Silicic Lavas Research Articles

Two-million-year eruptive history of Laguna del Maule volcanic field

The Laguna del Maule (LdM) volcanic field, which surrounds the 54-km2 lake of that name, covers ∼500 km2 of mountainous glaciated terrain with Quaternary lavas and tuffs that extend 40 km westward from the Argentine frontier and 30 km north-south from the Río Campanario to Laguna Fea. Complementing recent investigations of postglacial volcanism and the ongoing geophysical unrest around the lake, we here review the longer eruptive history that spanned the entire Quaternary.The distributed rear-arc LdM volcanic field is contiguous with the Tatara-San Pedro stratovolcano complex on the volcanic front of the Quaternary Andean arc. The LdM field has had only a few large edifices, but we identified at least 140 separate vents, from which >350 km3 of products have erupted since 1.5 Ma. Eruptive products of 14 (early and middle Pleistocene) stratocones and shields, and of ∼125 monogenetic cones, domes, and lava flows, were mapped on foot, studied petrographically, and chemically analyzed. More than 80 40Ar/39Ar and K-Ar ages have been determined to calibrate the Pleistocene eruptive sequence. An extensive welded ignimbrite erupted at 1.5 Ma and was followed by another at ∼950 ka, producing a 12 x 8 km-wide caldera that underlies the north part of the lake basin and the ruggedly eroded highlands north of it. Outside the caldera, the southern two-thirds of the lake basin is a drainage network cut on Tertiary andesites and dacites.A ring of ∼29 postglacial rhyolite and rhyodacite coulees and domes plus associated pyroclastic deposits that erupted from >30 separate vents (and together cover ∼100 km2) encircles the lake. The large number of postglacial silicic vents around the lake basin, several comagmatic multi-vent compositional arrays, and scarcity of mafic enclaves in the rhyolites are features that suggest growth of a latest Pleistocene to Holocene magma reservoir beneath the LdM Basin. The Barrancas center on the divide southeast of the Basin has an additional 21 lavas from 15 vents and represents a second independent postglacial rhyolitic reservoir. About 21 postglacial mafic and intermediate eruptive units accompany the rhyolites around the margins of the LdM Basin. Ongoing work by Fierstein et al. (this volume) has brought the total number of postglacial vents recognized to more than 73 and has determined ∼70 radiocarbon dates that bracket the abundant tephra deposits, thus providing a 17,000-year-long calibration of the postglacial eruptive sequence.In addition to the many postglacial silicic units, glacially eroded silicic lavas yield ages of 3.7, 2.5, 2.4, 2.0, 1.6, and 1.35 Ma, and 924, 880, 712, 695, 680, 460, 335, 240, 203, 114, 97, 83, and 25 ka, providing evidence of a prolonged history of explosive silicic eruptions from vents scattered throughout the volcanic field. Production of widely distributed rhyolites throughout the long history of the volcanic field demonstrates intensive crustal processing as well as the enduring potential for explosive eruptions.For the Quaternary LdM volcanic field, chemical analyses define an array continuous from 49% to 77.6% SiO2, medium-K toward its mafic end (1.5% K2O @ 55% SiO2) but high-K at its silicic end (4.5% K2O @ 75% SiO2). Quaternary eruptive units include 5 basalts, ∼30 mafic andesites (52–57% SiO2), 33 andesites, 11 dacites (63–68% SiO2), 25 rhyodacites, ∼27 rhyolites (>72% SiO2), and 6 ignimbrites (andesitic to rhyolitic). None of the basalts is primitive, and most of the mafic rocks display petrographic and/or chemical evidence for diverse crustal contributions (Hildreth et al., 2010).

Anatomy of a Lunar Silicic Construct—The Wolf Crater Complex, Mare Nubium and Implications for Early Silicic Magmatism on the Moon

AbstractSilicic lithologies on planetary surfaces indicate magmatic evolutionary processes in their interiors. The Wolf crater complex within Mare Nubium on the Moon is one such silicic construct associated with a high thorium anomaly. This study integrates morphological, compositional, chronological and gravity anomaly analyses of high‐resolution data from various lunar missions to establish this construct as a silicic volcanic caldera. Lobate flows with steeply sloping fronts indicate that the crater rims comprise high‐viscosity silicic lavas, while the structurally controlled inner crater walls suggest caldera collapse triggered by magma depletion. In the crater rims, low Christiansen Feature position values reaffirm the presence of silicic lithologies, consistent with the low gravity anomaly signature beneath the complex, while spectroscopic data reveal low mafic mineral abundances and negligible hydration features. Chronological analyses yield silicic volcanism ages coeval with surrounding mare basalts (3.8–3.6 Ga), while intra‐caldera basalts have 2.36–2.02 Ga ages, indicating prolonged magmatism in this region. Melting of suitable crustal protoliths like alkali gabbronorite/monzogabbro/troctolite by basaltic underplating is inferred to have generated silicic magmas that formed the Wolf volcanic complex, instead of basaltic magma fractionation or silicate‐liquid immiscibility processes. Large impacts during the Late Heavy Bombardment may have enhanced partial melting of the mantle and created crustal fractures that facilitated the ascent of viscous silicic melts through the lunar crust. Contemporaneous existence of suitable protoliths and adequate crustal pathways for magma ascent may have controlled silicic volcanism on the Moon, and can explain the sporadic occurrence and overlapping ages of the lunar silicic constructs.

