Ash-flow Tuffs Research Articles

Mechanisms of rapid plant community change from the Miocene Succor Creek flora, Oregon and Idaho (USA).

The fossil record of the U.S. Pacific Northwest preserves many Middle Miocene floras with potential for revealing long-term climate-vegetation dynamics during the Miocene Climatic Optimum. However, the possibility of strong, eccentricity-paced climate oscillations and concurrent, intense volcanism may obscure the signature of prevailing, long-term Miocene climate change. To test the hypothesis that volcanic disturbance drove Middle Miocene vegetation dynamics, high-resolution, stratigraphic pollen records and other paleobotanical data from nine localities of the Sucker Creek Formation were combined with sedimentological and geochemical evidence of disturbance within an updated chronostratigraphic framework based on new U-Pb zircon ages from tuffs. The new ages establish a refined, minimum temporal extent of the Sucker Creek Formation, ~15.8 to ~14.8 Ma, and greatly revise the local and regional chronostratigraphic correlations of its dispersed outcrop belt. Our paleoecological analysis at one ~15.52 Ma locality reveals two abrupt shifts in pollen spectra coinciding with the deposition of thick ash-flow tuffs, wherein vegetation dominated by Cupressaceae/Taxaceae, probably representing a Glyptostrobus oregonensis swamp, and upland conifers was supplanted by early-successional forests with abundant Alnus and Betula. Another ephemeral shift from Cupressaceae/Taxaceae swamp taxa in favor of upland conifers Pinus and Tsuga correlates with a shift from low-Ti shale to high-Ti claystone, suggesting a link between altered surface hydrology and vegetation. In total, three rapid vegetation shifts coincide with ash-flow tuffs and are attributed to volcanic disturbance. Longer-term variability between localities, spanning ~1 Myr of the Miocene Climatic Optimum, is chiefly attributed to eccentricity-paced climate change. Overall, Succor Creek plant associations changed frequently over ≤105 years timespans, reminiscent of Quaternary vegetation records. Succor Creek stratigraphic palynology suggests that numerous and extensive collection of stratigraphically controlled samples is necessary to understand broader vegetation trends through time.

Volcanic Glass as a Proxy for Paleotopography Suggests New Features in Late-Miocene Oregon

Volcanic glass has been used extensively as a paleoaltimeter. Deuterium (2H) concentrations in glass have been found to be stable over geologic timescales, making δ2H (also known as δD) a reliable proxy for ancient water chemistry. However, continued work revolves around better understanding how different factors affect preserved water in volcanic ash. Here, we analyze δD in the Rattlesnake Tuff (RST), a widespread ca. 7 Ma ash-flow tuff, and create a paleoisoscape to assess variations in δD across Oregon during that time. To this end, 16 ash samples were collected across central and eastern Oregon from various flow units within the RST. Samples were analyzed for δD using a temperature conversion elemental analyzer (TC/EA) connected to a mass spectrometer and elemental composition using a scanning electron microscope (SEM). We compared the isotopic results to modern water and published ancient water proxy data to better constrain changes in climate and topography across Oregon throughout the Neogene. We also estimated wt. % H2O by calculating excess (non-stoichiometric) oxygen from SEM elemental data. We did not observe significant variations in δD among the flow units from single locations, nor was there a significant relationship between the prepared glass shard composition and wt. % H2O or δD, supporting the use of volcanic glass as a reliable paleoenvironmental indicator. Our results show significant spatial variation in δDwater values of RST, ranging from −107‰ to −154‰. δD values of ancient glass were similar to modern water near the Cascade Mountains but became relatively negative to the east near the inferred eruptive center of the RST, suggesting that a significant topographic feature existed in the vicinity of the RST eruptive center that has since subsided.

Geochemical characteristics and mapping of Reşadiye (Tokat-Türkiye) bentonite deposits using machine learning and sub-pixel mixture algorithms

Reşadiye bentonite deposits, which play a significant role in Türkiye's bentonite production, are situated in Central Anatolia. Geochemical, mineralogical, and remote sensing data have been integrated to map the spatial distribution of clay minerals in the bentonite deposits and argillic areas. It is hypothesized that the bentonite samples occurred by the in-situ diagenetic alteration of rhyolite-dacite, trachyte, and andesite/basaltic andesitic composition pyroclastic rocks (ash-flow tuff). Biotite, clinoptilolite, calcite, dolomite, K-feldspar, opal-CT, quartz, and clay minerals are detected in most bentonite samples. The clay patterns determined in the bentonite samples in the X-ray diffraction (XRD) diagrams were 12.3–12.6 Å and were interpreted as being rich in Na-smectites. Mineral mapping in these deposits is essential for mining operations since the high-grade bentonite deposits can be affected by the other clay, gang, and ore minerals they contain in addition to the smectite. The sample spectra measurements matched montmorillonite and kaolin/smectite spectra. This study tests support vector machine (SVM) and artificial neural network (ANN) machine learning and MTMF subpixel algorithms in lithology and mineral mapping in Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite data. It combines the power of subpixel unmixing algorithms to determine the distribution of clay and high-grade bentonites in argillic areas discriminated by machine learning. The results showed that the SVM algorithm can map better than ANN for argillic areas. Additionally, the distribution of high-grade bentonite and kaolin/smectite bearing sites in the study area is discriminated by the mixture-tuned matched filtered (MTMF) spectral classification method. As a result, this study shows that remote sensing studies can be utilized for the exploration and monitoring of high-grade bentonite sites during and/or post-mining operations.

