Origin of the Cretaceous ore-bearing granitoids in the Beihuaiyang Zone, northern margin of the Dabie Orogen, Eastern China

摘要

The Beihuaiyang Zone (BHY) is one of the most important Mo-Pb-Zn polymetallic metallogenic belts in China, and the BHY deposits are genetically and geographically associated with Cretaceous magmatic rocks. In this article, we present new zircon U-Pb ages and Hf isotope data, whole-rock major and trace-element analytical results, and Sr-Nd-Pb isotope data for the granite porphyry of the Shapinggou (SPG) Mo deposit and the quartz monzonite porphyry of the Gongdongchong (GDC) Pb-Zn deposit. The high contents of SiO2, crust-like rare-earth-element and trace element patterns, and the enriched Sr-Nd-Pb-Hf isotopic compositions indicate that both porphyries originated from crustal melting. Inherited Neoproterozoic zircons are common in both porphyries, which implies that their crustal sources were the South China Block rather than the North China Block. Whole-rock epsilon(Nd)(t) values (-10.8 to -9.8 for the GDC deposit, -12.9 to -12.4 for the SPG deposit) and zircon epsilon(Hf)(t) values (-14.3 to -11.1 for the GDC deposit, -18.4 to -13.3 for the SPG deposit) for the ore-bearing rocks are significantly higher than the values found in the widespread and older ore-barren rocks, indicating that the magma sources of the ore-bearing rocks were younger than those of the ore-barren rocks. An integrated study of the Sr-Nd-Pb-Hf isotope contents shows that these younger source rocks are similar to the gneisses found in the South and Central Dabie units, which represent the upper crust of the subducted South China Block. Given the geochemical behaviour of molybdenum, a surface enrichment process would have been an essential prerequisite for the formation of the large Mo deposit. The early Paleozoic Mo-Pb-Zn-enriched black shales, which are widespread in the upper layers of the South China Block, might have been scraped off during Triassic subduction and then transported to deep-crustal levels below the BHY, thus forming an ideal source for the ore-bearing porphyries. An upper-crustal origin for the ore-bearing magmatic rocks is also consistent with the data for most other deposits distributed in the BHY of the Dabie Orogen.