PORPHYROBLAST-MATRIX RELATIONS IN WESTERN CONNECTICUT: INCLUSION TRAILS AND STRAIN-SHADOW TAILS

Porphyroblast-matrix relations in Rowe-Hawley pelitic schists of western Connecticut have recorded a complex Lower Paleozoic tectonic history of multiple orogenic episodes. Our group’s results of mapping and study of collected specimens have been presented at NEGSA (Bestine & Solar, 2002; Fissler & Solar, 2004; Murphy & Solar, 2013; McCarthy & Solar, 2016; Solar et al., 2016), focused on the spatial variations of rock types, and bulk mineral textures/kinematics. We document in this contribution microstructures of garnet and staurolite porphyroblasts and associated mica-dominated matrix mineral fabrics. Porphyroblasts are typically 1-3 cm in diameter with distinct inclusion patterns and strain-shadow tails that are draped by 10-30 micron-thick matrix minerals defining strongly developed linear and planar fabrics. As such, these rocks are well suited for assessing a long-duration metamorphic and structural record.

Forty thin sections were produced from 25 selected specimens. Thin sections were cut according to fabrics as mutually perpendicular thin sections pairs (both mounted for down-dip observation), one parallel to the lineation, but perpendicular to the foliation, and the other perpendicular to both (X-Z and Y-Z planes, respectively). Data sets collected included overall mineral composition (point counts); the shape, geometry, and orientation of matrix fabrics (Se); porphyroblast shapes, sizes, and inclusion trail (Si) geometry, and Si/Se relations; and mineral content, shape, and size of porphyroblast strain-shadow tails. Twenty-two data sets were correlated.

There are strong trends in these data. Mica-quartz tails around porphyroblasts range in length dependent on porphyroblast size, from 1-18 mm. The tails are uniformly asymmetric and sigma-shaped, with 74% sinistral kinematics in all sections - 77% in X-Z sections and 60% in Y-Z sections. Porphyroblast inclusions are dominantly quartz and graphite. Inclusion trails are mostly straight but curve both at rim/core and grain boundares. Inclusion trail orientations (139 Si/Se measurements) are within 3 sets, all discerned by differing geometry and in clearly separate porphyroblast rims and core patterns. Most have Si/Se angles in the low 20°-CCW-rake range relative to the matrix foliation. Two other groups are ~10° CW rakes and ~40° CCW rakes. Data consistency, representing a wide geographic area, supports conclusions of at least 2 (or 3) separable dynamic events recorded by both individual rock porphyroblast-matrix relations, and across our specimen suite.