Acidic volcanic products in Lower Cretaceous deposits of the Sorachi- Yezo Belt, Hokkaido, northeast Japan

Abstract

The Sorachi-Yezo Belt in Hokkaido is made up of three geological units, from bottom to top: an ophiolitic sequence, lower Cretaceous volcanogenic sediments, here named the lower Cretaceous acidic deposits, and the Yezo Supergroup. The lower Cretaceous acidic deposits, Berriasian to Hauterivian (Aptian?) in age, overlie the ophiolitic sequence of the lower part of the Sorachi Group and are composed of a basal conglomerate and an alternation of pelitic rocks, acidic tuffites and volcanogenic sandstones. These acidic volcanic rock fragments are the only remains in Hokkaido indicating the existence of an Early Cretaceous acidic volcanism that possibly started during the latest Jurassic. The Rebun-Kabato volcanic arc (ie Rebun-Kabato Belt), at least partly built upon continental crust, was active during the Early Cretaceous and constitutes the only possible source for this acidic material. -from Authors