UVM Theses and Dissertations
Format:
Online
Author:
Ashley, Kyle Thomas
Dept./Program:
Geology
Year:
2011
Degree:
MS
Abstract:
Geochemical, microstructural and petrological analyses were conducted on metapelites from the Strafford Dome, Vermont. Samples record metamorphic conditions from biotite to peak kyanite/staurolite grade and preserve microstructures related to two Acadian nappe emplacement events. The purpose of this study was to test the validity and application of the Ti-in-quartz ("TitaniQ") thermobarometer to constraining pressure-temperature-timingdeformation (P-T-t-D) paths in metamorphic tectonites. Due to the nearly ubiquitous presence of quartz in continental rocks, the ability to apply this method would have significant implications for improving our ability to resolve tectonic histories.
Cathodoluminescence (CL) imaging on quartz was conducted to qualitatively assess the distribution of Ti in a single grain and/or compare neighboring crystals. X-ray mapping of garnet porphyroblasts was conducted to estimate P-T conditions during garnet growth to provide a framework for included quartz grains. P-T-X contour diagrams (used in P-T calculations for garnet growth) were constructed from data obtained by X-ray fluorescence analysis on bulk-rock chemistries. Secondary ion mass spectrometry analysis was conducted to constrain Ti concentrations in quartz due to the low [Ti] present in the Strafford samples (<10 ppm).
Analysis of the samples revealed [Ti] in zoned quartz grains that can be grouped and associated with certain P-T-D conditions. A majority of quartz grains have dark cores in CL images with low [Ti] (~2.5-3.5 ppm) in both matrix quartz and inclusions. Quartz inclusions in garnets that grew syn-tectonically with D₂ have bright rims ~5.5 ppm. Matrix quartz, on the other hand, has rims with much higher [Ti] (~7.5-9.5 ppm). Comparing these Ti concentrations to summary P-T paths from previous studies suggests: quartz inclusions have rims recrystallized during the end of D₁ deformation, matrix grains have rims re-equiliprated at peak P-T conditions post-D₂ deformation, and the dark cores observed in CL images must be from early prograde or relics of the protolith.
The evaluation of the TitaniQ thermobarometer's application to constrain P-T-t-D histories has highlighted some potential problems and significant benefits. To use the thermobarometer, either T or P must be independently constrained, which is often difficult to do given the many microstructural contexts of quartz in a single sample. This study capitalized on the ability to determine the relative timing of quartz (re)crystallization relative to garnet growth. Using another trace element thermobarometer would be ideal (e.g. Zr-in-rutile) for greater precision, although the relevant accessory phases may not be present and constraining the timing of re-equilibration is challenging. The abundance of quartz in continental rocks and the various microstructural occurrences of quartz in a single metamorphic tectonite provides additional opportunities to constrain points on the P-T-D path than conventional thermobarometers. The TitaniQ thermobarometer has the potential to yield deeper insights into the tectonic history of crustal rocks than previously available. These findings further elucidate the potential of the method for use in studies of metamorphic tectonites,continental tectonics and rheology.
Cathodoluminescence (CL) imaging on quartz was conducted to qualitatively assess the distribution of Ti in a single grain and/or compare neighboring crystals. X-ray mapping of garnet porphyroblasts was conducted to estimate P-T conditions during garnet growth to provide a framework for included quartz grains. P-T-X contour diagrams (used in P-T calculations for garnet growth) were constructed from data obtained by X-ray fluorescence analysis on bulk-rock chemistries. Secondary ion mass spectrometry analysis was conducted to constrain Ti concentrations in quartz due to the low [Ti] present in the Strafford samples (<10 ppm).
Analysis of the samples revealed [Ti] in zoned quartz grains that can be grouped and associated with certain P-T-D conditions. A majority of quartz grains have dark cores in CL images with low [Ti] (~2.5-3.5 ppm) in both matrix quartz and inclusions. Quartz inclusions in garnets that grew syn-tectonically with D₂ have bright rims ~5.5 ppm. Matrix quartz, on the other hand, has rims with much higher [Ti] (~7.5-9.5 ppm). Comparing these Ti concentrations to summary P-T paths from previous studies suggests: quartz inclusions have rims recrystallized during the end of D₁ deformation, matrix grains have rims re-equiliprated at peak P-T conditions post-D₂ deformation, and the dark cores observed in CL images must be from early prograde or relics of the protolith.
The evaluation of the TitaniQ thermobarometer's application to constrain P-T-t-D histories has highlighted some potential problems and significant benefits. To use the thermobarometer, either T or P must be independently constrained, which is often difficult to do given the many microstructural contexts of quartz in a single sample. This study capitalized on the ability to determine the relative timing of quartz (re)crystallization relative to garnet growth. Using another trace element thermobarometer would be ideal (e.g. Zr-in-rutile) for greater precision, although the relevant accessory phases may not be present and constraining the timing of re-equilibration is challenging. The abundance of quartz in continental rocks and the various microstructural occurrences of quartz in a single metamorphic tectonite provides additional opportunities to constrain points on the P-T-D path than conventional thermobarometers. The TitaniQ thermobarometer has the potential to yield deeper insights into the tectonic history of crustal rocks than previously available. These findings further elucidate the potential of the method for use in studies of metamorphic tectonites,continental tectonics and rheology.