Rhyolitic Lava Flows Research Articles

Multi-proxy case study of a Neoproterozoic rhyolite flow in southernmost Brazil: Emplacement mechanisms and implications for ancient felsic lavas

Rhyolites compose an important record in the volcanic history of Earth, with significant occurrences in volcanic arcs, large igneous provinces and post-collisional terranes, often associated with explosive events. In several geologic provinces, rhyolites dominate as the most expressive geologic units (e.g., Silicic Large Igneous Provinces - SLIPs). Despite their importance, several aspects related to the emplacement of rhyolite flows are still enigmatic. Recent studies in modern rhyolite lavas suggest similar emplacement mechanisms to basaltic lavas, implying a more dynamic growth model for silicic flows, including outbreak lobes and outpour structures. Despite these advances, studies related to the recognition of these features in ancient flows are still rare. In this work we perform a multi-proxy study of an ancient (Neoproterozoic) rhyolitic lava flow combining fieldwork, petrography, geochemistry, rheology and magnetic fabric analysis. The Cerro do Perau outcrop (CP, southern Brazil) consists of a natural laboratory for the study of rhyolite lavas, presenting excellent exposure of a partially preserved flow with distinct flow features and folds. CP flow consists of a high-silica and low-crystal content rhyolite, suggesting its emplacement as an obsidian flow. Rheology data indicates high liquidus temperatures (>957 °C), with maximum viscosities of 108.5 Pa s and glass transition temperatures (Tg) of 750 °C. The absence of brittle features suggests little to none displacement below Tg. Structural analysis indicates the predominance of sub-vertical foliation planes, including axial planes of folds, indicative of proximal (near-vent) regions in rhyolite flows. The absence of lineations favors a predominantly planar accommodation of the flow-induced deformation, which is confirmed by the shape of the magnetic fabric ellipsoids. Several of these ellipsoids display a high degree of anisotropy, mostly related to an oblate fabric, indicative of the development of high-strain zones within the flow. Our data suggest that CP flow presents some similarities with recently proposed field-based emplacement models for rhyolitic flows, highlighting the significant data that can be extracted from a combination of magnetic fabrics and rheological analyses.

Physical volcanology of the Pavagadh rhyolites, northern Deccan Traps: Stratigraphic, structural, and textural record of explosive and effusive eruptions

Mount Pavagadh (840 m) in the northern Deccan Traps, India, is mainly composed of mafic lava flows (picrites, ankaramites, mugearites, basalts) and capped by rhyolites. Rhyolites also found at lower elevations, on and around the mountain, are thought to represent blocks of the eroded upper rhyolites or local eruptions in situ. Although the Pavagadh sequence has been studied for 150 years, the stratigraphy, structures, textures, and physical volcanology of the Pavagadh rhyolites have been poorly documented, and are discussed here at length. The upper rhyolite sequence contains tens of non-welded, crystal-bearing, glass- and pumice-rich ash beds of 47 m total thickness. These represent a primary fallout ash succession derived from distant Plinian eruptions, possibly corresponding with massive ignimbrite deposits recently recognised in Saurashtra several hundred kilometers to the southwest. The fallout ash succession is overlain by a 50 m thick lava flow showing well-developed flow banding and folding, basal and upper breccias, and a local basal vitrophyre. The lava flow is overlain by an ignimbrite with a basal vitrophyre, both these units showing excellent vitroclastic textures. The upper rhyolites thus represent alternating explosive-effusive-explosive eruptions. The lower-level rhyolites in the area, showing intense rheomorphic deformation in outcrop but lacking vitroclastic textures, are probably also lavas. However, they are not blocks of the eroded upper rhyolite lava flow but from separate eruptions. The Pavagadh rhyolites were probably far more laterally extensive than their present outcrops, with a minimum original volume of 9 km3; their source areas remain unknown. Geochemical and geochronological work on the Pavagadh rhyolites is needed, in the stratigraphic and volcanological context provided here, to understand their petrogenesis and potential relationship with the Cretaceous/Palaeogene (K/Pg) boundary mass extinction.

