Two mid-Miocene (16.5-15 Ma) rhyolite volcanic centers in eastern Oregon, the Buchanan rhyolite complex and Dooley Mountain rhyolite complex, were investigated to characterize eruptive units through field and laboratory analysis. Results of petrographic and geochemical analysis add to field observations to differentiate and discriminate the eruptive units. Additionally, new geochemical data are used to correlate stratigraphically younger and older basalt and ash-flow tuff units with regional eruptive units to constrain the eruptive periods with modern Ar-Ar age dates. Previous work at the Buchanan rhyolite complex was limited to regional mapping (Piper et al., 1939; Greene et al., 1972) and brief mention of the possibility of multiple eruptive units (Walker, 1979). Observed stratigraphic relationships and geochemical analysis were used to identify eight distinct eruptive units and create a geologic map of their distribution. Slight differences in trace element enrichment are seen in mantle normalized values of Ba, Sr, P, Ti and Nd-Zr-Hf and are used to differentiate eruptive units. New geochemical analyses are used to correlate the overlying Buchanan ash-flow tuff (Brown and McLean, 1980) and two underlying mafic units to the Wildcat Creek ash-flow tuff (~15.9 Ma, Hooper et al., 2002) and flows of the Upper Steens Basalt (~16.57 Ma, Brueseke et al., 2007), respectively, bracketing the eruptive age of the Buchanan rhyolite complex to between ~16.5 and ~15.9 Ma (Brueseke et al., 2007; Hooper et al., 2002). The Dooley Mountain rhyolite complex was thoroughly mapped by the U.S. Geological Survey (Evans, 1992) and geochemically differentiated in a previous Portland State University M.S. thesis (Whitson, 1988); however, discrepancies between published interpretations and field observations necessitated modern geochemical data and revisions to geologic interpretations. Field and laboratory studies indicate that the Dooley Mountain rhyolite complex consists of multiple eruptive units that were effusive domes and flows with associated explosive eruptions subordinate in volume. At least four geochemically distinct eruptive units are described with variations in Ba, Sr, Zr and Nb. Picture Gorge Basalt flows and Dinner Creek Tuff units found within the study area both overlay and underlie the Dooley Mountain rhyolite complex. These stratigraphic relationships are consistent with the one existing Ar-Ar age date 15.59±0.04 Ma (Hess, 2014) for the Dooley rhyolite complex, bracketing the eruptive period between ~16.0 and ~15.2 Ma (Streck et al., 2015; Barry et al., 2013). The findings of this study indicate that the Buchanan rhyolite complex and the Dooley Mountain rhyolite complex are the westernmost and northernmost rhyolite complexes among the earliest (16-16.5 Ma) mid-Miocene rhyolites associated with initiation of Yellowstone hot spot related volcanism.

