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

In the Besshi district of the Sambagawa metamorphic belt, thereare two types of eclogites: one occurring in the Sebadani metagabbromass 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 emplacementinto the Sambagawa terrain as a hot eclogite mass. Basic schistssurrounding the Sebadani mass, which had suffered the Sambagawametamorphism of albite epidote amphibolite facies, were contact-metamorphosedat high pressure by the emplacement of the Sebadani mass. Asa result, the basic schists were dehydrated to form eclogiticbasic schists, i.e. garnet and omphacite porphyroblast-bearingbasic schists. Thus, two types of ecologite, retrograde andprograde, converged into the same metamorphic condition, 610–650?C, 7–17 kb, in a part of the ecologite facies duringthe Sambagawa metamorphism. Correspondingly, the values of distributioncoefficients of Fe and Mg between garnet and omphacite increasefrom core pairs to rim pairs in the retrograde eclogites anddecrease from core pairs to rim pairs in the prograde ecologites.After this stage, both the prograde and retrograde eclogitesshared a common metamorphic history; they were retrograded viathe epidote amphibolite facies to the greenschist facies. The Sebadani metagabbro mass, as a large tectonic block, hadbeen emplaced into a m?lange zone in the Sambagawa metamorphicbelt after the peak of the Sambagawa metamorphism, probablyfollowing initiation of uplift of the metamorphic rocks fromtheir deep-seated environment.