Texture to decipher growth stage of albite-porphyroblasts in pelitic schists from the Sambagawa metamorphic belt

Abstract

Considering textures, chemical analyses and mineral assemblages of the Sambagawa metamorphic rocks, some insight to the growth stage of albite-porphyroblasts was obtained. There are two types of titanite in the pelitic schists studied: 1) minute grains (up to 0.35 mm in length) in the matrix and 2) euhedral coarse grains (up to 0.6 mm in long axis) included in albite porphyroblasts. The minute titanite formed during the prograde stage, because it is a member of the main foliation-forming minerals. It suggests that albite-porphyroblasts crystallized after the titanite had formed. The textures and chemical compositions of garnet suggest that its growth finished near the thermal climax of metamorphism. The euhedral titanites with inclusions of euhedral garnet formed at the culminating stage of metamorphic temperature, therfore the albite porphyroblasts including the euhedral titanites formed during a retrograde stage.