Geochronology and Geochemistry of Cretaceous Adakitic Rocks of the Dongguashan Cu Deposit in the Lower Yangtze River Belt: Insights into Petrogenesis and Mineralization

Abstract

The Lower Yangtze River Belt (LYRB) is a well-known and important base area with regard to Cu polymetallic resources in China. Large Cu polymetallic deposits in the LYRB are strongly associated with Cretaceous adakitic rocks. However, the petrogenesis of the Early Cretaceous adakites and the temporal–genetic relationship with mineralization are still disputable. The Dongguashan (DGS) Cu polymetallic deposit in the Tongling ore cluster is one of the largest Cu deposits in the LYRB. The DGS intrusion mainly comprises quartz monzodiorite, with SiO2 contents varying from 63.7 to 67.9 wt%. Zircons from the quartz monzonite yield a SIMS U-Pb age of 138.9 ± 1.8 Ma, which indicates that the Cretaceous magmatism is coeval with mineralization. The studied rocks show typical geochemical signatures of adakites, characterized by high Al2O3 (14.9–16.2 wt%) and Sr (800–910 ppm) and low Y (15.2–17.5 ppm) and Yb (1.37–1.52 ppm) contents, with consequently high Sr/Y (46–61) and (La/Yb)N (14.8–18.5) ratios. The zircon δ18O values of the DGS adakites range from 5.7‰ to 7.3‰, indicating a heterogeneous source. Whole-rock Sr-Nd isotopic compositions show an enriched character, with ISr ratios from 0.70783 to 0.70794 and εNd(t) values around −11.0, which fall intermediately in the area of MORB (mid-ocean ridge basalt), marine sediment, and the ancient lower crust. Comprehensively, whole-rock geochemical compositions and isotopic values suggest that the adakites are generated from the partial melting of the subducted oceanic crust and possibly with the involvement of sedimentary materials derived from the slab or continental crust. Moreover, the bulk-rock high-Cu composition, and the physical–chemical conditions (high oxygen fugacity and high volatile contents) revealed by apatites, plays critical roles in the formation of Cu mineralization in the DGS Tongling ore cluster, LYRB.