Melting experiments on hydrous low‐K tholeiite: Implications for the genesis of tonalitic crust in the Izu–Bonin – Mariana arc

Abstract

A series of water‐deficient partial melting experiments on a low‐K tholeiite were carried out under lower crustal P–T–H2O conditions (900–1200 °C, 0.7–1.5 GPa, 2 and 5 wt% H2O added) using a piston‐cylinder apparatus. With increasing temperature at 1.0 GPa, supersolidus mineral assemblages vary from amphibolitic to pyroxenitic. Garnet crystallizes in the higher pressure runs (> 1.2 GPa). Melt compositions show low‐K calc‐alkalic trends, and are classified as metaluminous or peraluminous tonalite. These features are similar to the felsic rocks in the Izu–Bonin – Mariana (IBM) arc, for example Tanzawa plutonic rocks. The anatectic origin of Tanzawa tonalites is consistent with geochemical modeling, which demonstrates that the rare earth element (REE) characteristics of Tanzawa plutonic rocks (which represent the middle crust of the IBM arc) can be generated by partial melting of amphibolite in the lower crust (∼ 50% melting at 1050 °C and below 1.2 GPa). Estimated densities of pyroxenitic restites (∼ 3.9 g/cm3) after extraction of andesitic melts are higher than that of mantle peridotite beneath the island arc (3.3 g/cm3). The high density of the restite could cause delamination of the IBM arc lower crust. Rhyolitic magmas in the IBM arc (e.g. Niijima) could be formed by low degrees of partial melting of the amphibolitic crust at a temperature just above the solidus (10% melting at or below 900 °C).