Vegetation responses to Quaternary volcanic and hydrothermal disturbances in the Northern Rocky Mountains and Greater Yellowstone Ecosystem (USA)

Volcanic and hydrothermal processes produce disturbances by diverse mechanisms and ecological responses are varied. New and published pollen records from the Northern Rocky Mountains and Greater Yellowstone Ecosystem document the response of vegetation to three different types of volcanic and hydrothermal disturbances: (1) Pleistocene rhyolite lava flows in the central Greater Yellowstone Ecosystem created infertile landscapes that have shaped vegetation since rhyolite emplacement. Nutrient-poor, well-drained soils that developed on these flows supported low-diversity grassland during late-glacial time and Pinus contorta forests in interglacial periods. (2) Ash layers from eruptions of Pacific Northwest stratovolcanoes are commonly preserved in lake-sediment records in the Northern Rocky Mountains, and associated pollen records show enhancement of steppe vegetation for years to decades. (3) Local hydrothermal explosions have resulted in vegetation changes in hydrothermal areas that indicate tree mortality following deposition of explosion debris, followed by recovery in years. Thus, the type and duration of the vegetation response to volcanic and hydrothermal disturbances are highly contextual and governed by the antecedent plant communities and the magnitude and mechanism of the volcanic or hydrothermal disturbance. Vegetation resilience varied between disturbances, ranging from enduring ecosystem parameter changes to short-lived state changes in resilient plant communities.

Volcanic SiO2-cristobalite: A natural product of chemical vapor deposition

Abstract Cristobalite is a low-pressure, high-temperature SiO2 polymorph that occurs as a metastable phase in many geologic settings, including as crystals deposited from vapor within the pores of volcanic rocks. Such vapor-phase cristobalite (VPC) has been inferred to result from silica redistribution by acidic volcanic gases but a precise mechanism for its formation has not been established. We address this by investigating the composition and structure of VPC deposited on plagioclase substrates within a rhyolite lava flow, at the micrometer to nanometer scale. The VPC contains impurities of the form [AlO4/Na+]0—coupled substitution of Al3+ charge-balanced by interstitial Na+—which are typical of cristobalite. However, new electron probe microanalysis (EPMA) element maps show individual crystals to have impurity concentrations that systematically decline from crystal cores-to-rims, and atom probe tomography reveals localized segregation of impurities to dislocations. Impurity concentrations are inversely correlated with degrees of crystallinity [observed by electron backscatter diffraction (EBSD), hyperspectral cathodoluminescence, laser Raman, and transmission electron microscopy (TEM)], such that crystal cores are poorly crystalline and rims are highly ordered tetragonal α-cristobalite. The VPC-plagioclase interfaces show evidence that dissolution-reprecipitation reactions between acidic gases and plagioclase crystals yield precursory amorphous SiO2 coatings that are suitable substrates for initial deposition of impure cristobalite. Successive layers of cubic β-cristobalite are deposited with impurity concentrations that decline as Al-bearing gases rapidly become unstable in the vapor cooling within pores. Final cooling to ambient temperature causes a displacive transformation from β→α cristobalite, but with locally expanded unit cells where impurities are abundant. We interpret this mechanism of VPC deposition to be a natural proxy for dopant-modulated Chemical Vapor Deposition, where halogen-rich acidic gases uptake silica, react with plagioclase surfaces to form suitable substrates and then deposit SiO2 as impure cristobalite. Our results have implications for volcanic hazards, as it has been established that the toxicity of crystalline silica is positively correlated with its purity. Furthermore, we note that VPC commonly goes unreported, but has been observed in silicic lavas of virtually all compositions and eruptive settings. We therefore suggest that despite being metastable at Earth's surface, cristobalite may be the most widely occurring SiO2 polymorph in extrusive volcanic rocks and a useful indicator of gas-solid reaction having occurred in cooling magma bodies.

Geochemistry of lavas from Aegina and Poros (Aegean Arc, Greece): Distinguishing upper crustal contamination and source contamination in the Saronic Gulf area

