Ash-flow Tuffs Research Articles

Volcanism at 1.45 Ma within the Yellowstone Volcanic Field, United States

Rhyolitic volcanism in the Yellowstone Volcanic Field has spanned over two million years and consisted of both explosive caldera-forming eruptions and smaller effusive flows and domes. Effusive eruptions have been documented preceding and following caldera-forming eruptions, however the temporal and petrogenetic relationships of these magmas to the caldera-forming eruptions are relatively unknown. Here we present new 40Ar/39Ar dates for four small-volume eruptions located on the western rim of the second-cycle caldera, the source of the 1.300 ± 0.001 Ma Mesa Falls Tuff. We supplement our new eruption ages with whole rock major and trace element chemistry, Pb isotopic ratios of feldspar, and paleomagnetic and rock magnetic analyses. Eruption ages for the effusive Green Canyon Flow (1.299 ± 0.002 Ma) and Moonshine Mountain Dome (1.302 ± 0.003 Ma) are in close temporal proximity to the eruption age of the Mesa Falls Tuff. In contrast, our results indicate a period of volcanism at ca 1.45 Ma within the Yellowstone Volcanic Field, including the eruption of the Bishop Mountain Flow (1.458 ± 0.002 Ma) and Tuff of Lyle Spring (1.450 ± 0.003 Ma). These high-silica rhyolites are chemically and isotopically distinct from the Mesa Falls Tuff and related 1.3 Ma effusive eruptions. The 40Ar/39Ar data from the Tuff of Lyle Spring demonstrate significant antecrystic inheritance, prevalent within the upper welded ash-flow tuff matrix, and minimal within individual pumice. Antecrysts are up to 20 kyr older than the eruption, with subpopulations of grains occurring every few thousand years. We interpret these results as an indicator for the timing of magmatic pulses into a growing magmatic system that would ultimately erupt the Tuff of Lyle Spring, and which we more broadly interpret as the tempo of crustal accumulation associated with bimodal magmatism. We propose a system whereby chemically, isotopically, and temporally distinct, isolated small-volume magma batches are periodically generated and erupted in a low magmatic flux state, which is punctuated by larger volume caldera-forming eruptions.

Stratigraphic revision and structure of Sierra Plomosa, Chihuahua

New geological mapping and U-Pb dating of detrital zircons and volcanic rocks permit stratigraphic revision and interpretation of pre- Late Permian deformation in the Sierra Plomosa area, located 95 km east of the city of Chihuahua and at comparable distance to the southwest of the border town Ojinaga. The Plomosas area forms the central part of a north-northwest trending uplifted block ranging from Placer de Guadalupe at the north, to the cerro Carrizalillo at the south. Along this elevated block, Precambrian crystalline basement rocks are exposed, as well as lower Paleozoic rocks belonging to the southern platform of Laurentia and upper Paleozoic to lower Mesozoic clastic and volcaniclastic rocks. The main structures in the area are north-northwest trending folds and thrust faults showing a west-southwest vergence or tectonic transport, being the age of this deformation, necessarily pre-Late Permian, given the occurrence of an erosional and angular unconformity between strongly deformed pre-Permian strata and less deformed Permian-Triassic clastic and volcaniclastic deposits which contain Archean (3,022–2,513Ma), Paleoproterozoic (2,461–1,601Ma), Mesoproterozoic (1,592–1,013Ma), Neoproterozoic (994–546Ma) and Paleozoic (533–252Ma) detrital zircon populations, with maximum depositional ages calculated between 252 and 244 Ma and isolated concordant zircons as young as 227 to 223 Ma. In the middle part of the permo-triasic succession, a Permian rhyolitic flow or ignimbrite was previously reported on the basis of a lead alpha age, however, during the present research this rock yielded an Early Jurassic age of 183.3+2.2/–2.7 Ma. On the basis of petrographic features and field relations, the rhyolite is interpreted as an ash flow tuff or ignimbrite, similar to other rhyolitic flows observed in the area. Such flows are concordant to bedding and several clasts of this rock are present in strata overlying the rhyolite. All Paleozoic rocks, including the top of the Permian and Lower Triassic, underlie clastic and volcaniclastic Jurassic rocks of shallow marine origin. In the area was also documented an erosional, locally angular unconformity which separates a Lower Jurassic unit from a Middle (?) to Upper Jurassic unit with previously reported Late Jurassic fauna in the upper part of the succession. Limestone layers in the upper part of the Jurassic succession, are recrystallized as marbles, especially in the area of the Plomosas mine, where both the metamorphism of these limestones and the presence of stratabound ore bodies of lead and zinc seem to be related to thrusting with a tectonic transport to the southwest and northwest, that placed very fractured Permian sandstones and siltstones over the Upper Jurassic rocks. The faulting is possibly associated with transpression along the so-called Plomosas lineament and transcurrent faulting, described as a left lateral shear of the basement whose regional trace lies along the eastern front of the sierra. Finally south of the Sierra Plomosa, in the northeast trending Sierra de Santo Domingo Carboniferous and Permian limestone are thrust over Jurassic sandstones and shale. This thrust fault also cuts a rhyolitic dyke of probable Oligocene age and can be considered as a very late laramide shortening contractive structure, possibly related to transpressive deformation along the Plomosas lineament and its interaction with the uplifted Cerro Carrizalillo block 15 km to the South of Plomosas.

