Rhyolitic Lava Flows Research Articles

Examining rhyolite lava flow dynamics through photo-based 3D reconstructions of the 2011–2012 lava flowfield at Cordón-Caulle, Chile

During the 2011–2012 eruption at Cordón-Caulle, Chile, an extensive rhyolitic flowfield was created (in excess of 0.5km3 in volume), affording a unique opportunity to characterise rhyolitic lava advance. In 2012 and 2013, we acquired approximately 2500 digital photographs of active flowfronts on the north and east of the flowfield. These images were processed into three-dimensional point clouds using structure-from-motion and multi-view stereo (SfM–MVS) freeware, from which digital elevation models were derived. Sequential elevation models—separated by intervals of three hours, six days, and one year—were used to reconstruct spatial distributions of lava velocity and depth, and estimate rheological parameters. Three-dimensional reconstructions of flowfronts indicate that lateral extension of the rubbly, 'a'ā-like flowfield was accompanied by vertical inflation, which differed both spatially and temporally as a function of the underlying topography and localised supply of lava beneath the cooled upper carapace. Compressive processes also drove the formation of extensive surface ridges across the flowfield. Continued evolution of the flowfield resulted in the development of a compound flowfield morphology fed by iterative emplacement of breakout lobes. The thermal evolution of flow units was modelled using a one-dimensional finite difference method, which indicated prolonged residence of magma above its glass transition across the flowfield. We compare the estimated apparent viscosity (1.21–4.03×1010Pas) of a breakout lobe, based on its advance rate over a known slope, with plausible lava viscosities from published non-Arrhenian temperature–viscosity models and accounting for crystallinity (~50vol.%). There is an excellent correspondence between viscosity estimates when the lava temperature is taken to be magmatic, despite the breakout being located >3km from the vent, and advancing approximately nine months after vent effusion ceased. This indicates the remarkably effective insulation of the lava flow interior, providing scope for significant evolution of rhyolitic flow fields long after effusive activity has ceased.

Petrogenesis of the Early Permian volcanic rocks in the Chinese South Tianshan: Implications for crustal growth in the Central Asian Orogenic Belt

The Paleozoic and Early Mesozoic magmatic suites in the Central Asian Orogenic Belt (CAOB) provide important insights on the crustal growth and reworking process associated with the construction of the largest Phanerozoic orogen on the Earth. Among the tectonic blocks of the CAOB, the South Tianshan Terrane (STT) occupies the southwestern margin and is located adjacent to the Tarim Craton. Here we investigate the Early Permian Xiaotikanlike Formation in the central part of the Chinese STT in Xinjiang in Northwest China. The formation is composed of a series of terrestrial volcanic lava flows and volcanic breccia, interbedded with siltstones, sandstones and sandy conglomerates. Zircon U–Pb and Lu–Hf isotopic analysis, whole-rock major oxide, trace element and Sr–Nd isotopic data are presented for the volcanic lava flows of the Xiaotikanlike Formation exposed in the Boziguo'er, Laohutai and Wensu regions. The new zircon ages from our study, together with those reported in previous investigations on the rhyolitic lava flow from the Wensu region, suggest that the volcanic rocks of the Xiaotikanlike Formation simultaneously erupted at ca. 285Ma. The lavas of the formation show a wide range of SiO2 (49.88 to 78.56wt.%). The basaltic rocks show SiO2 from 49.88 to 53.78wt.%, MgO from 3.73 to 7.01wt.% and Mg# from 41 to 61. They possess slightly enriched Sr–Nd isotope signature [(87Sr/86Sr)t=0.70495–0.70624 and εNd(t)=−0.5 to +0.6], and have trace and rare earth element patterns similar to those of oceanic island basalts (OIBs). Petrographic and whole-rock chemical characteristics indicate that the basaltic lava flows are dominantly tholeiitic, and were likely derived from a spinel-dominated peridotite asthenospheric mantle source. The felsic lavas of the Xiaotikanlike Formation show SiO2 in the range of 60.71 to 78.56wt.% and display overall similar immobile element pattern characterized by notable troughs at Nb–Ta, P and Ti and gently sloping REEs. Zircon Lu–Hf analysis yields εHf(t) values of −8.7 to −0.3 for the felsic lavas from the Boziguo'er region. Geochemical and isotopic data suggest that the felsic lava flows were likely derived from an ancient crustal source(s). Our study suggests that the Xiaotikanlike volcanic lava flows erupted after the collision between the Tarim Craton and the Kazakhstan–Yili–Central Tianshan Terrane, and that the South Tianshan Terrane was not affected by Permian Tarim mantle plume activities. Furthermore, our data also suggest that the lithosphere under the eastern part of the STT at ~285Ma had been considerably thinned, whereas the lithosphere under the western part of STT was still thick enough to allow the presence of a garnet-bearing source in the lower lithospheric mantle. Such distinction in the lithospheric thickness may be attributed to the oblique collisional tectonics. The widespread Late Paleozoic igneous rocks from the Western Tianshan were genetically associated with regional-scale post-collisional extension. These rocks provide robust evidence for net vertical continental growth at the final stage of evolution of the CAOB.