Pliocene (Phase 1) deposits from the volcanic centres of Aegina and Poros (western Aegean Arc) consist of andesitic-dacitic lava flows and domes with mafic enclaves, whereas basaltic-andesitic and rhyolitic lava flows were emplaced during the early Pleistocene Phase 2 and its precursor activity. Fifty-nine new whole rock analyses of lavas and enclaves show geochemical characteristics in keeping with their continental arc setting, with a significant spread in Sr-Nd-Hf-Pb isotope compositions and a strong crustal imprint (e.g. 87Sr/86Sr 0.7041–0.7076). Unlike other Aegean volcanic centres, the Phase 1 deposits show high Ba concentrations and high Ba/Th ratios. These are unlikely to be a signature of fluid-mediated metasomatism of the mantle wedge; subduction of barite-enriched sediments appears a more plausible option. The amount of sediment added to the mantle wedge is ca. 4%. High 208Pb/204Pb relative to 206Pb/204Pb, especially for the Poros lavas, points towards contamination with continental crust similar to the basement of Ios. First-order modelling shows that up to 35% crustal contamination took place. Additionally, crystal fractionation, magma mixing and crystal accumulation shaped the whole rock geochemistry. Similar to the neighbouring island of Methana, the widest isotopic variation is seen in the early Pleistocene deposits, reflecting remobilisation of magma from the previous volcanic episode by isotopically more primitive mafic magmas after a period of volcanic quiescence. The isotopic contrast between the volcanic centres of the Saronic Gulf and those of Santorini and Kos-Nisyros further west is likely related to the geometry and physical characteristics of the subducting slab and mantle wedge, rather than the actual amount of sediments subducted or their composition.

Contrasting tectonic settings in Northern Chon Aike Igneous Province of Patagonia: subduction and mantle plume-related volcanism in the Marifil formation

ABSTRACT The northern zone of the Chon Aike Igneous Province, located in the North Patagonian Massif, exhibits extensive outcrops of Jurassic volcanic rocks of the Marifil Formation. In the Arroyo Verde area, the initial volcanic stage of the Marifil Formation, that we denominate V0 (192.6 ± 2.5 Ma), includes coulées, megabreccias and lapilli tuffs assigned to plinian-type volcanism. This magmatism was generated by cold-wet-oxidized magmas that resemble those produced in active continental margins and volcanic arcs. The second stage, located unconformably over the first, includes welded lava-like ignimbrites, massive lapilli tuff and rhyolitic lava flow that resemble the Snake River-type volcanism. The magma that produces this volcanic stage exhibit the characteristics of hot-dry-reduced magmas emplaced in intraplate continental environments associated with continental rifting. This stage coincides with the V1 volcanic episode early recorded in the Marifil Formation.

Physicochemical models of effusive rhyolitic eruptions constrained with InSAR and DEM data: A case study of the 2011-2012 Cordón Caulle eruption

The 9 month long 2011-2012 eruption of Cordón Caulle (Southern Andes, Chile) is the best instrumentally recorded rhyolitic eruption to date and the first time that the effusion of a rhyolitic flow has been observed in detail. We use Interferometric Synthetic Aperture Radar (InSAR), with time-lapse digital elevation models (DEMs) and numerical models to study the dynamics of coupled magma reservoir deflation and lava effusion. InSAR recorded 2.2-2.5 m of subsidence after the first three days of the eruption, which can be modeled using a spheroidal magma reservoir at a depth of ∼5 km, ∼20 km long, and with a pressure drop of 20-30 MPa. The source is elongated in the NW-SE direction and its large dimensions imply a large plumbing system active throughout the eruption and spanning neighboring volcanoes, with a slight change in the geometry halfway through the effusive phase. TanDEM-X and Pléiades DEMs record the extrusion of both the rhyolitic lava flow and the intrusion of a shallow laccolith around the eruptive vent after the third day of the eruption, with a total volume of ∼1.45 km3 DRE. The laccolith was emplaced during the first month of the eruption, during both the explosive and effusive stages of the eruption. Both the reservoir pressure drop and the extruded volume time series follow quasi-exponential trends, and can be explained by a model that couples the reservoir pressure decrease, time- and pressure-dependent variations in the magma properties inside of the reservoir, and conduit flow. This model predicts both the temporal evolution and amplitude of both time series during the effusive phase, and a magma compressibility of ∼10−10 Pa−1, half the reported compressibility of the magma of the sub-Plinian explosive phase. Further, we estimate that the reservoir contained 1-3 wt.% dissolved H2O at the onset of lava effusion, with no exsolved CO2 and H2O in the reservoir throughout the effusive phase. This implies that the magma was significantly degassed after the explosive phase. The remaining volatiles in the magma after the explosive stage might have caused magma fragmentation, consistent with the hybrid explosive-effusive style observed during the waning of the eruption.

