Petrological and mineralogical contrasts of basic lithologies between eclogite and non–eclogite units along the Kokuryo River of the Sanbagawa belt, Central Shikoku, Japan

Abstract

The Tonaru epidote–amphibolite is one of the large metagabbro dominated bodies and occurs in schistose rocks of the Sanbagawa metamorphic belt, central Shikoku. This body locally retains mineral parageneses of eclogite facies equilibrium prior to the epidote–amphibolite facies stage. The lithologic boundary between the epidote–amphibolite and the surrounding schistose rocks was well observed along the Kokuryo River in the western part of the Besshi region in central Shikoku. Boundary zone of 1.5–2.5 m wide is developed between the epidote–amphibolite and pelitic schist. This zone is composed of a basic layer and alternating layers consisting of thin amphibole–rich and mica–rich bands, which occupy the epidote–amphibolite and pelitic schist sides, respectively. The basic layer has a chondrite–normalized rare–earth element (REE) pattern with slight enrichment of light REEs, which corresponds to epidote–amphibolite. By contrast, the amphibole–rich band has a flat REE pattern similar to the basic schist of the Sanbagawa belt.

 The basic layer in the boundary zone and epidote–amphibolite have composite–zoned garnet, showing a compositional discontinuity between the core and mantle parts, similar to that in the Sanbagawa eclogite unit. Garnet in the amphibole–rich and mica–rich bands of the alternating layers and pelitic schist shows simple normal zoning, which commonly occurs in the Sanbagawa non–eclogite unit. Sodic plagioclase occurs as inclusions in the mantle part of the composite–zoned garnet and normally zoned garnet as well as in a matrix phase. These lithologies belong to the oligoclase–biotite zone with equilibrium pressure/temperature conditions of 1.1–1.2 GPa/595–625 °C; discontinuity of metamorphic grade is not detected throughout the outcrop for the epidote–amphibolite facies stage.

 These data suggest that (1) the basic layer is the fractured part of the epidote–amphibolite and (2) the tectonic boundary between the eclogite and non–eclogite units corresponds to the lithologic boundary between the basic layer and alternating layers of thin amphibole–rich and mica–rich bands in the boundary zone.