Tower Mountain Caldera is the main feature of an Oligocene volcanic field located in the Umatilla National Forest, eastern Oregon. It is perfectly suited to investigate models of rhyolite petrogenesis as all of the important rock components for evaluating generation models are present in a single location and thus are presumably related; basalts, intermediate igneous rocks (which consist of older plutons and younger volcanic rocks, which are ~coeval with rhyolites), metamorphic basement rocks of significant grade, and rhyolites of varying composition. The formation of the caldera produced the Dale Tuff, which comprises the intra-caldera and outflow facies. 40Ar/39Ar dating places the age of the tuff at 32.66 ± 0.36 Ma. Post-caldera rhyolites erupted along apparent ring fractures and elsewhere. Radiometric U-Pb dating of zircons from three of these rhyolites yielded ages of 32.167 ± 0.020 Ma (#CH07a), 31.798 ± 0.012 Ma (#TM5), and 31.426 ± 0.016 Ma (#CH08a). All rhyolites at Tower Mountain range from low to high silica varieties. Some of the post-caldera rhyolites are chemically similar to the Dale Tuff, such as sample CH07a, and have compositions typical of rhyolites of calc-alkaline volcanic centers (I-type rhyolites), while others are similar to A-type rhyolites (CH08a and TM5). The ages indicate that the calc-alkaline rhyolites were followed by the A-type rhyolites. The petrogenetic relationships between the various rocks types were evaluated. Partial melt modeling based on experimental melts produced from crustal material indicates that batch partial melting of metamorphosed high silica crustal material modified by the addition of more primitive mafic material by assimilation/contamination is the most likely source for the Tower Mountain rhyolites.

The Dinner Creek Tuff erupted during a period of rhyolitic volcanism coeval to the flood volcanism associated with the Columbia River Basalt Group. The High Rock Caldera Complex, Lake Owyhee and McDermitt volcanic fields account for ~90% of the rhyolites erupted between 16.7-15.0 Ma. Situated at the northern end of the Lake Owyhee volcanic field, the Dinner Creek Tuff was originally mapped as a ~2,000 km2 single ignimbrite confined to the Malheur Gorge. Streck et al. (2015) correlated tuff outcrops previously mapped as generic Miocene welded tuff as well as local units such as the "Mascall" or "Pleasant Valley" tuff of eastern Oregon to individual cooling units that comprise the newly redefined Dinner Creek Tuff, enclosing an area of ~25,000 km2. Areal extents defined in this study show that all outcrops now determined to be Dinner Creek Tuff enclose an area of ~31,800 km2 not including any fallout deposits that likely extended beyond the defined area. Although Dinner Creek Tuff rhyolites have nearly identical compositions, different ages and subtle geochemical and mineralogical differences exist and were used to divide the Dinner Creek Tuff into four discrete cooling units. Except for unit 4, the units are lithologically very similar. Unit 1 is the Dinner Creek Tuff unit associated with the Malheur Gorge type section. The four cooling units have ages of 16.15-16 Ma (unit 1), 15.6-15.5 Ma (unit 2), 15.46 Ma (unit 3) and 15.0 Ma (unit 4). Areal extents were established for all four cooling units based on feldspar compositions along with lithological and bulk rock geochemical data. Minimal extents of individual units are as follows: ~22,590 km2 (unit 1), ~17,920 km2 (unit 2), ~14,170 km2 (unit 3) and ~8,370 km2 (unit 4). Using conservative thicknesses, determined erupted tuff volumes are ~170 km3 (unit 1), ~125 km3 (unit 2), ~99 km3 (unit 3) and ~46 km3 (unit 4), totaling ~440 km3 and dense rock equivalents are ~152 km3 (unit 1), ~96 km3 (unit 2), ~76 km3 (unit 3) and ~31 km3 (unit 4), totaling ~356 km3. These extents and volumes are the absolute minimum based solely on the locations of exposed tuff sections and the inclusion of the source. Centering eruptive units on source areas where they are known, expands the tuff extents into a more radial pattern as would be expected for low-aspect ratio, high energy ash-flow tuff eruptions. These probable extents increase the areal extents of the individual units to: ~36,900 km2 (unit 1), ~31,660 km2 (unit 2), ~17,290 km2 (unit 3) and ~10,150 km2 (unit 4) distributed over a ~43,490 km2 area. Likewise, erupted tuff volume and dense rock equivalents also increase: volume-- ~277 km3 (unit 1), ~222 km3 (unit 2), ~121 km3 (unit 3) and ~56 km3 (unit 4); DRE-- ~248 km3 (unit 1), ~170 km3 (unit 2), ~93 km3 (unit 3) and ~38 km3 (unit 4). New mapping confirms previous hypotheses that the Castle Rock caldera erupted unit 1 and identified the new Ironside Mountain caldera as the source for unit 2 while precise source areas for unit 3 and 4 are not yet known but are thought to lie within the Dinner Creek Eruptive Center. Minimal calculated caldera volumes for units 1 and 2 are ~98.5 km3 (unit 1) and ~31.1 km3 (unit 2). Adding the thick ponded intra caldera tuff volume to the determined and probable erupted tuff volumes determined in this study, increases the erupted volumes to ~268 km3 (determined) and ~375 km3 (probable) for unit 1 along with ~157 km3 (determined) and ~253 km3 (probable) for unit 2. DREs increase to ~251 km3 (determined) and ~347 km3 (probable) for unit 1 along with ~128 km3 (determined) and ~202 km3 (probable) for unit 2.