Evidence for the formation of silicic lava by pyroclast sintering

Silicic lavas can be produced by the sintering of pyroclasts in the volcanic sub-surface, and then advected out of the vent. Here, we provide evidence for this mechanism preserved in the exposed post-glacial remnants of a silicic volcanic conduit at Hrafntinnuhryggur, Krafla volcano, Iceland. We show that the conduit margins are a clast-supported pumice lapilli tuff deposit that grades continuously into dense obsidian and that the obsidian contains cuspate relict clast boundaries and country rock lithic fragments throughout. Transects of H2O concentrations across the conduit show that the magma was degassed to different degrees laterally with systematic spatial variation that is consistent with progressive conduit clogging and final gas pressurisation. Textures in the overlying effusive lavas record the variably sheared and brecciated remnant of the same in-conduit sintering. This record of a silicic conduit system connected to upper eruptive deposits provides support for the ‘cryptic fragmentation model’ for effusive silicic volcanism.

Obsidian clasts as sintered remnants of agglutination processes in volcanic conduits, evidence from the Pepom tephras (Sete Cidades), São Miguel, Azores

The youngest explosive eruptions of the Sete Cidades volcano, São Miguel, Azores, are recorded by a series of relatively small-volume (<0.03 to 0.13 km3 DRE) trachytic pyroclastic deposits termed the Pepom tephra deposits. While dominated by crystal-poor to crystal-moderate (<10%) pumice clasts, these deposits also contain a suite of dense glassy clasts of broadly similar crystallinity. The obsidian clasts from a single deposit vary in texture from entirely dense to those that are moderately vesicular and typically single clasts will contain multiple textural domains. The majority (∼71%) of these dense clasts have compositions both from bulk rock and in-situ glass measurements that are identical to those of the pumice clasts within the same deposit. We interpret these dense clasts to reflect sintering of previously fragmented magma at shallow levels in the conduit prior to being re-entrained and erupted with the vesicular magma, in agreement with recent studies focussing on textural observations. Notably, across the exposed volcanic stratigraphy of São Miguel obsidian domes, flows/coulees are not preserved, arguing against the idea that the dense glass clasts within the Pepom tephras are sourced from existing surficial rocks. In contrast, the neighbouring island of Terceira exhibits domes and coulees with large obsidian bands that cut through the crystalline groundmass. Most silicic rocks of Santa Bárbara and Pico Alto volcanoes on Terceira are peralkaline, comenditic to pantelleritic in composition, and at similar conditions (e.g., temperature and water content) have lower viscosities than the trachytic Pepom obsidian clasts. However, the Santa Bárbara silicic lavas on Terceira (the less peralkaline suite) have more obsidian than the more peralkaline Pico Alto domes and coulees indicating that while peralkalinity, developed during magmatic evolution in the crust, may play a role, sintering occurring at shallow levels within the conduit likely is more important in producing obsidian.

Age of the magma chamber and its physicochemical state under Elbrus Greater Caucasus, Russia using zircon petrochronology and modeling insights

Mount Elbrus, Europe's tallest and largely glaciated volcano, is made of silicic lavas and is known for Holocene eruptions, but the size and state of its magma chamber remain poorly constrained. We report high spatial resolution U–Th–Pb zircon ages, co-registered with oxygen and hafnium isotopic values, span ~ 0.6 Ma in each lava, documenting magmatic initiation that forms the current edifice. The best-fit thermochemical modeling constrains magmatic fluxes at 1.2 km3/1000 year by hot (900 °C), initially zircon-undersaturated dacite into a vertically extensive magma body since ~ 0.6 Ma, whereas a volcanic episode with eruptible magma only extends over the past 0.2 Ma, matching the age of oldest lavas. Simulations explain the total magma volume of ~ 180 km3, temporally oscillating δ18O and εHf values, and a wide range of zircon age distributions in each sample. These data provide insights into the current state (~ 200 km3 of melt in a vertically extensive system) and the potential for future activity of Elbrus calling for much-needed seismic imaging. Similar zircon records worldwide require continuous intrusive activity by magmatic accretion of silicic magmas generated at depths, and that zircon ages do not reflect eruption ages but predate them by ~ 103 to 105 years reflecting protracted dissolution–crystallization histories.

