Petrogenesis and high-pressure metamorphic overprint of deep-seated Cu-rich ultramafic cumulates in the Sulu orogen

Abstract The recycling of ultramafic–mafic cumulates formed at arc roots is widely regarded as a key mechanism for returning dense lower-crustal materials to the mantle during continental crust formation. However, direct petrological evidence constraining the nature and evolution of such recycled lithologies remains scarce. Here, we present an integrated petrographic, geochemical, and thermodynamic study of the Hujialin ultramafic body from the Sulu orogen. This body comprises refractory dunites alongside a coherent wehrlite-clinopyroxenite suite, offering insights into the formation and fate of arc lower-crustal cumulates. The dunites represent refractory mantle fragments of the North China Craton, characterized by depleted whole-rock and mineral major element compositions, including high olivine magnesium numbers (91.2–92.8) and spinel chromium numbers (66.1–79.1). In contrast, the Hujialin wehrlites and garnet clinopyroxenites exhibit enriched whole-rock signatures and mineral layering consistent with a cumulate origin. The decoupling between whole-rock incompatible element enrichment and Sr–Nd isotopic compositions, together with relatively low olivine magnesium numbers (86.7–88.3) and spinel characterized by high Al2O3 (58.2–66.6 wt.%) and low Cr2O3 contents (0.19–3.30 wt.%), also supports a cumulate origin. Forward fractional-crystallization modelling shows that a representative continental arc basaltic melt evolving at ~1 GPa can broadly reproduce the observed cumulate assemblages and major-element trends, supporting the interpretation of the wehrlite-clinopyroxenite suite as deep arc-related cumulates. Phase equilibrium modelling indicates that garnet was not part of the primary crystallizing assemblage but formed mainly during post-magmatic cooling at lower-crustal pressures. Early garnet coronas around spinel and abundant porphyroblastic and matrix garnet record this cooling evolution, whereas the spinel-free matrix assemblage, together with the low Al2O3 contents and consistently high magnesium numbers of matrix clinopyroxene, supports subsequent high-pressure re-equilibration during continental subduction. Notably, Cu contents in the wehrlite-clinopyroxenite suite are elevated relative to the bulk continental crust and locally reach 464 μg/g, while preserved sulfide inclusions provide petrographic evidence for sulfide retention in the deep cumulates. The Hujialin suite therefore provides petrological support for the formation of Cu-bearing arc-root cumulates and illustrates a plausible pathway by which dense cumulates may be removed from the arc root and subsequently undergo high-pressure metamorphism during continental subduction.