Tectonics of metamorphic crystallization.

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Tectonics of metamorphic crystallization.

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Metamorphic reactions occur in response to changes in the pressure-temperature-composition (e.g. fluid, bulk-rock) and deformation states of a system in an attempt to re-equilibrate to a new set of conditions. The development of reaction textures and deformation microstructures in rocks are a record of this response. The work presented in this thesis aims to investigate the processes and conditions of metamorphic reactions. In particular, the role of deformation, <italic>P-T</italic> evolution and evolving scales of mass transport in reaction initiation and evolution are considered in detail. The extent of interaction between deformation and metamorphism is difficult to determine because, under some circumstances, they may be mutually dependent. Metamorphic reactions can enhance rheological weakening, as is often the case with the development of discrete crustal and mantle shear zones. Deformation can, in turn, enhance the re-equilibration of a rock volume. Ductile deformation processes in rocks affect reaction kinetics through the generation and subsequent mobilization of dislocations and grain boundaries. Deformation experiments carried out in torsion on the Al<sub>2</sub>SiO<sub>5</sub> polymorphs illustrate the intimate relationship between deformation and metamorphic processes. Experiments carried out in the sillimanite stability field show near complete transformation of kyanite and andalusite to sillimanite. However, undeformed samples that experienced similar <italic>P-T</italic> conditions as deformation experiments show no evidence of polymorphic transformation - inline with the well-known sluggish kinetics of transformation in the Al<sub>2</sub>SiO<sub>5</sub> system. Polymorphic transformation was concentrated in low-angle shear bands, which developed even at very low strains. These data suggest that strain is required for polymorphic transformation to occur in the Al<sub>2</sub>SiO<sub>5</sub> system, at least at laboratory time-scales. The common preservation of metamorphic reaction textures and the preservation of compositional zoning in minerals is evidence that the attainment of equilibrium, in a strict sense, is rarely obtained throughout the <italic>P-T</italic> evolution of a metamorphic rock. Orthoamphibole-cordierite gneiss from the Thor-Odin dome, British Columbia, Canada preserves coronal reaction textures around Al<sub>2</sub>SiO<sub>5</sub> and garnet. Reaction textures associated with Al<sub>2</sub>SiO<sub>5</sub> are layered symplectitic coronas dominantly consisting of two-phase vermicular product assemblages. The distribution of these assemblages is heterogeneous with respect to their presence/absence within a reaction texture, location about the central Al<sub>2</sub>SiO<sub>5</sub> reactant, and their relative thicknesses. The heterogeneous nature of symplectitic coronas is explored by estimating differences in relative magnitudes of component material transport necessary to form the observed assemblages. Results are consistent with expansion of the chemical system through time, likely related to the progressive formation of symplectitic reaction textures during decompression. This result may reflect the difficulty of modeling the <italic>P-T</italic> evolution of a system using equilibrum phase diagrams in which absolute and effective bulk composition evolved during metamorphism as a result of fluid influx and associated increases in diffusion rates and length scales. In addition, petrographic and high-resolution X-ray computed tomography (HRXCT) data show the development of bulk rock-scale interconnected coronal networks linking reaction textures around garnet and Al<sub>2</sub>SiO<sub>5</sub>. These factors along with questions regarding the effective bulk composition controlling phase assemblage stability made quantitative determination of <italic>P-T</italic> conditions difficult using equilibrium phase diagram analysis in rocks containing evidence for multi-scale diffusion gradients.


University of Minnesota Ph.D. dissertation. May 2009. Major: Geology. Advisor: Professor Donna L. Whitney. 1 computer file (PDF); xiii, 285 pages., appendix.

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Goergen, Eric Thomas. (2009). Tectonics of metamorphic crystallization.. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/52003.

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