Characterisation and tectonic implications of the Early Cretaceous, Skeleton Coast Dyke Swarm, NW Namibia

The Early Cretaceous, Skeleton Coast Dyke Swarm (SCDS) was intruded into Permo-Carboniferous sediments of the Karoo Supergroup, Pan-African granites, Neoproterozoic metasediments, and Archean to Mesoproterozoic gneisses of the Kaoko Belt, NW Namibia. Aeromagnetic data and satellite imagery has been used to map the distribution of the generally coast parallel mafic dykes along with a significant number of dykes that cut across Pan-African structures of the Kaoko Belt. Dykes of the SCDS were mapped for some 500 km along the Skeleton Coast of NW Namibia and up to 200 km inland, and with a likely continuation into Angola. The geochemistry and published geochronological data suggest that the majority of the dykes are related to the Etendeka volcanics. These volcanics, together with the basalt and rhyolite lava flows and associated mafic dyke swarms of S-SE Brazil, form the Parana-Etendeka magmatic province of Early Cretaceous age. Geochemical analysis of 26 dykes show that the majority of the dykes are Low-TiO2 quartz tholeiites of two types: 1) qtz-tholeiites enriched in LREE [(La/Yb)N - 3.43 to 6.46]; & 2) qtz-tholeiites with REE patterns similar to E-MORB [(La/Yb)N

The origin and evolution of breakouts in a cooling-limited rhyolite lava flow

Understanding lava flow processes is important for interpreting existing lavas and for hazard assessments. Although substantial progress has been made for basaltic lavas our understanding of silicic lava flows has seen limited recent advance. In particular, the formation of lava flow breakouts, which represent a characteristic process in cooling limited basaltic lavas, but has not been described in established models of rhyolite emplacement. Using data from the 2011–2012 rhyolite eruption of Puyehue-Cordon Caulle, Chile, we develop the first conceptual framework to classify breakout types in silicic lavas, and to describe the processes involved in their progressive growth, inflation, and morphological change. By integrating multiscale satellite, field, and textural data from Cordon Caulle, we interpret breakout formation to be driven by a combination of pressure increase (from local vesiculation in the lava flow core, as well as from continued supply via extended thermally preferential pathways) and a weakening of the surface crust through lateral spreading and fracturing. Small breakouts, potentially resulting more from local vesiculation than from continued magma supply, show a domed morphology, developing into petaloid as inflation increasingly fractures the surface crust. Continued growth and fracturing results in a rubbly morphology, with the most inflated breakouts developing into a cleft-split morphology, reminiscent of tumulus inflation structures seen in basalts. These distinct morphological classes result from the evolving relative contributions of continued breakout advance and inflation. The extended nature of some breakouts highlights the role of lava supply under a stationary crust, a process ubiquitous in inflating basalt lava flows that reflects the presence of thermally preferential pathways. Textural analyses of the Cordon Caulle breakouts also emphasize the importance of late-stage volatile exsolution and vesiculation within the lava flow. Although breakouts occur across the compositional spectrum of lava flows, the greater magma viscosity is likely to make late-stage vesiculation much more important for breakout development in silicic lavas than in basalts. Such late-stage vesiculation has direct implications for hazards previously recognized from silicic lava flows, enhancing the likelihood of flow front collapse, and explosive decompression of the lava core.

Magmatic evolution of the Boku Volcanic Complex, Main Ethiopian Rift

The Boku volcanic complex is a Quaternary center situated on the axial segment of the Main Ethiopian Rift (MER), located 92 km South-east from Addis Ababa. The main objective of this study is to understand the magmatic evolution of the volcanic complex and to develop a model to answer some outstanding questions related to bimodal products of rift related volcanism, using geological mapping, petrographic and geochemical approaches. The Boku complex is characterized by two main phases of activity: pre-caldera/caldera forming eruptive activity and post-caldera eruptive activity. The volcanic stratigraphy consists from bottom to top of a sequence of rhyolitic lava flows, pumice flows, welded ignimbrite, pumice fall, rhyolitic lava dome, obsidian flow, lower basaltic lava flow, ash flow, basaltic scoria and upper basaltic lava flows. The lithologic varieties together with the geochemical results indicate that the Boku eruptive products are bimodal in composition; no intermediate compositions are found. The mafic rocks are transitional to weakly subalkaline basalts while the silicic rocks are predominantly peralkaline rhyolites. These two groups of rocks are co-genetic and related to each other by fractional crystallization processes starting from mantle-derived basaltic magma with a small component of crustal contamination. The available geophysical, geochemical and field data suggest that the evolution of the evolved silicic center which hosts a bimodal rock distribution can be explained as a result of prolonged stagnation of transitional basaltic melt (sourced from the mantle) at relatively high pressures, where these evolve to intermediate compositions. The transitional basaltic melts can occasionally erupt to the surface along weakness lines such as faults that infrequently cut the lower part of the shallow reservoir, generating basalts from the intra-caldera and lateral eruptive centers. We consider the intermediate magma is mechanically trapped at mid-crustal depths because of its higher crystal load (ca. 50%). Silicic magma is formed at shallow depth by prolonged fractional crystallization of the intermediate magma and minor assimilation of crustal material. These silicic magmas generate both effusive and explosive eruption products in the overall stratigraphy of the volcanic complex.

