高知県南西部柏島‐沖ノ島地域の花こう岩類の岩石学的研究

Abstract

From the mode of occurrence, texture, and mineral assemblage, granitic rocks in the Kashiwa-jima-Okinoshima district are classified into four groups; namely, the Tanijiri-type granodiorite (abbreviated to TGd), the Tanijiri-type granodiorite porphyry (TGdp), the Moshima-type medium-grained granite (MmGr), and the Moshima-type fine-grained granite (MfGr). The similarity of major and trace element chemistry of the TGd and TGdp, and of the MmGr and MfGr suggests that the petrogenetic processes operating were similar, respectively. The Rb-Sr isotopic analyses of nine whole-rock samples of the Moshima-type granitic rocks (MmGr and MfGr) yield an age Rb-Sr of 16±2 Ma with an initial 87Sr/86Sr ratio of 0.70740± 0.00018. From the data of Nd and Sr isotopic ratios (Fig. 9) and of initial 87Sr/86Sr ratios and reciprocals of the strontium concentration (Fig. 10), it is suggested that the Tanijiri-type granodioritic rocks have been formed by mixing of an original S-type magma and sedimentary rocks of the Shimanto belt, while the Moshima-type granitic rocks have been formed by fractional crystallization of a S-type magma, which has been more felsic than the original S-type magma. From the chemical composition of garnet in the TGdp, it is suggested that the original S-type magma of the Tanijiri-type granodioritic rocks may have been generated by the partial melting of metamorphic rocks of pelitic and psammitic origin at a depth of about 20km. The TGdp shows a distinct porphyritic texture compared with the TGd. Minerals, such as plagioclase, orthopyroxene, cordierite, and biotite in the TGd have slightly wider range of solid solution compared with the minerals in the TGdp. Therefore, it is suggested that the petrogra-phical difference between the TGd and TGdp may have been caused by a difference in the ascent and cooling rate of their magmas.