Ophiolite Metamorphism and Ocean-Floor Metamorphism.

Ophiolites of primary igneous origin have been more or less modified by later metamorphic processes, and there are several types of ophiolite metamorphism such as (1) the low-pressure type as recognized in ophiolites of the Mediterranean, Alps, Oman, Newfoundland, Chile, Taiwan, Borneo, California Coast Range, Hidaka Western Zone, and Horokanai, (2) the high-pressure type as recognized in dismembered ophiolites of Franciscan, New Caledonia, Alps, and Sanbagawa, (3) the medium-pressure type as recognized in Yakuno ophiolite, and (4) the polymetamorphosed (from low-pressure to high-pressure) type as recognized in ophiolites of Kurosegawa. Of these, the first, low-pressure type has the nature broadly similar to that of metamorphic rocks dredged or drilled from ocean-floor; both types of metamorphism are characterized by (a) the facies series includes the zeolite, prehnite-actinolite (partly, prehnite-pumpellyite), greenschist, amphibolite, and granulite facies, but the top sedimentary rocks of the ophiolite sequence exhibit no metamorphic feature, (b) the metamorphic grade increases down the ophiolite sequence, and the order of about 100°C/km is inferred for the geothermal gradient, (c) the igneous structure and/or texture are commonly preserved in the low-to mediumgrade rocks, and (d) the element migration mainly due to hydrothermal circulation of hot brine occurs generally in the low-grade rocks. It follows that there is a similar genetic environment between the low-pressure type of ophiolite metamorphism and ocean-floor metamor-phism. Therefore, the comparative study of both metamorphism may provide an excellent opportunity for understanding the origin of ophiolites, and it is proposed that the oceanic or backarc spreading ridge may be the most plausible tectonic setting for this type of metamorphism.