Double injection events of mafic magma into supersolidus Yucheon granites to produce two types of mafic enclaves in the Cretaceous Gyeongsang Basin, SE Korea

摘要

Petrographic and geochemical features of the Cretaceous Yucheon granites and their mafic microgranular/ magmatic enclaves (MMEs), SE Korea, reveal that the MMEs originated from magma mixing. Mesoscopic and microscopic features indicate that mechanical mixing operated heteroge-neously to produce the MMEs with a wide range of sizes and textures. Chemical compositions of amphibole, biotite, and plagioclase rims of both the MMEs and host granites are almost identical, indicating that chemical homogenization took place to some extent after the mechanical mixing. Plagioclase cores, however, have various compositions depending on the host rocks and/or sampling locations, suggesting their sluggish re-equilibration. The MMEs are divided into Type A (low TiO_2, very fine-grained, chilled margins) and Type B (high TiO_2, fine- to medium-grained, no chilled margins). The lower TiO_2 MMEs cooled more rapidly and interacted with granitic magma for a shorter period of time than the higher TiO_2 MMEs. Additionally, the former are less enriched in HREEs than the latter. Zoned plagioclase has two zones of increased An content. These features are indicative of double injection events of mafic magma. A previous model explains the magma mixing as resulting from the generation of a slab window due to Kula-Pacific ridge subduction. The model cannot, however, explain the eastward younging of the granites in Korea, necessitating a new, more elaborate model of Cretaceous geodynamics and magmatism in East Asia.