An X-ray computed tomography study of inclusion trail orientations in multiple porphyroblasts from a single sample

Inclusion trails in 58 garnet porphyroblasts in a single sample from the Cram Hill Formation in southeast Vermont were imaged using high-resolution X-ray computed tomography. Texturally the porphyroblasts have a large core with a sub-vertical foliation that has a well defined geometry with a mean orientation of 113/72E for a 95% confidence cone semi-angle of 5.3°. This steep foliation curves into a sub-horizontal foliation. The foliation intersection/inflection axes (FIAs) defined by hinge lines for this curvature are tightly clustered with a mean plunge of 11° towards 201° with a 95% confidence cone semi-angle of 4.1°. Eigenvalue analysis indicates that the clustered distribution of the foliation and FIA data are unlikely to be the result of a random event. There is evidence in the specimen for multiple foliation-forming events subsequent to garnet nucleation, and the preservation of these clustered distributions in their wake strongly suggests that the porphyroblasts have not rotated with respect to a geographic reference frame. FIAs represent a measurable structural element that can provide information on tectonic events at the time the porphyroblasts grew. A comparison of FIA data collected using high-resolution X-ray computed tomography with data collected using the asymmetry method demonstrates that the asymmetry method is a valid technique for defining the mean FIA orientations in a sample.