The Assemblage Quartz–K Feldspar–Biotite–Garnet–Sillimanite as an Indicator of –T Conditions

Abstract

The assemblage quartz–K feldspar–biotite–garnet–sillimanite is common in high-grade pelitic rocks. Diverse compositions of biotite and garnet from this assemblage reflect differences in Ptotal, [Formula: see text], and T. For rocks from a small area, Ptotal probably was nearly constant. In order to show the separate effects of [Formula: see text] and of T on mineral composition, a plot of conditions indicated by various rocks with this mineral assemblage on a [Formula: see text]–T diagram is presented. This is accomplished by treating the minerals and the fluid phase as ideal solutions, by assigning reasonable [Formula: see text]–T values to one rock, and by assuming entropies for two dehydration reactions. Rocks from the Thor-Odin gneiss dome in British Columbia reflect a temperature range of about 200 °C and relatively low water pressures.