A NanoSIMS Investigation on Timescales Recorded in Volcanic Quartz From the Silicic Chon Aike Province (Patagonia)

We compare quartz zonation and diffusion timescales of crystal-rich rhyolitic ignimbrites and crystal-poor rhyolitic lava flows from the Jurassic Chon Aike Province as exposed in Patagonia (Argentina). The timescales are assessed by using diffusion modelling based on nanoscale secondary ion mass spectrometry (NanoSIMS) analysis of titanium (Ti) concentration profiles in quartz crystals oriented by image analysis using micro-tomography. Quantitative Ti-data were acquired by SIMS to estimate crystallization temperatures. The textural and geochemical analysis revealed clear differences between crystal-poor rhyolitic lava flows and crystal-rich rhyolitic ignimbrites. Quartz crystals from rhyolitic lava flows display simple oscillatory cathodoluminescence (CL) zoning interpreted to be magmatic and diffusion chronometry suggest a short timescale for quartz crystallization from 5.6 ± 2.2 yr to 41.6 ± 9.8 yr. Resorption textures are rare, and hence crystals in rhyolitic lava flows recorded a simple, rapid extraction, transport and eruption history for these crystal-poor melts. Rhyolitic ignimbrites, in contrast, reveal complex zoning patterns, reflecting several episodes of partial resorption and growth throughout their crystallization history. The complex quartz zoning textures together with longer diffusion times (< 350 yr), rather suggest a storage in a mush with fluctuating pressure and temperature conditions leading to intermittent resorption. Yet, a final quartz overgrowth rim occurred over a much shorter timescale in the order of years (< 3 yr), which implies that crystal-rich ignimbrites can be re-mobilized very fast.

Emplacement of the Rocche Rosse rhyolite lava flow (Lipari, Aeolian Islands)

The Rocche Rosse lava flow marks the most recent rhyolitic extrusion on Lipari island (Italy), and preserves evidence for a multi-stage emplacement history. Due to the viscous nature of the advancing lava (108 to 1010 Pa s), indicators of complex emplacement processes are preserved in the final flow. This study focuses on structural mapping of the flow to highlight the interplay of cooling, crust formation and underlying slope in the development of rhyolitic lavas. The flow is made up of two prominent lobes, small (< 0.2 m) to large (> 0.2 m) scale folding and a channelled geometry. Foliations dip at 2–4° over the flatter topography close to the vent, and up to 30–50° over steeper mid-flow topography. Brittle faults, tension gashes and conjugate fractures are also evident across flow. Heterogeneous deformation is evident through increasing fold asymmetry from the vent due to downflow cooling and stagnation. A steeper underlying topography mid-flow led to development of a channelled morphology, and compression at topographic breaks resulted in fold superimposition in the channel. We propose an emplacement history that involved the evolution through five stages, each associated with the following flow regimes: (1) initial extrusion, crustal development and small scale folding; (2) extensional strain, stretching lineations and channel development over steeper topography; (3) compression at topographic break, autobrecciation, lobe development and medium scale folding; (4) progressive deformation with stagnation, large-scale folding and re-folding; and (5) brittle deformation following flow termination. The complex array of structural elements observed within the Rocche Rosse lava flow facilitates comparisons to be made with actively deforming rhyolitic lava flows at the Chilean volcanoes of Chaitén and Cordón Caulle, offering a fluid dynamic and structural framework within which to evaluate our data.

