Natural calcite c-axis fabrics: An alternate interpretation

Abstract

Microstructures and crystallographic fabrics in natural calcite mylonites from Newfoundland, Canada are discussed and compared with other natural and experimental examples. In the Newfoundland rocks, porphyroclasts deformed mainly by twinning and intracrystalline slip, and the recrystallized grains deformed by dislocation creep assisted by dynamic recrystallization by grain boundary migration ( T < 350° C). Porphyroclast c-axis fabrics consist of a broad point maximum oriented normal to the shear plane. Recrystallized grains in the ultramylonites have strong c-axis fabrics consisting of a single point maximum oriented either parallel or oblique to the normal to the shear plane. Experimental, simple shear, calcite c-axis fabrics have been divided by previous workers into low- (LT) and high-temperature (HT) fabrics. Twinning and intracrystalline r-slip have been identified as the dominant deformation mechanisms in low-temperature experiments whereas intracrystalline r- and f-slip dominate in high-temperature experiments. LT fabrics have a single point maximum which is oriented approximately parallel to the principal stress axis and is oblique to the normal to the shear plane. HT fabrics have three point maxima which are asymmetrical both in orientation and intensity with respect to the normal to the shear plane. The transition between LT and HT fabrics occurs at temperatures lower than 400°C in fine-grained limestones and lower than 700°C in marbles at laboratory strain rates. Natural calcite c-axis fabrics, in agreement with study, almost invariably consist of a point maximum oriented either normal or slightly oblique to the shear plane. Because their fabrics are similar to experimental LT fabrics, they have been interpreted as LT fabrics. By comparison with experimental and computer-simulated LT fabrics, the obliquity of their c-axis point maximum with respect to the shear-plane normal has been used as a shear sense indicator and as a measure of the degree of non-coaxiality of the deformation. Two lines of evidence mitigate against calcite c-axis fabrics as shear sense and strain path indicators in mylonite zones. First, the obliquity of the c-axis point maximum in natural examples is not consistent for a given sense of shear. Secondly, mylonite microstructures reported here and in the literature supports the contention that dynamic recrystallization and intracrystalline slip are responsible for the strong c-axis fabrics of most calcite mylonites. Therefore, natural calcite mylonite c-axis fabrics do not generally correlate with either LT or HT experimental fabrics. Instead, natural calcite mylonites have similar microstructures and fabrics to marbles that were deformed experimentally at very high temperatures (800–850°C) and underwent pervasive dynamic recrystallization by grain boundary migration. Consequently c-axis fabrics of calcite mylonites cannot be used as a measure of the degree of non-coaxiality of the deformation nor can they be used as shear sense indicators.