Tracking Crystal-Melt Segregation and Accumulation in the Intermediate Magma Reservoir

The genesis of intermediate intrusions is highly controversial, and one of the hot topics is whether they represent frozen melts or cumulates in the evolution of magmatic systems. Distinguishing accumulation from crystallization melt differentiated along the liquid line of descent is the key issue. The Paleoproterozoic intermediate intrusions in southern North China Craton provide an excellent case to decipher this issue. Multiple lines of evidence, including mineral textures, geochemistry as well as alphaMELTS modeling, indicate disequilibrium between whole-rock and minerals, with melt extraction occurring at temperatures of 760 degrees-820 degrees C and with 10-40 wt.% of trapped melts. Effective water storage, revealed by amphibole and clinopyroxene hygrometers, plays a crucial role in promoting crystal-melt segregation in pluton-sized reservoirs in the upper crust. This study demonstrates that the accumulation in intermediate magmas can be identified even without evident complementary initial and extracted melts and provides deep insights into the genesis of intermediate continental crust.