Origin of felsic magmas produced during the spreading of the back-arc basin

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The results obtained from this research project are summarized as follows.1) Miocene volcanic rocks from the Okkope area along the Okhotsk Sea coast, north Hokkaido can be divided into three groups based on their petrographical and geochemical characteristics ; 1) andesitic to dacitic rocks in the Toyono Formation, 2) Mitsuiyama lavas and dike rocks of type I, and 3) dike rocks of type II. The first rocks with much higher Sr/Y ratios compared to the ordinary island arc type andesitic rocks are defined as adakitic andesite and dacite, whereas the second and third rocks chracterized by high SiO_2 and high MgO contents correspond to high-Mg andesites. The adakitic intermediate rocks may result from a modification of the subducting oceanic crust derived felsic magma due to interaction with the overlying mantle peridotite during its rise to the surface. It is likely that the second rocks may be derived by mixing between the primary high-Mg andesitic magma and more felsic magma.2) Most of mi … More ddle Miocene to Quaternary basaltic rocks younger than around12 Ma from north Hokkaido show initial ^<87>Sr/^<86>Sr (SrI) and initial ^<143>Nd/^<144>Nd (NdI) in narrow ranges slightly undepleted, compared to MORB ; 0.70296-0.70393 in SrI and 0.51288-0.51307 in NdI, respectively, but no temporal variation in SrI and NdI, whereas the exceptional basalts from one location (Ohmu area) exhibit more undepleted Sr and Nd isotopic signature than other north Hokkaido basaltic rocks (the NH basaltic rocks). The generation of the NH basaltic rocks seems to be deeply related to the mantle evolution delineated as the asthenospheric injection into continental lithosphere beneath north Hokkaido, accompanying with the spreading of the Kurile back-arc basin ; the magma source for the Ohmu area basalts may correspond to the subcontinental lithospheric mantle material thinned by the injection of asthenospheric mantle material from which most of the NH basaltic magmas have been derived. These asthenospheric mantle materials are slightly different from MORB source with respect to major and trace element characteristics such as TiO2/K2O and Zr/Y ratios as well as Sr and Nd isotopes Less