CONTEXTUALIZING GEOCHRONOMETERS VIA THIN SECTION SCANS IN ORDER TO CONSTRAIN THE TIMING OF METAMORPHISM AND DEFORMATION DURING THE GRENVILLE OROGENY, LLANO UPLIFT, CENTRAL TEXAS

While the pressure-temperature (P-T) conditions of metamorphism and the history of deformation in the Llano uplift of central Texas have been well described, the timing of these events is poorly constrained. The Llano uplift, the only major exposure of the Grenville orogen west of Alabama, exhumes basement rocks that were polymetamorphosed, polydeformed, and then subsequently intruded and metamorphosed by the Town Mountain Granite. The timing of metamorphism and deformation is bracketed between 1301-1118 Ma, based primarily on relative age relationships between the metamorphic rocks and intrusions of known ages rather than on any direct dating of the metamorphism itself. This is due in part to the fact that most research on the uplift’s metamorphic history was conducted prior to the development and widespread application of techniques that allow for in situ analysis of zircon or monazite grains for geochronology, or for the simultaneous collection of geochronologic and geochemical datasets, analyzed in tandem (petrochronology), from such grains. While access to metamorphic rocks within the uplift is limited, metamorphic rocks that outcrop in Enchanted Rock State Natural Area are publicly accessible. Before geochronologic analysis of these rocks can begin, there must be 1) detailed assessment of the metamorphic minerals and deformation textures present, 2) determination of the relative timing of mineral growth and/or development of deformation textures, and 3) identification of zircon or monazite in thin section, and their relationship to metamorphic minerals and deformation textures. This study presents the results of preliminary and ongoing research to use full thin section scanning, accomplished using a high-resolution flatbed scanner modified with polarizing film, to identify geochronometers appropriate for geochronologic and/or petrochronologic analysis with the goal of better constraining the timing of metamorphism and potentially improving the pressure-temperature-time-deformation (P-T-t-D) history of metamorphic rocks in the Llano uplift.