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

a r t i c l e i n f o 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. Studying the history preserved in deformed metamorphic rocks can be complicated by the fact that only the most recent events are preserved in the matrix (Bell, 1986). Inclusion trails in porphyroblasts can be used to provide evidence of earlier deformation events and have been used in this way for many years (e. These inclusion trails are commonly curved; the term FIA (foliation inflection/intersection axes) has been introduced to describe the axis about which the inclusion trails curve (Fig. 1; Bell et al., 1995; Hayward, 1990). The formation of this curvature has been interpreted in two ways; as either the result of the inclusion of a foliation as the porphyroblast rotates due to non-coaxial flow (e. or the inclusion of a foliation as it rotates around a porphyroblast that is fixed relative to a geographic reference frame (e. It has been suggested that FIA are a particularly useful microstructure that can be applied in much the same way as other structural elements such as bedding strike, fold axes, and intersection lineations (e. Moreover, if the porphyroblasts have not …