ON THE MICROSTRUCTURE OF HIGH PRESSURE MAFIC ROCKS AND ASSOCIATED TECTONITES IN A LARGE, HOT OROGEN: OBSERVATIONS FROM THE WESTERN GRENVILLE PROVINCE

Rocks of mafic to ultramafic composition constitute a significant portion of the lower continental crust, and under certain conditions may accumulate significant ductile strain during subduction-collision events. The extensive section of orogenic lower crust exposed in the western Central Gneiss Belt (wCGB) Grenville Province, Ontario, Canada hosts a variety of mafic to ultramafic rock types distributed throughout the volumetrically dominant felsic orthogneisses. These include garnet pyroxenites, coronitic metagabbros, metanorthosites, and retrogressed eclogite occurring as km-scale composite complexes to discrete meter-scale pods. Most of these rocks exhibit a variety of metamorphic reaction textures, with some apparently representing prograde reactions arrested during transformation from the igneous protolith to a HP-HT assemblage and others representing retrograde reactions associated with hydration and/or decompression. Detailed analysis and modeling of intracrystalline zoning profiles in garnet porphyroblasts suggests that some of the mafic complexes have different thermal and exhumation histories, which appears to correlate with their size and position relative to orogen-scale structures (e.g. the upper Allochthon Boundary Thrust). Field and petrographic observations indicate that some mafic complexes are essentially undeformed, whereas others show evidence for penetrative deformation or localized marginal shearing. The development of leucosome-rich, high-strain margins is particularly common surrounding smaller, more strongly retrogressed mafic pods, where distinctive changes in mineral assemblage are present between the pod core and margin. This contribution presents petrological and microstructural data from HP-HT mafic rocks in different locations within the wCGB in an effort to identify the grain-scale deformation processes that have affected these rocks and their relationship to changing P-T conditions, metamorphic reactions, fluid activity, and melt presence. Coupling of microstructural evolution to temporally constrained metamorphic episodes should yield important insights into the mechanical behavior and rheology of mafic lower crust through an orogenic cycle.