Application and reliability of calcic amphibole thermobarometry as inferred from calc-alkaline products of active geothermal areas in the Andes

Igneous thermobarometry may contribute to locate the upper-crust heat source/s representing one of the main factors required to establish the potential of high enthalpy geothermal areas through models of the heat flux. To test this possibility, we applied amphibole thermobarometry to some of the youngest lavas and pyroclastics (andesite to rhyolite) of Apacheta and La Torta (Chile) and Chachimbiro (Ecuador) areas representing medium to high enthalpy geothermal systems located in two different crustal contexts along the Andean active continental margins of South America. We used the robust application of Ridolfi and Renzulli (2012) allowing to estimate the intrinsic physico-chemical parameters with reasonably low uncertainties (T ± 24 °C, P ± 12%, fO2 ± 0.4 log units) and the composition of the melt in equilibrium with Mg-rich calcic amphibole in a wide range of conditions, up to 1130 °C and 2.2 GPa. In addition, analyses of the intra-crystalline compositional and textural variations allowed us to propose a method to distinguish between chemically homogeneous and heterogeneous amphiboles, avoiding some incorrect applications of the Ridolfi and Renzulli (2012)'s equations and thus unreliable results.Thermobarometric constraints indicate a polybaric crystallization of magmas below the Chilean geothermal areas of Apacheta and La Torta. In the last 150 ka, the feeding systems of these areas mainly consisted of shallow silicic magma chambers at depths of 4–8 km (740–840 °C), giving rise to dacite domes (tortas). Amphiboles in the rhyolite lava flows, erupted earlier (Pliocene-Pleistocene) during the activity of the nearby Apacheta-Aguilucho Volcanic Complex, show a similar T-depth range (780–810 °C, 4–6 km). At deeper levels (10–20 km), the depths estimated using amphiboles from the magmatic enclaves (in the dacite domes) and from the andesite lavas are approximately aligned along the upper morphologic margins of the well geophysically constrained Altiplano-Puna Magma Body (APMB; a partially melted zone in the upper crust where the S-waves velocities are <2.9 km/s), with estimated temperatures of 850–1010 °C. Heat source contributions for the investigated Chilean geothermal areas should therefore be represented by both the shallower magma bodies and the huge APMB.The Holocene products of the Ecuadorian Chachimbiro geothermal area indicate amphibole crystallization from 7 to 17 km depth at temperature conditions in the range of 800–910 °C. The occurrence of two separated regions of magma storage at approximately 230 ± 30 MPa and 380 ± 60 MPa is discussed. They should be taken into account as heat sources for this Ecuadorian geothermal area.

Turmoil before the boring billion: Paleomagnetism of the 1880–1860 Ma Uatumã event in the Amazonian craton

The Uatumã silicic large igneous province (SLIP) has covered about 1,500,000km2 of the Amazonian craton at ca. 1880Ma, when the Columbia/Nuna supercontinent has been assembled. Paleomagnetic and geochronological data for this unit were obtained for the Santa Rosa and Sobreiro Formations in the Carajás Province, southwestern Amazonian craton (Central-Brazil Shield). AF and thermal demagnetizations revealed northern (southern) directions with high upward (downward) inclinations (component SF1), which passes a ‘B’ reversal test, and is carried by magnetite and SD hematite with high-blocking temperature. This component is present on well-dated 1877.4±4.3Ma (U-Pb zrn - LA-ICPMS) rhyolitic lava flows, providing the SF1 key paleomagnetic pole (Q=6) located at 319.7°E, 24.7°S (A95=16.9°). A second southwestern (northeastern) direction with low inclination (Component SF2) was obtained for a well-dated 1853.7±6.2Ma (U-Pb zrn - LA-ICPMS) dike of the Velho Guilherme Suite. This component also appears as a secondary component in the host rhyolites of the Santa Rosa Fm and andesites of the Sobreiro Fm at the margins of the dike previously dated. Its primary origin is confirmed by a positive baked contact test, where a Velho Guilherme dike crosscuts the 1880Ma andesite from the Sobreiro Formation. The corresponding SF2 key pole is located at 220.1°E, 31.1°S (A95=5°) and is classified with a reliability criterion Q=7. The large angular distance between the almost coeval (difference of ~25Ma) SF1 and SF2 poles implies high plate velocities (~39.3cm/yr) which are not consistent with modern plate tectonics. The similar significant discrepancy of paleomagnetic poles with ages between 1880 and 1860Ma observed in several cratons could be explained by a true polar wander (TPW) event. This event is the consequence of the reorganization of the whole mantle convection, and is supported by paleomagnetic reconstructions at 1880Ma and 1860Ma and also by geological/geochronological evidence.

