Petrogenesis of the Tanzawa plutonic complex, central Japan: Exposed felsic middle crust of the Izu–Bonin – Mariana arc

The Miocene Tanzawa plutonic complex, consisting mainly of tonalite intrusions, is exposed at the northern end of the Izu–Bonin – Mariana (IBM) arc system as a consequence of collision with the Honshu Arc. The Tanzawa plutonic rocks belong to the calc-alkaline series and exhibit a wide range of chemical variation, from 43 to 75 wt% SiO2. They are characterized by relatively high Ba/Rb and Ce/Nb ratios, and low abundances of K2O, LIL elements, and rare earth elements (REE). Their petrographic and geochemical features indicate derivation from an intermediate parental magma through crystal fractionation and accumulation processes, involving hornblende, plagioclase, and magnetite. The Tanzawa plutonic complex is interpreted to be the exposed middle crust of the IBM arc, which was uplifted during the collision. The mass balance calculations, combining data from melting experiments of hydrous basaltic compositions at lower-to-middle crustal levels, suggest that parental magma and ultramafic restite were generated by dehydration partial melting (∼ 45% melting) of amphibolite chemically similar to low-K tholeiitic basalt. Partial melting of hydrated mafic lower crust might play an important role in felsic middle-crust formation in the IBM arc.