Geological and Petrological Studies of the Mikabu Greenstones in Eastern Shikoku, Southwest Japan

This paper presents geological and petrological studies on the Mikabu greenstones and their associated rocks in three areas in eastern Shikoku, i.e., Kamiyama (Myozai-gun, Tokushima Pref.), Higashiiya (Miyoshi-gun, Tokushima Pref.) — Toyonaga (Otoyo-cho, Nagaoka-gun, Kochi Pref.) and and Jizoji (Tosa-cho, Tosa-gun, Kochi Pref.) areas. An emphasis is placed on the analysis of geological structures of the Mikabu belt and its relation to the Sambagawa belt in the north and to the Chichibu belt in the south. Petrological and petrochemical characteristics of the Mikabu greenstones are analyzed and examined in order to compare them with those of typical ophiolites and with those of similar rocks of various origins. The major results are listed below.
1) The Mikabu belt is a large-scale nappe (Mikabu nappe) which is covered by the sedimentary se-quences of the Chichibu belt and overlies the Sambagawa crystalline schists across a thrust. The Mikabu nappe appears to have been thrust over the Sambagawa belt during the Late Triassic. 2) Folding in a large scale occurred in two distinct stages in the Mikabu belt, and they correspond to the Nagahama-Ozu phase and the Hijikawa phase (the Latest Jurassic or the Early Cretaceous) of de-formation in the Sambagawa belt.
3) The Mikabu greenstones consist of pillow lava — hyalocalastite sequence interbedded with thin layers of sediments (chert, pelite and limestone), diabase dikes and gabbro sills intruded into the volcanic sequence, and peridotite-gabbro complexes. The peridotite-gabbro complexes occur as small lenses (less than about 500 m in major axis) mostly aligned in rows parallel to the thrusts within the Mikabu belt, and they seem to have been taken tectonically into the Mikabu belt in a solid state nearly contemporaneously with the movement of the Mikabu nappe. The Mikabu greenstones do not show the complete sequence of typical ophiolites.
4) Petrochemical analysis of the volcanic rocks reveals that picrite-basalts, tholeiitic basalts and alkali basalts coexist within the Mikabu greenstones. This is in marked contrast with typical ophiolites which consist of abyssal tholeiites. The origin of the Mikabu greenstones can not be explained as being derived from a single magma.
5) The tholeiitic basalts in the Mikabu greenstones are similar to abyssal tholeiites which have been depleted in incompatible elements. But the Mikabu tholeiites are poorer in A12O3 than are the abyssal tholeiites, and in this respect they are similar to oceanic-island tholeiites.
6) Based mainly on the petrochemical data of the Mikabu tholeiites combined with geological data on the Mikabu belt and its surrounding areas, it is argued that the Mikabu greenstones probably formed during the Early Pennsylvanian or slightly before in a marginal basin. This conclusion is still somewhat speculative and complete understanding of the origin of the Mikabu greenstones must await future studies.