Petrogenesis of Early Cretaceous adakites in Tongguanshan Cu–Au polymetallic deposit, Tongling region, Eastern China

Abstract

Adakites are closely related to Cu–Au deposits in many regions worldwide. The petrogenesis of the Early Cretaceous adakites in the Lower Yangtze River Belt (LYRB) of East China and their genetic relationship with Cu-Au polymetallic mineralization are still debated. The Tongguanshan (TGS) pluton is representative of the adakitic intrusions in the Tongling region and is composed of granodiorite and quartz monzodiorite, and contains quartz monzodiorite enclaves. Zircons from the granodiorite yield a SIMS U-Pb crystallization age of 136.7 ± 1.4 Ma, which suggests that the Cretaceous magmatism is related to the mineralization. The host rocks and enclaves have SiO2 contents of 61.2–65.3 wt% and 54.9–58.0 wt%, respectively. Both the host rocks and the enclaves are characterized by high Al2O3 and Sr, and low Y and Yb contents, as well as high Sr/Y and (La/Yb)N ratios, which are typical geochemical characteristics of adakites, and they can be further divided into high-silica (host rock) and low-silica (enclave) adakites. The large range of εHf(t) and δ18O values of the host rocks (−11.6 to − 21.9, 6.8‰–7.6‰, respectively) and of the enclaves (−13.8 to − 23.6, 6.9‰–7.3‰, respectively), suggest an enriched and heterogeneous source. The host rocks and enclaves have similar Sr-Nd isotopic compositions (εNd(t) = −9.73 to −11.4, ISr = 0.7068 to 0.7073), which are intermediate among those of MORB, marine sediment, and the Yangtze lower crust. Whole-rock element ratios suggest that the host rocks were generated by the partial melting of subducted oceanic crust mixed with sediments, followed by assimilation of the lower crust. In contrast, the enclaves were derived from partial melting of enriched mantle metasomatized by slab-derived melts. Magma mixing, crustal contamination, and metasomatism play roles in generating the geochemical variations in the TGS adakites. The high oxygen fugacity and high contents of slab-derived volatiles suggested by apatites are favorable for large-scale Cu mineralization in the Tongling area, LYRB.