Chemical factors controlling occurrence of staurolite in the Ryoke metamorphic rocks of the Hongu-san area, Aichi Prefecture

Abstract

Eleven localities of staurolite-bearing metapelites and pelitic bands in metachert, are reported from the Ryoke metamorphic belt in the Hongu-san area (Aichi Prefecture, Japan). Staurolite occurs as inclusions within andalusite porphyroblasts and within mica-quartz aggregates in metapelites. On the other hand, staurolite in pelitic bands in metachert, occurs not only as inclusions within mica-quartz aggregates but also as porphyroblast. Petrographic observations indicate that the mica-quartz aggregates were formed by replacing staurolite porphyroblasts during prograde stage of the Ryoke regional metamorphism. Based on the widespread spatial distribution of the mica-quartz aggregates, it is suggested that staurolite was widely produced in the area. During subsequent stages of prograde and peak metamorphism, staurolite became unstable in some rocks while remaining stable in others. Thermodynamic calculations reveal that the Ti content of biotite and the Zn content of staurolite affect the stability of staurolite, thus explaining its heterogeneous preservation in the study area.