Cooling and crystallization of rhyolite–obsidian lava: Insights from micron-scale projections on plagioclase microlites

To reveal the cooling process of a rhyolite–obsidian flow, we studied the morphology of plagioclase microlites in the Tokachi–Ishizawa lava of Shirataki, northern Hokkaido, Japan, where the structure of the lava can be observed from obsidian at the base of the flow to the innermost rhyolite. Needle-like micron-scale textures, known as “projections”, occur on the short side surfaces of the plagioclase microlites. Using FE–SEM we discovered a positive correlation between the lengths and spacings of these projections. On the basis of the instability theory of an interface between melt and crystal, and to understand the length and spacing data, we developed a model that explains the positive correlation and allows us to simultaneously estimate growth rates and growth times. Applying the model to our morphological data and the estimated growth rates and growth times, we suggest that the characteristics of the projections reflect the degree of undercooling, which in turn correlates with lava structure (the obsidian at the margin of the flow experienced a higher degree of undercooling than the interior rhyolite). The newly developed method provides insights into the degree of undercooling during the final stages of crystallization of a rhyolitic lava flow.

Lithological architecture and petrography of the Mako Birimian greenstone belt, Kédougou-Kéniéba Inlier, eastern Senegal

The new lithological and petrographic data obtained in the Mako sector are analyzed in the light of the geochemical data available in the literature. It consists of ultramaic, mafic rocks of tholeiitic affinities associated with intermediate and felsic rocks of calc-alkaline affinities and with intercalations of sedimentary rocks. The whole unit is intruded by Eburnean granitoids and affected by a greenschist to amphibolite facies metamorphism related to a high grade hydrothermalism. It consists of: (i) ultramafic rocks composed of a fractional crystallization succession of lherzolites, wehrlites and pyroxenites with mafic rock inclusions; (ii) layered, isotropic and pegmatitic metagabbros which gradually pass to metabasalts occur at the top; (iii) massive and in pillow metabasalts with locally tapered vesicles, completely or partially filled with quartzo-feldspathic minerals; (iv) quarzites locally overlying the mafic rocks and thus forming the top of the lower unit.This ultramafic-mafic lower unit presents a tholeiitic affinity near to the OIB or N-MORB. It represents the Mako Ophiolitic Complex (MOC), a lithospheric fragment of Birimian lithospheric crust.The upper unit is a mixed volcanic complex arranged in the tectonic corridors. From bottom to top it comprises the following: (i) andesitic, and (ii) rhyodacitic and rhyolitic lava flows and tuffs, respectively. They present a calc-alkaline affinity of the active margins.Three generations of Eburnean granitoids are recognized: (i) early (2215–2160 Ma); (ii) syn-tectonics (2150–2100 Ma) and post-tectonics (2090–2040 Ma). The lithological succession, geochemical and metamorphic characteristics of these units point to an ophiolitic supra-subduction zone.

A05 剪断応力による流紋岩質溶岩流の破砕組織 : 九州北東部,姫島火山群の例(噴火と噴煙のダイナミクス,口頭発表)

A05 剪断応力による流紋岩質溶岩流の破砕組織 : 九州北東部,姫島火山群の例(噴火と噴煙のダイナミクス,口頭発表)

Eruption mechanisms and short duration of large rhyolitic lava flows of Yellowstone

Large-volume effusive rhyolite lava flows are a common but poorly understood occurrence from silicic volcanic centers. We integrate characterization of lava flow topographic morphology and petrographic textures and zoning of crystals with physical models of viscous fluid flow in order to interpret the eruption durations and discharge rates for the most recent effusive volcanic eruptions from Yellowstone. These large-volume (10–70 km3) crystal-poor rhyolite lavas erupted within the Yellowstone caldera as 100–200 m thick flows and have a cumulative erupted volume of 650 km3 that is similar to less frequent caldera-forming events, but occur as individual eruptions spread over ∼100 ka. Most of this work is focused on the axisymmetric 124 ka, ∼50 km3 Summit Lake flow. We examined crystallinity, major and trace element concentrations, oxygen and hydrogen isotopic values, and quartz morphology and zoning in samples from the center to margin of this flow. Water contents down to 0.1 wt.% and δD values of −110‰ are low and require closed-system degassing until near-surface lithostatic pressure, while major elements are consistent with water-undersaturated pre-eruptive storage and crystallization at ∼4–8 km depth. We found some evidence for subtle km-scale zoning within the lavas but describe significant microscopic scale compositional diversity including sharp boundaries between high-Ti cores and ∼200 μm thick rims on quartz phenocrysts. Embayed quartz external morphology and rim growth may be the result of undercooling during coalescence of magma bodies during shallow transport between dikes and sills. Modeling the emplacement of the lava flow as a simple viscous fluid suggests that emplacement of rhyolite lava at ∼800 °C occurred over ∼2 to 5 yrs with high discharge rates >100 m3/s. Such high magma discharge rates are accommodated through ∼6 km-long fissures that allow for slower magma ascent velocities of <1 cm/s required for eruptions to remain dominantly effusive. Lower temperatures will result in >10 yr flow durations and significant cooling of the flow front that should result in a more complex compound flow morphology than observed. Higher temperatures require unrealistically wide (>50 m) dike widths to accommodate large discharge rates. Petrographic and isotopic evidence from crystals suggests recharge and merger of individual magma batches occurs on a similar timescale to the eruption duration and may directly cause overpressure and emplacement of these rhyolite lava flows from a shallow, ephemeral magma chamber. Large-volume rhyolitic lavas are able to erupt effusively through elongate fissures that utilize preexisting zones of crustal weaknesses such as ring fractures. Less-common explosive eruptions at Yellowstone may result when ascending magmas are forced through narrower conduits or when recharge rates are especially high. The results of this study provide a unique “top-down” constraint on effusive eruption rates, make new interpretations of common petrographic textures, and presents a comprehensive model for eruption control.