Trace element partitioning in strongly peraluminous rare-metal silicic magmas – Implications for fractionation processes and for the origin of the Macusani Volcanics (SE Peru)

The Macusani Volcanics and related rocks (SE Peru) are well known examples of erupted strongly peraluminous rare-metal rich silicic magmas. We used the phenocryst/matrix glass method to determine relevant mineral/melt partition coefficients to facilitate modeling of such systems. Concentrations of trace (Li, Be, B, Al, P, Ti, Mn, Zn, Ge, Rb, Sr, Zr, Nb, Sn, Cs, Ba, La, Sm, Eu, Gd, Yb, Ta, W, Pb and U) and major elements were measured in obsidians, matrix glasses, glass inclusions, phenocrysts and phases from biotite reaction zones by Laser-Ablation ICP-MS and Electron Microprobe Analysis. The derived phenocryst/melt partition coefficients for quartz, plagioclase, sanidine, biotite, muscovite, andalusite and ilmenite are compared with values from reference silicic magmas and literature. Mineral/biotite partition coefficients are calculated for tourmaline and hercynite. The database is then applied to model chemical fractionation in Macusani magmas. Fractional crystallization and partial melting trends (with the B concentration used as a proxy for the fraction of residual liquid) generate progressively more evolved (higher Rb, Nb, Sn, Cs and Ta and lower Sr, Ba and Pb) residual liquids. The trace element concentrations in obsidians are reproduced for 50–70% crystallization of matrix glasses from ash-flow tuffs. This demonstrates that differentiation of magmas parental to the ash-flow tuffs can generate the highly evolved obsidian-forming liquids. Based on this, we present an updated model for crustal melting and the generation of the Macusani magmas. A metapelitic component dominates the source region but calcic plagioclase cores enriched in Sr, Ba and La suggest an affiliation with mafic magmas, which were likely potassic to ultrapotassic. The presence of a mafic component is consistent with the high heat fluxes in the source region and accounts for specific magmatic variables (i.e., middle crustal anatectic zone, very reducing fO2, high F). Petrogenetic processes differ significantly between Macusani Volcanics suites and two-mica leucogranites despite both having nearly identical source rocks, mineral assemblages and compositions.

The Beacon Heights “Tinguaite” (Phonolite): A recrystallised agpaitic ignimbrite in the Mesoproterozoic Pilanesberg alkaline complex, South Africa

AbstractAn extrusive rock unit in the Mesoproterozoic Pilanesberg alkaline complex that has been referred to as the “Beacon Heights Tinguaite” since the work of Shand (1928) has been reexamined in the field and studied by petrographic microscopy, scanning electron microscopy, and whole-rock major and trace element analysis. The new observations indicate that the unit (here informally renamed the “Beacon Heights Phonolite” to comply with current petrographical nomenclature) is a laminated, volcanic rock in which mafic layers rich in sodic pyroxene alternate on any scale with discontinous, felsic layers consisting of alkali feldspar, nepheline, sodalite and their alteration products natrolite and analcime. Highly strontian apatite, fluorite, a manganiferous pectolite-group mineral, sphalerite and pyrrhotite are minor to accessory minerals. In addition, the rock carries indicator minerals typical of agpaitic rocks (including lamprophyllite and eudialyte-group minerals and their replacement products). The rock contains abundant crystal fragments of felsic minerals (alkali feldspar, nepheline and sodalite), fragments and elongated lenses of nepheline syenite, some of which contain primary magmatic lamprophyllite, and rare armoured lapilli. The overall structure of the rock is that of a welded ashflow tuff (igmimbrite) affected by post-magmatic recrystallisation processes. Whole-rock major and trace element compositions suggest a compositional affinity with members of the agpaitic, intrusive Green Foyaite Suite of the Pilanesberg Complex, and probably also a genetic relationship. The Beacon Heights Phonolite is a rare, possibly unique, example of an ashflow deposit of agpaitic, highly silica-undersaturated composition.

Lower Cambrian volcanism in the Hawker Group and the Billy Creek Formation, Arrowie Basin, Flinders Ranges, South Australia

Widespread tuffs are known from the lower Cambrian succession of the Arrowie Basin, Flinders Ranges, mainly from the lower parts of both the Mernmerna Formation and the Billy Creek Formation. Up to 2.5 m of potassium-rich (K2O 6.32–9.60 wt%) rhyolite tuffs are known from the Mernmerna Formation, with the ‘Big Green Tuff’ from Bunyeroo Gorge yielding a TIMS age of 515.4 ± 0.2 Ma (i.e. unnamed Cambrian stage 3). The petrography of the Big Green Tuff, with numerous glass shards, suggests an explosive subaqueous pyroclastic deposit or ash-flow tuff. The presence of several layers of identical green tuff indicates that these were produced by a multi-phase volcanic event. There are up to thirty tuff horizons (5–50 cm thick) within the Warragee Member of the Billy Creek Formation. The tuffs within the Billy Creek Formation occur over a wide area, with an east–west extent of 250 km and a north–south extent of up to 80 km. It is suggested that they are in part ash-flow tuffs. A previous TIMS age for the Billy Creek Formation was 511.87 ± 0.14 Ma (i.e. unnamed Cambrian stage 4). The source of the tuffs, both from the Mernmerna Formation and the Billy Creek Formation is unknown. Detailed study of the whole-rock and trace-element geochemistry is required to fully document them.

