Mélange Melting Generates Metasomatic Precursors for Post-Collisional Potassic Magmas

When tectonic plates collide, different types of crustal and mantle rocks are mixed together to form “mélanges”. While scientists know these mixed rocks contribute to magma generation in many settings, their specific role in creating potassium-rich magmas after continental collisions remains unclear. To investigate this, we conducted laboratory experiments simulating the melting of these mixed rocks under the high temperatures and pressures of the shallow Earth's mantle. We found that the proportion of different starting rocks controls the primary chemical makeup of the resulting melts, while the sedimentary component dictates their trace element signatures. More importantly, our experiments reveal a two-stage process. The melts generated at shallow depths do not directly erupt to the surface. Instead, they act as early-stage metasomatic agents that move upward and enrich the overlying mantle. Later, when geological forces cause this newly enriched mantle to melt again at greater depths, it produces the final potassium-rich magmas observed in nature. This two-stage model provides a reasonable mechanism for how crustal materials interact with the mantle to form specific post-collisional magmas.