A Middle Miocene (13.5–12 Ma) deformational event constrained by volcanism along the Puna-Eastern Cordillera border, NW Argentina

The features of Middle Miocene deposits in the Puna-Eastern Cordillera transition (Valles Calchaquíes) indicate that Cenozoic deformation, sedimentation and volcanism follow a complex spatiotemporal relationship. The intense volcanic activity recorded in the eastern Puna border between 14 and 11.5Ma coincides with the occurrence of one of the most important deformation events of the Neogene tectonic evolution in the region.Studies performed across the Puna-Eastern Cordillera transition show different relationships between volcanic deposits of ca. 13.5–12.1Ma and the Oligocene-Miocene Angastaco Formation. In this paper we describe the ash-flow tuff deposits which are the first of this type found concordant in the sedimentary fill of Valles Calchaquíes. Several analyses performed on these pyroclastic deposits allow a correlation to be made with the Alto de Las Lagunas Ignimbrite (ca. 13.5Ma) of the Pucarilla-Cerro Tipillas Volcanic Complex located in the Puna. Outcrops of the ca. 13.5Ma pyroclastic deposits are recognised within the Puna and the Valle Calchaquí. However, in the southern prolongation of the Valle de Hualfín (Tiopampa-Pucarilla depression) that separates the Puna from the Valle Calchaquí at these latitudes, these deposits are partially eroded and buried, and thus their occurrence is recorded only by abundant volcanic clasts included in conglomerates of the Angastaco Formation. The sedimentation of the Angastaco Formation was aborted at ca. 12Ma in the Tiopampa-Pucarilla depression by the Pucarilla Ignimbrite, which unconformably covers the synorogenic units. On the contrary, in the Valle Calchaquí the sedimentation of the Angastaco Formation continued until the Late Miocene.The different relationships between the Miocene Angastaco Formation and the ignimbrites with ages of ca. 13.5 and ca. 12Ma reveal that in this short period (~1.5m.y.) a significant deformation event took place and resulted in marked palaeogeographic changes, as evidenced by stratigraphic-sedimentological and chronological records in the Angastaco Formation.

Complex remanent magnetization in the Kızılkaya ignimbrite (central Anatolia): Implication for paleomagnetic directions

Pyroclastic flow deposits, known as ash-flow tuffs or ignimbrites, are invaluable materials for paleomagnetic studies, with many applications for geological and tectonic purposes. However, little attention has been paid to evaluating the consistency and reliability of the paleomagnetic data when results are obtained on a single volcanic unit with uneven magnetic mineralogy. In this work we investigate this issue by concentrating on the Kızılkaya ignimbrite, the youngest large-volume unit of the Neogene ignimbrite sequence of the Central Anatolian Volcanic Province in Turkey, bringing evidence of significant magnetic heterogeneities in ignimbrite deposits (magnetic mineralogy, susceptibility, magnetic remanence, coercivity, etc.) and emphasizing the importance of a stratigraphic sampling strategy for this type of volcanic rocks in order to obtain reliable paleomagnetic data. Six sections were sampled at different stratigraphic heights within the devitrified portion of the ignimbrite. Isothermal remanence measurements point to low-Ti titanomagnetite as the main magnetic carrier at all sites; at some sites, the occurrence of oxidized Ti-magnetite and hematite is disclosed. The bulk susceptibility (km) decreases vertically at two out of six sections: its value for the topmost samples is commonly one order of magnitude lower than that of the samples at the base. In most cases, low km values relate to high coercivity of remanence (BCR) values, which range from 25 to >400mT, and to low S-ratios (measured at 0.3T) between 0.28 and 0.99. These data point to the occurrence of oxidized magnetic phases. We therefore consider the km parameter as a reliable proxy to check the ignimbrite oxidation stage and to detect the presence of oxidized Ti-magnetite and hematite within the deposit. The characteristic remanent magnetization is determined after stepwise thermal and AF demagnetization and clearly isolated by principal component analysis at most sites. For these sites, the site-mean paleomagnetic direction is consistent with data from the literature. At a few other sites, the remanence is more complex: the direction moves along a great circle during demagnetization and no stable end-point is reached. The occurrence of oxidized Ti-magnetite or hematite as well as two remanence components with overlapping coercivity and blocking temperature spectra suggest that the Kızılkaya ignimbrite acquired first a thermal remanent magnetization and then, during the final cooling or a short time later, a secondary remanent magnetization component which is interpreted as a CRM acquired during post-emplacement devitrification processes. Notwithstanding the Kızılkaya ignimbrite is a single cooling unit, its magnetic properties suffered substantial variations laterally and vertically within the deposit. The Kızılkaya case shows that thick pyroclastic deposits should be sampled using a stratigraphic approach, at different sites and different stratigraphic heights at each individual sampling location, otherwise, under-sampling may significantly affect the paleomagnetic results. When sampling is performed on a short duration or on very poorly preserved deposits we recommend drilling the lower-central portion in the most strongly welded and devitrified facies. Such sampling strategy avoids complications arising from the potential presence of a pervasive secondary CRM masking the original ChRM.