Mecanismos eruptivos y procesos depositacionales del Grupo Choiyoi en el área de Las Caletas, Cordillera Frontal de San Juan, Argentina

The Permo-Triassic Choiyoi rhyolite province of Chile and Argentina is by its extension, volume and geological significance, the record of one of the most important magmatic events of the southwestern margin of Gondwana. However, aspects dealing with the physical volcanology and tectonic setting are poorly understood. In order to characterize the eruptive activity and style of deposition, a stratigraphic and litofacial study was carried out in the Horcajo Formation, a thick silicic volcanic succession representative of the upper section of the Choiyoi Group cropping out in the Cordillera Frontal of San Juan (western Argentina). A volcanic cycle defined by four eruptive episodes is recognized in the lower and middle sections of the unit: 1. pre-collapse effusive volcanism; 2. phreatomagmatic explosive volcanism; 3. explosive volcanism and volcanotectonic collapse; and 4. post-collapse effusive volcanism and syn-eruptive sedimentation. The emplacement of the eruptive units of the Horcajo Formation was controlled by volcano-tectonic subsidence processes associated with large magnitude ignimbrite forming eruptions. Although this eruptive mechanism would have been dominant, it alternate with important effusive phases which lead to the emplacement of voluminous rhyolite lava flows. Minor phreatomagmatic activity was responsible for the construction of silicic tuff cones or rings also related to the formation of local alluvial and lacustrine environments during brief intervals of syn-eruptive sedimentation. Stratigraphic and structural evidence suggests that the recognized deposits are remnants of an ancient caldera-like eruptive center, although without limits or geometry clearly exposed. The processes involved in the genesis of Horcajo Formation, a widespread unit of Cordillera Frontal, can be considered as some of the most common of the Choiyoi volcanism.

Cenozoic Eruptive Stratigraphy and Structure in Taiz area of Yemen

The present study focuses on the field description of a bimodal volcanic rock centre and fault types present in Taiz area located in the southern part of Yemen. Taiz area serves as one of the key areas to understand the Afro-Arabian bimodal volcanism and the emplacement of Afar plume and the relationship between extension tectonics and magmatism. Taiz area comprises bimodal volcanic rocks encompassing mafic and silicic lava flows and pyroclastic rocks. The lava flows were erupted in five phases – three major basic and two acid phases. Lower basalt sequence (Tb1) is the oldest and main phase of flood basalt volcanism, formed as part of a wide spread volcanism within the Afro-Arabian region. This is followed by lower silicic sequence phase (Tr1) consisting of varied assortment of lithologies such as rhyolite flows with subordinate ignimbrites, welded ash, pyroclastic breccia, and random pumice and obsidian. The middle basalt sequence phase (Tb2) has been formed from the flows fed by fissure-dyke systems and separated from Tr1 by red colored weathering band of saprolitic bole (paleosol). The latest silicic sequence phase (Tr2) with limited exposures in the region, comprises rhyolitic plug domes, rhyolitic lava flows and locally welded and unwelded volcaniclastic deposits. In places, the silicic volcanic rocks have been diagenetically altered to bentonitic clay minerals and zeolites. The youngest phase of volcanic rocks represented by the basaltic rocks in the region (Tb3) is exposed in few outcrops along the southeastern slope of the mapped area and conformably capping the Tr2. It includes the basaltic flows intercalated with mafic conglomerate and tuff layers. The study area has been subjected to tensional tectonic regime throughout much of the Tertiary and extensions led to volcanism, granitic rock intrusions and formation of structural elements such as normal faults and deep joints. NW, NE, and E-W are the three major trends of faults recognized and these are related to the progressive rifting of Red Sea and Gulf of Aden.

Exceptional mobility of an advancing rhyolitic obsidian flow at Cordón Caulle volcano in Chile

The emplacement mechanisms of rhyolitic lava flows are enigmatic and, despite high lava viscosities and low inferred effusion rates, can result in remarkably, laterally extensive (>30 km) flow fields. Here we present the first observations of an active, extensive rhyolitic lava flow field from the 2011-2012 eruption at Cordón Caulle, Chile. We combine high-resolution four-dimensional flow front models, created using automated photo reconstruction techniques, with sequential satellite imagery. Late-stage evolution greatly extended the compound lava flow field, with localized extrusion from stalled, ~35 m-thick flow margins creating >80 breakout lobes. In January 2013, flow front advance continued ~3.6 km from the vent, despite detectable lava supply ceasing 6-8 months earlier. This illustrates how efficient thermal insulation by the lava carapace promotes prolonged within-flow horizontal lava transport, boosting the extent of the flow. The unexpected similarities with compound basaltic lava flow fields point towards a unifying model of lava emplacement.

