Tectonic evolution of core zones of orogenic belts (Canadian Cordillera and Grenville Province)

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In order to formulate and test models for the formation of collisional mountain belts, it is essential to know the structural evolution, pressure (P) - temperature (T) history, and relative and absolute timing of deformation and metamorphism throughout a vertical crustal section in the internal zone. Such a section is exposed in the Cordillera of southern BC where the structural history, timing and P-T conditions of the last stages of mountain building events are well understood. Difficulties arise in placing age constraints on the P-T history during the early stages of construction, because minerals that are readily dated do not contain direct P-T information. The first goal is to set up a new technique that uses Sm-Nd isotopes to date intracrystalline domains of garnet, a mineral whose variations in chemical composition and textures may record the ambient P-T conditions during its growth and deformation history. Thus P-T conditions and structures that formed at different stages may be dated directly, a most powerful tool with applications to a wide spectrum of geological studies. The second goal is to apply this technique, in conjunction with other field, petrology and geochronology studies, to the Cordillera and Ontario Grenville where direct dates on early-stage deformation and P-T conditions are missing. Results will provide a test of models about mid-crustal processes and the construction and collapse of mountain belts, with applications to interpreting ancient geologic terranes in the Canadian Shield and refining models for resource exploration. Thirdly, methods used to determine P-T conditions of rocks from chemical compositions of minerals are based on thermodynamic principles, and do not account for the effects of deformation. The interplay between deformation and metamorphic processes will be evaluated by carrying out detailed studies in rocks where we can track differences in metamorphism as a function of variation in microstructures. The results of this study will have broad reaching implications for significance of P-T results in deformed rocks. Undergraduate and Graduate Student research involving all aspects of project design, fieldwork, analytical techniques and tectonic synthesis is an integral part of the proposed research program.