Remelting of Neoproterozoic relict volcanic arcs in the Middle Jurassic: Implication for the formation of the Dexing porphyry copper deposit, Southeastern China

The Dexing copper deposit in southeastern China is a typical non-arc porphyry deposit, the origin of which has been a topic of debate for several decades. Here we present new results from U–Pb geochronology, whole-rock chemistry and Sr–Nd–Hf–O isotopic investigations on the ore-forming granodioritic porphyry. LA-ICPMS zircon U–Pb data suggest that the granodioritic porphyry was formed in the Middle Jurassic (ca. 172.5 Ma) probably associated with lithospheric thinning driven by either sub-continental lithospheric mantle delamination or asthenospheric upwelling. The porphyry displays both arc-like and adakitic trace element signatures. The adakitic features suggest that HREE (heavy rare earth elements)-rich minerals such as garnet and hornblende, in the absence of plagioclase resided in the source region. The arc-like signatures are broadly comparable with those of the proximal Neoproterozoic island arc rocks including the keratophyre from Shuangxiwu Group and associated granitoids indicating a potential genetic relationship. The porphyry has chondritic εNd(t) of − 0.28 to 0.25 and radiogenic εHf(t) of 2 to 7, and correspondingly, uniform two stage depleted mantle Nd model ages of 940–980 Ma and Hf model ages of 800–1100 Ma (mean ~ 920 Ma). On Nd and Hf isotopic evolution diagrams, these values are markedly similar to those of the adjacent Neoproterozoic arc rocks when calculated forward to the Mid-Jurassic. Zircons of the porphyry show mantle-like oxygen isotope characters with δ18O values clustering in the range of 4.7–5.9‰, similar to the values for the Neoproterozoic arc rocks mentioned above. The geochemical and isotopic features recorded in our study suggest mantle-derived magmas with no significant supracrustal input for the source of the porphyry. With regard to the source of the Cu ore, we consider a model involving the remelting of sulfide-bearing arc-related lower crustal source. Furthermore, the occurrence of a Neoproterozoic VMS (volcanic massive sulfide) type copper deposit (the Pingshui Cu deposit) in the Shuangxiwu Group might suggest that the lower crustal rocks related to a Neoproterozoic relict island arc provided the source for copper during a second stage melting event. We propose a new geodynamic model for the Dexing porphyry Cu deposit which envisages that the sulfide-bearing arc lower crustal rocks were generated during oceanic slab subduction in the early Neoproterozoic, the remnants of which were preserved at the crust/mantle boundary. Subsequently, in the Middle Jurassic, these rocks were heated by asthenospheric upwelling and remelted to produce fertile magmas. The magmas ascended along the Northeast Jiangxi Fault and intruded into the Jiuling terrane where Cu precipitation occurred upon subsequent magma cooling and fluid exsolution.

► Ore-forming porphyry at Dexing has similar trace element patterns to nearby Neoproterozoic arc rocks.
► The Nd–Hf–O isotopes of the porphyry are comparable to the Neoproterozoic arc rocks.
► The Dexing Cu deposit is generated by melting of Neoproterozoic relict island arcs.
► A new genetic model is presented for the Dexing porphyry Cu deposit.