Magma-induced midcrustal flow and high-temperature metamorphism: Insights from the early Mesozoic Yunkai massif (South China) and tectonic implications

The way in which partial melting weakens the middle continental crust and triggers lateral flow is fundamental to understanding orogenic processes, yet the specific roles of magmatic heat and deformation remain debated. We addressed this issue through an integrated study of the early Mesozoic granitoids and migmatites in the Yunkai massif (South China). To better understand the early Mesozoic crustal evolution and geodynamic processes of South China, we conducted detailed studies on structural geology, pressure-temperature (P-T) estimates, and multimineral geochronology. The main deformation structure is represented by regional NE-SW stretching that was associated with widespread magmatism and partial melting under amphibolite-facies metamorphism. The strain partitioning at different locations shows dome-like structures, involving horizontal stretching at Fuhu Hill and central Hailing Island, subvertical shearing along the coast of Hailing Island, and dextral strike-slip shearing in the Jiahai area. Phase equilibrium modeling on high-grade metamorphic rocks defines peak P-T conditions at 6.8−7.0 kbar and 780−810 °C, indicating midcrustal high-temperature metamorphism dated between ca. 244 Ma and 232 Ma. The published data and our new results document that the high-temperature metamorphism occurred ∼10 m.y. later than magmatism dated between ca. 254 Ma and 234 Ma, indicating a magma-induced cause of the metamorphism. This midcrustal melting and high-temperature metamorphism were likely a consequence of either Indochina−South China collision or possible Paleo-Pacific subduction beneath the SE China continent. Our study provides a new magma-induced model for intracontinental partial melting and midcrustal flow, with broad implications for understanding crustal anatexis and rheology in orogens worldwide.