Partial melting of subducted paleo-Pacific plate during the early Cretaceous: Constraint from adakitic rocks in the Shaxi porphyry Cu–Au deposit, Lower Yangtze River Belt

Abstract

A large porphyry Cu–Au deposit associated with early Cretaceous intrusive rocks has been discovered and explored in the Shaxi area, Lower Yangtze River Belt (LYRB), eastern China. We studied two types of intrusive rocks in the Shaxi area: Cu–Au mineralization related diorites and quartz-diorites (adakitic rocks), and newly found high Sr/Y ratio biotite-gabbros. They were formed almost simultaneously with crystallization ages of ca. 130 to 129Ma, younger than the early stage shoshonitic rocks (Longmenyuan, Zhuanqiao and Shuangmiao Fm.) in the Luzong volcanic basin, ~10km south of the Shaxi area. These intrusive rocks show similar distribution patterns of trace elements (enriched in LILEs and depleted in HFSEs) and REEs (enriched in LREEs and depleted in HREEs, no Eu negative anomaly, flat HREE patterns). The diorites and quartz-diorites are adakitic rocks with calc-alkaline affinity, distinguished from other adakitic rocks in the LYRB which are high-K calc-alkaline series. The biotite-gabbros are not adakitic rocks, although they are characterized by high Sr/Y ratios.Shaxi adakitic rocks show positive zircon εHf(t) values, which may be attributed to the contribution of subducted oceanic crust, while the εHf(t) values of the biotite-gabbros are mostly negative, indicating the involvement of old crustal materials. Although Sr–Nd–Pb isotopes of Shaxi adakitic rocks are more depleted than those of other adakitic rocks in the LYRB, they are still slightly enriched, similar to continental arc adakites in the Andean Austral Volcanic Zone. The Shaxi adakitic rocks are characterized by high Sr contents and Sr/Y ratios, medium (La/Yb)N, MgO contents and Mg#, and low K2O/Na2O ratios, decoupling of Sr/Y and (La/Yb)N, low Th/U values, exhibiting characteristics of slab-derived adakitic rocks. They were not produced by fractional crystallization of basaltic magmas like adakitic rocks in Edong–Jiurui and Tongling but originally generated from partial melting of subducted paleo-Pacific Plate. Their enriched Sr–Nd–Pb–Hf isotopes indicate that the primary adakitic melts experienced assimilation of LYRB continental crust during ascending to the surface. While other high-K calc-alkaline “C-type” adakitic rocks in the LYRB were generated from mantle melting followed by crustal AFC processes. The Shaxi biotite-gabbros were generated from partial melting of enriched sub-continental lithosphere mantle (SCLM), which was previously metasomatized by sediment melts and adakitic slab melts. The LYRB adakitic rocks are characterized by high oxygen fugacities, thus favorable for Cu–Au mineralization.