P-T evolution of Abukuma metamorphic rocks in north-east Japan: metamorphic evidence for oceanic crust obduction

Abstract

The Abukuma metamorphic terrane of northeast Japan is well-known for the type terrains of andalusite-sillimanite progressive metamorphism of Miyashiro’s classic studies (1958, 1961). Later, however, relict kyanite was found in the high-grade pelitic gneisses (Kano & Kuroda 1968), and there have been arguments about whether the Abukuma metamorphism was plurifacial or polymetamorphic; earlier kyanite-sillimanite type and later andalusitesiUimanite type. Since 1984 we have been studying the geology and petrology of the Abukuma metamorphic rocks to reveal their P-T-t paths and the tectonic evolution of the terrane. The purpose of this paper is to outline our new insight into the P-T-t paths of rocks and their correlation with deformation events of the terrane. Detailed geology, mineralogy and petrology will be presented elsewhere. In this paper, mineral abbreviations of Kretz (1983) are used to represent compositions and reactions of minerals. Geological and petrological setting The Abukuma metamorphic terrane is one of the major geological units of north-east Japan, which is bounded from south-west Japan by the Tanakura tectonic line (Fig. 1). The Abukuma metamorphic rocks, which are extensively intruded by the Cretaceous plutonic rocks (Shibata & Uchiumi 1983, Shibata & Tanaka 1987), are lithologically divided into two, the Gosaisyo series to the north-east and the Takanuki series to the south-west (Fig. 1). The Gosaisyo series is composed mainly of metabasites and metacherts with subordinate amounts of meta-ultrabasites, metapelites and limestones, having affinities with oceanic crust. Original structures and textures, however, are poorly preserved owing to intense deformation. Hiroi et al. (1987) reported