Volcano-stratigraphy of the extension-related silicic volcanism of the Çubukludağ Graben, western Turkey: an example of generation of pyroclastic density currents

AbstractWestern Turkey's extension-related Cumaovası volcanic rocks (Lower Miocene, 17 Ma) are excellent examples of silicic eruptions. The sub-aerial silicic volcanism at Çubukludağ Graben between İzmir and Kuşadası in west–central Anatolia is mainly in the form of rhyolite domes, lava flows and pyroclastic deposits. The initial features of volcanism derived from phreatomagmatic explosive eruptions from silicic magma that came into contact with lake waters during Neogene times. Most of the volcanic succession represents pyroclastic density currents (PDCs), known as the Kuner ignimbrite. The deposits are fine grained and laminated at the base and pass laterally and vertically into deposits displaying well-developed traction structures, soft sediment deformation and/or erosion channels in the NE part of the region. Alternate deposits of massive, diffusely stratified lapilli and ash are the main products of the later explosive stage. Massive lithic breccias forming the top of the sequences are the proximal facies of the PDCs. The lava phase mainly consists of rhyolite extruded as dome and fissure eruptions of lavas, aligned along NE–SW-trending faults as well as from extensional cracks that are nearly perpendicular to the main graben faults. Considering the tectono-stratigraphical aspects and geochemical nature of the study area, we propose that the Cumaovası silicic volcanism was produced by extension-related crustal melting during the Late–Early Miocene period (17 Ma).

Paleomagnetic investigation of rhyolite lava: Is rhyolite with clearly marked flow structure a high-fidelity geomagnetic field recorder?

Rhyolite is a common volcanic rock; however, few studies have focused on the remanent magnetization of rhyolite lava, and few paleomagnetic studies have successfully investigated rhyolite lavas. We suspect that problems associated with paleomagnetic studies of rhyolite may be due to the nearly ubiquitous flow structure in rhyolite lava. In this study, we examined a thick rhyolite lava flow with clearly marked flow structure to assess its ability to record a consistent paleomagnetic direction, using material penetrated by two drill cores. Progressive thermal demagnetization isolated two magnetization components. A high-temperature component from each of the two cores yields inclinations that differ from each other. The low-temperature component had those that agreed with each other, and were also consistent with the direction expected from a geocentric axial dipole field. The modification of direction of the high-temperature component may be explained by post-magnetization acquisition tilting. The development of flow structure also leads to distortion of directions of the component, which is observed at stratigraphic positions where the volume fraction of light-colored parts of the flow structure >30%. In the case of silicic lava, the low-temperature component may retain directions parallel to the ambient field direction at the time of lava emplacement.

In situ U/Pb dating of impact‐produced zircons from the Vargeão Dome (Southern Brazil)

AbstractThe Vargeão impact structure was formed in the Serra Geral basaltic and rhyodacitic to rhyolitic lava flows of southern Brazil, that belong to the Paraná‐Etendeka large igneous province. The Chapecó‐type rhyodacites contain small baddeleyite crystals recently dated at 134.3 (±0.8) Ma, which is regarded as the age of this acid volcanism coeval to the flood basalt eruption. Inside the impact structure, a brecciated rhyodacitic sample displays fine veinlets containing numerous lithic fragments in a former melt. This impact breccia contains newly formed zircons, either in the veins or at the contact between a vein and the volcanic matrix. The zircons are 10–50 μm in length, clear and nearly unzoned. In situ laser‐ablation dating of the zircons provides a concordant Early Aptian age of 123.0 ± 1.4 Ma that is regarded as the age of the impact event. As in situ age determination ensures the best possible selection of the analyzed mineral grains, the methodology employed in this study also represents a promising method for dating other impact structures.

Microtexture development during rapid cooling in three rhyolitic lava flows

The effects of water concentration and degassing history on the development of spherulites and flow banding were examined in three middle Tertiary rhyolitic lava flows from the Atascosa Mountains of southern Arizona. The Hell’s Gate lava flow and the Atascosa lava flow host spherulites of strongly contrasting texture, and neither are flow banded. The Sycamore Canyon lava flow is a flow-banded rhyolite that hosts two populations of spherulites. Spherulites in the Hell’s Gate lava flow consist of two to four generations of bladed radiating alkali feldspar crystals that increase in water concentration along their length. Needle-like radiating feldspar crystals in spherulites in the Atascosa and Sycamore Canyon lava flows are in some cases punctuated by concentric rinds of glass that are reservoirs for water rejected by the feldspar crystals. The differences in spherulite crystal morphology between the Sycamore Canyon and Atasocosa flows (both needle-like) and the Hell’s Gate flow (bladed) may reflect a more rapid cooling rate of the Sycamore Canyon and Atascosa flows. Thick gray flow bands in the Sycamore Canyon lava flow host higher water concentrations than thin orange flow bands, suggesting that flow bands are zones of greater and lesser volatile concentration, deformed by stretching of the flowing magma. Temperature was uniform across the light and dark flow bands of the Sycamore Canyon flow, indicating that water concentration, one of the variables that controls diffusion coefficients, rather than temperature, controlled spherulite size in this case. Phenocrysts in the Hell’s Gate lava flow are strongly resorbed, probably as a result of magma ascent along a near-adiabatic gradient that resulted in exsolution of water from the melt, and subsequent dissolution of existing quartz phenocrysts by the water-rich melt. Resumption of crystallization of anhydrous phases such as quartz and feldspar would have further enriched the melt in water, facilitating the growth of spherulites. Spherulites in two of the lava flows (Sycamore Canyon and Hell’s Gate) increase in water concentration from core to rim, as would be expected in spherulites growing in melt enriched in water rejected by the growing feldspar crystals. Spherulites in the Atascosa rhyolite flow decrease slightly in water concentration from core to rim, possibly because the magma degassed during spherulite growth. Calculation of water concentration profiles in spherulites from all three rhyolite flows on the basis of Rayleigh fractionation of water between sanidine and rhyolitic melt shows that the very high water concentrations in spherulites (typically >0.6 × water concentration in surrounding glass) cannot be accounted for by Rayleigh fractionation. Instead it is likely that sanidine incorporated water as fluid inclusions and/or as water clusters during rapid crystal growth. Water concentration profiles in the glass surrounding spherulites do not preserve the high concentration zone at the spherulite boundary that has been observed in younger lava flows, so that spherulite growth rates cannot be calculated on the basis of mass-balance calculations of distribution of water during spherulite growth. Rather, the water concentration profile in the surrounding glass is a half plateau, the height of which is approximately equivalent to the far-field water concentration in the surrounding glass, indicating that water that accumulated at the spherulite/magma boundary diffused sufficiently rapidly to equilibrate with the surrounding magma as the lava flow cooled.

