|Budget Amount *help
¥2,200,000 (Direct Cost : ¥2,200,000)
Fiscal Year 1991 : ¥600,000 (Direct Cost : ¥600,000)
Fiscal Year 1990 : ¥1,600,000 (Direct Cost : ¥1,600,000)
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.