Potassium isotope fractionation during silicate-carbonatite melt immiscibility and phlogopite fractional crystallization

Abstract Potassium (K) isotopes have been used as a tracer of K recycling in the Earth, but K isotope fractionation during magma evolution is poorly constrained. Here, we present K isotope data for a magmatic suite of alkaline silicate-carbonatite affinity. The suite was formed from liquid-liquid immiscibility and subsequent phlogopite fractionation. The K isotopic signatures of different rock types are in the following order: alkaline silicate lavas (δ41K = –0.424 to 0.090‰) &amp;gt; carbonatitic silicate lavas (δ41K = –0.640 to –0.035‰) &amp;gt; carbonatites (δ41K = –0.858 and –0.258‰). Phlogopite phenocrysts in the silicate lavas are isotopically lighter (δ41K = –0.628 to –0.534‰) than the lavas in which they occur (Δ41KPhlogopite-whole rock = –0.502 to –0.109‰). Correlations between δ41K values and chemical proxies of melt immiscibility and phlogopite fractionation indicate that K isotopes are significantly fractionated by both processes at a ~0.6‰ magnitude. Such K isotope variation overlaps the range of δ41K in arc lavas. Compilations of literature data further confirm the critical roles of melt immiscibility and phlogopite fractionation in K isotope variations of high-K lavas (K2O &amp;gt;1 wt%) from post-collision orogenic and intra-continental settings. In comparison, basaltic arc lavas are depleted in K2O (mostly &amp;lt;1 wt%) and lack evidence of significant phlogopite fractionation. The K isotope variations of arc lavas are mainly controlled by their mantle sources, which were metasomatized by melt or fluid released from the subducting slab. Therefore, K recycling and K isotope variation are controlled by distinct mechanisms in different tectonic settings.

Emplacement dynamics of the plumbing system and lava pile of the Paraná Magmatic Province in Morro da Igreja, Santa Catarina, Brazil

The mechanisms of magma ascent, transport and emplacement of the volcanic pile from LIPs are key issues regarding the understanding of the complex construction of volcanic systems and magma transport through the crust. The integrated approach of morphology of the volcanic and subvolcanic bodies, whole rock geochemistry and ASM data provides a robust tool to unravel the flow dynamics of the volcanic bodies and the sequence of the magmatic events. Such approach allows building up a model of construction of the lava pile considering the role played by the plumbing system. This paper investigates the emplacement mechanism of sill and lava flows of the Serra Geral Group (SGG), in the southern part of Mesozoic Paraná-Etendeka Magmatic Province (PEMP) in Brazil. Geologic and geochemical analysis were integrated with anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and rock magnetism experiments on 14 sampling sites in the Morro da Igreja region, top of the lava pile in Santa Catarina. The area is composed of low-Ti basaltic (with predominance of rubbly pahoehoe) and silicic lava flows, both intruded by a tabular sill that reaches a few hundred meters wide. The mafic lava flows are composed of Gramado, Urubici and Esmeralda magma-type basalts and andesite basalts. The silicic lava flows are massive or foliated, classified as Palmas type dacites. The sill has regular columnar joints and is classified as Esmeralda type. Magnetic mineralogy data suggests magnetite or Ti-poor magnetite as the main magnetic mineral for all the igneous rocks, and AARM results show anomalous fabric for the lava flows and normal or intermediate fabric for the sill. Besides the anomalous fabric sites, AMS data provide reliable directional data to infer flow direction for the sill, with initial propagation towards NE, followed by a preferential SE direction of magma flow. For the silicic rocks, the presence of vertical and inclined magnetic foliation suggest a lava dome geometry. The dynamics of the magmatic flow of the sill and lava flows associated with the compositional characteristics of the rocks allows stablishing the stratigraphy of the magmatic events in the area as well as the proposition of an emplacement model for the sequence.

