Rhyolite Research Articles

Microstructure Investigation of Oil-Bearing Rhyolites: A Case Study from the Hailar Basin, NE China

Understanding the microstructure of rhyolites may greatly promote exploration efforts on rhyolitic hydrocarbon reservoirs; however, related studies are sparse. In this contribution, the microstructure and related porosity of oil-bearing rhyolitic lavas from the Hailar Basin (NE China) were investigated using a combination of optical microscopy, fluorescence image analysis, and scanning electron microscopy. The direct visual and quantitative analyses show that the rhyolites are heterogeneous and porous rocks and have complex microstructures. Phenocryst-rich rhyolitic lava, perlitic lava, and spherulitic rhyolite may be favorable targets for rhyolitic hydrocarbon exploration. For the phenocryst-rich rhyolitic lavas, embayment pores, cleavages, cavitational and shear fractures, and intracrystalline sieve pores are commonly observed in the phenocrysts; while flow-parallel laminar and micropores are ubiquitous in the groundmass. Perlitic lavas are characterized by the occurrence of numerous perlitic fractures which can also be produced in the glassy groundmass of other lavas. Spherulitic rhyolites mainly consist of small-sized (<1 mm) clustered or large-sized (>1 mm) isolated spherulites. Clustered spherulites are characterized by the development of interspherulite pores. Isolated spherulites contain numerous radiating micropores. Both types of spherulites may have water expulsion pores formed in the spherulite–glass border. The formation of the microstructure and related porosity of rhyolites is controlled by pre-, syn- (e.g., deuteric crystal dissolution, cavitation, ductile–brittle deformation, and high-T devitrification), and post-volcanic (e.g., hydration and low-T devitrification) processes. Although pores with diameters > 50 μm are often observed, small pores dominate in pore-size distribution. Small (<15 μm) and large (>300 μm) pores give the most volumetric contribution in most cases. Medium-sized pores with diameters ranging from ~150–300 μm are the least developed and contribute the least to the total volume. The results of this paper can be beneficial to further the understanding of the microstructure and pore system of rhyolites and may be applied to rhyolitic lava hydrocarbon reservoirs elsewhere.

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.

Volcanism of the La Quinta Formation in the Perijá mountain range

This study reports new data on the petrography, total rock chemistry and U-Pb zircon geochronology of volcanic rocks of the La Quinta Formation that outcrop the western flank of the Perijá mountain range and the Cesar and La Guajira departments. The volcanic rocks consist of basaltic, andesitic, dacitic and rhyolitic lavas, and the volcaniclastic rocks consist of crystal-vitric and crystal-lithic tuffs and agglomerates of calc-alkaline affinity, formed in a continental margin arc setting. Geochronological data suggest that the La Quinta Formation was volcanically active for approximately 25 Ma, during which its composition varied from basaltic trachyandesites to rhyolites. U-Pb dating suggests that the volcanism began in approximately 191 Ma (Sinemurian age) and continued until approximately 164 Ma, with at least three periods of increased volcanic activity. The inherited zircons contain Triassic, Permian, Neoproterozoic and Mesoproterozoic populations, indicating that this arc was emplaced on rocks of the Chibcha Terrane along the South American paleomargin and that it is part of the same arc that formed the Jurassic volcanic rocks of the Sierra Nevada de Santa Marta, Cocinas and San Lucas mountain ranges and the Upper Magdalena Valley.

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.

An early Jurassic graben caldera of Chon Aike silicic LIP at the southernmost massif of the world: The Deseado caldera, Patagonia, Argentina

Detailed geological mapping of 900 km2, newly defined volcanic and volcaniclastic lithofacies, multiple stratigraphic profiles, and a new zircon U/Pb eruption age (178.7 ± 1.9 Ma) describe an early Jurassic extensional-related piece-meal collapsed graben caldera in the eastern sector of Deseado Massif, at southern Patagonia. This volcano-tectonic structure, here referred to as The Deseado Caldera, collapsed at least twice controlled by two major NW-striking normal faults, during which a large acidic ignimbrite (predominantly moderate to extremely high-grade) and co-ignimbrite lag breccias were extruded through multiple NW-fissure vents. During the two post-collapse periods, alluvial fans, lagoons and laharic sedimentary environments re-worked the intra-caldera volcaniclastic deposits. Finally, numerous elongated and aligned rhyolitic lava domes and dykes were emplaced during the final magmatic resurgence stage. This caldera represents the first clear evidence of the extensional tectonic regime influence, associated with the inception of Gondwana breakup, on the eruptive mechanisms of the Jurassic Chon Aike Silicic LIP in the Deseado Massif.

