Browsing by Author "Sisson, Virginia B."
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Item Complex Garnet Zoning from Lawsonite Eclogite from South of the Motagua Suture Zone, Guatemala(2019-05) Bradley, Deborah 1985-; Lapen, Thomas J.; Sisson, Virginia B.; Robinson, Alexander C.; Khan, Shuhab D.; Harlow, George E.Serpentinite mélange of the Guatemalan suture zone hosts the exhumed remains of one of the coldest subduction zones in the world. The mélange contains several different metamorphic rocks, including lawsonite eclogite. Well-preserved lawsonite eclogite is particularly rare, and is preserved only in high-pressure, low-temperature metamorphism wherein exhumation is rapid. Though lawsonite eclogite is well-preserved, the full extent of the P-T-t path this HP-LT rock experienced is contentious, with two competing proposed P-T paths: one with a prograde path with garnet growth from 300 to 480 °C at 1.1-2.6 GPa, and another with a prograde path from 470 to 520 °C at 2-2.5 GPa. Combining major, trace, and rare earth element (REE) garnet concentrations with crystal size distribution analysis, X-ray element maps, REE modeling, and Sm-Nd age data, constraints are placed on the prograde P-T-t path of these HP-LT metabasites. My data reveal a complex zoning in major, trace, and rare earth elements. Core to rim concentration profiles define normal prograde zoning, but deviated from this zoning towards the rim, with restricted regions of complex zoning in garnet compatible elements (e.g., Mn and HREEs), with a decrease of these elements at the rim. This complex zoning is related to changes in bulk composition, fluid availability, and garnet growth rate, which indicates that garnet experienced different stages of growth over a protracted period, perhaps as long as 33 Ma. The overall variability of chemical zoning between eclogite samples in the mélange, it is also apparent that different P-T histories may be recorded in different eclogite blocks.Item Constraining the age of metamorphism in the Dora Maira, Western Alps: Implications of U-Th-Pb ages and REE concentrations in phosphates(2014-12) Tilghman, Katherine 1991-; Lapen, Thomas J.; Sisson, Virginia B.; Robinson, Alexander C.; Meen, James K.Understanding the rates of subduction, burial, and exhumation requires accurate and precise age determinations of various stages along a unit’s pressure-temperature path. Advances in geochronology result in age uncertainties that are significantly less than the duration of many geological processes such as prograde metamorphism and exhumation. Thus, it is more important than ever to precisely link the measured ages to the metamorphic conditions. This study presents U-Th-Pb and rare earth element data collected by LA-ICPMS for monazite, monazite-apatite symplectite, and florencite in pyrope quartzites from Parigi, Dora Maira nappe, western Alps, Italy. U-Pb and Th-Pb isochron ages of the monazite are 31.48 ± 0.22 and 32.67 ± 0.70 Ma, respectively (all uncertainties are reported at 2σ). The symplectite (+florencite) yielded U-Pb and Th-Pb isochron ages of 31.3 ± 1.7 and 31.8 ± 4.3Ma, respectively; these phosphate mineral assemblages are indistinguishable in age and initial Pb isotope composition from the monazite. The monazite ages of this study are younger than ages of peak metamorphic conditions as represented by a Lu-Hf garnet-matrix age of 34.1 ± 1.0, recalculated with 176Lu decay constant, and a 35.1 ± 0.9 Ma U-Pb age of UHP titanite. Monazite ages of this study are consistent with, but slightly younger than U-Th-Pb monazite ages from previous studies and in agreement with U-Pb ages of retrograde titanite. Given that the monazite from this study likely records REE liberated by garnet break-down associated with the reaction phengite+pyrope+H2O = phlogopite+kyanite+talc, ages from this study likely reflect the timing of this process. The P-T conditions of this reaction are estimated to lie above the quartz-to-coesite transition, so the monazite ages mayrecord UHP conditions of the retrograde path. Given these constraints, initial exhumation rates of the Dora Maira nappe may be significantly greater than the 3.4 cm/yr previously proposed.Item Ground-Based Hyperspectral Imaging at Various Scales(2019-12) Krupnik, Diana 1989-; Khan, Shuhab D.; Glennie, Craig L.; Iyer, Rupa; Sisson, Virginia B.; Zhou, Hua-WeiDetailed mineral mapping at millimeter