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UBC Theses and Dissertations

Petrologic studies of process interactions in metamorphic systems : deformation and metamorphism in the selkirk allochthon orogenic wedge; and feedback mechanisms during reactive fluid flow Marchildon, Marie-France Nathalie

Abstract

The first part of this thesis focuses on the tectono-metamorphic evolution of the Selkirk Allochthon, a welt of supracrustal rocks deformed and metamorphosed during Mesozoic to early Cenozoic time as a result of terrane collision at the western edge of North America. The studied area is part of the Selkirk Allochthon. Pelitic rocks in the area record a Barrovian metamorphic field gradient. Detailed mapping of metamorphic assemblage distribution and textural analysis of pelitic rocks has unraveled two distinct metamorphic events (M1 and M2). Emplacement of the Bigmouth Creek stock, which intrudes rocks of the Big Fish Creek area was dated as mid-Jurassic, with evidence for substantial reheating in early Cretaceous time. Based on regional correlation, M1 is assigned a mid-Jurassic age, synchronous with emplacement of the Bigmouth Creek stock. M2 is interpreted to have resulted from the early Cretaceous thermal event. The development of early-M2 kyanite and late-M2 andalusite indicates decompression over at least 3 kbar, implying at least 10 km of denudation. Detailed analysis of structural elements suggests that the area was part of a long-lived, mid-crustal zone of intense distributed strain. Uplift and denudation of metamorphic rocks by tectonic extension and coaxial thinning were synchronous with continued contraction. A tectono-thermal model is proposed for the Allochthon as a long-lived, dynamic orogenic wedge within which deformation accommodated contraction due to terrane convergence and accretion, and polycyclic metamorphism resulted from thickening and maintenance of the wedge over at least 20 My. Significant early Cretaceous thinning of the wedge recorded by extensional strain and late-M2 decompression resulted from waning of far-field contraction, and/or from rheological modification of the wedge due to thermal relaxation. The second part of this thesis reports on a study of heterogeneous reactive transport of H2O-CO2 fluids during contact metamorphism of siliceous dolomites. Comparison of mineral assemblage distributions resulting from numerical simulations of heterogeneous flow with assemblage distribution in the Alta (Utah) contact aureole indicates that mineral reactions record flow through highly heterogeneous permeability. This is interpreted to result from positive feedback between reaction enhancement of permeability and flow-focusing. Comparison with other contact aureoles suggests that this is a common mechanism of permeability evolution during contact metamorphism of carbonate rocks. More homogeneous permeability recorded in regional metamorphic rocks may reflect fundamental differences in the relative rates of fluid flow and compaction in regional and contact metamorphic environments.

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