The orientation of axes of sigmoidal, staircase or spiral inclusion trails within porphyroblasts provides an indicator of the direction of movement during deformation that is synchronous with metamorphism. A simple technique is presented in detail to find this axis in 3D by radially sectioning a horizontal slab cut from an oriented sample. When viewed from one direction, the switch in asymmetry of the porphyroblast inclusion trails in these sections defines the trend of the axis. Further radial sectioning of a vertical slab cut parallel to this trend determines the plunge of this axis. This technique is independent of the model adopted for the formation of sigmoidal or spiral‐shaped inclusion trails and can be used to evaluate the mechanism by which they form. It can also be used to evaluate theories of folding and orogenesis. Measurements of spiral and sigmoid axes in garnet porphyroblasts from the European Alps show that they reflect the movement of the African Plate relative to Europe better than linear indicators preserved within the same rocks.

A B S TR A CT In the Littleton Formation, garnet porphyroblasts preserve three generations of growth that occurred before formation of the Bolton Syncline. Inclusion trails of foliations overgrown by these porphyroblasts are always truncated by the matrix foliation suggesting that garnet growth predated the matrix foliation. In contrast, many staurolite porphyroblasts grew synchronously with formation of the Bolton Syncline. However, local rim overgrowths of the matrix foliation suggest that some staurolite porphyroblasts continued to grow after development of the fold during younger crenulation producing deformations. The axes of curvature or intersection of foliations defined by inclusion trails inside the garnet porphyroblasts lie oblique to the axial plane of the Bolton Syncline but do not change orientation across it. This suggests the garnets were not rotated during the subsequent deformation associated with fold development or during even younger crenulation events. Three samples also contain a diÂerent set of axes defined by curvature of inclusion trails in the cores of garnet porphyroblasts suggesting a protracted history of garnet growth. Foliation intersection axes in staurolite porphyroblasts are consistently orientated close to the trend of the axial plane of the Bolton Syncline on both limbs of the fold. In contrast, axes defined by curvature or intersection of foliations in the rims of staurolite porphyroblasts in two samples exhibit a diÂerent trend. This phase of staurolite growth is associated with a crenulation producing deformation that postdated formation of the Bolton Syncline. Measurement of foliation intersection axes defined by inclusion trails in both garnet and staurolite porphyroblasts has enabled the timing of growth relative to one another and to the development of the Bolton Syncline to be distinguished in rocks where other approaches have not been successful. Consistent orientation of foliation intersection axes across a range of younger structures suggests that the porphyroblasts did not rotate relative to geographical coordinates during subsequent ductile deformation. Foliation intersection axes in porphyroblasts are thus useful for correlating phases of porphyroblastic growth in this region.

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