Prograde and anatectic reactions in the deep arc crust exposed in the Hidaka metamorphic belt, Hokkaido, Japan

Abstract

The Hidaka metamorphic belt is a tilted crustal assembly of a magmatic arc, exposing two third of a total thickness of the arc crust. The igneous activities and metamorphism took place during the Eocene in a tectonically stacked and thickened accretionary prism of the latest Cretaceous to earliest Tertiary age. The lower part of the exposed section consists mainly of mafic rocks with thin intercalations of pelitic and psammitic rocks metamorphosed under amphibolite and granulite and granulite facies, while the upper part consists almost of pelitic and psammitic rocks metamorphosed under amphibolite and greenschist facies, which are gradually changed to very weakly metamorphosed sedimentary rocks toward the top of the section. Reexamination of prograde metamorphism of pelitic granulite facies rocks confirmed a path of increasing temperature with a constant pressure or with slight decrease of pressure, associated with moderate shearing. Norite and diorite veining as a part of mafic intrusives were followed to give rise to the peak metamorphism associating incipient anatexis of pelitic and psammitic gneisses in the basal part of the exposed section. Due to the anatexis, small leucocratic patches or viens are formed, with orthopyroxene and cordierite as residual phases, cutting strong foliations of biotite with garnet and cordierite porphyroblasts. The mafic intrusives played an important role in the metamorphic evolution in the deep part of the Hidaka crust.