Date of Award

2009

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Christopher Gerbi

Second Committee Member

Scott E. Johnson

Third Committee Member

Martin G. Yates

Abstract

The Whitestone Anorthosite of the Grenville Province, southwestern Canadian Shield, provides a natural laboratory for studying the relationship between strain localization and fluid infiltration in deep orogenic crust. The Whitestone Anorthosite is an anorthositic to leucogabbroic sill metamorphosed in the granulite facies during the Grenville orogeny. Margins of the Whitestone Anorthosite were deformed in the crustal scale Parry Sound Shear Zone following peak metamorphism. The core of the Whitestone Anorthosite preserves relict igneous textures and microstructures, whereas deformed margins are characterized by thinly banded layers progressively developed from the less deformed rock. A microstructural, petrologic, and geochemical study of rocks from the Whitestone Anorthosite was performed in order to investigate relationships between fluid infiltration and strain localization within deep orogenic crust. Microstructures coupled with the relative timing of the microstructural and chemical evolution of zones of low and high strain suggest that the Whitestone Anorthosite experienced four metamorphic events. Peak granulite facies metamorphism (Event I) was followed by the breakdown of diopside to amphibole during shear zone development concurrent with primary deformation and recrystallization of plagioclase (Event II). Event III involved the static growth of Na-rich rims on recrystallized plagioclase grains and the complete breakdown of diopside in slightly deformed rocks. Event IV corresponded to a second static event that allowed scapolite to form after plagioclase. Events II, III, and IV are coincident with three episodes of fluid infiltration. During the first episode, fluids infiltrated the anorthosite and focused along regions of high strain, coincident with the deformation associated with metamorphic Event II. The second episode occurred under static conditions and affected mostly weakly deformed rocks. This episode was synchronous with the rim growth on plagioclase and the complete breakdown of diopside in slightly deformed rocks associated with metamorphic Event III. The third episode of fluid infiltration, corresponding to Event IV, is characterized by the static infiltration of a compositionally new fluid that affected high strain regions more so than low strain regions. Based on the relative timing of metamorphic and fluid infiltration events, fluid infiltration correlates with strain localization. During the first fluid episode, synkinematic fluids localized in regions of the rock that were experiencing high strain, while the third fluid episode focused fluids into regions of previously localized strain. During the second fluid episode, fluids statically infiltrated the rock and affected the least strained regions. Microstructural and chemical evolution of the Whitestone Anorthosite suggests that fluid infiltration and strain localization correlated during active deformation, but that appreciable strain is not necessary for fluids to infiltrate and cause metamorphic changes in the rock.

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