Silicic lavas with no basal breccia: origin of the thinly jointed basal facies of low-Ti dacites in the Paraná-Etendeka Large Igneous Province

Silicic lavas with no basal breccia: origin of the thinly jointed basal facies of low-Ti dacites in the Paraná-Etendeka Large Igneous Province

Making sense of brittle deformation in rhyolitic lavas: Insights from Obsidian Dome, California, USA

AbstractThe scarcity of observed active extrusive rhyolitic lava flows has skewed research to extensively focus on prehistoric lavas for information about their eruptive and emplacement dynamics. The first ever witnessed silicic lava eruptive events, Chaitén (2008) and Cordón Caulle (2011–2012) in Chile, were illuminating to the volcanology community because they featured a range of emplacement processes (endogenous versus exogenous), movement limiting modes, and eruptive behaviors (explosive versus effusive) that were often regarded as acting independently throughout an eruptive event. In this study, we documented evidence of a continuum of brittle and brittle-ductile deformation and fracture-induced outgassing during the emplacement of the ~600-yr-old silicic lava from Obsidian Dome, California, USA. This study focused on mapping the textural-structural relationships of the upper surface of the lava onto high-resolution (&amp;lt;10 cm2/pixel) orthorectified color base maps. We found that the upper surface is characterized by small (&amp;lt;1 m) mode 1 tensile fractures that grew and initiated new cracks, which linked together to form larger tensile fractures (1–5 m), which in turn penetrated deeper into the lava. We recorded ornamentations on these fracture surfaces that allow snapshot views into the rheological and outgassing conditions during the lava’s effusion. The largest fractures developed during single, large fracture events in the final stages of the lava’s emplacement. Ornamentations preserved on the fractured surfaces record degassing and explosive fragmentation away from the vent throughout the lava’s emplacement, suggesting explosive activity was occurring during the effusive emplacement. Field-based cataloguing of the complexities of fracture surfaces provides qualitative constraints for the future mechanical modeling of effusive lavas.

Andesites and evolution of the continental crust: Perspectives from the Central Volcanic Zone of the Andes

Named for the Andes, andesites (53%–63% SiO2) are the archetypal magma erupted at magmatic arcs. They have been established as the average composition of continental crust and as such are integral to the growth and evolution of the continental crust. However, andesites are quite variable in trace element and isotopic composition reflecting disparate paths of origin. Herein we return to the original site of their identification, the Central Andes, and use a comprehensive dataset of published and unpublished trace elements and isotopes to show that during the past 6 Myr two distinct types of andesite have erupted in the Central Volcanic Zone (CVZ), which correspond with different geodynamic conditions. Consistent with previous work, we confirm that major composite cones and minor centers of the steady state (low magmatic flux) Quaternary CVZ arc have trace element and isotopic characteristics consistent with magma generation/fractionation in the lower crust. Within the Quaternary arc centers, there are also significant latitudinal variations that correspond with the age, composition, and P-T conditions of the lower crust. However, in contrast to this prevailing model, in the 21–24°S segment 6–1 Ma andesites from ignimbrites and lava domes associated with the peak of the regional Neogene ignimbrite flare-up have compositions that indicate these andesites are hybrids between mantle-derived basalts and upper crustal lithologies. Since ∼1 Ma, andesites in young silicic lava domes associated with the regional flare-up are compositionally indistinguishable from proximal Quaternary arc centers, indicating a return to steady-state magmatism and lower crustal production of andesites. We propose that the transition from upper crustal to lower crustal andesite production results from a decrease in mantle heat input and subsequent relaxation of the regional geotherm during the waning of the flare-up event. The two modes of andesite production have significant implications for the production and evolution of the CVZ arc crust. During the flare-up, prodigious amounts of basalt were emplaced into the mid-crust, resulting in the production of large volumes of hybrid intermediate magmas in the mid and upper crust. In contrast, the lower crustal differentiation recorded in the Quaternary steady state arc andesites would result in the formation of a dense crystalline residue in the lower crust and an overall densification of the lower crust. Over time, gravity instabilities associated with this densification may ultimately aid in the delamination of the dense lower crustal root, triggering flare-ups. These differences in andesite production may help explain the cyclicity (flare-up cycles) observed in mature continental arcs and emphasizes that andesite is not a monotonous composition and can vary with depth-dependent intra-crustal differentiation related to magmatic flux.

Emplacement dynamics of a crystal-rich, highly viscous trachytic flow of the Sancy stratovolcano, France

