Subduction-Induced Upwelling of a Hydrous Transition Zone: Implications for the Cenozoic Magmatism in Northeast China

The widespread Cenozoic basalts in northeast China are commonly thought to be related to the stagnation of the Pacific slab in the transition zone and its deep dehydration. By incorporating experimentally constrained phase diagrams of hydrous mantle and melting conditions at high pressures into two-dimensional petrological-thermomechanical models, here we model the interaction of a subducting slab with a hydrous transition zone (TZ) and examine its potential role in generating intracontinental magmatism. The model results show that descending of the oceanic slab first forces up the material in the TZ. Depending on the water content in the TZ, the upwelling hydrous material may undergo dehydration melting above the TZ. As a large slab stagnates within the TZ owing to the lower mantle resistance, the deep melting migrates progressively toward the overriding continent's interior, generating plutonic/volcanic rocks in the continental crust far away from the trench. The amount of deep melting and surface magmatism is obviously related to the amount of water stored in the TZ. In contrast, the water stored in the cold core of the subducting slab and released in the transition zone does not generate melting as it is entirely absorbed by mantle transition zone nominally anhydrous minerals. Comparing the model results with the distribution of the late Cenozoic basalts in northeast China, we suggest that the intracontinental magmatism there was likely caused by deep dehydration melting induced by the slab-transition zone interaction.