to centimeter scales can be useful for geological investigations, including sedimentologic and petrologic analysis, resource exploration, and other applications. This work presents case studies of close-range ground-based hyperspectral imaging from centimeter to sub-millimeter scales using sensors operating in the visible near infrared and short wave infrared spectral ranges. In a sedimentological study, ground-based hyperspectral imaging was combined with terrestrial laser scanning to produce mineralogical maps of Late Albian rudist buildups of the Edwards formation in Williamson County, Texas. The Edwards Formation consists of shallow water deposits of reef and associated interreef facies. Hyperspectral data were registered to a laser point cloud-generated mesh with sub-pixel accuracy and were used to map compositional variation by distinguishing spectral properties unique to each material. Calcitic flat-topped toucasid-rich bioherm facies were distinguished from overlying porous sucrosic dolostones, and peloid wackestones and packstones of back-reef facies. Ground truth was established by petrographic study of samples from the area. This research integrated high-resolution datasets to analyze geometric and compositional properties of this carbonate formation at a finer scale than traditional methods and modeled the geometry and composition of rudist buildups. Three case studies of economic deposits in active and abandoned mines are presented. Vertical exposures in a Carlin-style sediment-hosted gold deposit, an active Cu-Au-Mo mine, and an active asphalt quarry were studied to produce images that delineate mineral alteration at centimeter scale, to demonstrate a method of outcrop characterization that increases understanding of petrogenesis for mining applications. In the Carlin-style gold deposit, clay, iron oxide, carbonate, and jarosite minerals were mapped. In the copper porphyry deposit, different phases of alteration, some of which correspond to greater occurrence of ore deposits, were classified. A limestone quarry that contains bitumen deposits used for road paving aggregate was also imaged. Sub-millimeter scale laboratory imaging spectroscopy was applied to evaluate a sample suite of diagenetically altered limestones, sulfide-rich quartz veins, and eclogite. These samples were selected due to the availability of geochemical or petrographic data for mineralogical abundances to be compared to hyperspectral imaging results. Different classification techniques were evaluated for accuracy against ground truth data.Item Mapping Hydrocarbons and Rare Earth Elements at Various Scales through Imaging Spectroscopy(2023-12) Gadea, Otto C; Khan, Shuhab D.; Sisson, Virginia B.; Castagna, John P.; Krupnik, DianaRemote sensing techniques can play a critical role in geologic interpretation for mineral and energy exploration in unusual deposits. Hyperspectral cameras measure how a geophysical variable changes across wavelengths in the visible to short-wave infrared portion of the spectrum to map surface compositional variation. For two sites in this investigation, close-range ground-based imaging spectroscopy of hand samples with centimeter to sub-millimeter spatial resolution is used to establish preliminary information about the spatial distribution of natural resources within a geological formation. This information can then be compared with mineralogical maps of lateral rock exposures derived from airborne and spaceborne data. The first site is Fort McMurray in Alberta, Canada. The fluorescence characteristics of a bituminous sandstone depend on varying concentrations of light and heavy hydrocarbons, enabling imaging spectroscopy to distinguish zones of optimal yield for crude oil extraction. In the first chapter, multiple images of tar sand samples are collected under different wavelengths of ultraviolet illumination and normalized with respect to the fluorescence patterns of Spectralon diffuse reflectance material. Three classification methods are used to distinguish between bitumen, Spectralon, and a non-fluorescent slate background. Spectral indices useful for indicating concentrated bitumen in tar sands are proposed. The second site is the Sulfide Queen mine in Mountain Pass, California, which contains economic deposits of bastnäsite within a carbonatite body intruding a metamorphic host rock that have been mined for rare earth