Emplacement dynamics of highly viscous, silicic lava flows remain poorly constrained due to a lack of consideration of crystal-rich cases. Emplacement models mostly apply to glassy or microlitic, vesiculated rhyolitic flows. However, crystalline, vesicle-free silicic lava can flow differently. We studied the Grande Cascade unit, which is a vesicle-free, phenocryst-rich, trachytic flow in the Monts Dore massif, France. Field work was carried out to define internal structures, and oriented samples were collected for chemical, petrological, and anisotropy of magnetic susceptibility analyses, allowing us to estimate emplacement temperature and viscosity. These data allow us to define a new silicic lava flow subtype that is low in temperature (800−900 °C), high in silica content (up to 66.8 wt%), high in viscosity (109−1011 Pa s), rich in phenocrysts (∼35%), and lacks vesicles. Brittle deformation of the lava occurs upon extrusion, generating a cataclasite basal layer and thin (3-m-thick) shear zone that accommodates all of the stress, allowing most of the flow’s volume to slide over its base as a 40-m-thick plug in which there is no deformation. Blocks are rare, of a single size (10 ± 1 cm), and result from localized break-up of the basal shear zone. Emplacement dynamics are different from those of glassy, pumiceous lava flows. They are closer to glacier dynamics, where most of the volume slides over a thin basal shear zone and till is generated there by abrasion and milling of the underlying layer. For the Grande Cascade lava flow, abrasion means that the flow lacks its classical blocky crust and instead the flow base is marked by a layer rich in fine-grained material. The structures and emplacement dynamics of this crystal-rich flow are consistent with ideal, gravity-driven shear flow. We thus argue for a global reassessment of silicic-rich lava emplacement based on crystal content and using a multidisciplinary approach focused on well-exposed examples in the rock record.

Io’s Volcanic Activity and Atmosphere

Driven by tidal heating, Io’s extreme volcanism has created a young, impact crater–free surface dominated by hundreds of active volcanic centres. From these volcanoes erupt voluminous, low-viscosity, high-temperature silicate lavas. Volcanic plumes, from venting gas and mobilised surface ices (primarily SO2 and S), contribute to Io’s thin atmosphere. Away from volcanoes, SO2 ice on the surface alternately sublimes during the daytime and condenses during eclipses and at night, resulting in a strong day/night atmospheric dichotomy. Sunlight and radiation bombardment at high altitude breaks the gas molecules apart, leading to the formation of SO, O, O2, S, K, Na, and Cl. These atoms reside as both neutral and charged particles in clouds that are found along Io’s orbit around Jupiter.

A reappraisal of explosive–effusive silicic eruption dynamics: syn-eruptive assembly of lava from the products of cryptic fragmentation

Silicic volcanic eruptions range in style from gently effusive to highly explosive, and may switch style unpredictably during a single eruption. Direct observations of subaerial rhyolitic eruptions (Chaiten 2008, Cordón Caulle 2011–2012, Chile) challenged long-standing paradigms of explosive and effusive eruptive styles and led to the formulation of new models of hybrid activity. However, the processes that govern such hybrid explosive–effusive activity remain poorly understood. Here, we bring together observations of the well-studied 2011–2012 Cordón Caulle eruption with new textural and petrologic data on erupted products, and video and still imagery of the eruption. We infer that all of the activity – explosive, effusive, and hybrid – was fed by explosive fragmentation at depth, and that effusive behaviour arose from sticking and sintering, in the shallow vent region, of the clastic products of deeper, cryptic fragmentation. We use a scaling approach to determine that there is sufficient time available, during emplacement, for diffusive pyroclast degassing and sintering to produce a degassed plug that occludes the shallow conduit, feeding clastogenic, apparently effusive, lava-like deposits. Based on evidence from Cordón Caulle, and from other similar eruptions, we further argue that hybrid explosive–effusive activity is driven by episodic gas-fracking of the occluding lava plug, fed by the underlying pressurized ash- and pyroclast-laden region. The presence of a pressurized pocket of ash-laden gas within the conduit provides a mechanism for generation of harmonic tremor, and for syn-eruptive laccolith intrusion, both of which were features of the Cordón Caulle eruption. We conclude that the cryptic fragmentation models is more consistent with available evidence than the prevailing model for effusion of silicic lava that assume coherent non-fragmental rise of magma from depth to the surface without wholesale explosive fragmentation.

