Large-scale rare metal pegmatite formation via tectonic-induced pulsed injection of fractionated felsic magma in the Mufushan granite–pegmatite system, South China

Abstract

The evolving understanding of magma reservoirs has challenged traditional models of pegmatite formation. The mechanisms by which granitic crystal mush is remobilized to extract pegmatitic melt remain enigmatic. Here, an integrated petrological study of the Cretaceous Mufushan batholith—focusing on two-mica monzogranite, muscovite monzogranite, barren pegmatites, and rare metal pegmatites—reveals distinct associations: (1) co-occurrence of NYF and LCT pegmatites spatially linked to two-mica monzogranite in the Duanfengshan region, and (2) barren pegmatite crusts and dikes associated with muscovite monzogranite in the Guanyuan region. Based on combined Sm-Nd isotopic and mica geochemistry data, we propose a tectonically driven melt extraction model in which crystal mush is remobilized by tectonic activity. Faults and fissures generated by tectonic activities generate the pressure gradients to remobilize overcooled granitic crystal mush, facilitating the extraction of granitic melt. The high degree of overcooling conditions results in the formation of pegmatitic textures. Melt migration along faults and fissures promotes fractional crystallization, concentrating rare metals to form mineralized pegmatites. This multiphase melt extraction initiated chemical fractionation within pegmatites. The muscovite monzogranite and its associated pegmatite crusts/dikes represent the final products of the Mufushan magma reservoir, which interacted with exsolved magmatic volatile phases. Our findings highlight the critical role of tectonically controlled remobilization of crystal mush in both the generation and extraction of pegmatite melts.