Browsing by Subject "Metamorphic rocks"
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Item Constraining the exhumation history of high-pressure subduction zone rocks : insights from the Cycladic islands, Greece and the application of novel thermobarometry techniques(2018-12) Cisneros, Miguel; Barnes, Jaime Danielle; Behr, Whitney M; Stockli, Daniel; Ketcham, Richard; Ashley, Kyle; Befus, KennethThe mechanisms that lead to exhumation of high-pressure low-temperature (HP-LT) metamorphic rocks have remained enigmatic since the recognition of the petrologic significance of blueschist facies mineral assemblages. Much of our understanding about these mechanisms stems from constraints of the pressure-temperature history of exhumed metamorphic rocks. In this work I describe the results of four projects that focus on testing new thermobarometers that can be applied to metamorphic rocks, among other rock types, and then apply multiple integrative techniques to constrain the pressure-temperature-fluid histories of rocks from Syros and Tinos, Greece. Elastic thermobarometry is a rapidly expanding technique that can be used to constrain pressures and temperatures from inclusion-host mineral pairs. I present a compilation of new inclusion-host mineral pairs that can be used to constrain PT conditions, and an adaptive script that can be used for calculating entrapment pressure or temperature conditions from residual inclusion pressures. I further complete a case-study of the quartz-in-epidote barometer, because of its great abundance in metamorphic rock types and its potential utility for understanding geologic systems that currently lack barometers. Results from heating measurements of quartz inclusions in epidote from Alpine metamorphic rocks indicate two promising conclusions: 1) calculated entrapment pressures show good agreement with previously constrained pressure conditions, indicating that further investigation of the barometer is warranted, and 2) the quartz-in-epidote barometer may not require temperature-dependent entrapment pressure corrections. The Cycladic Blueschist Unit on both Syros and Tinos, Greece, preserves exhumed, HP subduction zone rocks that have an enigmatic history after having reached peak metamorphic conditions. I present a combination of thermobarometry and mineral chemistry techniques that include further application of the quartz-in-epidote barometer, oxygen isotope thermometry, and fluid chemistry as deduced from oxygen, hydrogen, and carbon isotopes, to better understand the protracted exhumation history of metamorphic belts. Results from PT-fluid data from select outcrops that record changing kinematics, indicate that retrograde metamorphic rocks from Syros Greece underwent cooling during decompression, and interaction with slab derived fluids during exhumation. Lower pressure samples from Tinos Greece and hydrogen isotopes possibly show evidence of interaction with meteoric derived fluids during later stages of exhumation along a low-angle detachment.Item Evidence from high-temporal-resolution strain rates for strain softening due to episodic fluid influx at Passo del Sole, Central Swiss Alps(2012-12) Stacy, Sarah Jean; Carlson, William, 1952-; Cloos, Mark; Ketcham, Richard ABerg (2007) determined hand-sample-scale high-temporal-resolution strain rates from rotated garnet for two samples of quartzose pelitic gneiss at Passo del Sole, Central Swiss Alps, documenting a correlation between dramatic increases in strain rate and compositionally anomalous garnet growth zones. Considering additional evidence that these anomalous zones resulted from externally derived ephemeral fluid flow, he concluded that increased strain rates at Passo del Sole are the result of strain softening caused by fluid influx. This study tests Berg's interpretation by calculating new hand-sample-scale high-temporal-resolution strain rates for two additional samples of the same gneiss: a control sample (Sample PDS 03-30) that shows no unusual zoning patterns, implying that it was unaffected by fluids; and another (Sample PDS 03-2) that features a prominent Ca spike, suggesting that it has been affected by fluid influx. Unique features of garnet from this locality--contemporaneity of chemical zones, near-simultaneous nucleation, size-proportional growth, and rock-wide chemical equilibrium--were exploited to calculate strain rates of unprecedentedly high temporal resolution. Thermodynamically modeled temperatures for several growth-zone boundaries in each garnet crystal were combined with a regional heating rate of 11.5 ± 3.5 °C/Myr (presumed constant) and measured deflections of inclusion trails in each zone to calculate strain rates for several discrete time increments during garnet growth. Sample PDS 03-2 displays a 2- to 16-fold increase in strain rate that correlates with growth of the high-Ca zone; strain rates are 0.4 x 10⁻¹⁴ s⁻¹ to 4.1 x 10⁻¹⁴ s⁻¹ for zones with normal Ca concentration and 9.1 x 10⁻¹⁴ s⁻¹ to 17.9 x 10⁻¹⁴ s⁻¹ for the high-Ca zone. Distinct amongst all analyzed samples from Passo del Sole, Sample PDS 03-30--which has not been affected by fluid influx--shows no fluctuations in strain rates, which remain low and similar (0.2 x 10⁻¹⁴ s⁻¹ to 2.6 x 10⁻¹⁴ s⁻¹) across all zones. Results from this study therefore further substantiate the correlation of high strain rates with compositionally anomalous zones, strengthening the interpretation that elevated strain rates at Passo del