Early Cretaceous bimodal magmatism related epithermal mineralization: A case study of the Gaosongshan gold deposit in the northern Lesser Xing’an Range, NE China

Low-sulfidation (LS) gold mineralization is partially associated with bimodal volcanism in origin. Sources of ore-mineralized fluids and relevant dynamic processes remain poorly understood. To better understand these issues, a study including deposit geology, petrology, zircon U–Pb dating, and element and Sr–Nd–Hf isotope geochemistry of bimodal volcanic rocks at Gaosongshan was conducted. The bimodal volcanic rocks are composed of andesitic rocks (basalt andesite–andesite–trachyandesite–andesitic pyroclastic) of the Meifeng Formation and overlying rhyolite lava of the Fuminhe Formation cut by equivalent porphyry stocks, which contain pyrite-bearing granular-textured quartz cavity with a sugary appearance, with trapped single-phase magmatic fluids. The andesitic suite formed at approximately 106–108 Ma, prior to eruption of the younger rhyolitic magmas (105.6 ± 1.7 Ma; n = 13, MSWD = 1.2). The andesites show calc-alkaline affinities, which is a subduction-related characteristic, and show moderate La/YbN (7.91–11.36) and weakly negative europium anomalies (0.80–1.02), indicating a source in the asthenospheric mantle with varying contamination by both subduction and continental materials. A-type rhyolitic rocks are geochemically comparable to the andesitic suite, and their higher La/YbN (13.01–30.05) and lower Eu/Eu* (0.66–0.73) indicate evolved products of andesitic magmas. The estimated oxygen fugacity of dry (＜1.2% water) rhyolite lava is FMQ −1.87, whereas rhyolite porphyry is more oxidized (FMQ + 0.12) and water rich (2.5–2.8%) with probably being water saturated at 2.0–2.4 km depths. Therefore, it is assumed that the essential constraints of bimodal magmatism on epithermal gold mineralization are sequential processes from ascending and gradual differentiation of crustal-level mantle-derived basaltic magmas to releases of hydrothermal solutions from the silica-rich magma reservoir at shallower levels. Most importantly, the oxygen fugacity of and the fluid fluxes from dry rhyolitic melts are critical restricting parameters for the limited reserves at Gaosongshan; thus, this research contributes objective evidence for investigations of deposit genesis and prospecting targets.

Geochemical and mineralogical studies of ca.12 Ma hydrothermal manganese-rich rocks in the Hokuroku district in Japan

Geological and geochemical studies were performed on ca.12 Ma Mn-rich rocks in the Hokuroku district in Japan to constrain the origin of Mn-rich hydrothermal fluids and precipitation pathways of Mn oxides. Mn-rich rocks occur in hyaloclastites of rhyolite lavas in the studied areas. Colloform textures of Mn oxides were common in those Mn-rich rocks. Carbonaceous layers uniquely alternate with Mn oxides in the colloform textures. The Mn oxides layers in the colloform textures were composed of aggregates of nano-scale todorokite, birnessite, and pyrolusite. The carbonaceous layers also host apatite nano-crystals. These findings imply that the carbonaceous layers most likely represent fossilized microbial mats of Mn-oxidizing bacteria. The close association of nano-scale Mn oxides with organic matter, clay, and silica minerals suggests that Mn oxides initially precipitated near venting areas of low-temperature hydrothermal fluids.The early Mn oxides were trapped in the hyaloclastites and experienced diagenetic modifications combined with hydrothermal alteration, seawater penetration, and microbial activities in the hyaloclastites. Such secondary processes further enriched Mn in the hyaloclastites, forming various Mn silicates (e.g., pennantite and shirozulite) and veins made of Mn (II and III) silicate and oxide/silicate (e.g., rhodonite and braunite).Later hydrothermal fluids also brecciated the massive Mn-rich rocks. Rhodochrosite and rhodonite occurred in such brecciated Mn-rich rocks or sheared hyaloclastites preferentially. The carbon isotope compositions of rhodochrosite ranged from −26.7‰ to −20.8‰ (VPDB). The mudstone and early colloform textures in the hyaloclastites contained appreciable amounts of organic matter. Such organic matter was degraded by microbial processes under the influence of the later hydrothermal activities, followed by the formation of rhodochrosite. These findings contribute not only to understanding the formation mechanisms of hydrothermal Mn-rich rocks, but also to future exploration of Mn deposits.