Metallogeny and exploration strategy for volcanic rocks hosting world class Be-U-F mineralization at Spor Mountain, Utah, U.S.A.

We report new geochemical data (major-elements, trace-elements, and Nd-Pb isotopes) for the Spor Mountain Formation (SMF), a volcanic sequence that hosts the largest deposit of beryllium in the world. Understanding the genesis of the deposit is critical for improved recognition of regional exploration targets containing additional volcanic-hosted beryllium resources. The deposit occurs in an area of extensive alkalic rhyolitic ash flow tuffs and calderas underlain by Paleozoic and older limestone, dolomite, shale, and quartzite. The lower fluorite and bertrandite-rich tuff and upper topaz-rich rhyolite of the SMF have overlapping zircon crystallization ages (25.59 + 0.29/−0.45 Ma). Enrichment of Be, U, and F is a key attribute of the SMF inherited from melting heterogeneous source materials and magma hybridization in the continental lithosphere. Two mineralization stages are implicated: magmatic, and hydrothermal/metasomatic. The magmatic stage occurred early, as indicated by the high beryllium content in early-crystallizing zircon. The post-magmatic/hydrothermal stage involved F-rich fluids that reacted with glass and carbonate nodules in the tuff to form bertrandite (and U-rich fluorite and opal) and high-grade mineralization. The rhyolite best displays the magmatic stage of enrichment. The SMF is alkaline. Chondrite-normalized REE patterns for tuff and rhyolite are regular, stacked subparallel, and imply a shared source derivation and evolution. The tuff and rhyolite plot along a sub-vertical array for Pb isotopes and suggest they are cogenetic; both have negative values of εNd. Pb isotopic ratios for SMF do not match the slopes, initial isotopic values, or estimated basement ages of precious and base metal mining districts in Utah; thus, they do not share a source region or evolution. Crustal ages from Nd isotopes (average TDM ~ 2.29 Ga) are older than basement ages obtained by Pb isotopes. The deposit occurs in a transitional area of thinned crust near the boundary between two basement domains, the eastern Great Basin and Colorado Plateau, which have contrasting ages. Many geochemical attributes are shared between SMF and the Ermakovka deposit in Russia: they represent the world's major commercial Be deposits. Rare-metal indices of granite-related mineralization show that SMF resembles other global beryllium-bearing granitic deposits. Despite this coincidence, none of these examples matches the mineralogy, geologic setting, or type of rare metal mineralization at Spor Mountain.

Environmental Geochemistry of the Round Mountain Gold Mine, Nevada, and Mineralogical Controls on Acid Generation

Abstract The Round Mountain mine in Nevada is one of the world’s largest volcanic-hosted precious metal deposits and has been commercially operated since 1976. The deposit is hosted by a thick sequence of Oligocene ash-flow tuffs and volcaniclastic rocks overlying a pre-Tertiary basement. Gold mineralization largely occurs as disseminated electrum in association with quartz, adularia, illite, smectite, pyrite, silver sulfosalts, manganese oxide, and iron oxides, with minor alunite, fluorite, and realgar in the upper portion of the deposit. The deposit was extensively oxidized during a period of Basin and Range faulting approximately 9 to 16 Ma. Multiple phases of geochemical characterization have been undertaken as part of project development, including over 740 acid-base accounting analyses and 38 humidity-cell tests operated for up to 193 weeks. These tests demonstrate that the Round Mountain host rock contains very little sulfide mineralization and show a low potential for acid generation and metal(loid) release. Where sulfide minerals are present, they are frequently partially or entirely encapsulated in nonreactive silicates or are present as coarse euhedral to subhedral pyrite grains that require more energy to oxidize compared to finer grained anhedral crystals. As such, there is likely to be a significant lag time to acid generation, and it is possible that acidic conditions may never develop under field conditions due to the combined effects of stable pyrite petrology and encapsulation, plus the lower leaching rates that would occur in the arid desert conditions in the field versus the laboratory. This prediction is supported by empirical field observations that indicate during the 40-yr period of operation there has been no acid generation associated with the waste-rock facilities. The multiple phases of geochemical characterization have been used to define procedures for operational waste-rock classification, segregation, and management at the Round Mountain mine based on lithology and oxidation.

Cenozoic drainage reversal on the southern margin of the Colorado Plateau, east-central Arizona, USA

