Petrological and geochemical evidences for magma mixing in the Palgongsan Pluton

Abstract

Cretaceous Palgongsan pluton in the Gyeongsang basin is composed of coarse-grained porphyritic granite, medium-grained equigranular granite and aplite. The granites are designated to cale-alkaline I-type granites. Mafic enclaves are easily found in the pluton, and they are textually classified into fine-grained microgranular mafic enclaves and medium-grained mafic enclaves. The mafic enclaves in the granites are igneous in origin and comprise plagioclase, amphibole, biotite, and small amount of apatite, opaque, pyroxene and quartz. The mafic enclaves characteristically show petrographical features such as fine-grained texture, mafic clots, acicular amphibole, acicular apatite, plagioclase xenocryst, rapakivi texture and ocellar quartz. This observation demonstrates that the mafic enclaves are derived from mafic magma injection into crystallizing granitic magma chamber. There are good linear correlations in major oxide contents vs. SiO2 content in the Palgongsan pluton. Particularly the compositional variation between the granites and the mafic enclaves is possibly ascribed to two-component mixing model. Highly linear compositional variation trends between the Palgongsan porphyritic granite and the fine-grained mafic microgranular enclaves can be best explained by the compositional mixing of granitic magma with mafic magma which injected into the crystallizing granitic magma chamber. However the compositions of the equigranular granite and the aplite are characteristically of highly fractionated rock, and they are caused by later stage fractionation of granitic magma independent on the two-component mixing