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 po rphyroblasts was analyzed by serial polishing and reflected-light microscop y. Most of the garnet porphyroblasts have straight inclusion trails of quar tz and some plagioclase (Si) oblique to an external foliation (S-e). Serial polishing of two samples normal to mineral/elongation lineation (A plane), and normal to S-e and parallel to the lineation (B plane), with twenty int ervals 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 Si orientation of individual garnets is fai rly constant in each sample. The attitude of S-i in geographic space, deter mined by using its pitch angles on the A and B planes, is more or less cons tant throughout the area (over 6.5 km traversing normal to the strike of re gional foliation). The sense of asymmetry of strain shadows at the margin o f garnet porphyroblasts and the S-i/S-e relationship indicate rotation of S -e rather than garnet porphyroblasts with respect to thee geographic refere nce frame. Rotation of S-e is also supported by a sense of shear inferred f rom other shear criteria, all indicating a top-up-to-the-south shearing. (C ) 1999 Elsevier Science B.V. All rights reserved.