Eruptive history of Mount Katmai, Alaska

Mount Katmai has long been recognized for its caldera collapse during the great pyroclastic eruption of 1912 (which vented 10 km away at Novarupta in the Valley of Ten Thousand Smokes), but little has previously been reported about the geology of the remote ice-clad stratovolcano itself. Over several seasons, we reconnoitered all parts of the edifice and sampled most of the lava flows exposed on its flanks and caldera rim. The precipitous inner walls of the 1912 caldera remain too unstable for systematic sampling; so we provide instead a photographic and interpretive record of the wall sequences exposed. In contrast to the several andesite-dacite stratovolcanoes nearby, products of Mount Katmai range from basalt to rhyolite. Before collapse in 1912, there were two overlapping cones with separate vent complexes and craters; their products are here divided into eight sequences of lava flows, agglutinates, and phreatomagmatic ejecta. Latest Pleistocene and Holocene eruptive units include rhyodacite and rhyolite lava flows along the south rim; a major 22.8-ka rhyolitic plinian fall and ignimbrite deposit; a dacite-andesite zoned scoria fall; a thick sheet of dacite agglutinate that filled a paleocrater and draped the west side of the edifice; unglaciated leveed dacite lava flows on the southeast slope; and the Horseshoe Island dacite dome that extruded on the caldera floor after collapse. Pre-collapse volume of the glaciated Katmai edifice was ∼30 km3, and eruptive volume is estimated to have been 57 ± 13 km3. The latter figure includes ∼40 ± 6 km3 for the edifice, 5 ± 2 km3 for off-edifice dacite pyroclastic deposits, and 12 ± 5 km3 for the 22.8-ka rhyolitic pyroclastic deposits. To these can be added 13.5 km3 of magma that erupted at Novarupta in 1912, all or much of which is inferred to have been withdrawn from beneath Mount Katmai. The oldest part of the edifice exposed is a basaltic cone, which gave a 40Ar/39Ar plateau age of 89 ± 25 ka.

The late Ediacaran (580–590 Ma) onset of anorogenic alkaline magmatism in the Arabian–Nubian Shield: Katherina A-type rhyolites of Gabal Ma’ain, Sinai, Egypt

The Katherina Volcanics of Gabal Ma’ain in the Sinai comprise an Ediacaran (580–590Ma) approximately 450m thick succession dominated by porphyritic rhyolite lava flows with subordinate related pyroclastics. These volcanics unconformably overlie the calc-alkaline Younger Granites (≥590Ma) and are intruded by alkaline granitoids (578±8Ma). The rhyolites have a potassic alkaline affinity and peraluminous to slightly metaluminous character. They exhibit many of the classic features of A-type magmas, including enrichment of incompatible elements, such as Zr, Nb, Y, Ga, Zn and Ce and total REE, as well as high FeO*/(FeO*+MgO) and 10,000*Ga/Al2O3 ratios. The A-type rhyolites have LREE-enriched patterns with pronounced negative Eu anomalies that are comparable with typical REE profiles for “hot-dry-reduced rhyolites”. Saturation thermometry has yielded zircon and apatite crystallization temperatures ranging between 913 and 925°C and 669 and 931°C, respectively. The investigated trace element patterns indicate that the Katherina A-type rhyolites were very likely to have evolved through simple fractional crystallization of a parental magma derived from an enriched (most probably asthenospheric) mantle source, supplemented by a crustal component inherited from pre-collision subduction events, or a ‘recycled component’ in the source. Katherina A-type rhyolites were likely erupted in a within-plate setting. The eruption of these rhyolites marks the onset of the anorogenic period during which the rigid massif (Arabian–Nubian Shield) was subjected to post-collisional tensional stresses and intra-plate rifting.

