Transition from Staurolite to Sillimanite Zone, Rangeley Quadrangle, Maine
Study of pelitic schists in the Rangeley area, Maine, by means of petrographic, x-ray, and electron-microprobe techniques enables definition of the isogradic reaction relating the Staurolite and lower sillimanite zones. The reaction is a discontinuous one and can be shown on an AFM projection as the tie line change from Staurolite + chlorite to sillimanite + biotite. This topology change, in conjunction with the mineralogical data provides the equation: Staur + M g -Chte + Na-Musc + (Garn?)→Sill + Bio + K-richer Musc + Ab + Qtz + H 2 O. This reaction should result in a sharp isograd in the field but in fact is found to be spread out over a zone which is called the transition zone. It is proposed that this zone results from buffering of f H 2 O by means of the equation above. Buffering of f H 2 O by continuous reactions also appears to be taking place in the lower sillimanite zone. Metamorphic conditions for the isogradic reaction are believed to be near 4 kb and 600°C. P fluid was probably equal to P total but somewhat greater than P H 2 O .