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

主要研究结果如下：1）北海道北部鄂霍次克海沿岸沿着中新世火山岩的岩石学和地球化学特征可分为3类：1）丰野组的安山质-英安质岩; 2）I型三井山熔岩和脉岩; 3）II型脉岩。第一类岩石的Sr/Y值远高于普通岛弧型安山岩，属于埃达克质安山岩和英安岩，而第二类和第三类岩石的高SiO_2和高MgO含量则属于高镁安山岩。埃达克质中间岩可能是俯冲洋壳源长英质岩浆上升至地表时与上覆地幔橄榄岩相互作用而发生变质的结果。第二类岩石可能是由原生高镁安山质岩浆和较长英质岩浆混合而成。 ...更多信息 与MORB相比，北海道北部晚于12 Ma的中新世至第四纪玄武岩显示出较窄范围的初始<87>Sr/Sr<86>（SrI）和初始<143>Nd/Nd<144>（NdI），但SrI和NdI没有随时间变化，SrI和NdI分别为0.70296-0.70393和0.51288-0.51307，而来自一个地点（Ohmu地区）的例外玄武岩比其他北海道北部玄武岩（NH玄武岩）显示出更多的未亏损Sr和Nd同位素特征。NH玄武岩的形成与地幔演化密切相关，即伴随着千岛弧后盆地的扩张，软流圈向北海道北部大陆岩石圈的注入; Ohmu地区玄武岩的岩浆源可能对应于被软流圈地幔物质注入而变薄的次大陆岩石圈地幔物质，已经衍生出来了。这些软流圈地幔物质在常量和微量元素特征如TiO 2/K2 O和Zr/Y比值以及Sr和Nd同位素方面与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