Subvolcanic subsidence and caldera formation during subaerial seafloor spreading in Iceland

Many caldera systems worldwide are interpreted to form as a result of catastrophic subsidence into space vacated by eruption. Some basaltic calderas, however, both associated with modern active volcanoes and in ancient, deeply eroded terranes, lack evidence for catastrophic formation and thus may have formed incrementally. The Vatnsdalur structural basin in northern Iceland is one such caldera-like volcanic depression. The Vatnsdalur structural basin is an elliptical depression with dimensions of ∼6 km × ∼3 km and a structural depth of ∼1.5 km. It occurs within the gently west-dipping, Tertiary basaltic lava sequences of the Skagi region, northern Iceland. Tilted basaltic to rhyolitic lava flows define a large-scale, rim monocline surrounding the basin. Exposures that form the southern rim of the Vatnsdalur structural basin reveal details of the pre-, syn-, and postsubsidence volcanic and tectonic processes during basin evolution. In these exposures, the dips of lava flows steepen continuously from horizontal to as much as 60°W, toward the center of the basin. Basaltic lava flows and mass-flow breccia units were deposited unconformably over the dipping lava flows and thicken substantially downdip, toward the basin center. The dip of bedding and flow contacts in these overlying units decreases up section, gradually becoming conformable with the regional lava-flow stratigraphy. Brittle folding of the lava flows within the rim monocline occurred by a combination of slip on minor faults and syntectonic dike intrusion. Based on structural evidence and comparisons to worldwide caldera systems, we conclude that the Vatnsdalur structural basin formed as a result of either (1) loading of the subsurface by dense, intrusive material or (2) shallow crustal magma redistribution accommodating many small subsidence events. In the context of crustal construction in Iceland, the Vatnsdalur structural basin and adjacent areas demonstrate significant along-strike variability in the mechanism(s) of crustal construction that occurs during subaerial seafloor spreading.

Tectonic control on hydrothermal circulation and fluid evolution in the Pietratonda–Poggio Peloso (southern Tuscany, Italy) carbonate-hosted Sb-mineralization

This paper deals with the development of an exhumed Sb-ore deposit associated with Neogene–Quaternary magmatism of southern Tuscany. This epithermal mineralization represents a fossil hydrothermal system that was probably active during the volcanic activity that produced rhyolitic lava flows (~ 2.3 Ma), presently exposed close to the study area (Roccastrada volcanic complex). Volcanism was coeval with strike-/oblique-slip and normal faults that controlled the hydrothermal circulation. Faults were active until the Pleistocene. Our study, based on fluid inclusion and structural data, demonstrates that fluids were channeled in damage zones of the Pliocene–Quaternary strike-/oblique-slip and normal faults and permeated within damage zones of Middle–Late Miocene extensional detachments, consisting of fractured carbonate rock masses sandwiched between successions with very low permeability. Fluids produced a diffuse hydrothermal alteration both in the cataclasite and in carbonate damage zones where jasperoid strata bounds developed. Fluid inclusion investigation allowed tracing the evolution of the hydrothermal fluids that is interpreted in terms of a progressive cooling of the hydrothermal system. Such evolution could be related to large infiltration of shallow waters from the early mineralizing stage (fluids at about 200–225 °C, salinity around 3 wt.% NaCl equiv.) down to the later post-ore stages, characterized by more diluted fluids (salinity of about 2.1 wt.% NaCl equiv) having lower temperatures (T h around 155 °C). The latest hydrothermal activity in the area is indicated by fluid inclusions hosted in late calcite veins that are characterized by T h values of about 125 °C. This fossil hydrothermal system may provide a close analog to the active geothermal systems occurring in southern Tuscany, thus the results are useful for better understanding the relationships between brittle structures and fluids migration.

Evidence from the Kyrenia Range, Cyprus, of the northerly active margin of the Southern Neotethys during Late Cretaceous–Early Cenozoic time