Mineral fabrics in high-level intrusions recording crustal strain and volcano–tectonic interactions: the Shellenbarger pluton, Sierra Nevada, California

The Minarets caldera is a volcano–plutonic complex in the Sierra Nevada, California, that exemplifies complex interactions between volcanism and tectonic deformation in continental-margin arcs. Caldera evolution commenced with emplacement of pre-collapse rhyolitic ash-flow tuff, followed by collapse and deposition of volcanic breccia and rhyodacitic ash-flow tuff. Subsequently, the volcanic rocks were deformed along the regional Bench Canyon shear zone. The caldera centre was then intruded by the resurgent c. 100 Ma steep-sided Shellenbarger granite pluton, which steepened the shear zone foliation. The pluton was overprinted by syn- to post-magmatic ∼NNE–SSW horizontal shortening; the same shortening was documented in several other Late Cretaceous syntectonic plutons in the Sierra Nevada and interpreted to record dextral transpression during convergence of the Farallon and North American plates. To explain the unusual tectonic fabric in the shallow-level Shellenbarger pluton, we develop a general model for strain partitioning in syntectonic magma bodies emplaced at various crustal levels. We propose that shallow intrusions, isolated within stiff crust, may tend to accommodate minor pure shear strain whereas simple shear dominates along weak faults and shear zones. By contrast, a rheological reversal is crossed deeper in the crust and magma bodies become the weakest, simple shear-dominated parts of the system. Supplementary material: Analytical methods and anisotropy of magnetic susceptibility and U–Th–Pb isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3582749

Large scale pantelleritic ash flow eruptions during the Late Miocene in central Kenya and evidence for significant environmental impact

In the area south-east of Mount Kenya, four previously unrecorded peralkaline rhyolitic (pantelleritic) ash flow tuffs have been located. These predominantly greyish welded and non-welded tuffs form up to 12m thick units, which are sometimes characterized by a basal vitrophyre. The four flow units yielded 40Ar/39Ar ages ranging from 6.36 to 8.13Ma, indicating a period of ~1.8Ma of pantelleritic volcanic activity during the Late Miocene in central Kenya. Tentative compositional and age correlations with other known tuff deposits suggest that the pantelleritic tuffs originally covered 40,000km2 in central Kenya, extending much further than earlier recorded Pliocene tuffs. This newly identified magmatic phase occurred between the phonolitic flood eruptions (16–8Ma) and the Pliocene tuff eruptions (6–4Ma). The occurrence of multiple ash flow tuff deposits up to 150km away from the inferred eruptive center(s) in the central sector of the Kenya Rift, indicates multi-cyclic peralkaline supereruptions during the Late Miocene. By analogy with more recent pantelleritic eruptions, the tuffs are thought to have been sulfur-rich; during eruption, they formed stratospheric aerosols, with significant environmental impact. The timing of the eruptions coincides with the shift towards more savannah-dominated environments in East Africa.

Magnetic Fabrics and Source Implications of Chisulryoung Ignimbrites, South Korea

The anisotropy of magnetic susceptibility (AMS) of late Cretaceous ash-flow tuffs in Chisulryoung Volcanic Formation, southeastern Korea was studied to define the primary pyroclastic flow azimuth. AMS data revealed a dominant oblate fabric with a tight clustering of k3 (minimum axis of magnetic susceptibility) and shallow dispersal of k1 (maximum axis of magnetic susceptibility) and k2 (intermediate axis of magnetic susceptibility). Dominance of oblate fabrics indicates clast imbrications imposed by compaction and welding. Flow azimuth inferred from AMS data indicates the nearby intrusive welded tuff (IWT) as the source of calderas for ignimbrites. Such an inference is supported by geologic investigations, in which the IWT displays eutaxitic textures nearly parallel to its subvertical contacts. The results are compatible with a unique prolate fabric and an anomalously high inclination observed for the IWT, possibly produced by rheomorphic flows as the welded tuff is squeezed along the rough-surfaced dyke walls due to agglutination.

Stratigraphy and structural development of the southwest Isla Tiburón marine basin: Implications for latest Miocene tectonic opening and flooding of the northern Gulf of California

Accurate information on the timing of earliest marine incursion into the Gulf of California (northwestern Mexico) is critical for paleogeographic models and for understanding the spatial and temporal evolution of strain accommodation across the obliquely divergent Pacific–North America plate boundary. Marine strata exposed on southwest Isla Tiburon (SWIT) have been cited as evidence for a middle Miocene marine incursion into the Gulf of California at least 7 m.y. prior to plate boundary localization ca. 6 Ma. A middle Miocene interpretation for SWIT marine deposits has played a large role in subsequent interpretations of regional tectonics and rift evolution, the ages of marine basins containing similar fossil assemblages along ∼1300 km of the plate boundary, and the timing of marine incursion into the Gulf of California. We report new detailed geologic mapping and geochronologic data from the SWIT basin, an elongate sedimentary basin associated with deformation along the dextral-oblique La Cruz fault. We integrate these results with previously published biostratigraphic and geochronologic data to bracket the age of marine deposits in the SWIT basin and show that they have a total maximum thickness of ∼300 m. The 6.44 ± 0.05 Ma (Ar/Ar) tuff of Hast Pitzcal is an ash-flow tuff stratigraphically below the oldest marine strata, and the 6.01 ± 0.20 Ma (U/Pb) tuff of Oyster Amphitheater, also an ash-flow tuff, is interbedded with marine conglomerate near the base of the marine section. A dike-fed rhyodacite lava flow that caps all marine strata yields ages of 3.51 ± 0.05 Ma (Ar/Ar) and 4.13 ± 0.09 Ma (U/Pb) from the base of the flow, consistent with previously reported ages of 4.16 ± 1.81 Ma (K-Ar) from the flow top and (K-Ar) 3.7 ± 0.9 Ma from the feeder dike. Our new results confirm a latest Miocene to early Pliocene age for the SWIT marine basin, consistent with previously documented latest Miocene to early Pliocene (ca. 6.2–4.3 Ma) planktonic and benthic foraminifera from this section. Results from biostratigraphy and geochronology thus constrain earliest marine deposition on SWIT to ca. 6.2 ± 0.2 Ma, coincident with a regional-scale latest Miocene marine incursion into the northern proto–Gulf of California. This regional marine incursion flooded the northernmost, >500-km-long portion of the Gulf of California shear zone, a narrow belt of localized strike-slip faulting, clockwise block rotation, and subsiding pull-apart basins. Oblique Pacific–North America relative plate motion gradually localized in the >1000-km-long Gulf of California shear zone ca. 9–6 Ma, subsequently permitting the punctuated south to north flooding of the incipient Gulf of California seaway.