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.

Rhyolite lava emplacement dynamics inferred from surface morphology

The morphology of a silicic lava is created by the fluid dynamic conditions that operated during flow. Careful analysis of these features can thus be used to reconstruct emplacement processes. We used unoccupied aerial system and structure-from-motion photogrammetry techniques to collect centimeter-scale spatial resolution imagery of the surfaces of South Coulee and Obsidian Dome rhyolite lavas (Mono-Inyo Craters, USA). We supplement the newly acquired orthomosaics and digital elevation models with existing imagery of other Holocene rhyolite lavas from across the western USA, including Rock Mesa Dome (South Sister, OR), Newberry Flow (South Sister, OR), Obsidian Flow (Newberry, OR), Interlake Flow (Newberry, OR), and Banco Bonito Flow (Valles Caldera, NM). Although many morphologic features exist, we restrict our quantitative analyses to the recurrent ridges and blocks of loose rubble that are common to the flows. Ridge spacing and ridge vergence indicate the ridges are likely folds produced by compression during emplacement. Pressure ridge spacings range from 27 to 45 m at South Coulee and 18 to 34 m on Obsidian Dome. Spacing generally decreases with distance from the vent across both lavas. Ridge amplitudes range from 4 to 17 m at both lavas and show little correlation with distance from the vent. The vergence of the crest of the ridges in the flow interiors points back to the source vent, which provides strong evidence for endogenous flow and the backward rotation of folds produced by undertow of the underlying lava. Ridges near flow margins verge towards the flow front, demonstrating that exogenous flow becomes increasingly important at the point of advance. In both South Coulee and Obsidian Dome block sizes are largest near the vent, and gradually decrease towards the flow front. The gradual decrease in block size with increasing distance from the vent likely reflects decreasing effusion rates from the conduit. We interpret our high-resolution field measurements under the lens of analytical solutions to fluid dynamic models to estimate emplacement timescales and associated rates. We calculate relatively abbreviated eruptive timescales ranging from 1 month to <2 years. Such values predict eruptive fluxes <135 m3 s−1 and velocities <100 m day−1, providing helpful criterion for volcanic monitoring or hazard forecasting efforts.

The enigmatic origin and emplacement of the Samosir Island lava domes, Toba Caldera, Sumatra, Indonesia

The 60 × 20-km Samosir resurgent dome within the 74-ka Toba Caldera features several clusters of rhyolitic lava domes. Because the previously known Tuk-Tuk and Samosir Fault lava domes occur near the base of a major resurgent dome fault, they appear associated with resurgent uplift, and dating them could provide constraints on the timing of resurgence. Towards that goal, we mapped and sampled the Tuk-Tuk and Samosir Fault lava domes, discovered two additional lava dome clusters in the uplifted interior of Samosir Island, then determined modal mineralogy, major and trace element geochemistry, and 40Ar/39Ar sanidine ages on several samples from each cluster. These quartz-bearing rhyolite lava domes have SiO2 contents ranging from ~ 70-76 wt%, crystallinities of 28-54%, and 40Ar/39Ar ages (~ 74-76 ka) that overlap with those of the Youngest Toba Tuff (YTT), ~ 74-75 ka. Our field observations and the remarkable similarity in mineralogy, geochemistry, and ages among the dome clusters and YTT support the interpretation that these domes represent remnant YTT magma that erupted shortly after the climactic YTT eruption. Later, during resurgence, these pre-resurgent lava domes were uplifted to their present locations, some near current lake level, others up to 440 m above lake level.