AbstractSedimentary geology and planktonic foraminiferal biostratigraphy have shed light on the geological development of the northern, active continental margin of the Southern Neotethys in the Kyrenia Range. Following regional Triassic rifting, a carbonate platform developed during Jurassic–Cretaceous time, followed by its regional burial, deformation and greenschist-facies metamorphism. The platform was exhumed by Late Maastrichtian time and unconformably overlain by locally derived carbonate breccias, passing upwards into Upper Maastrichtian pelagic carbonates. In places, the pelagic carbonates are interbedded with sandstone turbidites derived from mixed continental, basic volcanic, neritic carbonate and pelagic lithologies. In addition, two contrasting volcanogenic sequences are exposed in the western-central Kyrenia Range, separated by a low-angle tectonic contact. The first is a thickening-upward sequence of Campanian–Lower Maastrichtian(?) pelagic carbonates, silicic tuffs, silicic lava debris flows and thick-bedded to massive rhyolitic lava flows. The second sequence comprises two intervals of basaltic extrusive rocks interbedded with pelagic carbonates. The basaltic rocks unconformably overlie the metamorphosed carbonate platform whereas no base to the silicic volcanic rocks is exposed. Additional basaltic lavas are exposed throughout the Kyrenia Range where they are dated as Late Maastrichtian and Late Paleocene–Middle Eocene in age. In our proposed tectonic model, related to northward subduction of the Southern Neotethys, the Kyrenia platform was thrust beneath a larger Tauride microcontinental unit to the north and then was rapidly exhumed prior to Late Maastrichtian time. Pelagic carbonates and sandstone turbidites of mixed, largely continental provenance then accumulated along a deeply submerged continental borderland during Late Maastrichtian time. The silicic and basaltic volcanogenic rocks erupted in adjacent areas and were later tectonically juxtaposed. The Campanian–Early Maastrichtian(?) silicic volcanism reflects continental margin-type arc magmatism. In contrast, the Upper Maastrichtian and Paleocene–Middle Eocene basaltic volcanic rocks erupted in an extensional (or transtensional) setting likely to relate to the anticlockwise rotation of the Troodos microplate.

Eruptive history of South Sister, Oregon Cascades

South Sister is southernmost and highest of the Three Sisters, three geologically dissimilar stratovolcanoes that together form a spectacular 20 km reach along the Cascade crest in Oregon. North Sister is a monotonously mafic edifice as old as middle Pleistocene, Middle Sister a basalt–andesite–dacite cone built between 48 and 14 ka, and South Sister is a basalt-free edifice that alternated rhyolitic and intermediate modes from 50 ka to 2 ka (largely contemporaneous with Middle Sister). Detailed mapping, 330 chemical analyses, and 42 radioisotopic ages show that the oldest exposed South Sister lavas were initially rhyolitic ~ 50 ka. By ~ 37 ka, rhyolitic lava flows and domes (72–74% SiO 2) began alternating with radially emplaced dacite (63–68% SiO 2) and andesite (59–63% SiO 2) lava flows. Construction of a broad cone of silicic andesite–dacite (61–64% SiO 2) culminated ~ 30 ka in a dominantly explosive sequence that began with crater-forming andesitic eruptions that left fragmental deposits at least 200 m thick. This was followed at ~ 27 ka by growth of a steeply dipping summit cone of agglutinate-dominated andesite (56–60.5% SiO 2) and formation of a summit crater ~ 800 m wide. This crater was soon filled and overtopped by a thick dacite lava flow and then by > 150 m of dacitic pyroclastic ejecta. Small-volume dacite lavas (63–67% SiO 2) locally cap the pyroclastic pile. A final sheet of mafic agglutinate (54–56% SiO 2) – the most mafic product of South Sister – erupted from and drapes the small (300-m-wide) present-day summit crater, ending a summit-building sequence that lasted until ~ 22 ka. A 20 kyr-long-hiatus was broken by rhyolite eruptions that produced (1) the Rock Mesa coulee, tephra, and satellite domelets (73.5% SiO 2) and (2) the Devils Chain of ~ 20 domes and short coulees (72.3–72.8% SiO 2) from N–S vent alignments on South Sister's flanks. The compositional reversal from mafic summit agglutinate to recent rhyolites epitomizes the frequently changing compositional modes of the South Sister locus throughout its lifetime. South Sister is part of a reach of the Cascades unusually active in the last 50 kyr, characterized by high vent density, N–S vent alignments, and numerous eruptive units of true rhyolite (≥ 72% SiO 2) that distinguishes it from much of the Quaternary Cascade arc; these are eruptive expressions of the complex confluence of arc and intraplate magmatic–tectonic regimes.

High-K calc-alkaline to A-type fissure-controlled volcano-plutonism of the São Félix do Xingu region, Amazonian craton, Brazil: Exclusively crustal sources or only mixed Nd model ages?

Abstract In the Sao Felix do Xingu region, southern portion of Amazonian craton, voluminous well-preserved Late Paleoproterozoic volcanic and plutonic rocks are grouped in the Sobreiro and Santa Rosa formations. The basal Sobreiro Formation (1.88 Ga) is composed of massive andesitic to rhyodacitic flows and volcaniclastic facies that are high-K and metaluminous, geochemically similar to calc-alkaline granitoids of volcanic arcs. Fractional crystallization of magnetite + augite + magnesiohastingsite (up to 20 vol. %) and zircon + plagioclase in most evolved rocks controlled the magmatic evolution of this unit, under oxidizing condition. The Santa Rosa Formation (1.87 Ga) comprises: 1) massive, bedded, and foliated rhyolitic lava flows, and large dikes of banded rhyolite and ignimbrite; 2) felsic ignimbrite associated with thin unwelded ash-fall tuff and highly rheomorphic felsic ignimbrite; 3) felsic crystal tuff, lapilli-tuff, and breccia; and 4) stocks and dikes of granitic porphyry, and subordinate equigranular granite intrusions. This unit was in part erupted along major fissures and has A-type intraplate geochemical affinity, peraluminous composition, and transitional subalkaline to alkaline characteristics. The magmatic evolution was predominantly controlled by fractional crystallization of zircon + apatite + Fe-Ti oxides + feldspars (up to 30 vol. %) with subordinate biotite, probably under intermediate oxidizing conditions. Nd model ages (3.0–2.49 Ga) for the Sobreiro Formation strongly suggest its generation by mixing of mantle-derived and anatectic melts of Archean rocks beneath the volcanic sequences in the Sao Felix do Xingu region. The Santa Rosa Formation could have been originated from several Archean crustal sources (TDM 3.12–2.56 Ga), involving assimilation/contribution of juvenile mantle components. The integrated data point to a narrow transition from Andean-type subduction to a dominantly extensional tectonic setting for the volcanic and plutonic event of the Sao Felix do Xingu region. A stage of flattening in the subduction angle of the subducted plate during the Paleoproterozoic ocean–continent orogenesis in the Tapajos Gold Province might explain the volcanism and plutonism in the Sao Felix do Xingu region as consequence of the arc migration, resulting in the ∼2.0–1.87 Ga geochronological and metallogenetic zoning observed in southern Amazonian craton.

