Prograde and Retrograde Eclogites in the Sambagawa Metamorphic Belt, Besshi District, Japan

Abstract

In the Besshi district of the Sambagawa metamorphic belt, there are two types of eclogites: one occurring in the Sebadani metagabbro mass and retrograded from high-temperature anhydrous ecologite, and the other in the basic schists and produced by prograde, dehydration of epidote amphibolite. The Sebadani metagabbro mass was originally layered gabbro, which was once equilibrated in the ecologite facies before emplacement into the Sambagawa terrain as a hot eclogite mass. Basic schists surrounding the Sebadani mass, which had suffered the Sambagawa metamorphism of albite epidote amphibolite facies, were contact-metamorphosed at high pressure by the emplacement of the Sebadani mass. As a result, the basic schists were dehydrated to form eclogitic basic schists, i.e. garnet and omphacite porphyroblast-bearing basic schists. Thus, two types of ecologite, retrograde and prograde, converged into the same metamorphic condition, 610–650 °C, 7–17 kb, in a part of the ecologite facies during the Sambagawa metamorphism. Correspondingly, the values of distribution coefficients of Fe and Mg between garnet and omphacite increase from core pairs to rim pairs in the retrograde eclogites and decrease from core pairs to rim pairs in the prograde ecologites. After this stage, both the prograde and retrograde eclogites shared a common metamorphic history; they were retrograded via the epidote amphibolite facies to the greenschist facies. The Sebadani metagabbro mass, as a large tectonic block, had been emplaced into a mélange zone in the Sambagawa metamorphic belt after the peak of the Sambagawa metamorphism, probably following initiation of uplift of the metamorphic rocks from their deep-seated environment.