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.

Geochronology, geochemistry and zircon Hf isotope of the low Na rhyolite at Longling-Ruili belt, and its geological implications

龙陵-瑞丽早白垩世流纹岩带断续延伸约100km，其形成的构造背景是揭示怒江洋演化及腾冲与保山地块关系的重要证据。笔者在野外调研的基础上，对本区流纹岩开展了岩石学、地球化学、锆石U-Pb定年和Hf同位素等方面的研究。结果表明流纹岩具有高硅贫钠、高钾钙碱性和高分异特征；锆石LA-ICP-MS U-Pb年龄分布在129~123Ma区间，代表流纹岩的喷发时代。流纹岩锆石eHf(t)值分布于-12.3~-6.3之间，对应的地壳模式年龄为1591~1826Ma；其微量元素蛛网图、稀土元素配分模式图和Nb/Ta比值、Th/U比值等与大陆中上地壳相似；岩石中无斜长石斑晶、Sr含量极低以及在Al2O3/(MgO+FeOT)]-[CaO/(MgO+FeOT)]和Rb/Ba-Rb/Sr图解上，样品都分布在变泥质岩部分熔融区；表明流纹岩岩浆来源于古老地壳物质部分熔融的产物，其低钠是源区中斜长石含量低或斜长石未参与部分熔融造成的。流纹岩轻稀土元素和大离子亲石元素(Th、U和K)富集，Nb、Ta、Ti、P和Sr强烈负异常；在构造环境判别图解上样品全部落在活动大陆边缘区；在空间上，这些流纹岩呈带状分布，构成俯冲增生杂岩带的一部分，并与同时代侵入岩紧密相邻，成因上类似于区域内同期S型花岗岩。这些特征表明研究区内流纹岩与高黎贡构造带内同时代花岗岩类相似，形成于俯冲的大陆边缘弧环境，即幔源岩浆底侵至下地壳，这一过程提供了足够的热源使壳源物质(可能为富云母、贫斜长石的变沉积岩类)发生部分熔融生成酸性岩浆，它们在经历了高度的结晶分异后喷出地表形成低钠流纹岩，并在弧前或弧间盆地内被浅海相碎屑沉积物覆盖。

The Kumbis and Nagatis Formations and the Helmeringhausen Gabbro: Oldest undeformed rocks of the Sinclair Supergroup in Namibia

The Mesoproterozoic rocks of the Kumbis and Nagatis Formations and Helmeringhausen Gabbro in Southern Namibia are now recognised as the oldest rocks in the sedimentary and volcanic Sinclair Supergroup. The stratigraphic subdivision and plate tectonic setting are still debated and reliable ages are sparse. This work investigates the relatively undeformed felsic and mafic lava of the Kumbis Formation, rhyolitic lava of the Nagatis Formation and the intrusive Helmeringhausen Gabbro. Microscopic and field investigation reveals no evidence of regional metamorphism in these units. Ion probe U–Pb zircon dating yields ages of 1374 ± 7 Ma to 1358 ± 5 Ma (2σ) for the Kumbis Formation, 1363 ± 11 Ma for the Nagatis Formation and 1372 ± 12 Ma for the Helmeringhausen Gabbro. These dates are in contrast to the published stratigraphy in which the Helmeringhausen Gabbro was regarded as much older than the Kumbis Formation and both were excluded from the Sinclair Supergroup, whereas the Nagatis Formation was considered to be younger and was included as the oldest unit of the Sinclair Supergroup. Lu–Hf data on zircon indicates different magma sources for the Kumbis and Nagatis Formations, implying that although they are time equivalent, they are not co-magmatic. The geochemical data suggests that the most felsic rocks contain large amounts of older crustal material, from which their apparent subduction signatures were inherited. The more mafic rocks also contain minor crustal components mixed with material derived from depleted mantle. The Helmeringhausen Gabbro is similar to coeval hybrid gabbros at Neuhof Valley 150 Km to the north and they are grouped in the intrusive Helmering Suite. Taking the regional context into account, these oldest rocks of the Sinclair Supergroup probably formed in a rift environment which split the Rehoboth Province apart at the beginning of the Mesoproterozoic Namaqua-Natal Wilson Cycle.