Laramide northeast-flowing streams from the ancestral Mogollon highland beveled gently northeast-dipping Late Proterozoic to Cretaceous strata across the southern Colorado Plateau Transition Zone. Late Eocene renewed uplift rejuvenated northeast-flowing streams incising paleocanyons. Apache paleocanyon was incised into the Mogollon highland, including the north-trending Laramide Apache uplift bounded by major reverse faults/monoclines. The Mogollon Rim sequence aggraded in Apache paleocanyon and on a broad alluvial plain to the east. Middle Cenozoic tectonic subsidence of the Transition Zone, aridification, and volcanism combined to aggrade Apache paleocanyon with sedimentary and volcanic rocks from 37.6 to 18.63 Ma. Emplacement of the Mogollon-Datil caldera complex and Chuska erg on the southeastern Colorado Plateau forced streamflow to northwest-dispersal of fluvio-eolian sediment from 34 to 26 Ma. Following erosion by northwest-flowing streams on the southern Colorado Plateau from ~26 to 16 Ma, lake sediments of the lower Bidahochi Fm were deposited. Southwestward reactivation of Laramide faults was underway by ~25 Ma coeval with extensive 25 to 20 Ma Natanes Plateau basalt flows and extreme crustal thinning southwest of the Transition Zone. As northeastward streamflow gradually diminished, a Superstition field ash flow tuff ended northeastward flow at 18.63 Ma and was followed by a period of sluggish southwest stream flow and ponding until after ~14.84 Ma. Southwestward structural subsidence and possible spillover from ancestral Lake Hopi on the Colorado Plateau southern margin caused incision of the southwest-directed Dagger Canyon paleovalley after 14.84, which followed the path of the earlier Apache paleocanyon and possibly to the Sespe delta on the California coast before opening of Gulf of California. Structural collapse of the Tonto Basin to the west induced deposition of the Dagger Canyon Conglomerate in the Dagger Canyon paleovalley before the modern Salt River incised all previous deposits and became integrated with the Gila River during the Plio-Pleistocene.

Fracture Sustainability in Enhanced Geothermal Systems: Experimental and Modeling Constraints

Abstract Enhanced geothermal systems (EGS) offer the potential for a much larger energy source than conventional hydrothermal systems. Hot, low-permeability rocks are prevalent at depth around the world, but the challenge of extracting thermal energy depends on the ability to create and sustain open fracture networks. Laboratory experiments were conducted using a suite of selected rock cores (granite, metasediment, rhyolite ash-flow tuff, and silicified rhyolitic tuff) at relevant pressures (uniaxial loading up to 20.7 MPa and fluid pressures up to 10.3 MPa) and temperatures (150–250 °C) to evaluate the potential impacts of circulating fluids through fractured rock by monitoring changes in fracture aperture, mineralogy, permeability, and fluid chemistry. Because a fluid in disequilibrium with the rocks (deionized water) was used for these experiments, there was net dissolution of the rock sample: this increased with increasing temperature and experiment duration. Thermal-hydrological-mechanical-chemical (THMC) modeling simulations were performed for the rhyolite ash-flow tuff experiment to test the ability to predict the observed changes. These simulations were performed in two steps: a thermal-hydrological-mechanical (THM) simulation to evaluate the effects of compression of the fracture, and a thermal-hydrological-chemical (THC) simulation to evaluate the effects of hydrothermal reactions on the fracture mineralogy, porosity, and permeability. These experiments and simulations point out how differences in rock mineralogy, fluid chemistry, and geomechanical properties influence how long asperity-propped fracture apertures may be sustained. Such core-scale experiments and simulations can be used to predict EGS reservoir behavior on the field scale.

Geochronology of a long Pleistocene sequence at Kilombe volcano, Kenya: from the Oldowan to Middle Stone Age

We report a newly extended stratigraphic sequence with associated Palaeolithic sites from the area of the extinct Kilombe volcano in central Kenya. The extended archaeological sequence runs from Oldowan finds, through the Acheulean, and up to the Middle Stone Age. The sedimentary sequences within the Kilombe caldera and south flanks of the mountain have been dated through 40Ar/39Ar measurements and palaeomagnetic studies. A series of 40Ar/39Ar values date the geological sequence from 2.493 ± 0.095 Ma, near the beginning of the Lower Pleistocene, through to 0.118 ± 0.030 Ma near the Middle to Upper Pleistocene transition. It includes the first entirely new area of Oldowan localities in East Africa south of Ethiopia for thirty years, and the first in a rugged mountainous setting. Trachyte lavas of Kilombe mountain were extruded during and after c. 2.5 Ma, followed by formation of a caldera and subsequent caldera lake, and sedimentation of a sequence of tuffs, diamictites, sandstones, and claystones. Sections in the mid-part of this intra-caldera fill-sequence have produced dates of 1.8–1.7 Ma, associated with an Oldowan industry and fauna dated precisely at 1.814 ± 0.004, and overlain by Acheulean finds at higher level. On the southern outward flanks of Kilombe mountain, a second major sequence is bounded at the base by trachyphonolite and a tuff yielding dates in the range 1.58–1.50 Ma. The main Acheulean archaeological site (GqJh1) falls within the overlying sedimentary sequence and has an age of c. 1.0 Ma, on the basis of a new 40Ar/39Ar date for the Three-Banded Tuff and palaeomagnetic reversal stratigraphy. Further 40Ar/39Ar dates indicate an age of c. 0.48–0.46 Ma for a marker ashflow tuff (AFT), prominent across the area. At Moricho, west of Kilombe, sediments above the AFT have been dated in the range 270,000–120,000 years and are associated with Middle Stone Age assemblages. In total, these sites attest to hominin activity from an Oldowan horizon dated to 1.8 Ma up to Later Stone Age stone scatters within the last 100,000 years.

