Petrogenesis of the Eonyang granitoids, SE Korea: new SHRIMP-RG zircon U–Pb age and whole-rock geochemical data

Abstract

Petrogenesis of the Late Cretaceous Eonyang granitoids in the Gyeongsang Basin is considered in terms of field relations, petrography, geochemistry, Sr–Nd isotopes, and sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) geochronology. The plutons can be divided into two groups: Group I consists of quartz-monzodiorite (QMD), granodiorite (GD), enclave-rich porphyritic granite (ERPG), and enclave-poor porphyritic granite (EPPG), showing evidence of magma mixing/mingling. Group II comprises coarse-grained porphyritic granite (CPG), fine-grained micrographic granite (FMG), and equigranular granite (EG), displaying non-mixing and highly differentiated characteristics. The age of the EPPG, reflecting magma mixing, is 73.3 ± 2.0 Ma. The Eonyang granitoids show subalkaline, calc-alkaline, I-type, and high-K characteristics except for the mafic magmatic/microgranular enclaves (MMEs), which have medium-K calc-alkaline compositions. The mafic facies (QMD + GD + MME) are metaluminous, whereas the felsic facies (ERPG + EPPG + CPG + EG) are peraluminous. Two geochemical evolution trends are present, magma mixing/mingling in Group I and fractional crystallization in Group II. Based on the Sr–Nd isotope geochemistry, the various rock facies in Group I evidently resulted by hybridization of two magmas under different physical conditions. In contrast, the petrogenesis of Group II rock types involves two alternative possibilities: (1) mantle contamination after fractionation of a parental magma derived from the same source as Group I or (2) fractionation of a parental magma derived from a source different from that of Group I. The geochemistry of the Eonyang granitoids suggests mixed-origin magma generation in a Late Cretaceous subduction setting.