A Pleistocene diatomite from the western piedmont of Jebel Marra volcano, Darfur, western Sudan, and its hydrological and climatic significance

Diatoms collected at 10 cm intervals from a 6.3 m Pleistocene lacustrine diatomite section in the now arid piedmont west of Jebel Marra volcanic caldera, Darfur, western Sudan were analyzed. The lake probably formed as a result of a rhyolite lava flow damming a former river. The lake was initially shallow, but in the upper 5.3 m, diatom composition and a salinity reconstruction indicate persistent deep freshwater conditions. The diatomite is overlain by an Acheulian stone tool assemblage with a probable age range of ∼0.8 ± 0.3 Ma. The lake may be coeval with the oil palm fossils discovered ∼90 km to the southwest, which are underlain by Developed Oldowan/Early Acheulian artifacts, some in primary context, and so of Lower/Middle Pleistocene age. Both the diatomite and the oil palm fossils indicate a much wetter climate than today.

Pyroxene thermometry of rhyolite lavas of the Bruneau–Jarbidge eruptive center, Central Snake River Plain

The Bruneau–Jarbidge eruptive center of the central Snake River Plain in southern Idaho, USA produced multiple rhyolite lava flows with volumes of <10 km 3 to 200 km 3 each from ~11.2 to 8.1 Ma, most of which follow its climactic phase of large-volume explosive volcanism, represented by the Cougar Point Tuff, from 12.7 to 10.5 Ma. These lavas represent the waning stages of silicic volcanism at a major eruptive center of the Yellowstone hotspot track. Here we provide pyroxene compositions and thermometry results from several lavas that demonstrate that the demise of the silicic volcanic system was characterized by sustained, high pre-eruptive magma temperatures (mostly ≥950 °C) prior to the onset of exclusively basaltic volcanism at the eruptive center. Pyroxenes display a variety of textures in single samples, including solitary euhedral crystals as well as glomerocrysts, crystal clots and annealed microgranular inclusions of pyroxene ± magnetite ± plagioclase. Pigeonite and augite crystals are unzoned, and there are no detectable differences in major and minor element compositions according to textural variety — mineral compositions in the microgranular inclusions and crystal clots are identical to those of phenocrysts in the host lavas. In contrast to members of the preceding Cougar Point Tuff that host polymodal glass and mineral populations, pyroxene compositions in each of the lavas are characterized by single rather than multiple discrete compositional modes. Collectively, the lavas reproduce and extend the range of Fe–Mg pyroxene compositional modes observed in the Cougar Point Tuff to more Mg-rich varieties. The compositionally homogeneous populations of pyroxene in each of the lavas, as well as the lack of core-to-rim zonation in individual crystals suggest that individual eruptions each were fed by compositionally homogeneous magma reservoirs, and similarities with the Cougar Point Tuff suggest consanguinity of such reservoirs to those that supplied the polymodal Cougar Point Tuff. Pyroxene thermometry results obtained using QUILF equilibria yield pre-eruptive magma temperatures of 905 to 980 °C, and individual modes consistently record higher Ca content and higher temperatures than pyroxenes with equivalent Fe–Mg ratios in the preceding Cougar Point Tuff. As is the case with the Cougar Point Tuff, evidence for up-temperature zonation within single crystals that would be consistent with recycling of sub- or near-solidus material from antecedent magma reservoirs by rapid reheating is extremely rare. Also, the absence of intra-crystal zonation, particularly at crystal rims, is not easily reconciled with cannibalization of caldera fill that subsided into pre-eruptive reservoirs. The textural, compositional and thermometric results rather are consistent with minor re-equilibration to higher temperatures of the unerupted crystalline residue from the explosive phase of volcanism, or perhaps with newly generated magmas from source materials very similar to those for the Cougar Point Tuff. Collectively, the data suggest that most of the pyroxene compositional diversity that is represented by the tuffs and lavas was produced early in the history of the eruptive center and that compositions across this range were preserved or duplicated through much of its lifetime. Mineral compositions and thermometry of the multiple lavas suggest that unerupted magmas residual to the explosive phase of volcanism may have been stored at sustained, high temperatures subsequent to the explosive phase of volcanism. If so, such persistent high temperatures and large eruptive magma volumes likewise require an abundant and persistent supply of basalt magmas to the lower and/or mid-crust, consistent with the tectonic setting of a continental hotspot.

