Melting experiments on mixed crustal rocks of juvenile oceanic arc and mature continental arc: implications for genesis of diverse granitoid plutons in the Izu collision zone, central Japan

Abstract:

Neogene granitoid plutons are widely exposed in the Izu collision zone in central Japan, where the northern end of a juvenile oceanic arc (the Izu–Bonin–Mariana arc) has been colliding with a mature continental arc (the Honshu arc). The plutons in this area are composed of various types of granitoids ranging from tonalite through granodiorite to granite. Saito and Tani (2017, Lithos) has reviewed petrogenetic models proposed for the plutons and suggested that the chemical diversity among the plutons reflects heterogeneity of the ‘hybrid source’ comprising of both basaltic rocks of the Izu–Bonin–Mariana (IBM) arc and metasedimentary rocks of the Honshu arc.

In this study, high-pressure melting experiments are performed for mixtures of IBM arc basaltic rock and Honshu arc metasedimentary rock to evaluate the ‘hybrid source’ anatexis model proposed by Saito and Tani (2017, Lithos). The glass compositions produced by partial melting vary from tonalitic through granodioritic to granitic with increases of amounts of Honshu arc metasedimentary rock in starting material. The results are consistent with the model that the chemical diversity among the granitoid plutons reflects variable contributions of the Honshu arc component in ‘hybrid source’ of the Izu collision zone crust.

The tonalitic plutons in the Izu collision zone are characterized by low concentration of incompatible elements and shows geochemical similarity with modern juvenile oceanic arcs. In contrast, granodioritic and granitic plutons have incompatible element abundances comparable to the average upper continental crust. The previous petrogenetic studies together with the present experimental results collectively suggest that the collision with another mature arc/continent, hybrid lower crust formation and subsequent hybrid source anatexis are important processes for juvenile oceanic arcs to transform into mature continental crust.