徹底的な野外調査に基づく小規模深成岩体のマグマ進化および岩体成長プロセス解析

Abstract

Granitoids cover approximately 30% surface area of the Japanese island. Intense magma activities of the granitoids occurred during the Paleogene to Cretaceous (50-130 Ma). The Cretaceous granitoid batholith is exposed over an area of about 100 km from east to west and 50 km from north to south in northern Kyushu. Small mafic bodies occur sporadically in the batholith. Some of these mafic bodies represent high-Mg diorite (HMD) compositions derived from high-Mg andesite (HMA) magma, and their petrogenesis has been discussed by several researchers. The coexistence of both mafic and felsic rocks has been reported from volcanicplutonic sites of the active continental margin in the Cretaceous granitoid batholith of southwest Japan and the Cordillera Mountains in North America. In addition, mantle-derived mafic magma is strongly involved in the origin of granitic magma as a parent magma and a heat source of crustal melting. Therefore, clarification of the behavior of the HMA-derived magma in the crust play an important role in examining the genetic relationship with granitoids. Furthermore, plutonic rocks are often called "fossils of magma reservoirs" as they preserve magmatic or emplacement processes and the time scale of plutonic activity. Investigation of internal structures may help to clarify the lithological and chemical variation in plutonic rocks as well as their growth process, which is crucial for understanding the relationship between volcanic and plutonic activity. These studies will contribute to the understanding of crustal growth and evolution. Thus, in this study, the magmatic process, lithofacies variations and the growth process of a newly finding HMA-derived rock, the Shaku-dake body (a high-Mg diorite body), are discussed as a member of the Cretaceous granitoids batholith of northern Kyushu.