VARIATIONS IN PORPHYROBLAST-MATRIX FABRICS ASSOCIATED WITH HIGH-TEMPERATURE DEFORMATION, ROXBURY, CONNECTICUT, USA

Penetrative fabrics in rocks of western Connecticut have recorded regional deformation associated with the Devonian Acadian orogeny. We studied these fabrics at the meter scale along strike in garnet-staurolite schist as exposed in a single ~50 m long exposure in an abandoned garnet quarry, Roxbury Falls, CT (Ratlum Mtn. form). Matrix minerals are dominantly micas and quartz that define foliations and lineations of various intensities, but are consistent in orientation. Garnet and staurolite porphyroblasts range in diameter from 2 mm to 2 cm (garnet). Structural and petrographic analyses of porphyroblasts and matrix minerals were performed to document variations in grain size, shape, and abundance across the exposure in order to quantify the strain record.

Results show strain shadow tails of matrix minerals that drape porphyroblasts. The long aspect of the tails is sub-parallel to the matrix fabrics defined by elongated biotite and a highly schistose foliation. Deformation of the fabric apparently continued following formation of the porphyroblasts and the matrix minerals as shown by ‘pinched’ pressure shadow tails around porphyroblasts and crystal fragments that are also elongate sub-parallel to the main fabrics. Microstructural data show retrogression of garnet porphyroblasts in the form of pseudomorphic rims of biotite and/or muscovite around garnet cores. Inclusion trails within the porphyroblasts are discordant to the matrix foliation suggesting that more than one episode of deformation took place or porphyroblasts or matrix foliation has rotated with respect to each other. Variation in the dominant porphyroblasts suggests an apparent staurolite isograd is within the exposure along strike. Larger and more abundant garnet is found in the west part of the exposure, whereas comparable staurolite is in the east where a smaller proportion of garnet is observed or absent. Given the deformation textures, however, we conclude that the field gradient has been deformed into its current state as illustrated by the strain of existing fabrics in these rocks. This deformation of the local field gradient may be representative of the regional field gradient in this part of the northern Appalachians.