Projevy interakcí vulkanické aktivity Velkého Roudného s vodním prostředím

Velký Roudný scoria cone represents the most prominent volcanic edifice of the Plio-Pleistocene Bruntál Volcanic Field (according to Cajz et al. 2012). Various types of volcaniclastic deposits and volcanic features were interpreted in various ways in the past. However, several features remained overlooked, and the investigated ones required critical revision. Velký Roudný is an asymmetric horseshoe-shaped cone, with a crater open towards the southeast (Fig. 1a). The lava flows emitted from this cone flew southwards and also to the east/ southeast, where the lava filled a paleo-canyon of the Moravice River. In the paleo-canyon, the lava (up to 60 m thick) buried earlier fluvial sediments (Horský et al. 1972; Fig. 2). The best exposure of the lava flow is accessible in the active Bílčice quarry. The two levels of columnar jointing separated by a platy- to blocky-jointed domain represent the upper and lower colonnade of a single lava flow (Fig. 3a, b). The columnar jointing domains are interrupted by subvertically platy-jointed, piston-like features filled with breccia (Fig. 3c, d). These monomictic breccias are dominated by basaltic clasts with signs of abrupt quenching, like cauliflower structures (Fig. 3e, f ). The shapes of these breccia bodies, together with signs of magma/water interactions, suggest their origin from rootless explosions. Their extension through the complete thickness of the lava, as well as their distribution in the lava flow were earlier documented by geophysical survey (Štainbruch 2019), whose results are re-interpreted in this contribution (Fig. 4). Another effect of the lava/water interaction is represented by “sunburn” decomposition (spheroidal decomposition of the rock; Fig. 5a). The development of the spheroidal cracks seems to be restricted to the glass-containing groundmass (Fig. 5b). As the sunburn decomposition develops mainly at the base, around the rootless craters, and less frequently on the top of the lava. It seems that these irregular features reflect gentle hydration of glass-containing rock, similarly to perlitic cracks in silicic lavas (e. g., McPhie et al. 1993). Subsequently to cracking, the glass tends to analcime recrystallization (Fig. 5c, d), enhancing the decomposition of the rock. Pyroclastic deposits exposed in two small abandoned quarries near Razová and Karlovec, respectively, were interpreted as tuffites (mixed volcano-sedimentary deposits) in the past. Despite that these deposits contain abundant fragments of basement greywackes, the fragments display no signs of rounding or sorting (Fig. 6). Poorlysorted clast-supported deposits well correspond to the near-vent ballistic accumulation of phreatomagmatic eruption. At Karlovec, predominance of the finer-grained facies with diagonal bedding suggests that these rocks originated by base-surge deposition. The potential source of the phreatomagmatic eruption can be found in the circular depression underneath Velký Roudný (Fig. 1b, c), as supported by a magnetic anomaly (Fig. 7). The phreatomagmatic character of the volcanic activity possibly results from an eruption through a lake dammed by the Bílčice lava of the Velký Roudný scoria cone (Fig. 8).

Giant silicic ignimbrites and comagmatic domes at the southeastern end of the Sierra Madre Occidental, México

The Sierra Madre Occidental (SMO) ignimbrite province includes giant ignimbrites related to large silicic calderas and dome complexes. Previous work in the SMO has paid little attention to the ignimbrites and domes exposed in the southeasternmost portion of the volcanic province, in the Guanajuato and Querétaro States. This work focuses on two widespread Oligocene ignimbrite sheets and numerous lava domes of this part of the SMO, and provides data on their distribution, stratigraphy, geochronology, textures, and geochemistry. Two large ignimbrite sheets and several domes were identified, which according to the stratigraphy they were classified in pre- and post-ignimbrites. We have named the ignimbrites as Victoria I and Victoria II. Victoria I is purple to light grey, with strong welding, horizontal foliation, and well-developed columnar jointing. It contains quartz, plagioclase and sanidine; fiamme is a common feature. Victoria II ignimbrite conformably overlies Victoria I. It is dark pink to brown, densely welded but less welded than Victoria I; in most locations it has developed columnar jointing. Victoria II's mineral assemblage includes biotite and hornblende in addition to quartz, plagioclase and sanidine; it has a higher content of lithics fragments and fiamme than Victoria I. A black vitrophyre forms base of each ignimbrite and marks the contact between them. These ignimbrites are stratigraphic markers due to their wide distribution and distinctive field and textural characteristics. We have obtained zircon U–Pb ages of 31.65 ± 0.17 to 30.57 ± 0.14 Ma (1σ) for Victoria I ignimbrite and 31.31 ± 0.21 to 29.73 ± 0.18 Ma for Victoria II ignimbrite. Silicic lava domes identified in the study area are either pre- or post-ignimbrite; the former are dacitic, and the latter are rhyolitic or rhyodacitic. We focus on the post-ignimbrite domes with the hypothesis that they were comagmatic with the Victoria ignimbrites. The post-ignimbrite domes are devitrified rocks with phenocrysts of plagioclase, quartz, sanidine, biotite and hornblende; and zircon U–Pb ages yielded 30.81 ± 0.21 Ma to 28.02 ± 0.19 Ma.

