METAMORPHIC ROCKS OF THE HITACHI DISTRICT IN THE SOUTHERN ABUKUMA PLATEAU

Abstract

The Paleozoic rocks of the Hitachi district in the southern Abukuma Plateau suffered metamorphism ranging from the greenschist facies to the amphibolite facies through the epidote-amphibolite facies. This area can be divided into the following five zones by the stability relations of some metamorphic minerals. Zone I: chlorite zone Zone II: biotite zone Zone III: epidote-hornblende-sodic plagiocalse zone Zone IV, V: sillimanite zone The metamorphism is characterized by the behavior of the anorthite content in plagioclase, the chloritoid-Fe-chlorite-anorthite assemblage, the optical property of calciferous amphibole, the assemblage of cummingtonite-epidote-sodic plagioclase and the association of Al2SiO5 polymorphism. The association of andalusite-kyanite-sillimanite shows the following relation, These minerals probably grew metastably close by the triple point of these polymorphs. The chloritoid-Fe-chlorite-anorthite assemblage can be interpreted by the Po2 condition at the various stages of the metamorphism; chloritoid and Fe-chlorite are sensitive to Po2 and Fe-chlorite-chloritoid-magnetite assemblage occurs at the low Po2 condition. And as a result of dehydration reaction at the low Po2 condition, anorthite did not decompose to epidote but reacted to Fe-chlorite to produce chloritoid, ilmenite, quartz and calcite. The assemblage of cummingtonite-epidote-sodic plagioclasc indicate the higher PH2O condition of metamorphism; under the metamorphic condition such as the epidote-amphibolite facies, anorthite is unstable and the reaction between anorthite mole and vapor may begin easily to produce zoisite, so that under the higher PH2O and Po2 condition the reaction of producing cummingtonite-epidote-oligoclase may happen with progressive increasing of the metamorphic grade. The anorthite content in plagioclase is generally slower incrcasing with the rising of the metamorphic grade on account of the nature of metamorphism itself and peristerite may occur at a part of the lower metamorphic grade in this district. The optical property of calciferous amphibole is characterized by the small optic axial angle and the wide range of refractive index. Some amphiboles are plotted in field M by Miyashiro's classification. Reflecting on these characters, the metamorphism in the Hitachi district can be explained to be single progressive metamorphism without “polymetamorphism”. The Hitachi metamorphic facies series is of rather higher pressure and/or lower temperature than the central Abukuma Plateau and is characterized by heterogeneous PH2O or Po2 condition of the metamorphism.