TECTONIC INTERPRETATION OF P-T PATHS: INTEGRATING DEFORMATION, METAMORPHISM AND GEOCHRONOLOGY

Calculation of phase equilibria from metamorphic rocks is increasingly feasible and reliable over a broad range of P-T space. However, tectonic interpretations, as well as simple validation, of calculated P-T histories require: (1) integration of deformation on multiple scales and (2) relative and absolute timing constraints on the prograde, peak, and retrograde portions of the P-T path. Both linkages remain a significant challenge. Dynamic models can now be used to explore the thermal and rheological implications of crustal movements, both igneous and tectonic. Strain partitioning near igneous intrusions can complicate relationships, but thermal and rheological gradients (in space and time) are valuable for linking metamorphism, deformation, timing, and tectonics. On a small scale, microstructures link metamorphic and deformation events, but interpretations of porphyroblast inclusion relations are still problematical. Petrologic pseudosections are an increasingly essential tool for interpreting microtextures and for linking deformation, metamorphism, and large-scale tectonics. Finally, in-situ geochronology and petrological analysis of chronometer phases (i.e. monazite, xenotime, titanite, allanite, etc.) are a critical part of tectonic analysis of metamorphic rocks, and the electron probe plays an essential role. Compositional mapping of major and accessory phases and especially maps of larger thin section domains are particularly valuable for evaluating strain partitioning, scales of equilibrium, relationships between metamorphic textures and deformational fabrics, and in particular, for interpreting geochronologic data. Isobaric terranes in the southwestern USA (0.3 and 0.7 GPa) and northern Saskatchewan, Canada (1.0 GPa) provide excellent examples of deformation-metamorphism interactions and of the use of in-situ geochronology to illuminate interrelationships between extensive variables. In all areas, the principal role of strain involves providing access to fluids, sites for nucleation and growth of new phases, and especially promoting the removal of unstable (relict) phases. Feedbacks between metamorphism, igneous intrusion, and deformation exist at all scales, and can provide some of the strongest constraints on the tectonic history.