Dynamic recrystallization processes in plagioclase porphyroclasts

Recrystallization processes in plagioclase porpyroclasts were studied in samples from an anorthositic gabbro of the Jotun Complex, South Norwegian Caledonides. The deformation took place at a temperature of approximately 700°C and at pressures below 900 MPa. Two populations of plagioclase porphyroclasts can be distinguished based on microstructural observations. The crystallographic orientation and grain shapes for the two populations are interpreted as ‘soft’ and ‘hard’ orientations of the dominant slip system (010)[001] with respect to the kinematic reference system. The orientations of porphyroclasts and recrystallized grains were measured with a U-stage microscope. Analysis of misorientations between porphyroclasts and adjacent recrystallized grains yields different results for the two types of porphyroclasts. In type-1 porphyroclasts (soft orientation) the misorientation is inferred to be caused by subgrain rotation due to the activation of the (010)[001] slip system, resulting in a host control orientation relationship, whereas in type-2 porphyroclasts (hard orientation) no dominant misorientation relationship could be identified. The slip systems of the dislocations inferred from transmission electron microscopy (TEM) analysis correspond well with the results of the U-stage misorientation analysis. In type-2 porphyroclasts (hard orientation) micro-fracturing was observed in the TEM, whereas microfractures are absent in type-1 porphyroclasts (soft orientation). Micro-fracturing is likely to be the mechanism for nucleation of new grains in type-2 porphyroclasts, resulting in a no-host control orientation relationship between porphyroclasts and recrystallized grains.