The Mineralogy and Petrology of the Transition from the Lower to Upper Sillimanite Zone in the Oquossoc Area, Maine

Systematic changes in the assemblages and compositions of mineralssuggest that the transition from lower to upper sillimanitezone in the Oquossoc area, Maine, is marked by the reaction Staurolite+Sodic Muscovite+QuartzSillimanite+Biotite+K-richerMuscovite+Albite+Garnet+H₂O. Moreover, the mineralogic data for the silicates suggest thatH₂ O is buffered by some of the assemblages present but PH₂O<P_total. A consideration of the equilibria among the opaqueminerals enables a calculation of the composition of the fluidphase assuming specific PT conditions and that P_fluid = P_total.At several reasonable PT conditions the calculated fluid phaseis reasonable in terms of the model for P_H2O based upon thesilicate equilibria. Probable PT conditions for the metamorphism considered in thisstudy range from 575 to 630°C and 3 to 5 kb.