Lead isotope compositions have been obtained from five major volcanic and intrusive rock suites and several ore deposits on Vancouver Island. Lead, uranium and thorium concentrations and strontium isotope ratios have been obtained for a subset of these samples. The rock suites examined are the Paleozoic Sicker Group, Triassic Karmutsen Formation, Jurassic Island Intrusions and Bonanza Group volcanic rocks, and the Eocene Catface intrusions. Isotope geochemistry of the Sicker Group is consistent with the interpretation that it formed as an island arc. Relatively high 207pb/204pb ratios indicate sediment involvement in the subduction process, which suggests that the Sicker Group formed close to a continent. Buttle Lake ore deposits display decreasingly radiogenic lead isotope ratios with time, suggesting that the associated magmas become increasingly primitive. This supports the hypothesis that these deposits formed during the establishment of rifting in a back-arc environment. Karmutsen Formation flood basalts display isotopic mixing between an ocean island-type mantle source and average crust. Isotopic evidence is used to support a Northern Hemisphere origin for these basalts. Mixing is apparent in the lead and strontium isotope signatures of the Island Intrusions and Bonanza Group volcanic rocks, between depleted mantle and crustal (possibly trench sediments) components. This is consistent with formation of these rocks in an island arc environment. Eocene Catface intrusions have relatively high 207pb/204pb indicating that crustal material was involved in their formation. There are two groups of plutons corresponding to an east belt and west belt classification. Galena from the Zeballos mining camp related to the Eocene Zeballos pluton indicates that the mineralization was derived from the pluton. Galena lead isotope data from Vancouver Island may be interpreted in a general way by comparison with data from deposits elsewhere of known age and origin. No single growth curve model can be applied. Lead isotope characteristics of Vancouver Island are clearly different from those of the North American craton, reflecting the oceanic affinities of this terrane. A new technique has been developed to compare 207pb/204pb ratios between samples with differing 206pb/204pb ratios. The procedure projects 207pb/204pb ratios along suitable isochrons until they intersect a reference value of 206pb/204pb. This technique can be used for interpreting lead isotope data from old terranes, in which lead and uranium may have undergone loss or gain, and if lead and uranium abundances have not been measured. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Rhyolite lava flows are found at two dome complexes at Graveyard Butte and Gordon Butte, Mount Hood Quadrangle, Oregon. At Graveyard Butte, the White River has cut a winding canyon 150 m deep, exposing at its base, a 40-meter-thick outcrop of flow-banded rhyolite (73 wt.% SiO2, 3.67±0.01 Ma) that laterally extends along the canyon wall for about 1 km. Stratigraphically above the flow-banded rhyolite is locally-erupted iron-rich andesites (lava flows, agglutinate and other pyroclastic rocks as well as clastic debris), a rhyolitic ash-flow tuff (74 wt.% SiO2), and the 2.77±0.36 Ma tholeiitic basalt lava flows of Juniper Flat (Sherrod and Scott, 1995). Roughly 2 km downstream, a phenocryst-poor, maroon-colored rhyolite (3.65±0.01 Ma) is visible again, forming steep canyon walls for about 1.6 km. A compositionally similar silicic unit is found 18 km to the northwest of Graveyard Butte at Gordon Butte. Exposed units along Gordon Butte's Badger Creek (3.64±0.03 Ma) and the southeastern upper slopes of Gordon Butte include rhyolite flows (69.6-72.1 wt.% SiO 2). The rhyolite lava flows at Graveyard Butte and Gordon Butte's Badger Creek are nearly chemically indistinguishable and both contrast with the younger rhyolitic ash-flow tuff at Graveyard Butte and lava flows on Gordon Butte's Upper Slopes. The rhyolites of Graveyard Butte and Badger Creek are richer in Nb and Zr (30-40 ppm and 487-530 ppm, respectively) than the younger rhyolitic tuff and Upper Slopes flows (13-19 ppm and 235-364 ppm, respectively) and share characteristics with A-type granitoids. The rhyolite lavas are porphyritic (~7%) with the porphyroclasts comprising primarily individual feldspars (250-500 µm in length) with ragged margins, oscillatory zoning and less commonly, spongy cores. Other phenocrystic phases include fayalitic olivine, Fe-rich clinopyroxene, and Fe-Ti oxides. A-type-like incompatible trace-element-enriched compositions as well as mineralogical indicators suggest rhyolite lava flows at Graveyard Butte and Gordon Butte's Badger Creek are likely generated in an extensional tectonic setting. A possible geotectonic framework for generation of these rhyolite lavas is the northward propagating intra-arc rift of the Oregon Cascades.

No description available.

Abstract not available

Perched water zones have been identified in the fractured, welded tuff in the semiarid to arid environments of Yucca Mountain, Nevada and near Superior, Arizona. An understanding of the formation of such zones is necessary in order to predict where future perched water might form at Yucca Mountain, the proposed site of a high-level nuclear waste repository. The formation or growth of a perched zone above a repository is one factor of the factors to be considered in the risk assessment of the Yucca Mountain site. The Apache Leap Research Site (ALRS) near Superior, Arizona is a natural analog to the Yucca Mountain site in terms of geology, hydrology, and climate. Perched water has been identified over an area of at least 16 km² in the Apache Leap Tuff, a mid- Miocene fractured, welded ash-flow tuff. A primary goal of this investigation was to characterize the physical and hydrologic properties of the tuff in the region above and including the perched zone, and to evaluate those characteristics to develop a model for a perching mechanism in the tuff. A second goal was to determine what fraction of water entering a watershed reaches the subsurface, to potentially recharge the perched zone. The Apache Leap Tuff has been subject to considerable devitrification and vapor phase crystallization, which dominate the character of the rock. With depth to the perched zone, pumice fragments become increasingly flattened and segregated; the pumice fragments are the primary locations of porosity in the rock, therefore porosity also becomes greatly reduced with depth, to the extent that the rock matrix is virtually impermeable at the perched water zone. Fractures are the primary pathways by which water moves through the rock; fracture hydraulic conductivity values were determined to be nine orders of magnitude greater than measured matrix hydraulic conductivity at the perched zone. An increase in fracture filling by silica mineralization beneath the perched zone reduces the secondary permeability, enhancing the formation of perched water. Thus, the primary mechanisms for the formation of the perched zone include fracture flow bringing water into the subsurface, combined with extremely low matrix hydraulic conductivity at depth, and reduced secondary permeability by filled fractures and lower fracture density. Water budgets were calculated for two years in a 51.4-ha watershed. Direct measurements were made of precipitation and runoff', evapotranspiration was both directly measured, and modeled based on measurement of a number of weather parameters. Infiltration was calculated as the residual of precipitation after runoff and evapotranspiration were removed. Infiltration was determined to be less than 10% of the annual water budget; evapotranspiration removes on the order of 90% of precipitation on an annual basis.