An integrated geophysical imaging of the upper-crustal features in the Harney Basin, southeast Oregon

The Harney Basin is a relatively flat-lying depression in the northeast corner of the enigmatic High Lava Plains volcanic province in eastern Oregon. A thick blanket of volcanics including flood basalts, rhyolites, tuffs, ash flows, and distinct eruptive centers covers the basin, making it very difficult to study the upper-crustal features. In addition to a portion of the High Lava Plains active source seismic data in the Harney Basin area, we employed geologic, gravity, magnetic, digital elevation, and other geospatial data in our integrated study. We generated an upper-crustal 3D seismic tomographic model of the Harney Basin using a sparse grid of 2D seismic lines and constructed an integrated geophysical model of the upper-crustal structure, which reveals that the basin reaches as deep as 6 km in its central area. The tomographic inversion also detected some unusually high-velocity (>6.5 km/s) bodies in the upper crust near the central basin area. The presence of several ash-flow tuffs and voluminous rhyolites in the Harney Basin region indicates that the sources of these materials are nearby. We observe two major caldera-shaped features within the basin, which we interpret to be likely candidates for the source of some of these tuffs. These potential calderas are associated with low seismic velocities, low gravity anomaly values, and topographic depressions. We interpret the extent and evolution of these potential calderas based on our integrated analysis.

Catastrophic emplacement of the gigantic Markagunt gravity slide, southwest Utah (USA): Implications for hazards associated with sector collapse of volcanic fields

The newly recognized Markagunt gravity slide (MGS) represents gravitationally induced, catastrophic collapse, ca. 21–22 Ma, of the southwest sector of the Oligocene to Miocene Marysvale volcanic field in southwest Utah (USA). The ∼90-km-long slide mass consists of ∼1700–2000 km 3 of andesitic volcanic mudflow breccias and subordinate lava flows, sandstones, and intertongued regional ash-flow tuffs. The MGS extends over an area of >3400 km 2 and advanced >30 km to the south over the Miocene land surface. The low-angle basal slip surface is in Eocene–Oligocene Brian Head Formation, a weak, clay-rich, tuffaceous sedimentary unit at the base of the volcanic section. The presence of a basal zone of cataclastic and sheared breccia with associated clastic dikes, and pseudotachylyte along secondary shear planes, provides strong evidence of catastrophic emplacement. The uniformity of directional indicators, stratigraphy, and overall geometry of the MGS suggests that it represents a single-emplacement event coeval with late-stage magmatic doming of the volcanic field. The MGS thus represents one of the largest subaerial volcanic landslides on Earth, and along with the comparable Heart Mountain gravity slide in northwest Wyoming, constitutes a class of catastrophic collapse hazard not widely recognized within modern volcanic fields.

Evidence for Large-Magnitude, Post-Eocene Extension in the Northern Shoshone Range, Nevada, and Its Implications for the Structural Setting of Carlin-Type Gold Deposits in the Lower Plate of the Roberts Mountains Allochthon

