Reversal of the spiral direction of inclusion-trails in paratectonic porphyroblasts

Abstract

In certain circumstances, the trails of inclusions in a paratectonic porphyroblast may show one or two reversals in the spiral direction. A point of reversal of curvature in the spiral direction corresponds to a stage when the porphyroblast and the external foliation were rotating at the same rate. If the porphyroblast is spherical and the deformation is by combined pure- and simple-shears, then, from a double reversal of curvature, it is possible to calculate: (1) the ratio between strain-rates in pure shear and simple shear; (2) the initial orientation of the external foliation; (3) the amount of rigid rotation of the porphyroblast; and (4) the amounts of pure shear and simple shear corresponding to the period of paratectonic crystallization. If the porphyroblast is ellipsoidal and the bulk deformation is by simple shear, then from a single or a double reversal of spiral direction, one can calculate: (1) the initial orientation of the external foliation; (2) the rigid rotation of the porphyroblast; and (3) the finite simple shear corresponding to the period of paratectonic crystallization.