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.