The external morphology and internal fabric of staurolite from the Bolton syncline, eastern Connecticut: Implications for the interpretation of inclusion trails in porphyroblasts

Date of Completion

January 2006






Centimeter-size prismatic staurolite porphyroblasts occur in the Littleton Formation from the Bolton syncline in eastern Connecticut. They contain a captured, relict foliation defined by inclusions of quartz, ilmenite and graphite (type I inclusions) and smaller tube-shaped trains of quartz inclusions perpendicular to crystal faces (type 2 inclusions). The orientation of the type 1 inclusion surfaces in 132 staurolite porphyroblasts suggests their c-axes nucleated close to the plane of an early foliation. Where inclusion trails are sigmoidal-shaped, the axis of the curvature parallels the c-axis regardless of the staurolite orientation. The Hansen pole, defined by the switch in the asymmetry of sigmoidal-shaped inclusion trails, shows a consistent top-to-the-west sense of shear on the east limb of the syncline, but varies from top-to-the-south to top-to-the-east on the west limb at Box Mountain. Staurolite inclusion trail asymmetries observed in random vertical thin sections through the rock do not show a well-defined or consistent foliation intersection axis (FIA). ^ Fan-shaped staurolite porphyroblasts the western limb of the syncline and consist of intergrown planar splays of [001] elongated prisms, crudely radial from a single apex. Careful examination of the three-dimensional geometry of fans show these internal features reflect the rotation and deformation of a brittle rigid porphyroblast relative to syn-growth shear stresses, and provide additional evidence of local shear-induced porphyroblast rotation, independent of interpreting sigmoidal-shaped inclusion trails. ^ The size distributions of euhedral garnet porphyroblasts are identical in the matrix and in the staurolite porphyroblasts, suggesting they did not breakdown to grow the bulk of the staurolite. A second period of staurolite growth is preserved at the rims of the staurolite where it overgrew the external foliation. At these locations, garnet edges are ragged and compositional zoning is truncated, suggestive of a garnet breakdown reaction. ^ These observations are consistent with nucleation of staurolite in an early, fine foliation followed locally by a top-to-the-west shear-induced rotation. Subsequent eastward backfolding of the Glastonbury Gneiss produced the Bolton syncline, and southward extrusion of the Glastonbury Gneiss locally reoriented the west limb staurolite. Thermal effects of this backfolding and extrusion produced new staurolite overgrowths during the development of the new foliation. ^