Widespread, mid-Miocene rhyolite volcanism of eastern Oregon that are coeval or slightly postdate flood basalts of the Columbia River Basalt Province allows for mapping crustal domains using radiogenic and stable isotopes. Rhyolites are thought to be derived in large part by partial melting of the crust and thus yield direct information on the composition of the crust. Silicic volcanism is expressed in the form of numerous domes and tuffs exposed over a wide area (~300 km in N-S dimension and ~200 km in E-W dimension) west of the presumed craton boundary, which runs parallel but mostly east of the Oregon-Idaho state border as delineated by geophysical characteristics and isotopic transitions, including the 87Sr/86Sri = 0.7060 line (MSL) and 87Sr/86Sri = 0.7040 (CSL). 87Sr/86Sri of twenty-seven silicic units are variable and some are high. Sr isotopic ratios are inconsistent with the location of the traditional MSL and CSL boundaries. A primary control on the 87Sr/86Sri isotope variations may reflect changes in the crustal make-up of Paleozoic accreted terranes of a particular area rather than arising from a westward-dipping decollement that moved cratonic lithosphere below accreted terranes in eastern Oregon. A secondary control on observed isotopic ratios may be related to the amount and composition of basalt involved in the generation of rhyolites. This could lead to higher or lower 87Sr/86Sri relative to the surrounding crust because de facto coeval mafic magmas of the Columbia River Basalt Group have a wide range of Sr isotopic signatures. While Pb isotope data is incomplete for all samples of this study, the available data indicate a significant range in Pb isotopes. Yet, data of individual regions tend to plot close to one another relative to the entire data distribution. Comparison of samples from this study in a more regional view indicates the samples generally fall within the previously defined lead isotope boundaries of the main-phase Columbia River Basalt Group lavas. [lowercase delta]¹⁸O values range from below 2 parts per thousand to above 9 parts per thousand. In addition, there is a crude trend of rhyolites having lower [lowercase delta]¹⁸O and more radiogenic ⁸⁷Sr/⁸⁶Sr[subscript i] ratios. The lowest oxygen ratios (< 2 parts per thousand) are found in rhyolites ~80 km west of the cratonic margin, potentially reflecting remelting or assimilation of hydrothermally altered crust. Low [lowercase delta]¹⁸O of selected rhyolite flows cannot be explained by remelting of Cretaceous plutons of the Idaho Batholith and appear irreconcilable with remelting of altered silicic rocks at centers of multiple, confocal caldera cycles- both processes that have been proposed to explain low [lowercase delta]¹⁸O of rhyolites of the Snake River Plain-Yellowstone area.

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The Round Top laccolith is considered to be one of the youngest laccoliths in a series of five known as the Sierra Blanca peaks, located in Hudspeth county, Texas. The laccolith is anomalous within the region in that it is peraluminous and enriched in HREEs, F, and U, and is comprised of intermingled discrete packages of various rhyolite types. The laccolith rhyolite varies in color from gray, purple, red, and tan, which combine locally to form distinct geometric mottled textures. The general composition of the rhyolite is 48-52% potassium feldspar, 28-30% quartz, 8-14% plagioclase feldspar, 4-5% annite biotite, 2-3% magnetite-hematite, 1% zircon, and 1% trace phases. The morphology of the trace phases suggests quenching of a late-stage volatile-rich vapor phase at the time of the laccolith formation. The rhyolite displays a wide array of unique mineralogical characteristics indicative to rapid emplacement and metastable crystallization conditions, including three-part quartz phenocrysts, hourglass sector-zoned potassium feldspars, and late-stage anhedral zircons. Unique accessory and trace phases include cassiterite, cerianite-(Ce), changbaiite, columbite, cryolite, tantalite, thorite, yttrofluorite, yttrocerite, and two unidentified minerals named (W) and (X). Initial alteration of the laccolith by high temperature volatile-rich vapor during the late stages of crystallization caused the partial dissolution of the feldspars and quartz. Subsequent quenching of this high temperature vapor phase produced the abundant interstitial, and pore filling REE-fluorides common to the laccolith. The variation in rhyolite color and the presence of the mottled textures are a direct result of partial oxidation of the laccolith by secondary fluids. The oxidizing fluids migrated within the laccolith along an extensive fracture network, altering the adjacent wallrock by oxidizing magnetite phenocrysts to hematite. The gray, purple, and red rhyolite types reflect an increase in turbidity caused by hematitic inclusions primarily within the pore spaces of the potassium feldspar portions of the groundmass. The tan rhyolite is locally restricted to the base of the laccolith and has been subjected to an intense degree of alteration independent of the other rhyolite types, primarily indicated by the conversion of feldspars to clay. Petrographic, microbeam, and geochemical studies have determined little variation in REE concentration between the three rhyolites of similar alteration intensity, but have indicated a depletion in LREEs within the more altered tan rhyolite. The average REE+Y content for the rhyolites sampled (n=11) ranges between 249 ppm and 518 ppm. The REE+Y concentrations between rhyolite samples of the same type show some variation, possibly indicating a correlation between alteration and REE+Y abundance and/or innate heterogeneity in the vapor phase during the initial laccolith formation. The magma emplaced at Round Top underwent a prolonged evolutionary process of fractionation/differentiation as evident by the unusual mineral assemblage and geochemical enrichment associated with the laccolith (e.g. extremely negative europium anomaly, and the positive La/Yb correlation). Future exploration for Round Top style REE-deposits should center within long-lived, tectonically active and complex regions where laccoliths are likely to exist. Specifically, exploration should focus on identifying the youngest laccolith in a felsic series, as this is the most likely to contain the greatest abundance of incompatible elements within the laccolithic group. The early alteration of feldspars by the high temperature vapor phase was crucial in the development of the REE+Y enrichment at Round Top. The feldspar dissolution provided abundant open pore space that was subsequently filled by the REE-fluorides. Thus, exploration should additionally seek laccoliths that have undergone a similar early alteration process, and expand to potential laccolith groups not yet exposed by erosional processes. / text

