Study of origin of metabasites from high-pressure metamorphic terrains

对北海道高压Kamuikotan带和四国三巴川带的变质岩进行了野外调查和化学分析（主、次、稀土元素），以考察原岩的构造背景。Kamuikotan变质岩通常形成紧密堆积的枕状熔岩，很少形成孤立或角化的枕状熔岩和块状熔岩，在泥质基质中形成大小和形状各异的块体，形成混杂岩。三八川变质岩以密实堆积的枕状熔岩、孤立的或角化的枕状熔岩和块状熔岩等形式出现，与周围主要来源于泥岩的变质沉积岩普遍相序。在化学分析的基础上，Kamuikotan地区存在两类变质岩；一种是海洋高原型玄武岩，另一种是海洋海岛型玄武岩，其中前者在野外较为丰富。这表明Kamuikotan变质岩起源于海洋高原-岛屿杂岩。三巴川变质岩在化学上可分为两类；一种是具有弧后盆地亲缘关系的n型洋中脊玄武岩，另一种是在弧后盆地沉积物中以基底形式存在的e型玄武岩，其中e型玄武岩产于层序的最底层。这表明三八川变质岩起源于弧后盆地，如菲律宾海。Kamuikotan和Sanbagawa地区原岩构造环境的差异可能与变质条件，特别是温度条件的差异有关。因此，认为三巴川带的变质作用温度要高于龟影滩带。

Field surveyes and chemical analyses(major, minor and rare earth elemenys) were done for metabasites from the high-pressure Kamuikotan zone in Hokkaido and Sanbagawa zone in Shikoku in order to examine the tectonic setting of original rocks.The Kamuikotan metabasites usually form close-packed pillow lavas and rarely isolated or brecciated pillow lavas and massive lavas, and occur as various sized- and shaped-blocks within pelitic matrix, forming a melange. The Sanbagawa metabasites occur as close-packed pillow lavas, isolated or brecciated pillow lavas, and massive lavas, and are commonly coherent with surrounding metasediments mainly derived from mudstones.On the basis of the chemical analyses, there are two types of metabasites in the Kamuikotan zone ; one is an ocean plateau type of basalts and the other an ocean island type of basalts, of which the former is more abundant in the field. This suggests that the Kamuikotan metabasites were derived from an ocean plateau-island complex. Also, the Sanbagawa metabasites are chemically divided into two types ; one is a N-type MORB(Mid-Ocean Ridge Basalt)with a back-arc basin affinity and the other a E-type MORB commonly found as shills within sediments in back-arc basins, of which the former occurs in the lowest level of the stratigraphic sequence. This indicates that the Sanbagawa metabasites were derived from a back-arc basin such as the Philippine Sea.Such difference of the tectonic setting of original rocks from the Kamuikotan and Sanbagawa zones may be related with the difference of the metamorphic conditions, especially the conditions of temperature. It is, therefore, suggested that the temperature of metamorphism is higher in the Sanbagawa zone than in the Kamuikotan zone.