Matrix mosaics, brittle deformation, and elongate porphyroclasts: granulite facies microstructures in the Striding–Athabasca mylonite zone, western Canada

Abstract

The Late Archean Striding–Athabasca mylonite zone, western Canadian Shield, is a 400-km-long, linked system of granulite facies mylonite belts developed in the continental crust. The granulite facies mylonites (800–1000°C at 0.8–1.1 GPa) developed in subsets of the anhydrous mineral assemblage clinopyroxene–orthopyroxene–garnet–plagioclase–quartz±hornblende. Comparison of the microstructures of the Striding–Athabasca mylonites with published descriptions of natural and experimental deformation of geological and analogue materials at elevated homologous temperatures provides some insight into the role played by different processes in the development of the Striding–Athabasca mylonites. In addition to dislocation creep and dynamic recrystallisation, extensive mass transfer occurred contemporaneously with brittle fracture and cataclasis during granulite facies metamorphism. The microstructure and extreme phase dispersal in anhydrous polymineralic matrix mosaics is indicative of efficient grain-scale and aggregate-scale diffusion, grain boundary mobility, and mass transfer, at relatively slow strain rates. Despite their annealed appearance, the granoblastic matrix mosaics developed synkinematically; deformation-induced dilation may enhance metamorphic reactions where products are more voluminous than reactants. In the porphyroclast population, highly elongate (>20:1) monocrystalline orthopyroxenes appear to be fragments of dismembered, kinked parent grains, rather than stretched porphyroclasts. Granulite facies mylonites should not be treated as direct analogues of greenschist facies mylonites. In particular, it is essential to evaluate the potential positive feedback between structural and metamorphic processes in highly strained, high-temperature shear zone rocks.