MgO) values of 0.88-0.93. They have high concentrations of Nb, Ta, and Zr, and strongly negative Eu anomalies. Initial epsilon(Hf) values of zircon grains from the granodiorite and monzogranite range from - 8.01 to -6.71 and -11.40 to - 9.43, with 8180 values varying from 7.16 parts per thousand to 8.04 parts per thousand and 7.26 parts per thousand to 8.06 parts per thousand, respectively. Zircon grains from K-feldspar granites have relatively high initial epsilon(Hf) values of -7.62 to -4.78, but similar 8180 values of 7.03-8.84 parts per thousand. Geochemical features of granodiorite and monzogranite point to an I-type signature, whereas those of K-feldspar granite show an A-type affinity. We propose that the granodiorite and monzogranite originated from mixing melts produced by partial melting of lithospheric mantle metasomatized by fluids and crustal rocks induced by underplating basaltic magmas, related to subduction of the Paleo-Pacific oceanic crust, whereas the K-feldspar granite formed from crustal anatexis in an extension setting caused by rollback of the subduction.