Identification of the source caldera for a Pliocene ash-flow tuff in Northeast Japan based on apatite trace-element compositions and zircon U-Pb ages

In tephrochronology, trace-element compositions of apatite phenocrysts are useful because they change in response to various magmatic conditions as apatite crystallizes. Moreover, because apatite tolerates burial diagenesis and welding processes without chemical changes, its trace-element compositions are a powerful tool for correlating pre-Quaternary tephra; however, this method is not extensively applied. The Hirosegawa Tuff, composed of a pair of Pliocene ash-fall and ash-flow tuffs, is a distinctive marker tephra that is used in geological mapping and for correlating sedimentary rocks in the eastern part of central NE Japan. However, the source volcano of the Hirosegawa Tuff has not been identified because many calderas coexisted in NE Japan during the Pliocene. Analysis of the trace-element compositions of apatite, major element compositions of ilmenite, and U-Pb zircon ages demonstrate that, depending on the area, the previously mapped Hirosegawa Tuff comprises different volcanic ejecta. Furthermore, this study identified that the source volcano of the Hirosegawa Tuff in the type section was the Shiroishi Caldera, located 50 km south of Sendai City.

Zircon-hosted melt inclusion record of silicic magmatism in the Mesoproterozoic St. Francois Mountains terrane, Missouri: Origin of the Pea Ridge iron oxide-apatite-rare earth element deposit and implications for regional crustal pathways of mineralization

Voluminous silicic magmatism was coeval with iron ore mineralization in the St. Francois Mountains terrane in southeast Missouri, part of the broader Mesoproterozoic Granite-Rhyolite province along the eastern margin of Laurentia. Some of the iron deposits contain extraordinary endowments of critical elements, such as the Pea Ridge iron oxide-apatite (IOA) deposit, which has an average grade of ∼12 wt% rare earth oxides in breccia pipes that flank the ore body. To assess the role of silicic magmatism in the genesis of the Pea Ridge deposit, we present a high-spatial resolution study of zircon-hosted melt inclusions from rhyolitic ash-flow tuffs. Melt inclusion data are combined with textural, geochemical, and geochronological analyses of zircon hosts to elucidate the magmatic-hydrothermal evolution of the Pea Ridge system. Two contemporaneous silicic igneous centers in the St. Francois Mountains terrane, Bourbon and Eminence, were studied for comparison. Pea Ridge melt inclusions are trachydacitic to rhyolitic (∼63–79 wt% SiO2, ∼5.6–11.7 wt% Na2O + K2O) with very high Cl in the least-evolved and most alkaline melt inclusions (∼2000–5000 ppm Cl). Rare earth elements (REE) in melt inclusions have identical chondrite-normalized patterns to the mineralized breccia pipes, but with systematically lower absolute concentrations. Haplogranite ternary pressures range from ∼0.5 to 10 kbar, with an average of ∼2–3 kbar (7–12 km depth), and liquidus temperatures are ∼850–950 °C, with an average of ∼920 °C. Silicate and phosphate mineral inclusions have compositions that overlap minerals from the iron ore body and breccia pipes, recording a transition from igneous to hydrothermal zircon growth. Igneous iron oxide inclusions have compositions that indicate Pea Ridge magmas were reduced to moderately oxidized (log fO2 of −0.8 to −1.84 ΔNNO). Zircons from two Pea Ridge samples have 207Pb/206Pb concordia ages of 1456 ± 9 Ma and 1467 ± 13 Ma that overlap published ages for the breccia pipes and iron ore zones of the Pea Ridge deposit. A population of texturally and chemically disrupted zircons have discordant domains that correspond to high Fe, U, and REE concentrations, consistent with the unique geochemical attributes of the IOA-REE ore body. Inherited cores in Pea Ridge and Bourbon zircons have concordant 207Pb/206Pb dates of 1550–1618 Ma, providing direct evidence of cratonic basement beneath these centers. Oxygen isotope data for inherited and autocrystic igneous zircons span from mantle to crustal values (δ18Ozircon = 5.5–7.9‰). Our data are consistent with a model in which metasomatized mantle components were mixed with cratonic and accreted crustal material in a back-arc or rifted segment of a volcanic arc, with ore fluids derived from Cl-rich melts to transport Fe and REE in a long-lived (tens of Myr), pulsed, magmatic-hydrothermal system. Bourbon, which also possesses IOA mineralization, shares key petrologic similarities with the Pea Ridge system, whereas Eminence, which is not mineralized, has disparate geochemical and isotopic signatures that indicate it formed in a different crustal setting. The location of Pea Ridge and Bourbon along a cratonic margin may have been important in focusing silicic melts and mineralization in the upper crust, serving as a guide for future exploration efforts.

El Complejo Volcánico Bahía Laura en la zona del distrito minero Manantial Espejo

