Petrogenesis of the early Tertiary A-type Namsan alkali granite in the Kyongsang Basin, Korea

Abstract

The early Tertiary alkali granite in the Mt. Namsan near Kyongju city is well known as A-type granite. Whole-rock chemistry and Sr and Nd isotopic compositions were investigated in order to constrain the origin of the alkali granite. According to the degree of alumina saturation and the CIPW norm, the alkali granite is peralkaline and the assotiated Bulguksa granite is peraluminous to metaluminous. The element ratios of the alkali granite mostly belong to the A2 group of A-type granite, suggesting that the alkali granite may have emplaced in a post-orogenic environment. The Rb-Sr age of the alkali granite and the assotiated Bulguksa granite is 49.7 ±0.1 Ma. Both granites were derived from a common source of mantle-like, young lower crust, and their parental magma had little or no interaction with the sialic continental crust. According to rare earth element (REE) and other trace element data, the alkali-granite magma was produced by fractionational crystallization of feldspar from Bulguksa-granite magma. The alkali-granite magma contains high F and flourine played a major role in determining the geochemical characteristics of the alkali granite. In conclusion, the A-type nature of the Namsan alkali granite was developed mainly by differentiation of Bulguksa granite and by high F content.