KYANITE QUARTZITE IN THE WEST CHESTER MASSIF REVISITED: PETROGRAPHIC ANALYSIS AND EQUILIBRIUM ASSEMBLAGE MODELING

Kyanite-bearing quartzite was first described in Mesoproterozoic-aged gneiss of the West Chester massif, in the central Appalachian piedmont of Pennsylvania, by Wagner and Crawford (1975). During preparation of a new geologic map of the West Chester 7.5 minute quadrangle several samples from Wagner’s localities near Dutton’s Mill were examined. The kyanite quartzite consists of 45 – 70% quartz, 15 – 30% kyanite, 10 – 25% garnet, and ~1% rutile. The quartzite contains virtually no plagioclase and only trace amounts of biotite. A well-developed foliation is defined by elongate aggregates of garnet crystals and lens shaped clusters of kyanite. Although the clusters are flattened and elongate in the plane of foliation, the small kyanite grains are randomly oriented and occasionally occur as sprays within the elongate clusters. Rutile is a minor phase in the rock, but also occurs elongate parallel to foliation. While none have been observed to date in this study, Wagner and Crawford describe lenses of randomly oriented kyanite surrounding sillimanite interiors, and concluded that kyanite formed subsequent to sillimanite during a second metamorphic episode.

Equilibrium assemblage modeling using Theriak-Domino permits estimation of minimum pressure and temperature conditions of metamorphism. Bulk rock composition was determined by whole thin section EDS scan. In models using the Berman database (jun92.bs) the minimum PT conditions for the stability of the assemblage grt + ky + qtz + rt are 700C at 8.5 kb; minimum conditions for the same assemblage using the Thermocalc database (tcdb55c2d) are 650C at 9.5 kb. Temperature is constrained by the high T stability of staurolite, which is not present in the rock; pressure by the ilmenite to rutile transition. However, the textures described above indicate that sillimanite and rutile formed at the same time, prior to or during deformation and prior to the growth of kyanite. Minimum PT of the ky-sil transition with rt stable are 750 – 850C at 8.5 – 10 kb (jun92.bs yields lower P and T than does tcdb55c2d).

These results generally agree with Wagner and Crawford’s estimates for Grenvillian metamorphism in the West Chester massif, but these are minimum conditions – higher T requires corresponding higher P due to the positive slope of the ilm-rt boundary. Modeling in other rock types is on-going.