No description available.

No description available.

[Truncated abstract] The Neoarchean (2800 to 2600 Ma) Eastern Goldfields Superterrane (EGST) comprises elongated belts of deformed and metamorphosed volcanic and sedimentary rocks intruded by granitoids. The Superterrane is made up of five distinct tectonostratigraphic terranes. From west to east these are the Kalgoorlie, Gindalbie, Kurnalpi, Laverton and Duketon Terranes. The Kalgoorlie Terrane is characterised by 2720 to 2680 Ma marine mafic-ultramafic volcanic successions interlayered with, and overlain by, 2710 to 2660 Ma dominantly trondhjemite-tonalite-dacite (TTD) dacititic volcaniclastic rocks (Black Flag Group). The adjacent Gindalbie and Kurnalpi terranes are characterised by 2720 to 2680 Ma calc-alkaline volcanic successions representing oceanic island arcs. To the west of the EGST, the Youanmi Terrane is characterised by older, dominantly 3000 to 2900 Ma greenstone rocks and complex granitoid batholiths derived from older crustal sources. The southern Kalgoorlie Terrane comprises five elongate NNW-trending tectono-stratigraphic domains. Three principal marine komatiitic to basaltic suites, collectively referred to as the Kambalda Sequence, are present, including the wellpreserved massive to pillowed Lower and Upper Basalt Sequences, separated by the Komatiite Unit, as well as numerous dyke suites. The Lower Basalt Sequence comprises the Woolyeenyer Formation, Lunnon, Wongi, Scotia, Missouri Basalts and Burbanks and Penneshaw Formations, whereas the Upper Basalt Sequence contains the Paringa, Coolgardie, Big Dick, Devon Consols, Bent Tree, and Victorious basalts. ... Instead, the data suggest that discrete PGE-bearing phase (s) fractionated from the basaltic magmas. Such phases could be platinum group minerals (PGM; e.g. laurite) and/or alloys, or discrete PGE-rich nuggets. In summary, data on the three magmatic sequences record decompression melting of three distinct mantle sources: (1) long-term depleted asthenosphere for prevalent depleted tholeiitic and komatiitic basalts, and komatiites; (2) long-term enriched asthenosphere for Paringa Basalts and similarly enriched rocks; and (3) shortterm enriched continental lithospheric mantle (CLM) for HREE and Al-depleted dykes. Some of these rocks were contaminated by TTD-type melts. Taken with the existing geophysical and xenocrystic zircon data, the most straightforward interpretation is eruption of a zoned mantle plume at the margin of rifted continental lithosphere. The Kalgoorlie Terrane extensional basin was subsequently tectonically juxtaposed with the adjacent arc-like Gindalbie and Kurnalpi Terranes at approximately 2660 Ma at the start of orogeny in a Cordilleran-style orogen to form the EGST. Collectively, uncontaminated basalts have Nb/Th of 8-16, compared to 8-12 reported for the Lunnon basalts in a previous study. To a first approximation these asthenosphere melts are complementary to average Archean upper continental crust with Nb/Th =2, consistent with early growth of large volumes of continental crust rather than models of steady progressive growth.

Mémoire (M.Sc.A.)-- Université du Québec à Chicoutimi ; 1986. / "Mémoire présenté pour l'obtention du grade de maîtrise es sciences appliquées" CaQCU Document électronique également accessible en format PDF. CaQCU