The metamorphic paragenesis of cordierite in pelitic rocks

A petrogenetic grid is constructed for mineral assemblages occurring in metapelitic rocks, particularly those involved in the paragenesis of cordierite. The most useful assemblages for estimating pressures and temperatures are staurolite-cordierite, cordierite-biotite-Al₂SiO₅ and cordierite-hypersthene. Cordierite is stable with kyanite, sillimanite or andalusite. At high pressures cordierite is Mg-rich so that pelitic rocks typically do not contain the phase. Cordierite is stable at temperatures less than 500° C but does not commonly appear in metapelitic rocks until the garnet-chlorite, chlorite-staurolite or chlorite-Al₂SiO₅ tie-lines are broken. At high metamorphic grades, the assemblage garnet-hypersthene-cordierite indicates relatively low pressures, and the assemblage hypersthene-cordierite-sillimanite relatively high pressures. It is clear however, that the absence of cordierite is of little use in characterizing a metamorphic facies unless an alternate mineral assemblage can be shown to be more stable.