The Wildcat Creek Tuff is a thin (~3-12 m), rhyolite to andesitic ash-flow tuff with a minimal extent of 1500 km2 in Malheur county, eastern Oregon. The previously undated tuff yielded a single crystal, anorthoclase 40Ar/39Ar age of 15.49±0.02 Ma and thus is closely related to mafic and silicic volcanism of the Columbia River Province. The tuff texturally stands out by its high proportion of co-mingled mafic inclusions appearing as dark, scoriaceous, and phenocryst-poor fragments, and their proportion dictate bulk tuff compositions ranging from rhyolite (74% SiO2) to andesite (59% SiO2). Glass analyses confirm rhyolite end member at 74-75 wt.% SiO2 and two mafic members, one at 59-60 wt.% SiO2 and the other at 56-57 wt.% SiO2. Rare plagioclase and even rarer pyroxene phenocrysts with compositions clustering at An60-74 and An35-45, and Mg17-19 and Mg80-84, respectively, similarly suggest two andesitic magmas with the 60% member being the dominant mafic composition. It has distinctly lower TiO2 and CaO, slightly lower FeO, and comparable Al2O3, MgO, and alkalis. Eruption of crystal-poor dacitic to basaltic-andesitic cognate components is also observed in other Miocene ash-flow tuffs from eastern Oregon, like the Rattlesnake, Dinner Creek, and the Devine Canyon Tuffs, as well as other less voluminous tuffs. However, the high proportion of mafic components in the Wildcat Creek tuff seems currently unrivaled. The co-eruption of intermediate magmas with rhyolite implies that mafic magmas were tapped from a common reservoir, and these magmas increased in proportion during the course of the eruption(s). This continued up to the point where nearly all deposited tuff material consisted of andesite. This is consistent with progressively deeper magma withdrawal, in turn implying that mafic magmas resided below the rhyolites as a discrete magma batch. Dacitic components of voluminous rhyolitic tuffs have been recently interpreted as remelted samples of a crystal mush after crystal-poor rhyolites where extracted. Dacitic Wildcat Creek Tuff samples do not bear any evidence of this. To the contrary, small negative Eu anomalies, normal Ba and Sr concentrations, and nearly aphyric nature are consistent with a large portion of mixing between Wildcat Creek Tuff rhyolites and regional mid Miocene, Fe-rich, and crystal poor basaltic andesite magmas that occur ubiquitously as lava flows.

No description available.