A LA-ICP-MS study of minerals in the Rocche Rosse magmatic enclaves: Evidence of a mafic input triggering the latest silicic eruption of Lipari Island (Aeolian Arc, Italy)

The volcanic products of Lipari Island (Aeolian Arc, Italy) younger than 10 ka are mostly aphyric rhyolitic pumices and obsidians emitted during unusual strombolian-type eruptions, which ended with the emplacement of lava flows. The last volcanic activity on the island dates back to 1230 ± 40 AD, with the extrusion of Rocche Rosse (RR) obsidian lava flow. Recently, mafic enclaves of latitic to trachytic composition have been identified and an evolution process between these enclaves and the rhyolitic magma has been documented in detail [Davì, M., 2007. The Rocche Rosse rhyolitic lava flow (Lipari, Aeolian Islands): magmatological and volcanological aspects. Plinius, supplement to the European Journal of Mineralogy 33, 1–8]. In this work textural and trace-element investigation of mineral phases of the RR enclaves, such as crystals of clinopyroxene, olivine, plagioclase, alkali-feldspar and biotite, was carried out to delineate the most recent feeding system of the island, since such a reconstruction could be significant in terms of hazard forecasting. The results indicate that most of the mineral phases are reversely or oscillatory zoned with respect to both major and trace elements, suggesting an early crystallization under low fO 2 conditions from melts of intermediate composition, followed by a later growth from a more mafic (presumably shoshonitic–basaltic) magma than that from which their cores crystallized. Crystals of magnesium-rich pyroxene and forsteritic-rich olivine are indicative of the presence of this shoshonitic basaltic magma. Based on microanalytical data, it is suggested here that the feeding system of recent Lipari volcanic activity was characterized by a shoshonitic–basaltic magma originating from a deep reservoir, which may have evolved and stopped in the crust, generating zoned magma chambers at different depths, in which latitic and rhyolitic magmas reside. The sudden arrival of a new input of mafic melt may have interacted with these resident fractionated magmas and triggered the eruption. A plumbing system of this type has been shown to be active in the southern sector of the Aeolian Islands since the last 42 ka [Gioncada, A., Mazzuoli, R., Milton, A., 2005. Magma mixing at Lipari (Aeolian Islands, Italy): Insights from textural and compositional features of phenocrysts. J. Volcanol. Geotherm. Res. 145, 97–118; Peccerillo, A., Frezzotti, M.L., De Astis, G., Ventura, G., 2006. Modeling the magma plumbing system of Vulcano (Aeolian Islands, Italy) by integrated fluid-inclusion geobarometry, petrology and geophysics. Geology 34, 17–20]. It is concluded that the hazard assessment of Lipari Island should take into account the arrival of deep and never erupted mafic melts as eruption triggers of more evolved shallower magma bodies.

The viscosity of latitic melts from Lipari (Aeolian Islands, Italy): Inference on mixing–mingling processes in magmas

The viscosity of latitic melts occurring as enclaves in the rhyolitic lava flow of Rocche Rosse (RR, Lipari, Aeolian Islands, Italy) has been measured in the high T range (1323–1473 K) for a dissolved water content varying from 1.23 to 4.39 wt.%. Measurements were performed by the falling sphere method in an internally heated gas pressure vessel. As expected, the viscosity of latite decreases with increasing water content and temperature. No pressure effect was detected between 200 and 500 MPa. Combining the new viscosity data for hydrous melts with data for dry latite of similar base composition [Giordano, D., Mangiacapra, A., Potuzak, M., Russell, J.K., Romano, C., Dingwell, D.B., Di Muro, A., 2006. An expanded non-Arrhenian model for silicate melt viscosity: a treatment for metaluminous, peraluminous and peralkaline liquids. Chemical Geology 229, 42–56.] we propose an empirical equation to estimate the viscosity of latitic melts as a function of temperature and water content over the range 10 1 to 10 12 Pa s. The obtained relationship reproduces the experimental data with a 1σ standard deviation of 0.22 log units. However, the empirical model is not constrained by data for hydrous melts at high viscosity and, therefore, it can only be used at low temperatures for water-poor melts. The viscosity data were used to model mixing–mingling processes between latitic and rhyolitic magmas at conditions relevant for the Rocche Rosse (RR, Lipari, Aeolian Islands) and La Fossa Cone 1888–1890 (LFC, Vulcano, Aeolian Islands) eruptions. The results demonstrate that the ratio between mafic and silicic end-members is the main parameter governing mixing–mingling interactions between magmas. This study suggests a faster ascent of magma underneath Vulcano compared to Lipari, which may be taken into account in hazard forecasting.