Spiral-Shaped Inclusion Trails

Spiral-shaped inclusion trails are one of the most alluring and perplexing microstructures in metamorphic rocks, and are most commonly found in garnet porphyroblasts (two examples below).

Garnet porphyroblast with spiral-shaped inclusion trails from the Nepal Himalaya

Garnet porphyroblast with spiral-shaped inclusion trails from the Vermont Appalachians

On initial observation, they appear to provide an intuitively obvious indicator for the sense of shear experienced by deformed metamorphic rocks, and on this basis have also been used by some workers to calculate shear-strain rates. However, the origin of this microstructure has been the source of much debate in the recent literature (see Johnson, 1993b, for review). There are fundamentally two models for the origin of spiral-shaped inclusion trails. The first requires the garnet to grow while it rotates relative to an externally-fixed reference frame. The second requires the garnet to grow over surrounding foliations during sequential development of near-orthogonal foliations or crenulation cleavages, and in this instance the garnet need not rotate relative to a fixed external reference frame. Because this is such a controversial microstructure, I decided to have a close look at it to see if I could determine which of these models provided the best explanation. To start, I first determined the orientation of the spiral axes within a single sample, which involved cutting a number of systematically-oriented sections. I then cut 77 serial thin sections perpendicular to the determined spiral axes. Numerous garnet porphyroblasts were intersected, revealing the 3-D geometry of the trails; these trails are doubly-curved, non-cylindrical surfaces, as shown below. Details of the reconstruction methodology can be found here.

 

Reconstruction of the central inclusion surface within the first garnet porphyroblast above. The surface is doubly-curved and non-cylindrical. A QuickTime movie that shows this surface from different angles can be found here, but be aware that it is 360 K in size.

Armed with a clear understanding of the 3-D geometry of these trails, I thoroughly evaluated the two competing models for their formation (Johnson, 1993b). For better or worse, I concluded that geometries and microstructural features alone cannot distinguish between them, which illustrates a fundamental fact of structural geology: geometries very seldom provide unique information about strain paths. I concluded the same thing about oppositely-concave microfolds - or so-called "millipede" microstructure (Johnson & Bell, 1996). Thus, in my opinion, spiral-shaped inclusion trails are not well enough understood to be used as shear sense indicators. More discussion can be found in my Himalayan page.