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

本研究项目的研究结果总结如下：1)北海道北部鄂霍次克海沿岸奥科佩地区的新世火山岩根据岩石学和地球化学特征可分为3组；(1)东野组安山岩-英安质岩；(2)光山熔岩和ⅰ型脉岩；(3)ⅱ型脉岩。Sr/Y比值明显高于普通岛弧型安山岩的第一类岩石为埃达质安山岩和英安岩，而高SiO_2和高MgO含量的第二类和第三类岩石为高mg安山岩。易达质中间岩可能是俯冲洋壳衍生的长英质岩浆在上升过程中与上覆地幔橄榄岩相互作用而发生变质的结果。第二种岩石可能是原生高镁安山岩岩浆与较长英质岩浆混合形成的。(2)北海道北部年龄小于12 Ma的中新世—第四纪玄武岩，其初始^<87>Sr/^<86>Sr （SrI）和初始^<143>Nd/^<144>Nd （NdI）在较窄范围内略未枯竭；SrI值为0.70296 ~ 0.70393，NdI值为0.51288 ~ 0.51307，但SrI和NdI值无时间变化，而同一地点（Ohmu地区）的特殊玄武岩比北海道北部其他玄武岩（NH玄武岩）表现出更多的未贫Sr和Nd同位素特征。NH玄武岩的形成与北海道北部软流圈注入大陆岩石圈的地幔演化密切相关，并伴随千岛岛弧后盆地的扩张；欧姆地区玄武岩的岩浆源可能对应于软流圈地幔物质注入后变薄的次大陆岩石圈地幔物质，大部分NH玄武岩岩浆来源于软流圈地幔物质注入。软流圈地幔物质在TiO2/K2O和Zr/Y比值以及Sr和Nd同位素Less等主微量元素特征上与MORB源略有不同

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