El levantamiento geológico detallado junto con el análisis petrográfico y geoquímico de las unidades litológicas relevadas en la zona del distrito minero Manantial Espejo, permitió plantear la evolución del volcanismo Jurásico en esa zona del Macizo del Deseado (Provincia de Santa Cruz, Patagonia argentina). Se interpretaron tres ciclos de actividad volcánica, favorecidos por la tectónica distensiva que dominó esta provincia geológica durante el Jurásico Medio a Superior. El primero, el Ciclo Volcánico Inferior, corresponde a la efusión de lavas de composición andesítica (Formación Bajo Pobre) con una intercalación de areniscas tufíticas y niveles de aglomerados volcánicos producto de remoción en masa. La efusión de estas lavas se produjo sobre un basamento conformado por esquistos y granitos. El segundo, Ciclo Volcánico Medio, se inicia con una ignimbrita de composición dacítica a la que le siguen dos unidades ignimbríticas riolíticas, denominadas 1 y 2. La Ignimbrita dacítica y la Ignimbrita riolítica 2, son rocas con fuerte soldamiento, texturas reomórficas y muy alta participación de cristaloclastos, características que indican una elevada temperatura de depositación a partir del colapso de columnas eruptivas de baja altura, de relativa baja energía, elevada densidad y un bajo contenido de gases, originadas a partir de fontanas tipo ?boiling-over?. La Ignimbrita riolítica 1, intercalada entre las anteriores, se caracteriza por incluir grandes fragmentos líticos de naturaleza polimíctica, se correlaciona con las ignimbritas de extra-caldera formadas durante el colapso y erupción de la Caldera Cerro Primero de Abril, ubicada al norte del distrito Manantial Espejo, en las inmediaciones de Mina Martha. Continúa un período inter-eruptivo, durante el cual se depositan tufitas y travertinos que evidencian una actividad hidrotermal del tipo ?hot spring? controlada por estructuras de rumbo NNE y NNO a NO. El aporte de calor y la fuente de CO2 del sistema geotermal habrían estado vinculados a la desgasificación de un nuevo pulso de magma riolítico que marca el inicio del Ciclo Volcánico Superior, y que habría ascendido a través del mismo sistema extensional, evidenciado por diques riolíticos de rumbo NNO. El ascenso de lavas riolíticas dio lugar a la formación de dos centros volcánicos, el Centro Volcánico Norte, compuesto por un complejo extrusivo de domos riolíticos con depósitos de colapso tipo block and ash, y el Centro Volcánico Sur con el desarrollo de una compleja asociación de facies que sugieren que se trató de una erupción de tipo hidromagmática, con la formación de un aparato volcánico de tipo maar-diatrema al que se asocia un depósito de brecha constituida por fragmentos de lava acompañados por cantidades subordinadas de litoclastos de ignimbritas, tufitas y rocas laminadas silicificadas. Asociado a este evento explosivo se reconoció un depósito de tobas de caída con lapilli acrecionales que apuntan también a un probable origen hidromagmático. La interacción entre el magma y agua líquida sobrecalentada habría sido el responsable para la generación de esta erupción. Luego de un corto periodo de calma volcánica, la continuidad en el régimen deformacional distensivo permite nuevos asensos de magmas con la generación de nuevas unidades ignimbríticas riolíticas de similares características a las del segundo ciclo. El volcanismo andesítico a riolítico de la zona pertenece a una serie calcoalcalina de moderado a alto potasio, con afinidades de arco, mayoritariamente peraluminosas y subalcalinas. Dentro de este esquema, el Ciclo Volcánico Inferior es de composición netamente andesítica, el Ciclo Volcánico Medio registra la evolución de los magmas desde dacitas a riolitas y, por último, el Ciclo Volcánico Superior es típicamente riolítico en composición.

“Trachytes” from Sardinia: Geoheritage and Current Use

Sardinia was affected by an intense igneous activity which generated calc-alkaline products during the Oligo-Miocene period. The volcanic substance shows large variations, ranging from pyroclastic flow deposits, lava flows and domes. By composition, the deposits are all primarily dacites and rhyolites, with subordinate andesites and very scarce basalts. The rhyolite lavas show porphyritic and ash-flow tuffs. Ignimbrite structures are found in the dacitic domes and rhyolitic lavas. These rocks—commercially known as “Trachytes of Sardinia”—used to be quarried in all historical provinces, mainly in the central part of the island to be used as ornamental and building stone. They continue to be commonly used nowadays, but their use dates back to the prehistoric age. They are easily found in many nuraghi, “domus de janas”, holy wells, Roman works (mosaics, paving, roads, bridges), many churches built in Sardinia and practically in all kinds of structural elements in public and private buildings, such as walls, houses, and bridges. Contrary to the granitoid rocks, whose appearance is largely influenced by the mineralogical composition, the aesthetic feature of volcanic rocks is rather affected by the widest range of colors, structure and texture, i.e., shape, size and distribution of mineral components, porphyric index, etc. “Trachyte” is quarried opencast with the “single low step” method, with descending development, with prevalent use of double-disc sawing machines. Whenever the stone deposit allows higher steps, the chain cutting machine, in combination with diamond wire, becomes the preferred extraction solution. This study aims to at look Sardinian “trachytes” from a geoheritage perspective. After a geological-petrographic framework, the paper discusses the historical uses of “trachyte” in Sardinia. The current state of the art of “trachyte” quarrying, processing and usage in the Island is also described. An analysis of the “trachyte” production has been carried out. Finally, a consideration about how to enhance geotourism in the area is suggested.

