11:00 〜 13:00
[SCG47-P02] Mantle–crust interaction and crustal evolution in the Cretaceous Susuma–Nagao plutonic complex, Yamaguchi, southwest Japan
キーワード：分別結晶作用、玄武岩質マグマ、地殻の部分溶融
The mantle derived magma has formed the continental crust that is dominantly made up of granitic rocks. However, the granitic magma cannot be directly produced by the partial melting of mantle because granitic magmas are not equilibrium with a mantle peridotite. The crust forming granitic rocks, therefore, are thought to be generated by the following two mechanism; 1) differentiation through the fractional crystallization with or without crustal assimilation of mantle-derived basaltic magma and 2) partial melting of meta-igneous and/or meta-sedimentary rocks already existing as crustal materials. The former process contributes the growth of continental crust, whereas the latter mechanism proceeds the chemical differentiation of crust without any addition to crustal materials. Therefore, unraveling the respective contribution of two different mechanism in the genesis of granitic suites is necessary to understand the mechanisms of crustal growth and differentiation.
Above two processes ubiquitously occur in the plate convergent margins. The Cretaceous to Paleogene volcano-plutonic complexes related to the subduction of the Izanagi plate occurred as the magmatic flare-up in the Inner Zone of southwest Japan. The igneous complex in this zone consists mainly of granitoids, divided into the San-in, San-yo, and Ryoke belts from north to south in terms of their lithologies, intrusive ages, and associated ore deposits. The voluminous granitic suites are accompanied by small amounts of gabbro to diorite stocks. It means that the Cretaceous to Paleogene magmatism is thought to promote both of crustal growth and differentiation.
The Cretaceous gabbro, granodiorite, and granite suite is exposed in the Susuma-Nagaho area, western part of San-yo belt, west Chugoku district, southwest Japan. We address petrography, and mineral and whole-rock geochemical analyses to reveal their petrological characteristics and the petrogenesis of the complex as the highlighting following three points; 1) relationships between gabbro and granodiorite, 2) petrogenesis of granitic magma, and 3) origin of basaltic magma and mantle dynamics.
Magmatic activities occurred at 90Ma, which corresponds to the time for magmatic flare-up in southwest Japan. The hydrous basaltic magma evolved into granodiorite by fractional crystallization. Such basaltic magma penetrated the middle to lower crust underneath the Susuma-Nagaho area, and supplied heat energy to the already presented continental curst. As a result, partial melting took place in the middle to lower crust, giving rise to granitic magma. Differentiation of basaltic magma and recycling of the crust occurring at the same time should, therefore, play an important role for evolution of continental crust in an active continental margin. The Susuma-Nagaho plutonic complex, therefore, provides us opportunity for the study of the crustal growth and differentiation processes. This study essentially contributes to the evolution of continental crust and mantle dynamics above the subducting oceanic plate.
Above two processes ubiquitously occur in the plate convergent margins. The Cretaceous to Paleogene volcano-plutonic complexes related to the subduction of the Izanagi plate occurred as the magmatic flare-up in the Inner Zone of southwest Japan. The igneous complex in this zone consists mainly of granitoids, divided into the San-in, San-yo, and Ryoke belts from north to south in terms of their lithologies, intrusive ages, and associated ore deposits. The voluminous granitic suites are accompanied by small amounts of gabbro to diorite stocks. It means that the Cretaceous to Paleogene magmatism is thought to promote both of crustal growth and differentiation.
The Cretaceous gabbro, granodiorite, and granite suite is exposed in the Susuma-Nagaho area, western part of San-yo belt, west Chugoku district, southwest Japan. We address petrography, and mineral and whole-rock geochemical analyses to reveal their petrological characteristics and the petrogenesis of the complex as the highlighting following three points; 1) relationships between gabbro and granodiorite, 2) petrogenesis of granitic magma, and 3) origin of basaltic magma and mantle dynamics.
Magmatic activities occurred at 90Ma, which corresponds to the time for magmatic flare-up in southwest Japan. The hydrous basaltic magma evolved into granodiorite by fractional crystallization. Such basaltic magma penetrated the middle to lower crust underneath the Susuma-Nagaho area, and supplied heat energy to the already presented continental curst. As a result, partial melting took place in the middle to lower crust, giving rise to granitic magma. Differentiation of basaltic magma and recycling of the crust occurring at the same time should, therefore, play an important role for evolution of continental crust in an active continental margin. The Susuma-Nagaho plutonic complex, therefore, provides us opportunity for the study of the crustal growth and differentiation processes. This study essentially contributes to the evolution of continental crust and mantle dynamics above the subducting oceanic plate.