PETROCHEMICAL CHARACTERISTICS OF THE JURASSIC GRANITES IN THE MIDDLE PART OF THE KOREAN PENINSULA

The northern Gyeonggi massif in the middle part of the Korean Peninsula is mostly composed of Precambrian gneiss, Jurassic granites, Cretaceous tonalite and dykes. The Jurassic granites can be largely divided into two types: biotite granite (Gb) and garnet biotite granite (Ggb). Most of them are plotted in the monzo-granite field in the modal QAP diagram. They are medium- to coarse-grained with light grey and light grey to light pink colors, respectively. Mineral constituents are similar except the presence of allanite and opaques in Gb and muscovite and garnet in Ggb. They belong to acidic, peraluminous and calc alkaline series. With increasing silica contents, TiO2, MgO, CaO, Al2O3, K2O and P2O5 have moderately negative trends, and the first three oxides show two linear ones. These trends and linear patterns in AMF and Rb-Ba-Sr diagrams indicate that they were originated from different granitic magma sources and that Ggb was differentiated later than Gb. REE concentrations normalized to chondrite value have trends of parallel LREE enrichment and HREE depletion. Ggb shows a gradually enriched HREE trend, which may be caused by garnets. Ggb has more negative anomalies than Gb, showing that plagioclase fractionation in Ggb occurred much stronger than that in Gb. Results of EPMA analysis show that opaques of Gb are magnetite and ilmenite, and Ggb is magnetite-free and ilmenite is occasionally observed. Distribution of magnetic susceptibility values reveal statistically bimodal between the two granites, Gb (332.6 SI) and Ggb (2.3 SI). Based on the paleomagnetic and modal analysis, the susceptibilities of Gb and Ggb are mainly caused by ferromagnetic magnetite and paramagnetic mafic minerals, respectively. However, petrochemical data (i.e., SiO2 contents, K2O/Na2O, molar A/CNK and the results of ACF plots) show that they all belong to S-type granites.