Remelting of the Songpan-Ganze slab: Contribution to elevated magmatic oxygen fugacity and reactivation of copper in the Yulong porphyry copper deposits belt

Abstract

The factors contributing to the high magmatic oxygen fugacity (fO2) of post-subduction porphyry Cu deposits (PCDs) remain controversial. The prevailing hypothesis says that this high fO2 is inherited from the lithospheric mantle modified by previous seafloor subduction. However, recent discoveries pertaining to the prevalence of anoxic oceans, particularly the well-documented Paleo-Tethyan Ocean, would challenge this hypothesis. In order to address this matter, we studied representative primitive high-Mg igneous rocks (HMIRs) from the Yulong PCDs belt in Northern Qiangtang terrane of Tibet. These alkaline rocks are emplaced in a post-subduction setting after the closure of Paleo-Tethyan Ocean. In this study, we use newly generated in-situ mineral analyses on representative samples, together with meticulously reviewed and recalculated literature data, to elucidate the mechanism by which the oxidation process operated in the mantle source of these Cenozoic rocks. Our findings suggest that these post-subduction HMIRs exhibit characteristic compositions resembling those of continental crustal rocks and carbonatites. They display elevated fO2 levels and contain high concentrations of Cu ligands (S and Cl) derived from a source with mantle-like δ18O (4.09 to 5.39‰) and moderately low ɛHf(t =37Ma) values (4.0 to 9.3). This differs from the igneous rocks from the preceding Paleo-Tethyan arc remnants that exhibit lower fO2 levels and Cu ligand concentrations, sourced from a modified mantle with higher δ18O (5.6 to 7.9‰) and ɛHf(t =37Ma) values (9.3 to 13.4). Furthermore, through the application of zircon oxybarometry and existing experimental constraints, we have successfully identified the significance of oxidized mafic basement and overlying carbonates within the Songpan-Ganze terrane as influential oxidizing agents. Through detailed examination of geochemical and seismic data, we explore that these oxidizing agents are responsible for the formation of the oxidized HMIRs and copper mineralization. Our study thus provides significant insights into the role of subducted continental materials in generating carbonated silicate melts and causing oxidation of the mantle sources in a post-subduction setting for the petrogenesis of PCDs.