Sole result from strain softening caused by episodic, externally controlled flow of fluids through the system during synkinematic garnet growth.Item Tectonic evolution of Aegean metamorphic core complexes, Andros and Tinos Islands, Greece(2014-05) Shin, Timothy Andrew; Stockli, Daniel F.; Catlos, Elizabeth J., 1971-The Aegean is a classic setting for studying exhumation of high-pressure (HP) metamorphic rocks. Two end-member models are proposed to explain the uplift of these rocks: core-complex style extension along low-angle normal faults and extrusion-wedge uplift. Extrusion-wedge underplating is the mechanism that exhumed HP rocks on Evia whereas Tinos hosts several detachments varying in age from 30-9 Ma. Andros, situated between them, may be the geological manifestation of the interplay of these processes and provides an opportunity to test these models. Detachments on NW Tinos and on Andros and the enigmatic low-angle Makrotantalon Unit contact on Andros were insufficiently dated prior to this study. Geo- and thermochronometrycombined with structural observations from sampling transects in the transport direction from (1) lower plate Cycladic Blueschist Unit on Andros and Tinos, (2) middle plate Makrotantalon Unit on Andros, and (3) hanging wall Upper Unit address these issues. Maximum depositional ages from detrital zircon U-Pb geochronometry and structures reveal Paleocene-Eocene syn-HP metamorphism thrusting resulted in an inversed-age relationship between the Permian Makrotantalon Unit and the underlying Triassic-Eocene Cycladic Blueschist Unit on Andros. The Makrotantalon Unit has an internal inversed stratigraphy whereas the Cycladic Blueschist Unit on Andros and Tinos appear stratigraphically intact. Structures and zircon and apatite (U-Th)/He ages in transects from NW Tinos (~12-8 Ma) and central Andros Cycladic Blueschist Unit (~13-7 Ma) indicate rapid cooling due to exhumation associated with the Livada Detachment. Older cooling ages (~16-10 Ma) and structures in the Makrotantalon Unit indicate later brittle strain localization on the Makrotantalon Thrust contact is accommodated by rheologically weaker serpentinites and calc-schists, resulting in slivering of the footwall under the Livada Detachment on Andros. Estimated mean cooling slip rates of the Livada Detachment on Andros of ~3.8 (+1.2/-1.3) km/Myr and 2.1 (+0.2/-0.2) km/Myr on NW Tinos resulted in minimum vertical exhumations of 15 km and 4 km, respectively. The NCDS here accommodated ~12-25% of 60 km of HP-rock exhumation from ~30-7 Ma. We present a tectonic model to elucidate the evolution of the Makrotantalon Unit and the magnitude, temporal, and spatial variability of exhumation via detachments on these islands.Item Three-dimensional quantitative textural analysis of metamorphic rocks using high-resolution computed X-ray tomography : methods, techniques and application to natural samples(1995) Denison, Cambria; Carlson, William, 1952-Quantitative three-dimensional textural analysis of metamorphic rocks using high-resolution computed X-ray tomography and automated image mensuration produces data on the size and position of thousands of crystals in a rock volume. Part I of this dissertation reviews some fundamentals of computed X-ray tomography and describes techniques of three-dimensional quantitative textural analysis that determine, from tomographic imagery, the extent of ordering, clustering, intergrowth and isolation of porphyroblasts within a rock volume. These textural data can be interpreted in light of numerical models of nucleation and growth that predict quantitative textural features of porphyroblastic rocks for different crystallization mechanisms. Comparison of the predictions of these models to natural textures, in conjunction with analysis of compositional zoning, establishes the relative importance of possible atomic-scale mechanisms of porphyroblast crystallization. In Part 11, these methods are used to assess the mechanisms governing crystallization of garnet porphyroblasts in rocks from diverse regional metamorphic environments. In every case, spatial dispositions, crystal size distributions, and compositional zoning patterns of porphyroblasts indicate the dominance of diffusionally influenced nucleation and diffusion-controlled growth mechanisms. Nine samples from three geologic areas were studied: a suite of pelitic rocks from the Picuris Mountains, New Mexico (USA); a suite of mafic samples from the Llano Uplift, Texas (USA); and a kyanite schist from Mica Dam, British Columbia (Canada). For each sample, the centers and radii of thousands of garnet crystals were located and measured in three dimensions from the tomographic images. Statistical measures of the degree of ordering and clustering of nucleation sites, and estimates of crystal isolation for each porphyroblast, were then computed from the measured spatial dispositions. These measures can be reproduced in simple numerical models only by diffusionally influenced nucleation and diffusion-controlled growth mechanisms. Normalized radius-rate relations computed from compositional zoning patterns in the garnets require thermally accelerated diffusion-controlled growth, providing independent confirmation of the conclusions based on textural analysis. The unexpected similarity of results from all samples indicates that diffusionally influenced nucleation and growth mechanisms may govern porphyroblast crystallization in many metamorphic regimes.