Numerical simulations of spiral‐shaped inclusion trails: can 3D geometry distinguish between end‐member models of spiral formation?

Abstract

Numerical 3D simulations of the development of spiral inclusion trails in porphyroblasts were conducted in order to test the proposals that (a) 3D spiral geometry differs between the rotation and nonrotation end-member models of spiral formation proposed in the literature, and (b) 3D spiral geometry can be used as a criterion to distinguish between the two end-member models in rocks. Four principal differences are identified between the two sets of simulations: smoothness of spiral curvature; spacing of foliation planes; alignment of individual foliation planes either side of the sphere representing the porphyroblast; and spiral asymmetry with respect to matrix shear sense. Of these differences, only spiral asymmetry and possibly the alignment of individual foliation planes are diagnostic criteria for distinguishing between the end-member models. In the absence of a readily applied test to distinguish the end-member models, interpretation of spiral inclusion trails is problematic. It is necessary to determine complementary evidence to distinguish porphyroblast rotation or nonrotation during spiral formation.

Garnet: A Rock-Forming Mineral Petrochronometer

“…Shear stress can also deform the matrix and cause garnet to rotate (Spry 1963;i.e., 'snowball garnets';Rosenfeld 1968) or record changing metamorphic fabrics during porphyroblast growth (i.e., Ramsay 1962;Bell 1985). Controversy still exists between these two end-member interpretations (i.e., Johnson 1993;Ikeda et al 2002;Stallard et al 2002) but in any case, these common and often vivid patterns of inclusions inside garnets (e.g., Fig. 1c) clearly record rock deformation that may reflect localized structural-(e.g., Robyr et al 2009) or regional tectonic-scale (e.g., Aerden et al 2013;Sayab et al 2016) processes.…”