Prograde Metamorphism of the Horokanai Ophiolite in the Kamuikotan Zone, Hokkaido, Japan

The Horokanai ophiolite is a segment of metamorphosed oceanic crust and upper mantle, tectonically replaced into the Kamuikotan Zone of Hokkaido, Japan. Metamorphic grade, ranging from the zeolite faciles (Zone A) through the greenschist facile (Zone B) and the greenschist-amphibolite transitional facile (Zone C), to the amphibolitic and granulizes facile (Zone D) increases progressively downwards with zone boundaries subparallel to the ophiolite pseudostraitigraphy. The granulite facile rocks include both metagabbros and their underlying ultranafic rocks. Coexisting minerals from several tens of samples covering all the minerals zones were analysed by means of an electronprode microanalyser; the results are presented, along with brief consideration of their compositional variation with metamorphic grade. The faciles series of metamorphism of the Horokanai ophiolite corresponds to the low-pressure type with a temperature range of 100-750 °C, which is broadly comparable to that inferred for ocean-floor metamorphism. The major difference is the presence of the granulite facile rocks in the Horokanai ophiolite and its absence in ocean-floor metamorphism.