四国カルストの緑色岩類 中国帯，秋吉・帝釈石灰岩台地の基底緑色岩類との岩石化学的対比

Abstract

Shikoku Karst, the mountainous limestone plateau extending from west to east in central Shikoku, is one of Upper Paleozoic limestone plateau in the southwest Japan outer belt. Basal greenstones, overlain by the limestone bed are composed of weakly porous pillow to massive lavas and hyaloclastites which are intercalated with thin chert and clastic sediments. Petrochemical affinity of the greenstones from Shikoku Karst with arc basalt is well exhibited by MORB normalized Nb-depleted spidergram. The greenstones from Akiyoshi-dai and Taishaku-dai, the coeval limestone plateaus in the southwest Japan inner belt, have the similar petrochemistry to seamounts or oceanic islands of intra-oceanic plate. REE abundance of the greenstones from Shikoku Karst is evidently lower than that of Taishaku-dai and Akiyoshi-dai. Chondrite normalized patterns of the rocks from Shikoku Karst characterized by low and mild downward inclination from LREE to HREE or mild upward convex of intermediate REE are similar to that of N-MORB and/or island-arc tholeiites. Chondrite normalized patterns of the greenstones from Akiyoshi-dai and Taishaku-dai represent tholeiitic and alkaline basalts patterns of within oceanic plate. CeN/YbN to CeN relation supports the same magmatic and geotectonic interpretation mentioned above. It is concluded that the greenstones from Shikoku Karst represent island arc related origin, and the greenstones from Taishaku-dai and Akiyoshi-dai express intra-plate seamounts or oceanic islands.