Research Article| November 01, 2014 Evidence for Large-Magnitude, Post-Eocene Extension in the Northern Shoshone Range, Nevada, and Its Implications for the Structural Setting of Carlin-Type Gold Deposits in the Lower Plate of the Roberts Mountains Allochthon Joseph P. Colgan; Joseph P. Colgan † 1U.S. Geological Survey, Denver Federal Center, Bldg. 25, PO Box 25046, Lakewood, Colorado 80225 †Corresponding author: e-mail, jcolgan@usgs.gov Search for other works by this author on: GSW Google Scholar Christopher D. Henry; Christopher D. Henry 2Nevada Bureau of Mines and Geology, Mail Stop 178, University of Nevada, Reno, Nevada 89557 Search for other works by this author on: GSW Google Scholar David A. John David A. John 3U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information Joseph P. Colgan † 1U.S. Geological Survey, Denver Federal Center, Bldg. 25, PO Box 25046, Lakewood, Colorado 80225 Christopher D. Henry 2Nevada Bureau of Mines and Geology, Mail Stop 178, University of Nevada, Reno, Nevada 89557 David A. John 3U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, California 94025 †Corresponding author: e-mail, jcolgan@usgs.gov Publisher: Society of Economic Geologists Received: 25 Jun 2013 Accepted: 08 Mar 2014 First Online: 09 Mar 2017 Online ISSN: 1554-0774 Print ISSN: 0361-0128 © 2014 Society of Economic Geologists. Economic Geology (2014) 109 (7): 1843–1862. https://doi.org/10.2113/econgeo.109.7.1843 Article history Received: 25 Jun 2013 Accepted: 08 Mar 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Joseph P. Colgan, Christopher D. Henry, David A. John; Evidence for Large-Magnitude, Post-Eocene Extension in the Northern Shoshone Range, Nevada, and Its Implications for the Structural Setting of Carlin-Type Gold Deposits in the Lower Plate of the Roberts Mountains Allochthon. Economic Geology 2014;; 109 (7): 1843–1862. doi: https://doi.org/10.2113/econgeo.109.7.1843 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The northern Shoshone and Toiyabe Ranges in north-central Nevada expose numerous areas of mineralized Paleozoic rock, including major Carlin-type gold deposits at Pipeline and Cortez. Paleozoic rocks in these areas were previously interpreted to have undergone negligible postmineralization extension and tilting, but here we present new data that suggest major post-Eocene extension along west-dipping normal faults. Tertiary rocks in the northern Shoshone Range crop out in two W-NW–trending belts that locally overlie and intrude highly deformed Lower Paleozoic rocks of the Roberts Mountains allochthon. Tertiary exposures in the more extensive, northern belt were interpreted as subvertical breccia pipes (intrusions), but new field data indicate that these “pipes” consist of a 35.8 Ma densely welded dacitic ash flow tuff (informally named the tuff of Mount Lewis) interbedded with sandstones and coarse volcaniclastic deposits. Both tuff and sedimentary rocks strike N-S and dip 30° to 70° E; the steeply dipping compaction foliation in the tuffs was interpreted as subvertical flow foliation in breccia pipes. The southern belt along Mill Creek, previously mapped as undivided welded tuff, includes the tuff of Cove mine (34.4 Ma) and unit B of the Bates Mountain Tuff (30.6 Ma). These tuffs dip 30° to 50° east, suggesting that their west-dipping contacts with underlying Paleozoic rocks (previously mapped as depositional) are normal faults. Tertiary rocks in both belts were deposited on Paleozoic basement and none appear to be breccia pipes. We infer that their present east tilt is due to extension on west-dipping normal faults. Some of these faults may be the northern strands of middle Miocene (ca. 16 Ma) faults that cut and tilted the 34.0 Ma Caetano caldera ~40° east in the central Shoshone Range (<5 km south of Mill Creek), but further mapping is necessary to trace the faults through the highly deformed Paleozoic rocks that surround the isolated Tertiary outcrops. Significant post-Eocene extensional faulting in the northern Shoshone Range may have important implications for both the structure of the Roberts Mountains allochthon and the exposure of potentially mineralized rocks in its lower plate, both of which were likely east-tilted and repeated by west-dipping faults together with overlying Tertiary rocks. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Major element and oxygen isotope geochemistry of vapour-phase garnet from the Topopah Spring Tuff at Yucca Mountain, Nevada, USA

AbstractTwenty vapour-phase garnets were studied in two samples of the Topopah Spring Tuff of the Paintbrush Group from Yucca Mountain, in southern Nevada. The Miocene-age Topopah Spring Tuff is a 350 m thick, devitrified, moderately to densely welded ash-flow tuff that is zoned compositionally from high-silica rhyolite to latite. During cooling of the tuff, escaping vapour produced lithophysae (former gas cavities) lined with an assemblage of tridymite (commonly inverted to cristobalite or quartz), sanidine and locally, hematite and/or garnet. Vapour-phase topaz and economic deposits associated commonly with topaz-bearing rhyolites (characteristically enriched in F) were not found in the Topopah Spring Tuff at Yucca Mountain. Based on their occurrence only in lithophysae, the garnets are not primary igneous phenocrysts, but rather crystals that grew from a F-poor magmaderived vapour trapped during and after emplacement of the tuff. The garnets are euhedral, vitreous, reddish brown, trapezohedral, as large as 2 mm in diameter and fractured. The garnets also contain inclusions of tridymite. Electron microprobe analyses of the garnets reveal that they are almandinespessartine (48.0 and 47.9 mol.%, respectively), have an average composition of (Fe1.46Mn1.45Mg0.03Ca0.10)(Al1.93Ti0.02)Si3.01O12and are comparatively homogeneous in Fe and Mn concentrations from core to rim. Composited garnets from each sample site have δ18O values of 7.2 and 7.4%. The associated quartz (after tridymite) has δ18O values of 17.4 and 17.6%, values indicative of reaction with later, low-temperature water. Unaltered tridymite from higher in the stratigraphic section has a δ18O of 11.1% which, when coupled with the garnet δ18O values in a quartz-garnet fractionation equation, indicates isotopic equilibration (vapour-phase crystallization) at temperatures of ∼600°C. This high-temperature mineralization, formed during cooling of the tuffs, is distinct from the later and commonly recognized low-temperature stage (generally 50−70°C) of calcite, quartz and opal secondary mineralization, formed from downward-percolating meteoric water, that locally coats fracture footwalls and lithophysal floors.

