INTERACTIONS BETWEEN DEFORMATION AND METAMORPHISM: NEW TECHNIQUES FOR INTERPRETING P-T PATHS FROM METAMORPHIC ROCKS

Deformation can have two broad types of influences on metamorphism, in addition to facilitating changes in P-T-fluid conditions: (1) strain energy may directly alter the thermodynamic stability of minerals, and (2) strain and strain partitioning can affect the sites of growth and particularly the break-down of metamorphic minerals, thus influencing which parts of a P-T path are preserved in metamorphic rocks. The second type of interaction is more common and provides an important means of interpreting tectonics from metamorphic rocks, especially through the use of high-resolution compositional mapping. In rocks of the Moretown Fm., Mass., Grt, Pl, and Ms grew during development of crenulation cleavage. New Pl preferentially grew in crenulation hinges and Ms grew in crenulation limbs. Grt growth stages can be closely tied to deformation stages allowing equilibrium assemblages to be selected with confidence for petrologic calculations. In the East Athabasca area, N. Saskatchewan, strain partitioning occurred on all scales (mm to km). Late-stage equilibrium assemblages are well developed in higher-strain or higher-H2O regions; reaction textures (coronae, symplectites) and disequilibrium compositions are common in lower strain domains. Retrograde assemblages are only developed in very localized, late shear zones. Because there are multiple deformation events, it is critical to time deformation via in-situ dating methods. Once constrained, detailed P-T paths can be extracted involving multiple thermal events, juxtaposed terranes, and diachronous exhumation. Obviously, there are important feed-backs. In Proterozoic rocks of the Southwest, deformation and dynamic recrystallization have preferentially occurred in higher-T regions (i.e. near plutons), resulting in highly diachronous deformation and metamorphic events. Especially at mid-crustal conditions, significant weakening can occur over narrow T intervals. Although a direct influence of deformation on mineral stability is certainly possible, the energy contribution appears to be small. The most important interactions may be of the type described here. Especially with emerging techniques for in-situ age mapping and dating, these interrelationships allow a new resolution of tectonic history to be extracted from metamorphic rocks.