Genesis of the low-Sr Granitoid from the Cretaceous Granitic rocks in Northern Kyushu

Abstract

Widespread Cretaceous granitic rocks in north Kyushu have been geochemically classified into low-Sr and high-Sr granites by Izawa et al. (1990, 1994). The Kokura-Tagawa Tectonic Line (KTT) in the region separates high-Sr granite to the west from low-Sr granite to the east. The Hirao granodiorite and a related granite dike are exposed on the east side of the KTT and have low Sr contents of 213-291 ppm. In this study we consider the petrogenesis of the Hirao granodiorite and other low-Sr granites, in comparison with the nearby high-Sr Ushikiri-yama granodiorite. The Hirao granodiorite shows similar rare earth element(REE) patterns and Sr-Nd isotopic values to the granite dike. Mass balance calculations reveal that the granite dike magma could have been formed by the removal of plagioclase and hornblende from the Hirao granodiorite magma. The geochemical characteristics of the Hirao granodiorite, combined with the results of the melting experiments, confirm that the low-Sr granite magma could have been produced by relatively low-pressure melting, although the composition of the source of the Hirao granodiorite is similar to that of the high-Sr Ushikiri-yama granodiorite.