Zeolites in acid vitreous rocks, southern Mendoza, Argentina

A mordenite-rich deposit located in the south of the province of Mendoza (Argentina) was studied. The host rock in this area has not been previously described and corresponds to rhyolitic lavas of high-K calc-alkaline series. Petrographic, mineralogical and geochemical analyses of fresh and altered rocks are shown, and a proposal for the alteration process and the stratigraphic position of the protolith is presented. The main mordenite-rich sectors developed in zones of autobreccias of high permeability, with minor clinoptilolite, smectites, secondary K-feldspars, opal-CT, and colloform silica. The proportion of relict glass vs. alteration minerals is variable and mainly depends on the texture (permeability) of the host rock. The studied rhyolites overlie the Upper Cretaceous sediments of the Neuquén Group and are covered by Tertiary basalts of the Molle Eruptive Cycle and Quaternary sediments. Geochemically, they present similarities with rhyolitic rocks of Cordón del Burrero Volcanic Complex of Lower to Middle Miocene age.

A zircon U-Pb geochronology for the Rotokawa geothermal system, New Zealand, with implications for Taupō Volcanic Zone evolution

A U-Pb zircon geochronology for the Rotokawa geothermal system, central Taupō Volcanic Zone (TVZ), New Zealand, provides new constraints on the chronostratigraphy and volcanic and structural evolution of this area and the broader TVZ. The 3-km-thick volcanic sequence at Rotokawa is mainly composed of rhyolitic ignimbrites linked to large caldera-forming events from sources outside the field area, but locally sourced andesite and rhyolite lavas and intrusions are also present. Crystallisation age spectra (and consequent estimates of eruption age) have been obtained on zircons from hydrothermally altered magmatic rocks by Secondary Ion Mass Spectrometry techniques using a SHRIMP-RG instrument. The oldest rock dated is a Tahorakuri Formation ignimbrite (eruption age estimate of 1.87 ± 0.03 Ma) which, along with comparable-age units at other nearby TVZ geothermal systems (Ngatamariki, Ohaaki), is among the oldest silicic volcanic deposits known in the TVZ. These ignimbrites collectively onlap a basal andesite lava pile, up to 1.2 km thick at Rotokawa, that in turn rests on the Mesozoic basement greywacke. The base of the lava pile is more faulted than its top surface, implying that rifting and graben formation had started along the line of the modern TVZ arc prior to 1.84 Ma and is not a younger feature. Between ~1.8 Ma and 700 ka, there are no rocks represented at Rotokawa, with the next oldest lithology being a 720 ± 90 ka rhyolite lava. At 350 ka, the Rotokawa area was buried by regionally extensive ignimbrites of the Whakamaru Group, which have since subsided by ~700 m but not been greatly faulted. Ignimbrites and sediments of the Waiora Formation were then emplaced, coevally with widespread and volumetrically greater volcanism in the Maroa and Ngatamariki areas 13 km northwest and 8 km north of Rotokawa, respectively. Local rhyolites of the Oruahineawe Formation, dated at ~100 ka, were emplaced both as extrusive domes and shallow intrusions below the area. Sedimentary rocks of the Huka Falls Formation and deposits of the 14C-dated 25.4 ± 0.2 ka Oruanui eruption capped and sealed the system, which has since been disrupted by hydrothermal eruption events. The largest of these occurred at ~6.8 ka (14C date) broadly coincident with a resumption of eruptive activity at Taupō volcano, 20 km to the south-southwest. Notable aspects of the evolution of the Rotokawa area are the early onset of rifting and subsidence along the line of the modern arc, the lack of volcanic activity for >1 Myr from 1.84 Ma to 720 ka, the lack of faulting and only modest subsidence since 350 ka, and the contrasts in volcanic and subsidence histories with other, nearby geothermal systems.

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.

