Quartz as an archive of deformation and metamorphism of the continental crust
2015-12
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Quartz as an archive of deformation and metamorphism of the continental crust
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2015-12
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Quartz is perhaps the most rheologically-significant mineral in the continental crust. It occurs in many different rock types, it is susceptible to deformation over a range of crustal conditions, and it yields plastically at stresses lower than other rock-forming minerals. Consequently the deformation behavior of quartz – its rheology, microstructures, and recrystallization fabrics – has been well-studied in both natural and laboratory settings and is commonly used by structural geologists to interpret the direction and magnitude of forces in exhumed shear zones. In the last decade, it was discovered that quartz can be used as a quantitative indicator of metamorphic conditions. The solubility and diffusivity of trace concentrations of Ti in quartz was determined experimentally, establishing a technique that uses quartz geochemistry as a thermometer, barometer, and geospeedometer to estimate temperatures, pressures, and rates of crystallization in natural rocks. However, the experiments that determined these parameters were conducted under hydrostatic stress. Given the propensity for quartz to deform ductiley in the Earth, it is important to investigate the influence of dynamic recrystallization on Ti substitution to determine if quartz geochemistry provides reliable information on the conditions and rates of ductile deformation. This dissertation combined field and experimental approaches to study the effect of deformation on Ti exchange in quartz. Observations of quartzite mylonites from extensional shear zones in the North American Cordillera inspired the design of rock deformation experiments to simulate natural deformation processes under controlled laboratory conditions. Multiple quantitative (electron-, ion-, and laser-beam techniques) and semi-quantitative (cathodoluminescence) analytical methods were used to measure the Ti content of fine-scale microstructures, which were compared with crystallographic fabric analysis (electron back-scattered diffraction) to correlate the extent of recrystallization with Ti exchange. Results of this research show that dynamic recrystallization is effective at resetting Ti concentrations in quartz, indicating that Ti-in-quartz is capable of recording the conditions of ductile deformation. With the documented importance of quartz for accommodating ductile strain during orogenesis, findings presented in this dissertation demonstrate that quartz is capable of linking metamorphic petrology with deformation microstructures to contribute a new perspective on interpreting deformation in continental shear zones.
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University of Minnesota Ph.D. dissertation. December 2015. Major: Earth Sciences. Advisors: Donna Whitney, Christian Teyssier. 1 computer file (PDF); xx, 346 pages.
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Nachlas, William. (2015). Quartz as an archive of deformation and metamorphism of the continental crust. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/194566.
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