elements (REEs) critical to components in high technology devices. In the second chapter, a new spectral index based on reflectance measurements from hyperspectral data collected under visible illumination is created to map the relative REE abundances across three museum samples known to consist primarily of bastnäsite from the mine. In the third chapter, that index and its two new modified versions are applied to eleven ore samples of varying composition from known geolocations across the mine. To determine how changes in spectral patterns across different scales affect the perceived distribution of rare earth elements, these three indices are also applied to data collected by eight airborne and spaceborne imaging spectrometers over the Sulfide Queen mine and the nearby Ivanpah Dry Lake.Item MICROSTRUCTURAL AND METAMORPHIC ANALYSIS OF THE MALTON GNEISS DOME, SOUTHERN CANADIAN CORDILLERA(2013-08) Kabazi, Hana Najed 1986-; Robinson, Alexander C.; Sisson, Virginia B.; Murphy, Michael A.; Guynn, Jerome H.The Malton Gneiss Dome is located in the Southern Canadian Cordillera, at the northern most tip of the Shuswap Metamorphic Core Complex (SMCC). It is one of four domes within the SMCC, and is located closest to the foreland. Although deformation fabrics within the SMCC are dominated by E-W/NE-SW verging deformation fabrics, deformation fabrics within the Malton Gneiss Dome show top-NW sense of shear, parallel to the orogenic front. One possible explanation for this difference is the tectonically forced orogen parallel flow model, where foreland directed flow of low-viscosity mid-lower crust encounters a barrier, either thermal or structural, forcing it to flow laterally/parallel to the orogen. This model predicts that orogen parallel fabrics are synchronous with peak metamorphic conditions, during the late-Cretaceous to early-Cenozoic. Peak metamorphic conditions were investigated though quantitative thermobarometry and the relationship between deformation and metamorphism assessed through quartz deformation fabrics. My results show all samples analyzed have experienced amphibolite facies pressure and temperature conditions, with ranges of 600- 775 OC and 400-900 MPa. These conditions are similar to the other domes within the complex. Petrofabric analysis confirmed the orogen parallel movement within the dome coincided with peak metamorphic conditions, indicated by both syn-kinematic garnet and feldspar porphyroblasts which record peak metamorphic conditions.Item Paleocene Turbidite Deposition in the Central American Seaway (NW Costa Rica): Geochemical Analysis and Provenance of Detrital Spinel and Clinopyroxene(2015-08) Giblin, Allegra Catherine Thibodaux 1979-; Snow, Jonathan E.; Gazel, Esteban; Sisson, Virginia B.; Saylor, Joel E.The Central American Land Bridge is the crucial connection between North and South America, and the Miocene closure of the Panama seaway led to a change in global oceanic circulation patterns. Modern Costa Rica is part of the island arc that formed over the western Caribbean subduction zone, and the Santa Elena peninsula is on the northwest coast of Costa Rica next to the Sandino forearc basin. This study focuses on the origin and provenance of the Paleocene deep water Rivas and Descartes turbidites that crop out on the northern part of the Santa Elena peninsula in northwestern Costa Rica. Understanding the sedimentary fill of the Sandino Basin that contributed to the closing of the seaway may lead to a better understanding of the Late Cretaceous-Paleogene arcs. Provenance studies of the Santa Elena Peninsula turbidite sandstone bodies constrain the history of the paleogeography and tectonics of the region. Petrographic analyses of rock thin sections constrain source areas; geochemical analysis of individual detrital heavy minerals from rock samples give indications of sediment sources and tectonic setting during deposition. This study is a provenance analysis based on (i) semi-quantitative energy-dispersive spectrometry analysis of heavy minerals, (ii) quantitative wavelength-dispersive spectrometry for major elements of detrital clinopyroxene and spinel grains, (iii) trace element analysis through laser ablation of single detrital clinopyroxene grains, and (iv) comparative analysis of the different potential source rocks to clearly identify the most likely sediment sources. The detrital spinel and clinopyroxene