Abstract The Izu-Bonin-Mariana (IBM) arc system extends 2800 km from the Izu Peninsula to Guam Island (Fig. 2f.1) and provides an excellent example of an intra-oceanic convergent margin where the effects of crustal anatexis and assimilation are considered to be minimal (Stern et al. 2003; Tatsumi Stern 2006). The current IBM activity is caused by subduction of the Pacific Plate beneath the Philippine Sea Plate, which dips at 35° at the northern tip of this system and ia nearly vertical at Mariana. Evolution of the IBM Arc since 50 Ma has strongly in?uenced the present architecture of the Japanese archipelago. One important event during this arc system’s evolution is back-arc rifting from 15-25 Ma that separated the IBM Arc system from the remnant Kyushu-Palau Ridge by c. 500 km and created the young oceanic lithosphere of the Shikoku Basin that is now being subducted beneath the SW Japan Arc. It should be further stressed that recent geophysical and geological survey results suggest this intra-oceanic arc is an active site of both creation and growth of the continental crust. This chapter will highlight the tectonic and structural evolution of the IBM arc system.

Abstract Subduction zones are major sites of magmatism on the Earth. Dehydration processes and associated element transport, which take place in both the subducting lithosphere and the down-dragged hydrated peridotite layer at the base of the mantle wedge, are largely responsible for the following characteristics common to most subduction zones: (1) the presence of dual volcanic chains within a single volcanic arc; (2) the negative correlation between the volcanic arc width and the subduction angle; (3) selective enrichment of particular incompatible trace elements; and (4) systematic across-arc variations in incompatible trace element concentrations. The occurrence of two types of andesites, calcalkalic and tholeiitic, typifies magmatism in subduction zones. Examination of geochemical characteristics of those andesites in the NE Japan arc and bulk continental crust reveals marked compositional similarity between calc-alkalic andesites and continental crust. One of the principal mechanisms of generation of calc-alkalic andesites, at least those on the NE Japan arc, is the mixing of two magmas, having basaltic and felsic compositions and being derived from partial melting of the mantle and the overriding basaltic crust, respectively. It may be thus suggested that this process would also have contributed greatly to continental crust formation. If this is the case, then the melting residue after extraction of felsic melts should be removed and delaminated from the initial crust into the mantle in order to form ‘andesitic’ crust compositions. These processes cause accumulation in the deep mantle of residual materials, such as delaminated crust materials and dehydrated, compositionally modified subducted oceanic crusts and sediments. Geochemical modelling suggests that such residual components have evolved to form enriched mantle reservoirs.