Sublacustrine hydrothermal seeps and silicification of microbial bioherms in the Ediacaran Oued Dar'a caldera, Anti‐Atlas, Morocco

AbstractThis paper presents a case study of the sublacustrine precipitation of hydrothermal silica ± TiO2 in the Ediacaran Mançour Group of the Saghro inlier, Anti‐Atlas, Morocco. Lacustrine carbonates containing stromatolitic mats and bioherms occur in ephemeral ponds developed within the Oued Da'ra caldera. Its syn‐eruptive infill consists of pyroclastites, ashflow tuffs, and subsidiary lava flows and sills, whereas inter‐eruptive deposition is mainly represented by slope‐related debris‐flow breccias and landslides, alluvial fans and fluvial channels. Carbonate production took place in a mosaic of differentially subsiding, fault‐bounded intra‐caldera blocks controlled by episodic collapse‐induced drowning, pyroclastic blanketing and migration of alluvial/fluvial environments. After microbial carbonate production, the carbonates recorded several early‐diagenetic processes, punctuated by polyphase fissuring (controlling secondary permeability) locally linked to hydrothermal influx. Three generations of carbonate cements are recognisable: (i) fibrous, botryoidal and blocky/drusy mosaics of calcite; (ii) idiotopic mosaics of dolomite caused by flushing of hypersaline Mg‐rich brines; and (iii) euhedral to drusy calcite via dedolomitization. The δ13C and δ18O values from carbonate cements broadly become successively isotopically lighter, as a result of meteoric and hydrothermal influence, and were probably overprinted by the Panafrican‐3 phase that affected the top of the Mançour Group. Two mechanisms of silicification are involved: (i) early‐diagenetic occlusion of interparticle pores at the sediment/water interface of pyroclastic substrates and reefal core and flanks; and (ii) hydrothermal precipitation of silica ± TiO2 lining fissures and vuggy porosity encased in the host rock. Silica conduits cross‐cutting lacustrine mats and bioherms exhibit high potential of preservation in collapsed volcanic calderas. Primary fluid inclusions of hydrothermal silica contain brine relics with NaCl/CaCl2 ratios of 2·1 to 4·4, representing minimum entrapment temperatures of about 142 to 204°C, and abiotic hydrocarbons (heavy alkanes) related to serpentinization of the volcanic and volcanosedimentary basement of the Oued Dar'a caldera.

The Miocene Goldfield-Superstition volcanic province, Central Arizona, USA: Geochemically distinct rhyolite sources, 20.5 to 19 Ma

The Miocene Goldfield-Superstition volcanic province (G-SVP) in central Arizona is composed of silicic pyroclastic and lava flows, and, to a small extent, silicic domes, and mafic and intermediate lavas. Volcanism began ~20.5 Ma with small-scale silicic domes, followed by ≤1.5 m.y. of basaltic, andesitic, and dacitic lavas, after which large-scale pyroclastic flows and lavas created the bulk of this large province from ~19 to 18.5 Ma (~8000 km2).Rhyolite compositions dominate the G-SVP and our observations are that the earliest expressions, small-scale domes, had crustal sources geochemically different from those for the main mass of lavas and ash-flow tuffs. In particular, three early domes, while rhyolites, have notably low REE and HFSE abundances for high-SiO2 rocks (e.g., La 8–34 ppm; Zr 32–92; Nb 8–17, Th 6–7), generally lower than the abundances in overlying alkalic basalt lavas. They form two groups: one is higher in SiO2 (~77–79 wt%) than the other (~70–73 wt% SiO2). Both groups are crystal poor, from ~3 to 8 vol%. Modal mineralogy is mainly oligoclase, alkali feldspar, and quartz in the high-SiO2 group, and andesine and biotite in the low-SiO2 group. The high-SiO2 rhyolite domes have REE patterns that are nearly flat (La/Yb(n) 9–16), whereas the low-SiO2 domes have steeper REE patterns (La/Yb(n) 46–56).In comparison, the lavas and tuffs that erupted beginning ~1.5 m.y. later have REE patterns and higher REE and HFSE abundances (e.g., La largely 45–80 ppm; Nb 17–30; Th 9–20) characteristic of rhyolites in other western North American Oligocene-Miocene silicic large provinces. Their modal mineralogy (~2–13 vol% tuffs; ~5 to 26 vol% lavas) is andesine, oligoclase, alkali feldspar, quartz, biotite, edenitic amphibole, +/− pyroxene, titanite, zircon, and REE-rich perrierite.Our interpretation is that the earliest magmatism in the G-SVP – namely the domes – represents small percent (e.g., ≤10%) partial melts of lower crust amphibolite that had low trace element abundances, both with and without garnet (to influence the REE pattern slopes). Melting is attributed to heat from early stages of basaltic magmatism. The younger rhyolite lavas and pyroclastic flows are products of middle to upper granitoid crust anatexis (e.g., 40–60%) resulting from continual basalt magma injections up to 2 m.y. following dome emplacement (i.e., from ~20.5 to 19–18.5 Ma). The G-SVP therefore chronologically and geochemically tracks the change in sources for rhyolites over ~2 m.y. of volcanism to create a central Arizona large igneous province.