are possibly sourced from: mantle ophiolites, mid-ocean ridge gabbros, or volcanic arc tholeiitic basalts or calc-alkaline andesites. Spinel and clinopyroxne geochemistry suggests a possible peridotitic source, linked to mantle rocks that are now covered by Tertiary volcanic rocks or have completely eroded. The character of the crustal minerals indicates sources from mid-ocean ridge gabbros, and island arc tholeiites and andesites. This suggests that during the early history of the gateway uplift and seaway closure, sediment sources were dominated first by older ophiolites and gabbroic sources, then by volcanic inputs from the arc.Item Petrography, Geochemistry, and Geochronology of Cretaceous Porphyry Intrusives near Red Lode, Montana(2020-05) French, LoganThere are many localities of Cretaceous porphyry intrusives in and around the Beartooth Range and particularly along the Beartooth Front near Red Lodge, Montana. Although Rouse et al. (1937) carried out a study devoted to them, they have been almost ignored since then and today new methods are available in order to further our understanding of their history. In recent decades, there has been some confusion as to the age of these rocks due to their proximity to Paleogene porphyry felsic to intermediate intrusives. In contrast, Cretaceous porphyry intrusive rocks represent intrusion prior to the main Laramide thrusting of the region. These are felsic porphyries ranging in composition from andesite to dacite and all have large plagioclase phenocrysts with prominent zoning. The initial geochemical data suggests that these intrusives are the result of fractional crystallization. Geologic observations of the region have shown through relative geochronology that these rocks are late Cretaceous in age and this has been confirmed by preliminary geochronology by Barry Shaulis which provided an age of 93 Ma, and by this study which yielded an age of 96.7 +/- 1.77 Ma.Item Reconciling North American Tectonics with the Deep Mantle through Structurally Unfolding Subducted Slabs and Mantle Convection Modeling(2022-04-27) Fuston, Spencer Lamar; Wu, Jonny; Colli, Lorenzo; Lapen, Thomas J.; Sisson, Virginia B.; Tikoff, BasilThe western margin of North America has contributed to many geological paradims such as exotic terrane accretion, continental arc magmatism, subduction accretionary complexes, and Cordilleran strike-slip faulting. Yet, despite decades of research, the tectonics of this accretionary margin remain uncertain. The aim of this dissertation is to use an approach of structurally unfolding subducted slabs, imaged by mantle tomography, back to Earth’s surface paired with mantle convection modeling to address several uncertain aspects of western North American tectonics. We structurally unfolded upper mantle slabs below the Pacific Northwest and Alaska to build a “tomographic” plate reconstruction of Paleocene to recent western North America. These unfolded slab lengths, tested against near-trench magmatism, require an additional “Resurrection” tectonic plate between the Farallon and Kula plates offshore Paleocene-Eocene western North America. We identify fragments of the wholly subducted Resurrection plate in the upper mantle below Yukon, Canada. These unfolded slab lengths also constrain a Late Cretaceous margin-parallel slab break off event that we interpret as the collision of the Wrangellia Composite terrane (WCT), along an east-dipping intra-oceanic subduction zone, and a North American active margin. We use mantle convection modeling to generate predicted mantle structures for our new unfolded-slab reconstruction and those involving Jurassic WCT collision. Comparison to observed mantle structure demonstrates that our unfolded slab-derived plate reconstruction is the only model that fully reproduces all observed lower mantle slabs. We also show strong correlation between our reconstruction and a new restoration of Cordilleran strike-slip faults, WCT paleomagnetic data, and upper plate shear zone deformation. Our results show that it is possible to reconcile North American geology with the mantle record of subduction if lateral slab advection is allowed. Finally, I review the uncertainties inherent to building plate tectonic reconstructions from mantle tomographic constraints and propose several practical considerations that frame future western North American plate reconstructions.