Building zoned ignimbrites by recycling silicic cumulates: insight from the 1,000 km3 Carpenter Ridge Tuff, CO

The ~1,000 km3 Carpenter Ridge Tuff (CRT), erupted at 27.55 Ma during the mid-tertiary ignimbrite flare-up in the western USA, is among the largest known strongly zoned ash-flow tuffs. It consists primarily of densely welded crystal-poor rhyolite with a pronounced, highly evolved chemical signature (high Rb/Sr, low Ba, Zr, Eu), but thickly ponded intracaldera CRT is capped by a more crystal-rich, less silicic facies. In the outflow ignimbrite, this upper zone is defined mainly by densely welded crystal-rich juvenile clasts of trachydacite composition, with higher Fe–Ti oxide temperatures, and is characterized by extremely high Ba (to 7,500 ppm), Zr, Sr, and positive Eu anomalies. Rare mafic clasts (51–53 wt% SiO2) with Ba contents to 4,000–5,000 ppm and positive Eu anomalies are also present. Much of the major and trace-element variations in the CRT juvenile clasts can be reproduced via in situ differentiation by interstitial melt extraction from a crystal-rich, upper-crustal mush zone, with the trachydacite, crystal-rich clasts representing the remobilized crystal cumulate left behind by the melt extraction process. Late recharge events, represented by the rare mafic clasts and high-Al amphiboles in some samples, mixed in with parts of the crystal cumulate and generated additional scatter in the whole-rock data. Recharge was important in thermally remobilizing the silicic crystal cumulate by partially melting the near-solidus phases, as supported by: (1) ubiquitous wormy/sieve textures and reverse zoning patterns in feldspars and biotites, (2) absence of quartz in this very silicic unit stored at depths of >4–5 km, and (3) heterogeneous melt compositions in the trachydacite fiamme and mafic clasts, particularly in Ba, indicating local enrichment of this element due mostly to sanidine and biotite melting. The injection of hot, juvenile magma into the upper-crustal cumulate also imparted the observed thermal gradient to the deposits and the mixing overprint that partly masks the in situ differentiation process. The CRT provides a particularly clear perspective on processes of in situ crystal-liquid separation into a lower crystal-rich zone and an upper eruptible cap, which appears common in incrementally built upper-crustal magma reservoirs of high-flux magmatic provinces.

Paleomagnetic results from the eastern Caliente-Enterprise zone, southwestern Utah: Implications for initiation of a major Miocene transfer zone

The Miocene Caliente-Enterprise zone (CEZ) in southwestern Utah (USA) is a 20–50-km-wide east-northeast–trending left-lateral transfer zone that displaces north-south–trending crustal blocks of the eastern Basin and Range Province to the west. Previous paleomagnetic results from the central and western CEZ show significant counterclockwise vertical axis rotations of strike-slip–bounded fault blocks, and these rotation estimates vary in magnitude both across and along the strike of the zone. Results of recent detailed geologic mapping and new geochronologic data in the area east of previous studies allow us to extend paleomagnetic studies into the easternmost CEZ. New paleomagnetic results include data from 4 regionally extensive latest Oligocene to early Miocene (ca. 24–22 Ma) ignimbrite sheets and from 3 ca. 22–20 Ma Iron Axis laccoliths. These data reveal significant magnitudes and similar spatially variable components of counterclockwise vertical axis rotation. Rotation estimates from the ignimbrites are assessed relative to what we interpret to be a nonrotated to only minimally rotated reference section just north of the Colorado Plateau (Grass Valley) and from several low-extension areas in southeast Nevada. Accepted tilt-corrected paleomagnetic data from sites away from the reference areas are discordant in declination from the expected individual ash-flow tuff directions, with rotation (R) and flattening (F) estimates that range from R = −2° to −84° and F = +15° to −14°. Many of the rotation estimates are large and statistically significant. For example, site P-18 from the Bauers Tuff yields an R = −61.1° ± 5.3° and F = −0.6° ± 5.0°. Relative to the expected Miocene direction, in situ paleomagnetic data from the Iron Axis laccoliths, specifically the Three Peaks laccolith, yield a mean that is discordant in declination, with estimated R = −22.2° and F = −8.8° values. These rotation and flattening estimates, although consistent with the overall data set from volcanic rocks, must be considered of lesser quality, as we are unable to accurately correct these data for possible effects of local tilting. If the rotation estimate is viable, then we suggest that this component of deformation involves much of the upper crust, and we furthermore propose that the boundary of the eastern CEZ extends farther east than previously envisioned, to within a few kilometers of the breakaway with the Colorado Plateau. The transitional zone between the eastern CEZ and Colorado Plateau is therefore abrupt and occurs within a narrow zone near Cedar City, Utah.

