FORMATION OF SPIRAL INCLUSION TRAILS IN GARNET DURING FOLDING – A MINIMUM ROTATION MECHANISM

Petrographic, field, and x-ray computed tomography studies indicate that spiral inclusion trails in garnet porphyroblasts from the Laramie Range of SE Wyoming were produced by growth of garnet over a crenulation foliation and concomitant rotation of the garnet and the included crenulation foliation during progressive folding. The spiral garnet porphyroblasts occur in folded interlayers of Archean metapelite, psammitic schist, amphibolite, and hornblende garbenschiefer. The rocks have undergone a period Archean deformation (D1) and two periods of Paleoproterozoic deformation (D2 & D3). The spiral garnets formed during D2 at conditions of ~10.5kbar at 670°C (Goergen & Bauer, this meeting). They locally overgrew a zonal S2 crenulation foliation that is axial planar to large-scale recumbent to reclined F2 folds. S1 in the microlithons is well defined and continues to be well defined in quartz-rich areas within the rotated microlithons in the spiral garnets.

Spiral symmetries and inclusion trail geometries were analyzed in detail across one of the F2 folds that closes to the left as viewed down plunge. The symmetry of the spiral garnets varies on adjacent fold limbs from sinistral viewed down the fold plunge on the upper limb, to dextral viewed down plunge on the lower limb. Both symmetries are consistent with relative rotation of the garnet and the included crenulation foliation toward the fold’s hingeline. A significant number of the spiral garnets are elongate with their long axes parallel to those of the local F2 fold hinges. X-ray computed tomography (CT) images of several of the spiral garnets illustrate their axis-parallel elongation and also display an external “candy cane” spiral morphology that is consistent with the axial symmetry of the spiral. Down-axis movies through entire elongate garnet CT images indicate that the garnets’ full-length internal pattern is typically noncylindrical; however, garnet-rich parts of the curved pattern tend to be cylindrical over much of the central part of the porphyroblast.

The proposed mechanism was able to produce spiral inclusion trails in garnets in response to low partitioned shear strain during flexural flow folding, because as much as 180º of the apparent rotation of the inclusion trails was a result of overgrowing a pre-existing crenulation foliation.