Green Tuff metamorphism of the Miocene basaltic rocks in the northern part of Northeast Japan.

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This project was undertaken in order to understand better the secondary mineralization of the Miocene submarine lavas and pyroclastic rocks with basaltic to andesiiic compositions during the "Green Tuff alteration". The Hanaoka area of the Akita Prefecture, and the Okiura, Hakamagoshi-dake and Kanita areas of the Aosorl Prefecture were selected for this purpose in the northern part of the Northeast Japan. Drilling has confirmed that the Miocene basaltic piles developed in the Hanaoka and Okiura areas are at least 700 meters thick. Their thickness in the other areas are estimated to be more than a few hundred meters. The metabastes in these four areas well preserved their original textures and primary phases such as clinopyroxene and plagioclase, but they have been recrystallized to varying degrees at low temperatures, resulting in the formation of various secondary minerals.In the Hanaoka and Okiura areas, the metabastes are characterized by the assemblage of chlorite + epidote + pumpellyite + prehnite at the surface exposures, but chlorite + epidote + prehnite + actinolite in the deep drill core, in addition to albite and quartz. Zeolites occur only in veins and amygudules of the samples from the surface exposures. Ca-zeolites are laumontite and yugawaralite in the Hanaoka area and wairakite, laumontite and yugawaralite in the Okiura area. In the Hakamagoshi-dake and Kanita areas, chlorite, epidote, prehnite, and pumpellyite in the matrices and in pseudomorphs after plagioclase. Actinolite rarely occurs as overgrowth on relict clinopyroxene crystal in the Kanita area. Characteristic Ca-zeolites are laumontite, stilbite and heulandite in veins and amygudules in both areas.These Miocene rocks have been extensively subjected to the "Green Tuff" hydrothermal metamorphism. Their fossil geothermal gradients of more than 100ﾟC/km would be estimated in these four areas.