Talc-Phengite: a Widespread Assemblage in High-Grade Pelitic Blueschists of the Western Alps

Talc-phengite, an assemblage hitherto believed to be rare, is found in regional distribution in the Gran Paradiso area, where it occurs in the characteristic mineral association chloritoid-talc-phengite (Si3·4–3·5). Talc contains up to 15 mole per cent minnesotaite, and chloritoid up to 45 mole per cent of the magnesium end member. The talc-phengite stability results basically from the disappearance of chlorite + quartz in rocks with low and moderate MgO/FeO ratios through the divariant reactions first recognized here: Fe-Mg-Chlorite+quartz →talc + garnet + H2O and Fe-Mg-chlorite + quartz →talc + Chloritoid + H2O These reactions imply the disappearance of the join biotite-chlorite in the presence of quartz and thus open a talc-phengite stability field (±garnet or chloritoid or Mg-chlorite) which extends, with increasing P and T, toward Mg-richer compositions. Whether or not it reaches the magnesian subsystem in the Gran Paradiso area cannot be ascertained. However, the sporadic occurrence of the high-pressure assemblage talc-kyanite-chloritoid 50 to 70 km further northeast in the vicinity of the Monte Rosa massif within the same lithological unit (Zermatt-Saas Fee zone s.l.) indicates the instability of any chlorite in quartz-bearing rocks, and implies that talc-phengite must also be stable for purely magnesian compositions in that area. This progressive stabilization of talc-phengite with increasing metamorphic grade supports Abraham & Schreyer's (1976) hypothesis of a high-pressure field for this assemblage, and rules out Chernosky's construction (1978) implying a low-pressure field. The following paragenetic sequence is proposed for pelitic compositions with intermediate Mg/Fe ratios and excess quartz subjected to high-pressure metamorphism with maximum temperatures near 400–500 °C: chlorite-illite → chlorite-phengite → chloritoid-talc-phengite. The absence of biotite is a compositional effect due to the high degree of phengite substitution in the white mica.