Chloritoid-bearing basic schists from the Sanbagawa metamorphic belt, central Shikoku: their petrologic significance and tectonic implications

Abstract

Chloritoid- and taramite-bearing basic schists occur on the streambed at Ikadadu in the garnet zone of the Sanbagawa metamorphic belt, central Shikoku. Their major constituent phases are chloritoid, taramite, chlorite, paragonite, phengite, epidote, ilmenite, titanite, albite and quartz. Chloritoid occurs only as inclusions in garnet. The appearance of chloritoid in the basic schists is strongly dependent on their restricted FeO-rich and high Al₂O₃/(CaO + Na₂O + K₂O) bulk-rock compositions. The metamorphic pressure (P) and temperature (T) conditions, 0.9 GPa and 505-515°C, are estimated by using equilibrium relationships among chloritoid, taramite, chlorite, paragonite, garnet and quartz. These P-T conditions are distinctly lower than those estimated for a chloritoid-bearing assemblage at the eclogite facies stage of pelitic schists in the Seba eclogite unit (1.7 GPa/525-555°C), 0.5-1 km north of the Ikadadu outcrop. This implies that there is a significant difference in the peak metamorphic pressure of the chloritoid-forming stages between the Ikadadu and Seba chloritoid-bearing schists, indicating the existence of a tectonic discontinuity between the two localities. Chloritoid and amphibole-bearing assemblages are stable at various metamorphic P-T conditions within suitable bulk rock compositions and their phase equilibria are helpful in discussing P-T conditions for various metamorphic rocks.