The age and composition of the pre-Cenozoic basement of the Jalisco Block: implications for and relation to the Guerrero composite terrane

The Jalisco Block is thought to be part of the Guerrero terrane, but the nature and age of the underlying crystalline basement are largely unknown. We have collected a suite of schists, granitoids, and weakly metamorphosed marine sediments from various parts of the Jalisco Block including Atenguillo and Ameca, Mascota and San Sebastian, Cuale, Puerto Vallarta, Punta Mita, Yelapa, and Tomatlan. The schists range in age from 135 to 161 Ma, with many exhibiting Proterozoic and Phanerozoic zircon ages. The granitoids range in age from 65 to 90 Ma, and are calc-alkaline compositionally—similar to granitoids from the Puerto Vallarta and Los Cabos batholiths. The Jalisco granitoids also experienced similar uplift rates to granitoids from the regions to the north and south of the Jalisco Block. The marine sediments yield a maximum depositional age of 131 Ma, and also contain a significant zircon population with ages extending back to the Archean. Granitoids from this study define two age groups, even after the effects of thermal resetting and different closure temperatures are considered. The 66.8-Ma silicic ash flow tuff near Union de Tula significantly expands the extent of this Cretaceous–Paleocene age ash flow tuff unit within the Jalisco Block, and we propose calling the unit “Carmichael silicic ash flow tuff volcanic succession” in honor of Ian Carmichael. The ages of the basement schists in the Jalisco Block fully overlap with the ages of terranes of continental Mexico, and other parts of the Guerrero terrane in the south, confirming the autochthonous origin of the Jalisco Block rather than exotic arc or allochthonous origin. Geologic data, in combination with geochronologic and oxygen isotopic data, suggest the evolution of SW Mexico with an early 200–1,200-Ma passive margin, followed by steep subduction in a continental arc setting at 160–165 Ma, then shallower subduction by 135 Ma, and finally, emplacement of granitoids at 65–90 Ma.

Magmatism, ash-flow tuffs, and calderas of the ignimbrite flareup in the western Nevada volcanic field, Great Basin, USA

The western Nevada volcanic field is the western third of a belt of calderas through Nevada and western Utah. Twenty-three calderas and their caldera-forming tuffs are reasonably well identified in the western Nevada volcanic field, and the presence of at least another 14 areally extensive, apparently voluminous ash-flow tuffs whose sources are unknown suggests a similar number of undiscovered calderas. Eruption and caldera collapse occurred between at least 34.4 and 23.3 Ma and clustered into five ∼0.5–2.7-Ma-long episodes separated by quiescent periods of ∼1.4 Ma. One eruption and caldera collapse occurred at 19.5 Ma. Intermediate to silicic lavas or shallow intrusions commonly preceded caldera-forming eruptions by 1–6 Ma in any specific area. Caldera-related as well as other magmatism migrated from northeast Nevada to the southwest through time, probably resulting from rollback of the formerly shallow-dipping Farallon slab. Calderas are restricted to the area northeast of what was to become the Walker Lane, although intermediate and effusive magmatism continued to migrate to the southwest across the future Walker Lane. Most ash-flow tuffs in the western Nevada volcanic field are rhyolites, with approximately equal numbers of sparsely porphyritic (≤15% phenocrysts) and abundantly porphyritic (∼20–50% phenocrysts) tuffs. Both sparsely and abundantly porphyritic rhyolites commonly show compositional or petrographic evidence of zoning to trachydacites or dacites. At least four tuffs have volumes greater than 1000 km3, with one possibly as much as ∼3000 km3. However, the volumes of most tuffs are difficult to estimate, because many tuffs primarily filled their source calderas and/or flowed and were deposited in paleovalleys, and thus are irregularly distributed. Channelization and westward flow of most tuffs in paleovalleys allowed them to travel great distances, many as much as ∼250 km (original distance) to what is now the western foothills of the Sierra Nevada, which was not a barrier to westward flow of ash flows at that time. At least three tuffs flowed eastward across a north-south paleodivide through central Nevada. That tuffs could flow significant distances apparently uphill raises questions about the absolute elevation of the region and the elevation, relief, and location of the paleodivide. Calderas are equant to slightly elongate, at least 12 km in diameter, and as much as 35 km in longest dimension. Exceptional exposure of two caldera complexes that resulted from extensional faulting and tilting show that calderas subsided as much as 5 km as large piston-like blocks; caldera walls were vertical to steeply inward dipping to depths ≥4–5 km, and topographic walls formed by slumping of wall rock into the caldera were only slightly outboard (≤1 km) of structural margins. Most calderas show abundant post-collapse magmatism expressed as resurgent intrusions, ring-fracture intrusions, or intracaldera lavas that are closely related temporally (∼0–0.5 Ma younger) to caldera formation. Granitoid intrusions, which were emplaced at paleodepths ranging from <1 to ∼7 km, are compositionally similar to both intracaldera ash-flow tuffs and post-caldera lavas. Therefore in the western Nevada volcanic field, erupted caldera-forming tuffs commonly were the upper parts of large magma chambers that retained considerable volumes of magma after tuff eruption. Several calderas in the western Nevada volcanic field hosted large hydrothermal systems and underwent extensive hydrothermal alteration. Different types of hydrothermal systems (neutral-pH alkali-chloride and acid or low-pH magmatic-hydrothermal) may reflect proximity to (depth of) large resurgent intrusions. With the exception of the giant Round Mountain epithermal gold deposit, few known caldera-related hydrothermal systems are strongly mineralized. Major middle Cenozoic precious and base metal mineral deposits in and along the margins of the western Nevada volcanic field are mostly related to intrusive rocks that preceded caldera-forming eruptions.

