X-ray computed micro-tomography of spiral garnets: A new test of how they form

摘要

Garnet porphyroblasts with spiral shaped inclusion trails of two micaschist samples have been investigated with X-ray computed tomography. For each garnet crystal, the orientation of its internal Foliation Intersection/Inflexion Axes (FIA) was measured, its volume and shape orientation, the latter via calculation of best-fit ellipsoids. Additionally, image analysis of inclusion trails was performed in slices passing through garnet centres oriented normal to FIA. Our data demonstrate subvertical and subhorizontal preferred orientations of the inclusion trails and of the porphyroblast crystals themselves in both samples. This supports a genetic model in which garnets periodically overgrew a succession of vertical and horizontal foliations during alternating crustal shortening and gravitational collapse without significantly rotating. The fact that both samples come from areas previously identified as regional shear zones supports the idea that porphyroblasts are able to maintain stable positions even in such environments. Our study exemplifies the strength of X-ray computed micro-tomography compared to previous techniques based on serial thin sections for distinguishing multiple FIA sets present in single samples.