Collaborative Research: Constraining P-T-t-D paths of metamorphic tectoniteswith the TitaniQ thermobarometer
Intellectual merit: The recent development of several trace-element thermometers has provided powerful new tools for geologists to constrain pressure-temperature-time-deformation (P-T-t-D) histories of many crustal rocks to understand their tectonic and petrologic histories. Key to this approach is the recent development of a Ti-in-quartz (TitaniQ) thermobarometer that can ideally provide T or P information from the content of Ti dissolved in quartz. This approach remains to be tested in a well-characterized environment. To this end, an integrated microstructural, petrologic, and geochemical study is proposed to evaluate P-T-t-D paths of metamorphic tectonites from well characterized and previously dated gneisses, schists, and mylonites from the Strafford Dome complex (Vermont). These rocks record progressive deformation during crenulation cleavage development and, locally, complete transposition of the S1 fabric and mylonitization. S1 is preserved as quartz-rich inclusions suites in minerals such as garnet, staurolite, and kyanite, suggesting that CL imaging combined with in situ analyses of Ti-in-quartz in different microstructural contexts from the same sample may elucidate discrete segments of the P-T-t-D path. Electron backscatter and CL imaging will be used to investigate the strain field associated with P-T conditions and provide insight into the physical processes associated with quartz reequilibration with respect to Ti distributions. What is unique about this study is the potential to determine not just the temperature, but also the depth at which a fabric is developed.
Broader impacts: The proposed research efforts will foster collaboration at the interface of tectonics, petrology, and geochemistry. The project will support the research programs of two early career geoscientists (Webb and Thomas) and enhance graduate and undergraduate student education and training through the use of cutting-edge technology and hands-on field and laboratory experiences. Students will gain experience in linking observations from micron to plate scales. The results of this work will be broadly applicable to many problems in continental tectonics and disseminated at international meetings and in peer reviewed journals. Samples, field data and analytical data collected during the course of this study will be used in L. Webb's class, Microstructural Analysis and P-T-t-D Paths, for senior level undergraduate and graduate students at UVM. Photomicrograghs, CL images, and analytical data sets will be available to the scientific community for research and teaching through an NSF supported Database System for Metamorphic Geochemistry 'MetPetDB'.
Intellectual merit: This study investigates the ability of quartz, one of the most abundant minerals in the continental crust, to record information on a rock’s pressure-temperature-deformation (P-T-D) history through the application of the Ti-in-quartz thermobarometer (TitaniQ). Metapelites from the Strafford Dome and Rowe Hawley Belt of central Vermont were selected for this study because they record progressive fabric development during prograde metamorphism and have well-constrained P-T-D histories. Analyses were conducted on quartz in different microstructural contexts such as fold hinges, inclusion suites defining internal foliations in garnet, and pressure shadows. In each generation of foliation, metapelites display pressure solution seams, flattened quartz grains neighboring micas in cleavage domains, and more equant quartz grains with foam textures in quartz-rich microlithons and hinges of microfolds. Cathodoluminescence (CL) imaging (415 nm) reveals zoning patterns of quartz that are typically complex and associated with sharp boundaries. Documented zoning patterns include: 1) rounded-to-anhedral dark cores with bright rims; 2) bright cores with dark rims; 3) dark cores with bright mantles, and dark rims; 4) patchy zoning; and 5) striated grains. Results of ion microprobe analyses confirm that brighter intensities in CL images are associated with higher [Ti] than darker regions. Based on data from both sample suites, quartz grains are recording multiple segments of the prograde/peak P-T-D history and the observed variations in zoning patterns are consistent with that expected for constrained segments of the P-T-D path. Dark cores with low [Ti] are interpreted as quartz growth/precipitation during burial metamorphism and cleavage development via pressure solution. Brighter mantles and rims can be linked to pressure solution or quartz-producing metamorphic reactions, depending on microstructural context. Likewise, dark rims relative to cores or mantles may record tectonic burial. Relics of detrital quartz appear to be absent from the majority samples and are only locally preserved. The degree of geochemical recycling of quartz in both sample suites inferred from the results of all analyses cannot be accounted for by dynamic recrystallization, volume diffusion, or changes in the volume of quartz predicted by petrologic models. The results of our study imply that pressure solution creep was the dominant deformation process throughout the prograde P-T path for both sample suites and thus the primary control on quartz dissolution, precipitation, and growth. TitaniQ may be an effective tracer of this process when coupled with petrologic modelling, yielding important insights regarding the rheology of orogenic belts. Diffusion modeling of zoned quartz inclusions in garnet from the Strafford Dome further demonstrates that TitaniQ may also be an effective record of the timescales of orogeny and metamorphism. Broader impacts: This project fostered collaboration at the interface of tectonics, petrology, and geochemistry. The project supported the research programs of two early career geoscientists (Webb and Thomas) and enhanced graduate and undergraduate student education and training through the use of cutting-edge technology and hands-on field and laboratory experiences. Two graduate students completed Master’s theses focused on this research (Ashley, 2011; Dyess, 2013) and gained experience in linking observations from micron to plate scales. The results of this work were disseminated at international meetings (eight published abstracts) and in peer-reviewed journals (Spear et al., 2012; Ashley et al., 2013). Samples, field data and analytical data collected during the course of this study are used in two Geology classes at UVM. Photomicrograghs, CL images, and analytical data sets are available to the scientific community for research and teaching through an NSF supported Database System for Metamorphic Geochemistry "MetPetDB".
