GRANITIC MIDDLE CRUST OF THE NORTHERN IBM ARC: CONSTRAINTS FROM GEOCHEMICAL AND GEOPHYSICAL STUDIES OF THE MIOCENE GRANITOIDS IN THE IZU COLLISION ZONE, CENTRAL JAPAN

Recent seismic experiments defined the presence of a middle-crustal granitic layer having Vp = 6.0-6.3 km/sec throughout the IBM arc (Suyehiro et al., 1996; Takahashi et al., 2007; Kodaira et al., 2007). Oceanic arcs such as the IBM arc, therefore, are recognized as ‘factories' where granitic crustal layers are formed either by differentiation of arc basaltic magmas or by anatexis of arc basaltic crusts. The Miocene Tanzawa (TPC) and Kofu plutonic complexes (KPC) exposed in the Izu Collision Zone have been considered as the granitic middle-crust of IBM arc exhumed by tectonic uplift during the ongoing Izu arc-Honshu arc collision. The petrological, geochemical and isotopic characteristics of these granitoids and associated hlb-gabbros suggest that these have been derived from discrete intermediate to felsic parental magmas generated by anatexis of lower crustal materials of the Izu arc (Kawate & Arima, 1998; Nakajima & Arima, 1998; Saito et al., 2007). Thus, comparison of the measured Vp of TPC rocks with the seismic structure of northern Izu arc provides an excellent opportunity to examine the architecture of IBM arc crust. To construct petrological model for island arc crusts having relatively higher geothermal gradient, we developed experimental techniques capable for simultaneous measurement of Vp and Vs of an encapsulated rock specimen up to 1 GPa and 1000°C. On the basis of our ultrasonic velocity data measured for TPC rocks, we suggest the IBM arc crust consists of petrologically distinct layers; a basaltic upper crust, a tonalitic middle crust, a hlb-bearing gabbroic uppermost lower crust, and a lowermost lower crust composed of pyroxenite or gabbroic rocks. Our data suggest that the low-velocity domains probably represent mixtures of various grt-px-rich ultramafic rocks of crustal origin (restites after lower crustal anatexis and/or cumulates after magmatic differentiation) and mantle peridotites components.