海洋性島弧から大陸地殻へ：伊豆衝突帯花崗岩質岩体の成因研究

Abstract

Granitic rocks (sensu lato) are major constituents of the upper continental crust. Recent reviews that provide estimates of the composition of continental crust have established that the average composition of the upper continental crust is granodioritic. Although the oceanic arcs are regarded as a site producing continental crust material in an oceanic setting, intermediate to felsic igneous rocks occurring in the modern oceanic arcs are dominantly tonalitic to trondhjemitic in composition and have lower incompatible element contents than the average upper continental crust. Therefore, the juvenile oceanic arcs require additional processes to transform into the mature continental crust enriched in incompatible elements. Neogene granitoid plutons are widely exposed in the Izu Collision Zone in central Japan, where the northern tip of the Izu-Bonin arc (juvenile 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 and has chemical characteristics of juvenile oceanic arcs. In contrast, Kaikomagatake pluton and Kofu granitic complex have chemical compositions comparable to the average upper continental crust. The petrogenetic models of the Izu Collision Zone granitoid plutons collectively suggest that collision with another mature arc/continent, hybrid lower crust formation and subsequent hybrid source anatexis are required for juvenile oceanic arcs to produce granitoid magmas with compositions comparable to the average upper continental crust. The Izu Collision Zone granitoid plutons provide an exceptional example of the collision-induced transformation of a juvenile oceanic arc into the mature continental crust.