Deformation of rhyolite lava crust associated with intermittent inner flow of lava: palaeomagnetic evidence

SUMMARY A palaeomagnetic study has been conducted to examine the deformation of thick crusts of rhyolite lava while its inner portions continue to flow. The Sanukayama rhyolite lava, which erupted in the Pleistocene in Kozushima Island, Japan, was chosen as the investigation site because of its well-exposed vertical lithofacies variations classified into three distinct zones (pumiceous, obsidian and crystalline). The targets of this study are the pumiceous and obsidian zones, which constitute the crust of the lava. Thermal demagnetization reveals three remanent magnetization components from the pumiceous and obsidian samples but only a single magnetization component from the inner crystalline rhyolite samples. Alternating field demagnetization is ineffective in isolating the magnetization components in the pumiceous and obsidian samples. The multiple components of remanent magnetization of the crust are interpreted to have been acquired during cooling as thermoremanent magnetizations. We suspect intermittent lava transport of the inner portions, the primary mode of rhyolite lava advancement, to be responsible for the presence of multiple components in pumice and obsidian of the lava crust. When the inner portions of the lava retain mobility to flow out of the crust, the solidified crust of the lava surface below the magnetite Curie temperature remains susceptible to deformation. Analysis of palaeomagnetic directions from the crust allows the deformation of the crust to be described in terms of rotation. Although the mode of rhyolite lava advancement is not well understood, because of its infrequent occurrence, our observations offer an important insight on how the mobile part of the lava is associated with the deformation of the crust during continued lava advance.

Structural control of mass movements on slopes formed of magmatic and metamorphic rocks: the case study of Wielisławka Mt. (SW Poland, Sudetes Mts.)

We indicate the structural controls on, and provide an evolutionary model of, mass movements which developed on the slopes of a rhyolitic lava dome built of massive, sub-intrusive Permian rhyolites and its low-grade metamorphic cover, comprising Ordovician and Silurian sericite schists and metacherts (greenschist facies). The phenomena studied occur on the low-altitude, dome-like Wielislawka Mt. (370 m a.s.l.) in the Western Sudetes, SW Poland. A multidisciplinary approach involving geological and geomorphological fieldwork, LiDAR-based geomorphometric analyses, as well as analyses based on data obtained from terrestrial laser scanning (TLS), performed within old adits and shafts in the landslide area, have allowed determination of the origin and recent extent of the landslide phenomena. The geometry and development of the slip surface are closely linked with measured, existing discontinuities within the massif. As they enable observation of the initial stages of mass movement in the excavations within the cover rocks of the rhyolitic massif, the old adits and shafts are unique objects for the observation and reconstruction of landslide processes.

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.

The Permeability Evolution of Tuffisites and Implications for Outgassing Through Dense Rhyolitic Magma

AbstractThere is growing evidence that outgassing through transient fracture networks exerts an important control on conduit processes and explosive‐effusive activity during silicic eruptions. Indeed, the first modern observations of rhyolitic eruptions have revealed that degassed lava effusion may depend upon outgassing during simultaneous pyroclastic venting. The outgassing is thought to occur as gas and pyroclastic debris are discharged through shallow fracture networks within otherwise low‐permeability, conduit‐plugging lava domes. However, this discharge is only transient, as these fractures become clogged and eventually blocked by the accumulation and sintering of hot, melt‐rich pyroclastic debris, drastically reducing their permeability and creating particle‐filled tuffisites. In this study we present the first published permeability measurements for rhyolitic tuffisites, using samples from the recent rhyolitic eruptions at Chaitén (2008–2009) and Cordón Caulle (2011–2012) in Chile. To place constraints on tuffisite permeability evolution, we combine (1) laboratory measurements of the porosity and permeability of tuffisites that preserve different degrees of sintering, (2) theoretical estimates on grainsize‐ and temperature‐dependent sintering timescales, and (3) H2O diffusion constraints on pressure‐time paths. The inferred timescales of sintering‐driven tuffisite compaction and permeability loss, spanning seconds (in the case of compaction‐driven sintering) to hours (surface tension‐driven sintering), coincide with timescales of diffusive degassing into tuffisites, observed vent pulsations during hybrid rhyolitic activity (extrusive behavior coincident with intermittent explosions), and more broadly, timescales of pressurization accompanying silicic lava dome extrusion. We discuss herein the complex feedbacks between fracture opening, closing, and sintering and their role in outgassing rhyolite lavas and mediating hybrid explosive‐effusive activity.