Ascent of magma results in drastic drops of pressure and temperature during eruption. Exsolution or dissolution of water changes the physical and chemical properties of the magma and can promote or inhibit the formation of bubbles, crystals and cracks. The microstructural relations between bubbles, crystals and cracks are important records of processes immediately before and during volcanic eruptions and during deposition of volcanic products. This is an integrated study of analyses, conceptual and numerical models of textural relations, and water distribution patterns of natural and experimentally altered samples. Synchrotron Fourier transform infrared spectroscopy and focal plane array detectors open new possibilities for the analysis of the spatial distribution of volatiles in volcanic rocks. New ways of sample preparation, measurements and data analyses helped to create water distribution maps with spatial resolutions that are close to the diffraction limit (~3 μm). In order to constrain eruptive processes and mechanisms of lava emplacement, I describe textural features in volcanic glasses including bubbles, flow bands of crystals or bubbles, spherulites and different generations of cracks. In experiments, bubbles were grown under isobaric conditions, at one or two cooling steps, their textures were described and volume changes tracked. Water distribution patterns in the glass around the textures were described and categorized, and where possible, diffusion modeling was used to infer temperature- and timescales of formation. Rocks that are quenched within short periods of time after bubble growth preserve negative gradients of water toward the bubble margins. These gradients are generally not observed if the sample is kept at high temperatures for extended periods. If, however, a second step of cooling is added, water may be re-dissolved into the surrounding melt, which may lead to the complete resorption of bubbles. A conceptual of water redistribution during bubble resorption or collapse is used to interpret water heterogeneities across linear flow banding. These heterogeneities can be caused by shearing of bubbly magma, leading to collapse, degassing and resorption of water into the melt, creating a bubble free melt. Anhydrous spherulitic crystals grow both above and below the glass transition temperature (Tg) redistributiong water into the surrounding melt. Below Tg, cracks form and are successively hydrated by magmatic water from crystal growth or by meteoric water at temperatures far below Tg. The hydrated perlitic cracks in the samples of this study formed at elevated temperatures and are distinct from cracks formed at ambient temperatures without hydrated margins. This study shows that the heterogeneous distribution of water in volcanic rocks preserves the complex and non-linear degassing and cooling history of eruptive products. The timescales and temperatures discovered here provide new ways to interpret textural observations, water distribution patterns and signals of shallow volcanic unrest.

Thèse--Sc. nat.--Dijon, 1973. / Bibliogr. p. 364-377.

xix, 189 p. : ill. (some col.), maps (some col.) / Caldera-forming eruptions are dramatic and destructive natural phenomena, causing severe and sustained consequences to society. This dissertation presents new geochemical and geochronologic data for caldera-forming tuffs and pre- and post-caldera rhyolites of the two youngest caldera complexes in the Snake River Plain (SRP) in the western USA: Heise (6.6-4.5 Ma) and Yellowstone (2.1-0.6 Ma). Caldera complex evolution at Heise and Yellowstone can be described by formation of 3-4 spatially overlapping "nested" calderas, successive collapse of intracaldera fill, and development of a large hydrothermal system. Comparison between Heise and Yellowstone reveals that late-stage rhyolite eruptions have drastic depletions in 18 O that require remelting of large volumes (1,000's of km 3 ) of hydrothermally altered rock. Archean xenoliths and Phanerozoic rocks of the crustal basement beneath the SRP province are not depleted in 18 O and therefore cannot be a source of these rhyolites. Isotopic mixing models indicate that early large-volume rhyolites are produced by melting and hybridization of the crust by mantle-derived basalt, and late-stage rhyolites tap hydrothermally altered portions of intracaldera rocks from previous eruptions. Caldera-forming eruptions at Heise culminated 4.45 Ma with eruption of the 1,800 km 3 Kilgore Tuff, the most voluminous 18 O-depleted rhyolite in the SRP and worldwide. O, Sr, and Nd isotope geochemistry, zircon ages, mineral and whole-rock geochemistry, and liquidus temperatures for Kilgore Tuff samples erupted >100 km apart are similar and/or overlapping within error, indicating derivation from a remarkably homogeneous low-δ 18 O magma reservoir (δ 18 O=3.4[per thousand]). Caldera-wide batch assembly and homogenization of variably 18 O-depleted melt pockets with diverse zircon populations can explain the Kilgore Tuff's genesis. Central Plateau Member (CPM) rhyolites at Yellowstone have the same timing (∼2 million years after the initiation of volcanism), magnitude of δ 18 O depletion (∼3[per thousand] depleted relative to normal rhyolites), and cumulative eruptive volume (∼4,000-4,500 km 3 ) as the Kilgore Tuff of the Heise volcanic field. Isotopic, age, and geochemical data for CPM rhyolites show that they become progressively more homogeneous and evolved from 260 ka to 75 ka. Whereas the Kilgore Tuff erupted climactically as an explosive caldera-forming tuff, CPM rhyolite eruptions record sequential, predominantly effusive, "snapshots" of magma assembly, homogenization, and differentiation. This dissertation includes co-authored materials both previously published and submitted for publication. / Committee in charge: Ilya Bindeman, Chairperson; Gregory Retallack, Member; Mark Reed, Member; W. Andrew Marcus, Outside Member

