30 Igneous and Metamorphic Petrology

Wednesday, 27 March 2002
8:00 AM - 12:00 PM

ORIENTATION AND THREE-DIMENSIONAL GEOMETRY OF INCLUSION TRAILS IN STAUROLITE PORPHYROBLASTS FROM THE EAST LIMB OF THE GREAT HILL SYNCLINE, EASTERN CONNECTICUT

Paper number: 30-2
Program: Technical Sessions
Time: 8:45 AM – 9:05 AM

Authors

Abstract

Examination of the three-dimensional morphology of the internal foliation in oriented sections of staurolite porphyroblasts from the east limb of the Great Hill syncline (GHS) of eastern Connecticut confirms that the "fold axes" defined by the curved inclusion trails is, in all cases, parallel to the porhyroblast's c-axis. Contrary to the observations by Bell, Hickey and Wang (1997) the quartz inclusions, which define the trails are disk rather than tubular or cigar-shaped. Bell, Hickey and Wang's contention that the staurolites lack a preferred orientation is also contradicted by our observations. Using the information on the sense and orientation of the inclusion trails in 25 differently oriented porphyroblasts we can predict the distribution of apparent trail asymmetries that would be observed in any vertical section. The foliation intersection axis (FIA) identified by this means is consistent with the NNE-trending FIA observed by Bell, Hickey and Wang. However, the actual orientation distribution of internal foliations in the staurolites is more complex and inconsistent with Bell's hypothesis that the porphyroblasts did not rotate relative to a geographical reference frame.

On the eastern limb of the GHS the internal foliation in staurolite porphyroblasts is unconformably truncated by a westward dipping external foliation that appears to have developed as a consequence of simple irrotational flattening associated with the overturning of the GHS. The angle between the external foliation and the internal foliation at the rim of porphyroblasts having near equant cross sections ranges from 0° in staurolites plunging to the NW up to 70-80° in crystals plunging at a low angle to the NE. These relationships suggest that staurolite preferentially nucleated with its c-axis parallel to an early foliation, which dipped steeply to the SE. Top to the west shear during staurolite growth generated the curved inclusion trails. Post-staurolite flattening rotated and reoriented many of the porphyroblasts complicating the three-dimensional orientation distribution of the inclusion trails.

Bell, T. H., Hickey, H. A., and J. Wang, 1997, J.Met.Geology, v. 15, 467-478.