Age, geochemistry, and tectonic significance of Neoproterozoic alkaline granitoids in the northwestern margin of the Gyeonggi massif, South Korea

Alkaline meta-granitoids, ranging in composition from syenite to alkali granite, occur in the northwestern Gyeonggi massif. Ion microprobe U-Pb zircon analyses indicate that the granitoids were emplaced at 742±13 Ma, and are corroborated by a Rb-Sr whole rock age of 770 ± 40 Ma. Major and trace element characteristics, together with Sr and Nd isotopic data, suggest that the granitoid magma was derived from ancient (T_DM = 2.6-2.2 Ga) continental crust with addition of juvenile mantle-derived basaltic magma. The generation of the alkaline granitoid is attributed to crustal thinning induced by deep-seated thermal activity such as mantle upwelling or mafic magma influx. Alkaline igneous activity at 742 Ma is coeval with Neoproterozoic rift-related magmatism prevalent in South Korea and the South China Block but lacking in the North China Block. Thus, we suggest that the Gyeonggi massif is correlative with the South China Block and has experienced a rifting event during Rodinia breakup.