Petrogenesis of Early Cretaceous granitoids and mafic microgranular enclaves from the giant Tonglushan Cu–Au–Fe skarn orefield, Eastern China

Abstract

Late Mesozoic high-K calc-alkaline and high Sr/Y granitoids related to the skarn–porphyry–stratabound Cu–Au polymetallic deposits are widespread in the Middle–Lower Yangtze River Belt (MLYRB), Eastern China. However, the role of mantle-derived magmas in the petrogenesis of these granitoids remains controversial. In this study, we present new zircon U–Pb–Hf isotopes, and whole-rock geochemical and Sr–Nd isotopes data for quartz monzodiorites (porphyry), their hosting mafic microgranular enclaves (MMEs) and associated dykes from Tonglushan orefield in the MLYRB. Field relationships and geochronological data show that these intrusive rocks cluster around 141–139 Ma, indicating a short duration of magmatism, rather than a prolonged magmatism (~30 My) as previously thought. Crystallization age of the MMEs is identical to that of their quartz monzodiorite host. Petrographical and geochemical observations suggest that the MMEs represent mafic melts that injected into the quartz monzodioritic magmas, and magma mixing and (previously) fractional crystallization were involved in producing their compositional diversity. All the Tonglushan granitoids and MMEs show trace element geochemical and Sr–Nd–Hf isotope compositions similar to the coeval K-rich mafic rocks in the MLYRB. This indicates that these granitoids and MMEs were mainly sourced from the enriched subcontinental lithospheric mantle (SCLM) that metasomatized by slab derived fluids/melts, and had experienced significant fractional crystallization. The high oxygen fugacity and water content of the early Tonglushan quartz monzodiorite (porphyry) indicate melting of metasomatized SCLM and subsequently fractional crystallization and recharging/mixing in magmatic system have the potential to Cu–Au polymetallic mineralization. This study also highlights that the SCLM beneath the eastern Yangtze plate is one of the likely sources for Late Mesozoic magmatic rocks in the MLYRB.