Non-rotation of garnet porphyroblasts and 3-D inclusion trail data: An example from the Imjingang belt, South Korea

We describe non-rotation of garnet porphyroblasts in non-coaxially deformed metapelites of the Imjingang orogenic belt in the middle Korean Peninsula. The three-dimensional geometry of inclusion trails within 1-2 mm garnet prophyroblasts was analyzed by serial polishing and reflected-light microscopy. Most of the garnet porphyroblasts have straight inclusion trails of quartz and some plagioclase (S(i)) oblique to an external foliation (S(e)). Serial polishing of two samples normal to mineral/elongation lineation (A panel), and normal to S(e) and parallel to the lineation (B plane), with twenty intervals of about 0.1 mm, reveals that the straight S(i) is a planar structure. Measurement of the angles between S(i) and S(e) on both planes from samples of sixteen outcrops shows that S(i) orientation of individual garnets is fairly constant in each sample. The attitude of S(i) in geographic space, determined by using its pitch angles on the A and B planes, is more or less constant throughout the area (over 6.5 km traversing normal to the strike of regional foliation). The sense of asymmetry of strain shadows at the margin of garnet porphyroblasts and the S(i)/S(e) relationship indicate rotation of S(e) rather than garnet porphyroblasts with respect to the geographic reference frame. Rotation of S(e) is also supported by a sense of shear inferred from other shear criteria, all indicating a top-up-to-the-south shearing.