伊豆衝突帯に分布する新第三紀花崗岩質岩体の地球化学的多様性とその意義：未成熟海洋性島弧から成熟大陸地殻へ

Abstract

Neogene granitoid plutons are widely exposed in the Izu Collision Zone in central Japan, where the northern tip of the Izu-Bonin arc (juvenile intra-oceanic arc) has been colliding with the Honshu arc (mature island arc) since middle Miocene. The plutons in this area are composed of various types of granitoid ranging from tonalite to trondhjemite, granodiorite, monzogranite and granite (sensu stricto). Three main granitoid plutons are distributed in this area: Tanzawa plutonic complex, Kofu granitic complex, and Kaikomagatake pluton. Tanzawa plutonic complex is characterized by low concentration of incompatible elements such as K2O and has chemical characteristics of juvenile oceanic arc. In contrast, Kaikomagatake pluton and Kofu granitic complex have chemical compositions comparable to the average upper continental crust. Previous petrogenetic studies on the plutons have been suggested that (1) the Tanzawa plutonic complex formed by lower crustal anatexis of juvenile basaltic rocks occurring in the Izu-Bonin arc, (2) the Kofu granitic complex formed by anatexis of ‘hybrid lower crust’ comprising of both basaltic rocks of the Izu-Bonin arc and metasedimentary rocks of the Honshu arc, and (3) the Kaikomagatake pluton formed by anatexis of ‘hybrid lower crust’ consisting of K-rich rear-arc crust of the Izu-Bonin arc and metasedimentary rocks of the Honshu arc. These studies suggest that the chemical diversity within the Izu Collision Zone granitoid plutons reflects the chemical variation of basaltic sources (i.e., across-arc chemical variation in the Izu-Bonin arc) as well as variable contribution of the metasedimentary component in the source region. Contrasting to the rocks of Kofu granitic complex and Kaikomagatake pluton, the intermediate to felsic igneous rocks occurring in modern oceanic arcs and the Tanzawa plutonic complex show lower concentration of incompatible elements than the average upper continental crust. Petrogenesis of the Izu Collision Zone granitoid plutons together with other oceanic arc collision-related magmatism collectively suggest that collision has played an important role in transforming juvenile oceanic arc into mature continental crust.