Borehole LLE1 – an intra-caldera succession of the Goedgenoeg and Makwassie Formations, Ventersdorp Supergroup

Abstract During the late 1980s a deep exploration borehole (LLE1) was collared on the Makwassie Formation outcrop at Makwassie Hills, near Leeudoringstad. The borehole intersected a 2420 m thick succession of rhyolitic quartz porphyries and interbedded andesitic mafic lavas of the Makwassie Formation, with a further 1301 m of dacitic feldspar porphyries and andesitic mafic lavas of the Goedgenoeg Formation, before it was stopped at 3721 m, still in the Goedgenoeg Formation. The mafic lavas in the Goedgenoeg Formation are low viscosity basaltic flows, as are the mafic lavas interbedded in the Makwassie Formation. The Goedgenoeg Formation feldspar porphyries and the Makwassie Formation quartz porphyries are of extrusive origin, emplaced as ignimbrites and ash-flow tuffs, ascertained by macroscopic and petrographic characteristics. Individual ignimbrite or ash-flow units are several tens of metres to hundreds of metres thick, regularly separated by volcanoclastic sandstone and calcareous shales. Geochemically the lower part of the Goedgenoeg Formation feldspar porphyries in borehole LLE1 correlates with the low-Zr dacite group of the Bothaville area, and the upper part with the high-Zr dacites. The lower part of the Makwassie Formation quartz porphyries correlates with the high-Zr/Cr rhyolite group of the Bothaville area, and the upper part with the low-Zr/Cr rhyolite group. The Bothaville area’s Makwassie Formation dacite unit is absent from the LLE1 profile. The interbedded andesitic mafic lavas are similar in composition to those in the Bothaville area. There is no structural or stratigraphic indication (through lithological or geochemical means) of duplication in this borehole’s stratigraphic profile. The borehole drilled into a caldera succession, part of a cluster of calderas associated with half-graben structures aligned from west-northwest to east-southeast, from the Makwassie Hills towards Bothaville. These and other Platberg caldera clusters are developed outside the Central Rand Basin, mainly to the west of it.

Investigating Dismal River obsidian use in central Nebraska

Nineteen obsidian artifacts from surface contexts of probable Dismal River complex sites in the Sand Hills of Nebraska were analyzed via energy dispersive X-ray fluorescence (EDXRF) spectrometry. The artifacts derive from six distinct geological volcanic glass sources. One of the artifacts was made from ash-flow tuff obsidian of the Brown’s Bench chemical type and another from Wild Horse Canyon, Utah obsidian, representing the first documented use of these western source materials in Nebraska. Southwestern obsidian sources are less well represented than those to the west of Nebraska in this study sample, indicating that more than one pattern of obsidian use and conveyance may have existed during the Dismal River complex.

A Late Paleozoic Snake River-type ignimbrite (Planitz vitrophyre) in the Chemnitz Basin, Germany: Textural and compositional evidence for complex magma evolution in an intraplate setting

An Early Permian >VEI 6 eruption (ca. 42 km3) deposited an ash flow tuff into the Chemnitz Basin (southeastern Germany), which exhibits typical characteristics of a Snake River-type ignimbrite. The peculiarity of the Planitz ignimbrite are well-preserved vitroclastic domains with fresh minerals, which provide a unique opportunity for investigation of a relatively crystal-poor (~20 vol% crystals) magmatic system of Late Paleozoic Central Europe. Combined textural and chemical analyses on whole rock, glass shards and mineral phases were acquired to unravel the physical characteristics of the vitrophyre and the pre-eruptive evolution of its multistage magma system.The greyish-black vitrophyre is classified as a well-sorted coarse ash flow tuff (95 vol% of crystals <2 mm) with predominant cuspate and microvesicular fabrics in the groundmass. The latter comprise colorless glass shards; subordinately, brown glass shards (≤1 vol%). The felsic modal mineralogy comprises Ab-rich plagioclase (~7 vol%) followed in abundance by sanidine (~5 to 6 vol%) and quartz (~2 vol%), whereas the ferromagnesian cumulate fragments are dominated by pyroxene (3 to 4 vol%). Bulk ignimbrite composition corresponds to a peraluminous rhyolite that shows characteristics of crustal assimilation by pronounced negative Nb and Ta anomalies.The ferromagnesian silicates have compositions indicating crystal growth in alkaline to calc-alkaline magma (diopside, augite, annite) as well as peraluminous one (siderophyllite). Some glomerocrysts contain alkaline diopside with high Mg-numbers from 80 to 84 and plagioclase with An-content from 44 to 56 mol%, which are evidence for input of mafic melts in the Planitz magma reservoir. Oscillatory zonation in plagioclase crystals points to a repetition of this process. Melt inclusions in pyroxene (57 to 67 wt% SiO2) indicate the presence of intermediate to acidic magmas in mid crustal reservoirs. Thermo- and barometry calculations based on pyroxene suggest crystallization depths around 25 km for lower crustal dry magma, whereas the occurrence of biotite, sanidine and quartz has been interpreted as entrained material from a rhyolitic cap of an upper crustal batholithic reservoir at depths around 6 km.A genetic connection between the Planitz vitrophyre and the intracaldera Rochlitz ignimbrite of the North Saxon Volcanic Complex, as it was suggested in previous studies, is unlikely because (1) biotite in the Planitz vitrophyre is Fe-rich annite showing AlIV-Mn exchange trends, whereas the Rochlitz biotite is siderophyllite with FeMg substitution and (2) the Planitz vitrophyre lacks textures like granophyric intergrowth, which are abundant in the Rochlitz ignimbrites.During and after deposition of the ash flow tuff (≤ 30 m thick), high-temperature processes like moderate welding compaction and formation of lithophysae were followed by high-temperature perlite formation and glass hydration. The latter was associated with depletion of Na and K in colorless glass shards when compared to the whole rock composition. Moreover, the hydration reduced Mg, Fe, and Ca in evolved brown glass shards.