Golden Gate Mountain appears as a spur projecting westward from the Tucson Mountain range. It is made up of the capping Cat Mountain Rhyolite, the slope - forming Amole Formation, and a variety of intrusions of differing compositions. The emplacement of the andesitic portion of the intrusions occurred during, and probably lasted long after, the deposition of Amole Formation. The hot magma fluidized the wet sediments. Part of the fluidized materials formed pipes and dikes of tuffisites and part was brought up into the basin and contributed to the sedimentation of Amole Formation. During upper Amole time the intrusion of andesite increased in intensity. Part of the basin rapidly subsided and thick deltaic sediments and graywacke were formed. The development of a hinge line accompanied this subsidence. The hinge line controlled the occurrence of fluidization which undercut the Amole beds. The beds slumped into the fluidized parts. The process culminated in forming a large orifice through which the Cat Mountain Rhyolite welled up. The orifice is reflected in the sedimentary beds by the development of a funnel- shaped structure in the central part of which the capping of Cat Mountain Rhyolite is located. The bordering brecciated Amole beds represent the associated slump effects.

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.

The study (MS, diploma thesis) is based on more than 1500 rudist samples from the Upper Cretaceous sediments of localities Písečný vrch and Na Stínadlech (near Teplice). Investigated samples come mainly from the collection of A. H. Fassel and were collected at the end of the 19th century, currently stored in the Regional museum in Teplice and National museum in Prague. The rudist shells were determined by generic and species levels, including five genera and eight species. The stratigraphic age of both localities has been a subject of discussion since the second half of the 19th century. In the recent decades, the opinion on the lower turonian age prevailed. However, five of the eight species present in the studied localities demonstrate the upper cenomanian age of the assemblages, making them probably the oldest known rudist-corals assemblages of its kind. The palaeoecological analysis and the ecological relationships proposal for the studied localities is based on the research in literature on palaeoecology and evolution of the rudists, also presented in the study. Key words: Bohemian Cretaceous Basin, Cenomanian - Turonian, rudists, corals, palaeoenvironment, Teplice rhyolite

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.

Two adjacent caldera systems, the Mahogany Mountain and the Three Fingers caldera constitute voluminous rhyolitic volcanic deposits on the eastern margin of the Oregon-Idaho graben during the middle-Miocene. Both calderas are part of the Lake Owyhee volcanic field that in turn is part of widespread rhyolite deposits associated with the Columbia River Basalt province. We focus on establishing relationships between intracaldera units of Three Fingers caldera and caldera-forming tuff of Spring Creek and surveying the distribution of entrained mafic clasts which often display anomalous concentrations of rare earth elements. Previous mapping identified two intra-caldera facies and one outflow facies of the tuff of Spring Creek, in addition to a younger rhyolite within the caldera (Trp). New 40Ar/39Ar dates show these units are nearly time equivalent at 15.64 ± 0.08 Ma for Trp and 15.64 ± 0.09 Ma for tuff of Spring Creek. Field evidence shows extensive coverage of Trp and associated facies emplaced after a period of sedimentation within the caldera. The main reinterpretations are: i) the mostly devitrified units of Trp are time equivalent to flows and domes of glassy, vesicular, or brecciated rhyolite previously mapped as intra-caldera tuff of Spring Creek; and ii) mafic clasts present in dense glass and porous rhyolite are fragments of mafic lava flows entrained by the subsequent eruptions. New geochemical and mineralogical evidence clearly distinguish the outflow tuff of Spring Creek and intracaldera rhyolites. Compared to the outflow tuff, intracaldera rhyolite flows are less Fe-rich, (2 vs. 3 wt.% FeO), and higher silica (77 vs. 74 wt.% SiO2) rhyolites that lack vitrophyric texture. I interpret the investigated area as a rhyolite dome field, erupted subsequent to caldera collapse. The proximity of vents resulted in a complex stratigraphic overlap of rhyolite flows and clastic debris issued from coalescing domes. The predominance of high-standing dome interiors reflects the more resistant nature of dense devitrified rhyolite as compared to pumiceous, glassy, or brecciated facies of intra-caldera rhyolite. Enrichment of REE in mafic clasts is highly variable, and does not correlate with their entrainment in a specific facies of intra-caldera rhyolite. Individual clasts contain up to 2400 ppm Nd, 1800 ppm Ce, and 1400 ppm La in the most enriched samples. Linear regression shows these highly anomalous concentrations are not correlated with variations in major element chemistry between enriched and un-enriched clasts. The geographic extent of mafic clast-bearing units is limited to less than 5 percent of the area mapped, and their distribution within these units is typically volumetrically insignificant, limiting their economic potential. Mechanisms for enrichment of REE within these rocks is however significant to our understanding of a yet unexplained phenomenon and may lead to further discoveries with greater economic potential.