>Magister Scientiae - MSc / The Saldania Belt in southern Africa, a product of the Pan-African Saldanian Orogeny, forms part of a system of Neoproterozoic mobile belts that border and weld older cratons on the African continent. It is a low-grade orogenic belt situated along the southwestern margin of the Kalahari Craton and is composed of several inliers of greenschist facies metasedimentary and metavolcanic rocks (Malmesbury Group), unroofed in megaanticlinal hinges of the Permo-Triassic Cape Fold Belt. The Malmesbury Group rocks were syn- and post-tectonically intruded in a pervasive transpressive regime between 555 Ma and 515 Ma by Neoproterozoic to early Cambrian S-, I- and A-type granites, monzodiorites, gabbros and quartz syenites, which collectively constitute the rocks of the Cape Granite Suite (CGS). Along the south-western coastline of South Africa, the Saldanha Bay Volcanic Complex (which forms part of the CGS) is divided into two eruption centres both of which have been identified as “intra-caldera pyroclastic ignimbrites”. The Postberg eruption centre is situated to the south of the Saldanha Bay entrance and the Saldanha eruption centre is situated to the north of the entrance. Both eruption centres display distinct geochemical signatures, the most apparent being the greater TiO2 concentrations (> 0.25 wt. %) of the Saldanha centre ignimbrites when compared to its Postberg centre counterparts. The Postberg eruption centre consists of S-type rhyolitic ignimbrites which are subdivided into the two geochemically distinct Plankiesbaai and Tsaarsbank Ignimbrites. Small amounts of the Jacobs Bay and Saldanha Ignimbrites (less felsic tephra from the Saldanha eruption centre) are also present in the Postberg eruption centre. A robust geochemical analysis of both the Plankiesbaai and Tsaarsbank magma groups display high SiO2 content (>76 wt. %), a lack of variation in TiO2 and Zr, high Al2O3 and ASI (aluminium saturation index) values (> 1.0 and generally >1.1 which, on average, is higher than the Saldanha eruption centre ignimbrites), low CaO and Na2O, and a highly ferroan character. The Plankiesbaai ignimbrite also display lower #Mg concentration compared to the Tsaarsbank ignimbrite. Typical geochemical trends in the Postberg eruption centre include the lack of variation in Zr content, higher Rb content and lower Sr, Ba, V and Zn concentrations when compared to the tephra of the Saldanha eruption centre found in the Postberg area. The study’s main aim is not only to assess the geochemistry of the ignimbrites relative to the previous phases of magmatism originally proposed by Scheepers (1995) for the magmatism of the Cape Granite Suite, but also their age distribution. Previously defined phases of magmatism include Phase I (S-type granites subdivided into Sb, Sa1 and Sa2 all of which are dated to 555 - 540 Ma), Phase II (I-type granites subdivided into Ia and Ib both dated to 540 – 520 Ma), Phase III (A-type granites subdivided into Aa and Ab dated to ~ 520 Ma) and Phase IV (S-type volcanic and subvolcanic rocks dated to 515 Ma). Re-examination of the geochronology displays a U-Pb age for Postberg Centre Jacobs Bay Ignimbrite (tephra from the Saldanha eruption centre) of 538 ± 2.2 Ma: and for the Postberg Centre Tsaarsbank Ignimbrite between 536 ± 2 Ma – 540 ± 3.4 Ma. These new dates, in combination with the geochronological work done in the Saldanha Centre (particularly in light of the Clemens and Stevens (2016) and Clemens et al. (2017) studies that reclassify these rocks differing from the original and previous studies), place all the ignimbrites of the Saldanha Bay Volcanic Complex securely within the age bracket for the initial S-type magmatism of the CGS. This thesis presents a revised order for the phases of magmatism of the Saldania Belt, and by extension, of the Cape Granite Suite. All S-type magmatism, including that of the Saldanha Bay Volcanic Complex (Sv), forms part of the Phase I magmatism of the Saldania Belt (Sa1, Sa2, and Sb) emplaced between 555 – 540 Ma.

