Northeastern Section - 37th Annual Meeting (March 25-27, 2002)

The "log-jam" schist member of the Rowe schist in western Connecticut is a locally distinct unit marked by porphyroblasts of kyanite of unusually large size (up to 55cm in length) enclosed within a coarse matrix of varying amounts of Bt, Chl, ±Pl, ±Qtz. The unit is limited to a single exposure 500 x 300 m in area, and is defined by two distinct layers alternating throughout the outcrop. One layer type is dominated by kyanite + Bt at the 10 cm scale, the other layers are kyanite + Pl (~1 m scale). Layers contain a matrix fabric parallel to layer contacts in all exposures, particularly so for the kyanite + Pl layers. By visual inspection, kyanite has an apparent bulk preferred orientation in all layers, so this study was designed to test the intensity of the preferred orientation of kyanite porphyroblasts and evaluate whether there is a relation between such fabrics and the matrix mineral fabric. Using 11 stations in the field, each spaced 5 to 20 m, both across and parallel to fabrics, we have documented the orientations of the crystallographic axes of 479 porphyroblasts. Attitudes of b and c crystallographic axes were measured separately, correlated, and the a axes orientations were deduced as the pole to b-c axis planes for each crystal. Results show that ~90% of the total c-axis data defines a generally tight girdle on a stereogram, whose best fit great circle is 000°, 30°W, most data that produced the 10% scatter are from one station only. The stereograms for each station show tight girdles, closely resembling the pattern of the total data sets. Highest concentrations of c-axes trend shallowly north and south. Total b-axis data show a similar girdle pattern with the highest concentration trending 260°. Total a-axis data form a general cluster plunging 45° to 080°. The axes of the longer crystals tend to fall in the areas of highest concentration. The matrix foliation generally dips 35°W, and strikes 010°, generally 10-15°E relative to the kyanite b-c planes. By comparison, the regional fabrics in the area surrounding the "log-jam" schist locality strike 000° to 350° with 30-45°W dips, consistent with these kyanite data. Therefore, we interpret that fabric these data are important to the elucidation of the Appalachian tectonic history as recorded in western Connecticut.