Noble gas variation during partial crustal melting and magma ascent processes

Noble gas isotopes, although present in trace amounts, are generally more reliable and less ambiguous recorders of their source than the major volatile species. In volcanic settings in particular, this advantage derives from their chemical inertness, as noble gas isotopic and elemental fractionations are strongly coupled to their source and modified only by physical processes during magma ascent and eruption. The Neogene volcano El Hoyazo (Betic Cordillera, SE Spain) is a highly favourable natural laboratory to study the links between partial crustal melting processes occurring at depth and the eruptive products at the surface, because partially melted crustal xenoliths are preserved in silicic lavas. Comparing the noble gas isotopic compositions of xenoliths and lavas has the potential to yield new insights into volatile behaviour during melting processes at inaccessible depths in the crust.At El Hoyazo, noble gases trapped in lava glasses, and the fluid/melt inclusions within xeno- and phenocrysts, provide novel information on: (i) their response to the crustal melting process including mechanisms such as magma mixing (and crustal assimilation) of two endmembers: i.e. the extracted felsic melt from the country metapelitic crust, and the basic-intermediate magma from the underplating in the region. The results reveal significant modification of magmatic noble gases by the interaction with the partially melted crust; (ii) noble gas variations during degassing and magma ascent, showing higher atmospheric influence in the lava samples from shallower depths than in the deeper lavas and minerals; and (iii) higher magmatic influence in crystals of garnet from deeper lava than in both shallower crystals of amphibole, and garnet crystals within the crustal xenoliths. In addition, we find that noble gases in melt inclusions are also likely accumulating in their shrinkage bubbles, and not only remaining dissolved in the melt.

Geochemical evolution of the Paradise Mountain Caldera complex, Davis Mountains: Implications for the tectonic and magmatic evolution of Trans-Pecos Texas and adjacent Mexico

The Paradise Mountain Caldera (~20 × 14 km), located west of Fort Davis, Texas, produced two major ignimbrite/lava complexes at ~36.2 and 35.9 Ma. The older unit (Fort Davis Tuff) is composed of rheomorphic tuff and overlying chemically similar lava. The younger unit (Wild Cherry Tuff) forms thick intracaldera deposits and an outflow sheet that spread over an estimated 2000 km2. Near the caldera, the two ignimbrites are separated by trachytic lava of the Mount Locke Formation. The 5-km diameter Point of Rocks intrusion, which crops out within the eastern margin of the caldera, may have fed the Mount Locke flows. Wild Cherry Tuff is overlain by silicic lava (Casket Mountain Formation, ~35.8 Ma), with which is shares chemical similarities. Intracaldera Wild Cherry Tuff was intruded by silicic intrusions and locally intensively silicified and kaolinized. In a revision of stratigraphic nomenclature, we assign all of the above volcanic units to a Barrel Springs Group.Collectively, the above units form a series ranging from trachyte to alkali rhyolite. Glomeroporphyritic trachyte of Mount Locke lavas represents silicic melt incompletely segregated from more mafic material within a source magma chamber. Variation between Mount Locke Formation trachyte and Fort Davis Tuff rhyolite can be modelled by fractionation of a mineral assemblage of plagioclase, clinopyroxene, FeTi oxides, and apatite; variation within the two major tuff units can be modelled almost exclusively by alkali feldspar. Central eruption within the caldera produced intracaldera tuff more evolved than most of its outflow. Alkali rhyolite of the Wild Cherry Tuff and Casket Mountain lavas, although similar in major element chemistry and mineralogy, show wide variation in incompatible trace elements, suggesting that evolution of silicic magmas in terms of major element chemistry had reached an impasse due to fractionation of a mineral assemblage whose bulk composition was similar to that of parental rhyolite.Paradise Mountain units represent a transition from alkalic magmatism in northeastern Trans-Pecos Texas to metaluminous to peraluminous magmatism in southwestern Trans-Pecos Texas and adjacent Mexico. Over a span of 10 Ma, silicic magmatism terminated southwestward over this region at a rate of about 24 km/Ma. This termination may reflect continued foundering of a sinking Farallon slab and associated asthenospheric mantle upwelling. Mafic magma derived by melting of lithospheric mantle formed plutons that, through filter pressing of silicic magma from crystalline mush, generated silicic melts. The transition across the Trans Pecos region from alkalic to metaluminous magmatism may have resulted through preferential modification of lithospheric mantle along the easternmost extent of Laramide subduction.

Extremely high-grade, lava-like rhyolitic ignimbrites at Osham Hill, Saurashtra, northwestern Deccan Traps: Stratigraphy, structures, textures, and physical volcanology

