Pseudosection analysis for talc-Na pyroxene-bearing piemontite-quartz schist in the Sanbagawa Belt, Japan

Various modes of occurrence of talc were identified in piemontite-quartz schists collected from schist and eclogite units in the Kotsu area of the Sanbagawa Belt, eastern Shikoku, Japan. They can be classified into the following types: (A) matrix and (B) pull-apart talc. The matrix talc is associated with aegirineaugite or glaucophane in the eclogite unit and with albite or chlorite in the schist unit. The pull-apart talc is developed at the pull-apart of microboudin structures of Na-amphibole, along with albite or chlorite in samples from both units, suggesting that the pull-apart talc was formed by Na-amphibole consuming reactions in both units. The talc-aegirineaugite-phengite association is found in a thin layer (a few millimetres thick), with higher Na2O/(Na2O + Al2O3 + MgO) ratio in the ANM (Al2O3-Na2O-MgO) diagram projected from phengite, epidote and other minerals, in the eclogite unit. Crystals of aegirineaugite have decreased jadeite content [= 100 x Al/(Na + Ca)] and increased aegirine content [= 100 x (Na - Al)/(Na + Ca)] from the core (ca Jd(40)Aeg(40)Di(20)) to the rim (ca Jd(23)Aeg(53)Di(24)), and are replaced by winchite and albite in varying degrees at the crystal margins. Na-amphibole is glaucophane/crossite, commonly rimmed by Al-poor crossite or winchite at the margin in the eclogite unit, although it is relatively homogeneous crossite in the schist unit. These textures suggest that the talc-phengite-(aegirineaugite or glaucophane) assemblage equilibrated during an early stage of metamorphism and the pull-apart talc was formed at a later stage in the eclogite unit. A plausible petrogenetic grid in the NCKFe(3+)MASH system with excess piemontite (regarded as epidote), hematite, quartz and water, pseudosection analysis for the aegirineaugite-bearing layer and the observed mineral assemblages suggest that the talc-aegirineaugite-phengite assemblage is stable under high pressure conditions (ca 560-580 degrees C and 18-20 kbar). The pull-apart talc was formed at ca 565-580 degrees C and 9.5-10.5 kbar by the reaction of glaucophane/crossite + paragonite = talc + albite during the decompression stage, suggesting that the piemontite-quartz schist in the eclogite unit experienced high-pressure metamorphism at ca 50-60 km depth and was then exhumed to ca 30 km depth under nearly adiabatic conditions.