Petrogenesis of coeval shoshonitic and high-K calc-alkaline igneous suites in the Eopyeong granitoids, Taebaeksan Basin, South Korea: Lithospheric thinning-related Early Cretaceous magmatism in the Korean Peninsula

The Early Cretaceous was the initial period of prolonged Cretaceous-Tertiary magmatic episode in the Korean Peninsula, during which a tectonic transition from compression to extension occurred. The Early Cretaceous Eopyeong granitoids in the Taebaeksan Basin consist of two distinct series of magmas: shoshonitic and high-K calc-alkaline series. We investigated the petrology, whole-rock major and trace element geochemistry, and Sr-Nd-Pb isotope compositions of the Eopyeong granitoids to understand their magma evolution processes and source characteristics. Sensitive high-resolution ion microprobe zircon U-Pb analysis was performed on the Eopyeong granitoids to constrain their intrusion ages. Zircon U-Pb ages of the shoshonitic and high-K calc-alkaline series are 110.63 ± 0.52 and 110.65 ± 0.66 Ma, respectively, indicating that the two series formed during the same magmatic event. The mafic mineral assemblages of the shoshonitic series change from two-pyroxene+biotite to clinopyroxene+amphibole+biotite through magma evolution, whereas those of the high-K calc-alkaline series consist of more hydrous assemblages of amphibole+biotite. The shoshonitic series is also distinguished from the high-K calc-alkaline series by its higher La/Yb ratio and negative anomalies of Rb, Ba, and Eu. Mineral geothermobarometry and hygrometry data and geochemical mass balance modeling results show that the two series evolved mainly by fractional crystallization from two distinct parental magmas containing different water contents. Primitive rocks from each series have high compatible element contents such as MgO, Cr, and Ni with high Mg# compared to crustal melts in a similar range of SiO2, suggesting their mantle origin. They also have abundant incompatible elements and ‘crust-like’ isotopic compositions [(87Sr/86Sr)i = 0.7093–0.7098; εNd(t) = −7.1 to −8.8; (206Pb/204Pb)i = 18.133–18.327; (207Pb/204Pb)i = 15.621–15.665; (208Pb/204Pb)i = 38.691–38.862] with negative Nb-Ta anomalies in the primitive mantle-normalized patterns. The enrichment in both compatible and incompatible elements can be best explained by partial melting of phlogopite-bearing lithospheric mantle metasomatized by isotopically enriched upper crustal sediment-derived melt or fluid. The two magma series may have been generated by different degrees of partial melting of the mantle sources with varying water contents. Changes in subduction angle and direction of the paleo-Pacific plate during the Early Cretaceous may have led to lithospheric thinning and upwelling of asthenospheric mantle, which triggered the partial melting of fusible domains of metasomatized lithospheric mantle. The small igneous bodies of the Early Cretaceous shoshonitic to alkaline mafic rocks occur sporadically throughout the southern part of the Korean Peninsula in close spatial association with the Cretaceous basins. They also show crust-like geochemistry and isotopic compositions, similar to the Eopyeong granitoids, implying that the lithospheric thinning-related melting of the metasomatized lithospheric mantle may have been widespread in the southern part of the Korean Peninsula.