In many continental flood basalt (CFB) provinces of the world, widespread silicic lavas and ignimbrites overlie the mafic lavas. Our knowledge of this post-flood basalt silicic volcanism in the Deccan Traps CFB province of India remains limited. Here we describe the stratigraphy, structures, and textures of the rhyolites of Osham Hill in Saurashtra, in the northwestern Deccan Traps. Basaltic lavas forming the lower parts of the hill are overlain by the “Lower Rhyolite” (LR), which is tens of meters thick and shows intense rheomorphic deformation (flow banding and folding) at microscopic, outcrop, and tens of meters scales. The LR is overlain by a “Green Tuff and Pitchstone” (GTP) band 20–40 m thick, containing massive green tuff to strongly rheomorphic, commonly spherulitic pitchstone, both types showing sharp but irregular contacts. The GTP is overlain by the “Upper Rhyolite” (UR), with an ~100 m preserved thickness. This resembles the LR in many structural features, and also contains a thin (~1 m), laterally discontinuous basal layer of unbedded, nonwelded lapilli tuff. The LR and UR locally contain basal autobreccias which are unlike crumble breccias of rhyolitic lava flows. In thin section the LR and UR are microcrystalline and lava-like, though they contain many relict vitroclasts (glass shards and pumice fragments). The LR also shows rare millimeter-size basaltic lithic clasts. The GTP contains abundant vitroclasts. We interpret the LR and UR as extremely high-grade, intensely welded, lava-like ignimbrites, and the GTP as a low-grade ignimbrite with varying degrees of welding and rheomorphic deformation. The absence of sedimentary interbeds and soil horizons suggests that the whole Osham ignimbrite sequence was deposited subaerially and rapidly. The eruptive vent locations are unknown. However, a rhyolite dyke intruding basalts at the southeastern end of Osham Hill is pyroclastic, showing a eutaxitic to microcrystalline interior and margins of welded and rheomorphic tuff-pitchstone. Basaltic lithic fragments found in the dyke margins suggest that it may be the feeder of the LR. The LR and UR show many close similarities to “Snake River-type” ignimbrites (large-volume, metaluminous, high-temperature, lithic- and crystal-poor, intensely welded, rheomorphic and lava-like ignimbrites). These render Osham Hill a newly recognised and important locality of such ignimbrites.

Characteristics and significance of intravolcanic saprolite paleoweathering and associate paleosurface in a silicic effusive volcano: The case study of Monte Amiata (middle Pleistocene, Tuscany, Italy)

A paleoweathering saprolite mantle and an associated paleosurface have been recognized into the stratigraphic volcanic succession of the silicic effusive (trachydacite) middle Pleistocene Monte Amiata volcano (Italy). This element has been used for applying UBSU (Unconformity Bounded Stratigraphic Units) subdivision to the volcano stratigraphy. All the lavas emplaced in the onset of the volcanic activity (Bágnore Synthem, BAS) have been indistinctly affected by a saprolite in-situ alteration. Saprolite facies varies from poor lithoid consistency but with conservation of the original texture, to fractured rock with spheroidal weathering, to incoherent rock consisting of a prevalent sandy fraction, up to accumulations of residual lava blocks (core-stones, mainly in coincidence with the arcuate ridges on top of the main lava flows). Unaltered lavas of the younger phase of volcanic activity (Monte Amiata Synthem, MAS) have preserved the paleosurface within its own saprolite. Where the MAS cover is absent, the saprolite mantle represents a relict paleosurface on the present landscape. Weathering of the silicic lavas was favored by the vitrophyric texture of the parent rock and by moderate topographic reliefs formed by extensive lava plateaus positioned at various elevations. The palaeowathering saprolite is indicative of warm and relatively humid temperate climatic conditions and relative tectonic and morphogenetic stability.

Chlorine Heterogeneity in Volcanic Glass as a Faithful Record of Silicic Magma Degassing

AbstractDegassing processes occurring within silicic magma, such as bubble growth, bubble resorption, the welding of magma fragments, and open‐system gas loss are crucial in the control of volcanic eruption and lava emplacement, yet their details are still debated. To examine the possibility that these degassing processes are recorded in volcanic glass as heterogeneous Cl distribution patterns, we experimentally simulated these processes by heating rhyolitic obsidian and analyzed the distribution of Cl content in the recovered sample. The results showed that, for bubble growth, Cl diffused toward the bubble interface, leading to Cl depletion around the bubble. For bubble resorption, Cl was discharged from the bubble to the melt, leading to Cl enrichment in the ambient melt. For welding of magma fragments, Cl was depleted near the welded interface because each fragment had degassed Cl at the surface before the welding took place. For open‐system gas loss, Cl exsolved at the bubble interface while the bubble itself was being collapsed into a chain of small bubbles and a Cl‐depleted tail. These results indicate that Cl distribution is a reliable record of the experienced degassing processes. We then analyzed the Cl distribution in silicic lava from Naruko volcano, Japan, to study the gas flow mechanism. We observed that the glassy matrix was progressively corroded into porous crystalline material. The interface of the glass was highly enriched in Cl. We conclude that a Cl‐rich gas fluxed through hot lava and corroded the glass, developing a porous, gas‐permeable region.