COMPOSITION AND FABRIC ANALYSIS OF GARNET MICA SCHIST IN THE ROWE FORMATION, WESTERN CONNECTICUT

The garnet-mica schist (GMS) unit of the Rowe Formation is the result of a metamorphosed Ordovician and Cambrian oceanic shelf sequence. The metaturbidite that metamorphosed to staurolite zone, in part, during the Acadian orogeny may also indicate multiple metamorphic events. Evidence of progressive metamorphism is shown by idioblastic centimeter-sized garnet and similarly sized staurolite porphyroblasts in an otherwise muscovite-dominant matrix. The turbidite protolith is reflected by the five-to-ten-centimeter interlayers of metapsammite and metapelite; the latter being the layers that contain most garnet and all staurolite. Garnet makes up approximately 95% of the porphyroblasts. Outcrop studies have been extensive, however commensurate microstudies are few or lacking.

Samples were collected from a steep, single large exposure of the Rowe Formation in Roxbury, Connecticut, extending approximately 500m along strike of consistently similar rock. Mapping had been completed by previous workers in our group and a sample suite was collected representing both the pelitic and psammitic components. Regardless of the layer, amphibolite-facies metamorphic minerals define fabrics. The focus of this work is on samples of the pelitic layers where thin sections were prepared parallel and perpendicular to fabrics within each specimen. Petrographic analysis included mineral composition of the matrix and porphyroblasts (point counting), matrix grain sizes, phase alignment, and microstructures. The matrix of the GMS is dominated by muscovite-quartz aggregates with biotite, plagioclase, K-feldspar, and chlorite. Point counting results show mineral composition to be consistent between thin sections. Fabric analysis reveals that all matrix phases demonstrate preferred orientation, and that they are all internally consistent across all thin sections. Point counting results were also used to calculate bulk wt.% of major elements that was tested using the scanning electron microprobe. Likewise, porphyroblast chemical zones were investigated. These data are consistent with outcrop data and regional data.