Post-collisional Ediacaran volcanism in oriental Ramada Plateau, southern Brazil

Ediacaran volcanic sequences in southernmost Brazil are related to intense post-collisional magmatism of the Brasiliano Orogeny. A portion of this volcanism occurs in the oriental Ramada Plateau located in the center part of the Rio Grande Sul State and is correlated with Hilário and Acampamento Velho formations. The first one is represented dominantly by lava flows and dikes of shoshonitic andesitic composition, besides of volcanogenic sedimentary deposits. The acid rocks of the Acampamento Velho Formation are expressive in the area, comprising high-silica ignimbrites, usually densely welded. Dikes and domes are common too and rhyolitic lava flows occur at the top and intercalated to ignimbrites in the middle of the sequence. The acid rock association has a sodic alkaline affinity. In this unit we mapped a subvolcanic sill of trachyte showing evidence for magma mixing with the rhyolitic magma. It has sodic alkaline affinity, and FeOt/FeO + MgO ratios and agpaitic index lower than those recorded in the rhyolites/ignimbrites. The Acampamento Velho Formation includes in this area, subordinately, basalts as àà flows and dikes intercalated with acid rocks. They have sodic alkaline nature and characteristics of intraplate basic rocks. New zircon U-Pb dating indicates crystallization age of 560 ± 2 Ma in a densely welded ignimbrite, 560 ± 14 Ma for a mafic trachyte and 562 ± 2 Ma for a subvolcanic rhyolite. The sodic alkaline rocks in this region evolved by fractional crystallization processes and magma mixing with major crustal contribution at approximately 560 Ma. The chemical characteristics are similar to those of A-type granites associated with Neoproterozoic post-collision magmatism in the Sul-rio-grandense Shield.

Dynamics and time evolution of a shallow plumbing system: The 1739 and 1888–90 eruptions, Vulcano Island, Italy

In this work we analyze the morphology of latitic enclaves occurring in the rhyolitic lava flow of Pietre Cotte (Island of Vulcano, Italy). We show that enclave morphology is a feature inherited from the shallow plumbing system of the volcano during the invasion of the latitic magma into the rhyolitic magma. The complexity of enclave morphology is quantified by fractal analyses. Using the empirical relationship given by Perugini et al. (2005) relating the fractal dimension and viscosity ratio, the range of viscosity ratios that developed during the injection of the latitic magma into the rhyolitic one is estimated.Thermodynamical and rheological modeling indicates that the most plausible scenario to explain the variability of observed viscosity ratios is represented by a plumbing system where a large volume of latitic magma intruded a smaller volume of rhyolitic magma. The comparison of volume proportions of magmas on the outcrop with those estimated by the modeling allows us to infer that most of the latitic magma remained in the plumbing system after the ending of the 1739 eruptive cycle.The strong similarity of compositions from the Pietre Cotte lava flow and products erupted during the last volcanic activity of Vulcano (1888-1890) is interpreted as the reactivation of the latitic magma, that possibly evolved through fractional crystallization to produce trachy-rhyolitic compositions, during the 1888–1890 eruptive cycle.We suggest that the methodological approach presented here can represent a further tool for hazard mitigation in volcanic areas. In particular, it allows obtaining information about the dynamics of plumbing systems of active volcanoes and their time evolution.