MELTING EXPERIMENTS ON HYDROUS LOW-K THOLEIITE - IMPLICATIONS FOR THEGENESIS OF TONALITIC CRUST IN THE IZU-BONIN-MARIANA ARC

A series of water-deficient partial melting experiments on a low-K tho leiite were carried out under lower crustal P-T-H2O conditions (900-12 00 degrees C, 0.7-1.5 GPa, 2 and 5 wt% H2O added) using a piston-cylin der apparatus. With increasing temperature at 1.0 GPa, supersolidus mi neral assemblages vary from amphibolitic to pyroxenitic. Garnet crysta llizes in the higher pressure runs (> 1.2 GPa). Melt compositions show low-K calc-alkalic trends, and are classified as metaluminous or pera luminous tonalite. These features are similar to the felsic rocks in t he Izu-Bonin-Mariana (IBM) are, for example Tanzawa plutonic rocks. Th e anatectic origin of Tanzawa tonalites is consistent with geochemical modeling, which demonstrates that the rare earth element (REE) charac teristics of Tanzawa plutonic rocks (which represent the middle crust of the IBM: are) can be generated by partial melting of amphibolite in the lower crust (similar to 50% melting at 1050 degrees C and below 1 .2GPa). Estimated densities of pyroxenitic restites (similar to 3.9 g/ cm(3)) after extraction of andesitic melts are higher than that of man tle peridotite beneath the island are (3.3 g/cm(3)). The high density of the restite could cause delamination of the IBM are lower crust. Rh yolitic magmas in the IBM are (e.g. Niijima) could be formed by low de grees of partial melting of the amphibolitic crust at a temperature ju st above the solidus (10% melting at or below 900 degrees C).