Study of origin of metabasites from high-pressure metamorphic terrains

为了研究原岩形成的大地构造环境，对北海道高压区龟田带和四国三百川带中的变质基性岩进行了野外调查和化学分析(主要、次要和稀土元素)，这些变质基性岩通常形成紧密堆积的枕状熔岩，很少形成孤立或角砾状的枕状熔岩和块状熔岩，在泥质基质中呈不同大小和形状的块状产出，形成混杂岩。三巴川变质基性岩以密实的枕状熔岩、孤立或角砾状的枕状熔岩和块状熔岩的形式产出，并与周围主要来源于泥岩的变质沉积相一致。根据化学分析，Kamuikotan带有两种类型的变质基性岩，一种是大洋高原型玄武岩，另一种是大洋岛型玄武岩，其中前者在野外更为丰富。这表明Kamuikotan变质基性岩起源于大洋高原-岛屿杂岩。此外，Sanbagawa变质基性岩在化学上分为两种类型，一种是具有弧后盆地亲和性的N型MORB(洋脊玄武岩)，另一种是E型MORB，通常在弧后盆地的沉积物中发现，前者产于地层层序的最低水平。这表明三巴川变质岩起源于菲律宾海等弧后盆地，这种原岩构造环境的差异可能与变质条件的差异，特别是温度条件的差异有关。因此，三巴川带的变质作用温度高于Kamuikotan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.