Melt–mush reaction during magmatic conduit assembly: Evidence from the Silurian Lanpai and Huxiang mafic–ultramafic intrusions, South China

Deciphering the transition from diffuse to channelized melt flow of crystal mush remains challenging because direct natural constraints are scarce. The Silurian Lanpai and Huxiang mafic–ultramafic intrusions in the Wuyi-Yunkai Orogen, South China, preserve a fossilized magmatic conduit system that records multistage melt migration. These small composite bodies are composed of peridotites, pyroxenites, and gabbros with irregular contacts. Their cumulates preserve two disequilibrium assemblages recording an early crystal framework and subsequent interstitial melt products in a partially solidified mush. The crystal framework consists of resorbed Cr-rich magnetite (0.58–12.3 wt% Cr2O3), low-forsterite olivine (Fo68–77), Mg-rich orthopyroxene (Mg#73–76), and high-anorthite plagioclase (An74–90). Clinopyroxene crystallized from interstitial melt and occurs as primocrysts in pyroxenites and as oikocrysts in peridotites and gabbros. While homogeneous, unzoned primocrysts are consistent with near-equilibrium fractional crystallization, the oikocrysts exhibit variable incompatible element ratios (Zr/Nd, Ce/Y) and complex Cr-Zr zoning (locally positive co-variation), reflecting extensive melt–mush interaction. These features are supportive of a two-stage polybaric evolution of conduit assembly. An initial mantle-derived magma differentiated in a deep reservoir and subsequently ascended to a shallow conduit, where continued crystallization formed a crystal mush and reduced permeability. A later clinopyroxene-saturated magma then infiltrated this framework and reacted with and percolated through the mush. Feedback between the melt flow and dissolution focused porous flow into localized channels, ultimately generating the observed composite mafic–ultramafic bodies. These findings demonstrate that melt–mush reactions are widespread during conduit assembly, playing a critical role in regulating melt transport and differentiation within transcrustal magmatic systems.