Petrogenesis of the Late Cretaceous–Paleogene I- and A-type granite association from the Gyeongsang Basin, southeastern Korean Peninsula

Rare highly fractionated A-type granites are spatially associated with Late Cretaceous–Paleogene I-type granitoids in southeastern Korea. We present new zircon U-Pb ages and Hf isotopic compositions and whole-rock geochemical and Sr–Nd–Pb isotopic data for Late Cretaceous–Paleogene granitoids that crop out in the Gyeongsang Basin, SE Korea, to constrain their origin and the genetic relationship between the I- and A-type granites. The samples can be divided into three groups: (1) I-type low-silica granitoids (SiO₂ = 61–65 wt%), (2) I-type high-silica granites (SiO₂ = 71–78 wt%), and (3) A-type alkali feldspar granites (SiO₂ = 77–82 wt%). SHRIMP zircon U-Pb dating yields ages of 77–52 Ma for the I-type granites and 55–53 Ma for the A-type granites. Both I- and A-type granitoids have juvenile whole-rock Sr–Nd–Pb and zircon Hf isotopic compositions that are distinct from those of the Precambrian basement in the Yeongnam Massif. The I-type granitoids were most likely produced by adding <15 % of ancient-crust-derived melts to the magma derived from juvenile Paleozoic crust. In contrast to the I-type granitoids, the A-type granites are characterized by high SiO₂ and heavy rare earth element contents and large negative Ba, Sr and Eu anomalies. We propose that the A-type granites were produced by partial melting of dehydrated charnockitic rocks in the middle–lower crust. Both the I- and A-type granitoids were emplaced during crustal extension induced by rollback of the subducting paleo-Pacific plate. Starting during the Early Cretaceous, a retreating arc system led to increased extension, lithospheric thinning, and underplating of mantle-derived magmas. This caused the heat-flux melting of both juvenile and ancient basement rocks, leaving dehydrated charnockitic material above the I-type source. During the late stages of the Late Cretaceous magmatism, the charnockitized crustal rocks melted at high temperatures, leading to the formation of the A-type granites.