Petrogenesis of the mafic microgranular enclaves (MMEs) and their host granodiorites from the Zijinshan intrusion along the Middle-Lower Yangtze River Valley: Implications for geodynamic setting and mineralization

Abstract

The Zijinshan intrusion is located to the south of the Tongling ore cluster of the Middle-Lower Yangtze River Valley metallogenic belt (YRB) and even south to the Yangxing–Changzhou Fault (YCF), which separates the YRB and the Jiangnan Massif. It is an I-type high-K calc-alkaline granodiorite with abundant MMEs. Zircon LA-MC-ICP-MS U–Pb ages of 144.0±1.0Ma and 142.5±1.0Ma for the host granodiorite and MMEs indicate coeval formation. Biotites from the host granodiorites and MMEs have similar composition, with low FeOt/(FeOt+MgO) values of 0.60–0.65 and 0.60–0.65, and high MgO contents of 10.51–11.70wt.% and 10.89–12.37wt.%, respectively. All the biotite samples are between the NNO and the MH buffers. The host granodiorites are metaluminous (A/CNK=0.94–0.95), enriched in Cs, Rb, U and Pb, depleted in Ba, Sr, Nb, Zr, P, and Ti, with moderate negative Eu anomalies of 0.80–0.87, (87Sr/86Sr)i of 0.70874 to 0.70887, εNd(t) of −5.63 to −5.36 and εHf(t) of −8.68 to −2.76. The MMEs have lower SiO2 and higher K2O, and they are also enriched in Cs, Rb, U, Ta and Pb, and depleted in Ba, Sr, Nb, Zr, P and Ti, with (87Sr/86Sr)i of 0.70889 to 0.70927, εNd(t) of −5.52 to −5.20 and εHf(t) of −6.46 to −3.59. All these characteristics demonstrate that partial melting of both the lower crust and an enriched lithospheric mantle metasomatized by subducted slab fluid and their subsequent mixing led to the origin of the I-type granodiorite magmas of the Zijinshan intrusion, which took place in the geodynamic context of the subduction of the Izanagi plate. Combined with the field investigations, we suggest that the Zijinshan intrusion has potential for porphyry Cu–Au mineralization and the Cu–Au metallogenic belt of the YRB may have extended to the south of the YCF.