Petrology and geochemistry of the Youngju and Andong granites in the northeastern Yeongnam Massif, Korea

Abstract

Two late Paleozoic, partly deformed granitic batholiths, the Youngju and Andong granites, regionally occur in the northeastern Yeongnam Massif. The deformed granites generally occur along the shear zone trending northeast in the central part of the study area, and their foliations are sub-parallel to the trend of shear zone, indicating post-emplacement deformation. Both granites belong to tonalite-granodiorite-granite association, but have dominantly granodiorite composition. They have similar chemical features, corresponding to metaluminous, calc-alkaline, I-type, and volcanic arc granites. Most chemical variation trends suggest that mineral fractionation patterns in the two granites should be similar except for the plagioclase fractionation. Relative to the Andong granites, the Youngju granites contain significantly higher FeO, MnO, MgO, K2O, Rb, Th, U, Sc, and Pb, and lower Al2O 3, Na2O, P2O5, TiO2, Sr, Zr, and Ga. The Youngju granites also have higher K2O/Na 2O, Rb/Sr, and Th/Y ratios, and a lower K/Rb ratio than the Andong granites. The geochemical differences between the two granites are prominent, particularly, in mafic facies. The more incompatible nature of the Youngju granites may be attributed to higher degree of mixing or assimilation with old crustal materials during ascent or emplacement of magma. Taking into account that the tonalitic compositions are the most mafic fades of the two granites, the protoliths seem to have been mafic igneous rocks or their metamorphic equivalents distributed in the lowermost crust.