Browsing by Subject "Grenville"
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Item An evaluation of quartz-inclusion barometry by laser Raman microspectrometry : a case study from the Llano Uplift of central Texas(2010-08) McDowell, Emily Allen 1985-; Carlson, William, 1952-A new barometric technique measuring stored stress in quartz inclusions via laser Raman microspectrometry was employed in an attempt to elucidate the extent of highpressure (HP) metamorphism in the Llano Uplift of central Texas. Rare lithologies within the Llano Uplift contain mineralogical evidence of HP metamorphism (pressures from 1.4 to 2.4 GPa at temperatures from 650 to 775°C), but much of the uplift is composed of felsic gneisses lacking any HP signature; these felsic gneisses may never have transformed to HP assemblages, or they may have been thoroughly overprinted by later low-pressure events. Barometry via laser Raman microspectrometry computes entrapment pressure for a quartz inclusion in garnet from measurement of the displacements of its Raman peak positions from those of a quartz standard at atmospheric pressure. Quartz inclusions in garnets that grew in felsic gneisses under HP conditions should retain HP signatures, despite later overprinting. Application of the Raman microspectrometry technique should therefore allow barometry of previously uncharacterizable rocks. For two localities in the Llano Uplift, entrapment pressures from Raman barometry (0.6-0.7 GPa and 0.2-0.3 GPa) were substantially lower than pressures expected based on conventional barometers (1.4 GPa and 1.6-2.4 GPa). This absence of any HP signatures in the Llano rocks contrasts with more successful applications of the Raman technique by previous workers in high P/T blueschist-facies rocks. A key difference in the Llano rocks is that they reached peak temperatures at which intracrystalline diffusion in garnet, driven by compositional gradients produced during growth, had noticeable effects: complete homogenization of growth zoning had occurred in the locality that produced the greatest discrepancies between Raman and conventional pressures, and modest relaxation of zoning occurred in the locality with the smaller discrepancies. The failure of the Raman technique to recover pressures consistent with conventional barometry in the Llano Uplift is therefore attributed to relaxation of stress on the quartz inclusions as the result of intracrystalline diffusion within the garnet. This conclusion suggests that use of the Raman barometric technique must be restricted to rocks whose time-temperature histories produce only very limited intracrystalline diffusion in garnet, typically those rocks whose peak metamorphic temperatures fall at or below upper amphibolite-facies conditions.Item Geophysical investigation of the post Grenville Orogen lithosphere: Texas Gulf Coast(2013-05) Harrington, Tom; Gurrola, Harold; Nagihara, Seiichi; Asquith, George B.Receiver function and Pn tomography were used in conjunction to characterize the seismic velocity structure of the lower crust and upper mantle of the Laurentian Margin of Texas. The velocity interfaces modeled are the Mohoroviçiç Discontinuity (Moho), and the Hales Discontinuity. Observations resulted in crustal thicknesses from 26 to46 kilometers, and a Hales Discontinuity depth from 60to 88 kilometers. Pn tomography provided differential velocity variations of ±0.24 km/s, with the average velocity approximately 8.1 km/s. The Sabine and San Marcos Arches were not imaged in the upper mantle indicating they are crustal features only. A northwest-southeast trending anomaly was observed in observed in receiver function shallow Moho depths, Pn tomography low Moho velocity, and low aeromagnetic data was interpreted to be a relic of strike-slip motion attributed to the Ouachita Rift System. The anomaly crosses the Ouachita Orogenic Front without truncation or offset, inferring that the younger Ouachita Front was a, “thin-skinned,” collision along the Laurentian Margin