Palaeomagnetism of the upper volcanic supergroup, southern part of the Sierra Madre Occidental, Mexico

Ash flow tuffs, or ignimbrites have been recently proposed to be a good material for palaeointensity determination. In this paper, we present a multidisciplinary study, combining geochronology, petrology, rock magnetism and palaeomagnetism, carried out on Oligocene to Early Miocene ignimbrites and related flows from the southern part of the Sierra Madre Occidental. Two new 40Ar/39Ar ages were determined for ignimbrites; 20.4 ± 0.2 Ma (the youngest age obtained so far in this area) and 29.2 ± 0.5 Ma. Density measurements, as a proxy for welding, proved to be extremely useful to estimate the emplacement temperature and the origin of the magnetizations carried by the ignimbrites. After alternating field and thermal demagnetizations, the mean palaeomagnetic pole (Lat = 66.8°N; Long = 180.5°E; Kappa = 142; A95 = 6.3°), calculated for the period 28–31 Ma, is in close agreement with our only Miocene determination. Comparison with the North America Synthetic Apparent Polar Path indicates a net counter-clockwise vertical axis rotation of about 10 ± 4° compared to stable North America, which occurred likely during the last extensional episode in the Late Miocene (ca. 12–9 Ma). Palaeointensity estimates, obtained with the Thellier–Coe method, are mainly questionable and should not be used for global interpretation. Therefore, these ignimbrites are not a viable material for reliable palaeointensity determinations.

Geochemical composition of source obsidians from Kenya

Here we provide a reference resource to archaeologists interested in the sources of obsidian in Kenya, through electron microprobe analyses of 194 obsidian samples from 90 localities. Averaged analyses of each sample and eleven published analyses are categorized into 84 compositional groups of which only about 21 are known to have been used to produce artifacts, possibly because studies of artifactual material in the region are lacking. We also provide trace element analyses determined by XRF and LA-ICP-MS for these same obsidians. In northern Kenya 27 distinct compositions of obsidian have been found, including some of Miocene age, but the source of the most abundant obsidian found in archaeological sites in this part of Kenya remains obscure. The Baringo region contains at least 13 varieties of low-silica obsidian. The Naivasha–Nakuru region contains an abundance of obsidian with 38 compositional types recognized, and is the only region in Kenya apart from the Suregei (northern Kenya) that contains rhyolitic obsidian. Nine compositionally distinct types of obsidian are known from southern Kenya. Although Kenyan obsidians span the compositional range from phonolite to rhyolite, low-silica, nepheline-normative obsidians occur only south of 1°N latitude. One obsidian type, the Lukenya Hill Group, appears to have been derived from a regionally extensive ash flow tuff with a distribution of over 8000 km2. From previous studies it is known that obsidians of lowest (Mundui) and highest iron content were used for tool manufacture, as were some obsidians (e.g., Kisanana) with the highest alkali content, and obsidians with both high (Njorowa) and low (Kisanana) silica content.

Introduction: The 36-18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervolcanoes

During the middle Cenozoic, from 36 to 18 Ma, one of the greatest global expressions of long-lived, explosive silicic volcanism affected a large segment of southwestern North America, including central Nevada and southwestern Utah in the southern Great Basin. The southern Great Basin ignimbrite province, resulting from this flareup, harbors several tens of thousands of cubic kilometers of ash-flow deposits. They were created by more than two hundred explosive eruptions, at least thirty of which were super-eruptions of more than 1000 km3. Forty-two exposed calderas are as much as 60 km in diameter. As in other parts of southwestern North America affected by the ignimbrite flareup, rhyolite ash-flow tuffs are widespread throughout the southern Great Basin ignimbrite province. However, the province differs in two significant respects. First, extrusions of contemporaneous andesitic lavas were minimal. Their volume is only about 10% of the ignimbrite volume. Unlike other contemporaneous volcanic fields in southwestern North America, only a few major composite (strato-) volcanoes predated and developed during the flareup. Second, the central sector and especially the eastern sector of the province experienced super-eruptions of relatively uniform, crystal-rich dacite magmas; resulting deposits of these monotonous intermediates measure on the order of 16,000 km3. Following this 4 m.y. event, very large volumes of unusually hot and dry trachydacitic magmas were erupted. These two types of magmas and their erupted volumes are apparently without parallel in the middle Cenozoic of southwestern North America.

Timing and Rates of Flysch Sedimentation In the Stanley Group, Ouachita Mountains, Oklahoma and Arkansas, U.S.A.: Constraints from U-Pb Zircon Ages of Subaqueous Ash-Flow Tuffs

Abstract Uranium-lead geochronology of zircon from five subaqueous ash-flow tuffs distributed throughout the Stanley Group of the Ouachita Mountains in Oklahoma and Arkansas define eruption ages that range from 328.5 ± 2.7 Ma near the base to 320.7 ± 2.5 Ma near the top. Biostratigraphy indicates that the oldest age of the Stanley Group is likely to be between 345.3 and 338 Ma. From these age constraints and stratigraphic positions of the tuff units, calculated deposition rates are 1–40 m/My in the Lower Tenmile Creek Formation (< 326 to 322.4 ± 2.4 Ma) and increase to 300–1400 m/My for the overlying Moyers and Chickasaw Creek formations (322.4 ± 2.4 to 320.7 ± 2.5 Ma). Ages of zircons representing detrital components in the tuffs indicate that they are derived mainly from Laurentian sources. The flysch units in the Stanley Group were deposited in a remnant ocean basin flanked by the Alleghenian orogen to the east, which supplied west-southwest-flowing submarine fan systems. These fans likely incorporated material cycled through a growing offshore allochthonous prism in front of a northward-encroaching volcanic arc system to the south of the Laurentian margin in the late Mississippian.