Plutonic rocks in the Mineoka–Setogawa ophiolitic mélange, central Japan: Fragments of middle to lower crust of the Izu–Bonin–Mariana Arc?

Abstract

A Paleogene accretionary complex, the Mineoka–Setogawa Belt, is distributed around the Izu Collision Zone, central Japan. Plutonic rocks of gabbro, diorite and tonalite compositions are included as fragments and dykes in an ophiolitic mélange in this belt. Zircon U–Pb dating of the plutonic rocks indicates that they were formed at ca. 35Ma simultaneously. These ages are consistent with Eocene–Oligocene tholeiite and calc-alkaline arc magmatism in the Izu–Bonin–Mariana (IBM) Arc and exclude several previous models for the origin of the Mineoka–Setogawa ophiolitic rocks. The geochemical characteristics of these plutonic rocks are similar to those of the Eocene–Oligocene IBM tholeiite and calc-alkaline volcanic rocks as well as to the accreted middle crust of the IBM Arc, the Tanzawa Plutonic Complex. Moreover, their lithology is consistent with those of the middle and lower crust of the IBM Arc estimated from the seismic velocity structure. These lines of evidence strongly indicate that the plutonic rocks in the Mineoka–Setogawa ophiolitic mélange are fragments of the middle to lower crust of the IBM Arc. Additionally, the presence of the Mineoka–Setogawa intermediate to felsic plutonic rocks supports the hypothesis that intermediate magma can form continental crust in intra-oceanic arcs.