PETROLOGY AND GEOCHEMISTRY OF LAVAS FROM THE SUMISU AND TORISHIMA BACKARC RIFTS

Thirteen dredge hauls from the active Sumisu and Torishima rift grabens west of the Izu-Bonin arc at about 30°-31°N, 140°E, recovered a suite of tholeiitic basalts to sodic rhyolites. Volcanism occurs along tensional faults within and bounding the rift grabens and along the transfer zones between adjoining rift segments. The Sumisu and Torishima rift lavas differ significantly from the lavas of the adjacent arc volcanic centers in having lower Al2O3 Na2O, Ba Zr, V Ti, and Ba Ce and higher abundances of the rare earth elements. The rift lavas also have characteristics of backarc basin basalts, in that they are enriched in Al2O3, and depleted in total iron and TiO2 relative to mid-ocean ridge basalt, characteristics which are consistent with a higher water content in the source. Thus, the model of a progressive change in backarc basin basalt composition from arc-like to mid-ocean ridge-like, as a function of evolution of the basin, as has been suggested from many backarc regions, is not generally applicable. The comparison of the Sumisu and Torishima rift lavas with Mariana backarc basin lavas indicates that backarc basin basalts differ in composition from one basin to another. The comparison of these backarc basin suites with mid-ocean ridge suites from similar axial depths indicates that the overall control over the spectrum of backarc basin basalt compositions may be different extents of melting of the mantle. The Sumisu and Torishima rift lavas formed by a slightly higher extent of melting than the Mariana backarc basin basalts, a phenomenon which is related to the depth of the ridge segments. Furthermore, the data suggest a systematically higher extent of melting in the arc lavas than in the backarc lavas for both of these arc/backarc systems, consistent with a greater flux of water beneath the arc than beneath the backarc region. © 1990.