>Magister Scientiae - MSc / The Saldania Belt in southern Africa, a product of the Pan-African Saldanian Orogeny, forms part of a system of Neoproterozoic mobile belts that border and weld older cratons on the African continent. It is a low-grade orogenic belt situated along the southwestern margin of the Kalahari Craton and is composed of several inliers of greenschist facies metasedimentary and metavolcanic rocks (Malmesbury Group), unroofed in megaanticlinal hinges of the Permo-Triassic Cape Fold Belt. The Malmesbury Group rocks were syn- and post-tectonically intruded in a pervasive transpressive regime between 555 Ma and 515 Ma by Neoproterozoic to early Cambrian S-, I- and A-type granites, monzodiorites, gabbros and quartz syenites, which collectively constitute the rocks of the Cape Granite Suite (CGS). Along the south-western coastline of South Africa, the Saldanha Bay Volcanic Complex (which forms part of the CGS) is divided into two eruption centres both of which have been identified as “intra-caldera pyroclastic ignimbrites”. The Postberg eruption centre is situated to the south of the Saldanha Bay entrance and the Saldanha eruption centre is situated to the north of the entrance. Both eruption centres display distinct geochemical signatures, the most apparent being the greater TiO2 concentrations (> 0.25 wt. %) of the Saldanha centre ignimbrites when compared to its Postberg centre counterparts. The Postberg eruption centre consists of S-type rhyolitic ignimbrites which are subdivided into the two geochemically distinct Plankiesbaai and Tsaarsbank Ignimbrites. Small amounts of the Jacobs Bay and Saldanha Ignimbrites (less felsic tephra from the Saldanha eruption centre) are also present in the Postberg eruption centre. A robust geochemical analysis of both the Plankiesbaai and Tsaarsbank magma groups display high SiO2 content (>76 wt. %), a lack of variation in TiO2 and Zr, high Al2O3 and ASI (aluminium saturation index) values (> 1.0 and generally >1.1 which, on average, is higher than the Saldanha eruption centre ignimbrites), low CaO and Na2O, and a highly ferroan character. The Plankiesbaai ignimbrite also display lower #Mg concentration compared to the Tsaarsbank ignimbrite. Typical geochemical trends in the Postberg eruption centre include the lack of variation in Zr content, higher Rb content and lower Sr, Ba, V and Zn concentrations when compared to the tephra of the Saldanha eruption centre found in the Postberg area.

The 7.7 Ma Kimberly Member of the Cassia Formation is part of a succession of A-type rhyolites associated with the Yellowstone hotspot track. It was sampled by the Kimberly core that was drilled on the Snake River Plain as part of Project HOTSPOT (Shervais, et al., 2013). The Kimberly Member is a 170 m thick high-silica rhyolite lava flow containing quartz, plagioclase, anorthoclase, sanidine, augite, pigeonite, magnetite, ilmenite, zircon, and apatite. δ 18O of zircon ranges from 0 to 4.9‰ (Colón et al., 2018), typical low values for the Snake River Plain. Quartz is intensely embayed. Exsolved and resorbed pigeonite cores are mantled by augite. REE-poor apatite cores are resorbed and oscillatory zones truncated by rims with SiO2 as high as 12.8 wt% and LREEtot up to 4.7%. There are three chemically distinct feldspars. Rounded and pitted anorthoclase (Or21 Ab64 An15) mantles plagioclase (An20 to An40) cores. Sanidine (Or47 Ab48 An05) forms thin, subhedral drapes on the outer edges of anorthoclase. Sanidine also fills some of the sieved holes in plagioclase and anorthoclase. There are two chemically distinct glasses, a light glass (~95%) and a dark glass (~5%). Relative to the light glass, the dark is enriched in Al2O3 , CaO, and Na2O and depleted in Fe2O3 and K2O. The dark glass is depleted in Rb and enriched in Sr and Ba, but they have similar concentrations of the high field strength elements (Y, Zr, Nb, Hf, and Ta). LREE are slightly more enriched in the dark glass than in the light glass. Temperatures of 926°C (magnetite-ilmenite thermometry with QUILF), 894°C (pigeonite-augite pairs with QUILF), and 889°C (zircon-saturation) are calculated for the magma. Although Fe-Ti oxides appear to have equilibrated with melt before eruption, most of the other phases preserve strong evidence of disequilibrium. These complex mineral textures also indicate assimilation and mixing processes. We propose a pigeonite-bearing, dry, metasomatized, A-type granite was fragmented and assimilated by the Kimberly member, mantling exsolved pigeonite with augite. Also incorporated into the Kimberly member were volcanic xenocrysts indicative of rhyolite assimilation or magma mixing. These components are embayed volcanic quartz, and composite plagioclase-anorthoclase grains (mantled by sanidine upon assimilation). Complex zircon grains could be sourced from metasomatized rhyolite or intrusion, and complex apatite grains could be due to mixing or assimilation. We propose the distinct glass types are caused by mingling of the Kimberly magma with the melted metasomatized assimilant. This scenario demonstrates the complexity of open system processes involved in some Snake River Plain magmas.