PETROGRAPHIC AND FABRIC ANALYSIS OF THE LOG-JAM SCHIST AND SURROUNDING ROCKS, WESTERN CONNECTICUT

The ‘Log-Jam’ schist (LJS) in the Rowe schist, W Connecticut, is locally distinct, confined to 10-cm-thick Ky+Bt layers concordant with fabrics in quartzofeldspathic gneiss. LJS layers are spaced ~1m making the gneiss the dominant rock (Ky and St porphyroblasts are local). The gneiss and LJS layers are within a single set of exposures that form a sub-elliptical, 500m x 300m area. The LJS name is from the famously-large Ky (up to 60cm long), enclosed within a coarse matrix of varying amounts of Bt, Chl, Pl, and Qtz. On inspection, the Ky layers in the gneiss are subparallel and shallowly-W-dipping as is the gneiss fabrics. Rocks surrounding these exposures are Rowe schists with matrix of Qtz, Pl, Kfs, Ms, Chl, Bt, Sill with Grt and/or St porphyroblasts. The LJS Ky, regardless of layer, define a foliation sub-parallel to the structures defined by the layers in the outcrops, but also at the regional scale, and statistically define a consistent sub-down-dip lineation (moderately W-plunging; Bestine and Solar, NEGSA 2002). Questions have remained about the petrography and structure (if any) defined by the matrix minerals in and around the LJS.

Fifteen thin sections from rocks collected at 11 stations representing LJS, the gneiss and Rowe schist were analyzed for mineral composition and textures. Petrographic analysis involved matrix grain size, apparent arrangement, boundary geometries, evidence of mineral reactions, and microstructures. Results show that in the LJS layers the matrix mineral composition is consistent with residual enrichment of Al (metasomatism), whereas in the gneiss layers no such evidence is found. Textural data support growth of Ky after Al enrichment (post-metasomatism). The Inverse SURFOR Wheel (Panozzo, JSG, 1987) was used to document fabric ellipses and preferred orientations of the bulk matrix fabrics and the individual Qtz, Bt, and Ms fabrics. Results show consistency in all fabric ratios (1.5:1 to 3.4:1), whose axes are all consistently-oriented with both each other, and the outcrop and regional structures. We conclude that the fabrics have regional relevance. Petrographic and fabric data together suggest that syntectonic metasomatism in structurally-controlled horizons is responsible for the Al enrichment in the ‘Log-Jam’ layers in an otherwise sheared and metamorphosed granite pluton.