Andesitic Lava Flows Research Articles

Intermittent Growth of a Newly-Born Volcanic Island and Its Feeding System Revealed by Geological and Geochemical Monitoring 2013–2020, Nishinoshima, Ogasawara, Japan

The island-forming Nishinoshima eruptions in the Ogasawara Islands, Japan, provide a rare opportunity to examine how the terrestrial part of Earth’s surface increases via volcanism. Here, the sequence of recent eruptive activity of Nishinoshima is described based on long-term geological and geochemical monitoring of eruptive products. Processes of island growth and temporal changes in the magma chemistry are discussed. The growth of Nishinoshima was sustained by the effusion of low-viscosity andesite lava flows since 2013. The lava flows spread radially with numerous branches, resulting in compound lava flows. Lava flows form the coherent base of the new volcanic edifice; however, pyroclastic eruptions further developed the subaerial volcanic edifice. The duration of three consecutive eruptive episodes decreased from 2 years to a week through the entire eruptive sequence, with a decreasing eruptive volume and discharge rate through time. However, the latest, fourth episode was the most intense and largest, with a magma discharge rate on the order of 106 m3/day. The temporal change in the chemical composition of the magma indicates that more mafic magma was involved in the later episodes. The initial andesite magma with ∼60 wt% SiO2 changed to basaltic andesite magma with ∼55 wt% SiO2, including olivine phenocryst, during the last episode. The eruptive behavior and geochemical characteristics suggest that the 2013–2020 Nishinoshima eruption was fueled by magma resulting from the mixing of silicic and mafic components in a shallow reservoir and by magma episodically supplied from deeper reservoirs. The lava effusion and the occasional explosive eruptions, sustained by the discharge of magma caused by the interactions of these multiple magma reservoirs at different depths, contributed to the formation and growth of the new Nishinoshima volcanic island since 2013. Comparisons with several examples of island-forming eruptions in shallow seas indicate that a long-lasting voluminous lava effusion with a discharge rate on the order of at least 104 m3/day (annual average) to 105 m3/day (monthly average) is required for the formation and growth of a new volcanic island with a diameter on km-scale that can survive sea-wave erosion over the years.

Geomorphological, morphometric, and spatial distribution analysis of the scoria cones in the Negros de Aras monogenetic volcanic field, northern Chile

The Negros de Aras monogenetic volcanic field in the Central Volcanic Zone of the Andes displays the highest concentration of scoria cones in northern Chile (0.23 per km2). It contains 66 vents, 22 of which are emission points of lava flows without an associated recognizable volcanic edifice, whereas 44 are typical scoria cones. Most vents are associated with basaltic andesite and andesite lava flows of up to 4.6 km in length. A few scoria cones show signs of either initial or final phreatomagmatic eruptions recorded in their pyroclastic successions. A geomorphological, morphometric, and spatial distribution analysis of the scoria cones of this volcanic field was carried out with the purpose of identifying the factors and processes that governed their emplacement, growth, and final morphology. The morphometric analysis was performed on 16 well-delimited scoria cones applying the MORVOLC algorithm using the TanDEM-X 12 m resolution digital elevation model. Analysis of the distribution and elongation of the craters of the scoria cones defines four alignments inferred to be related to the local and regional structural elements of the area. Volcanic activity seems to have been related to a N-S fault system with an E-W maximum stress orientation, favoring magma ascent and scoria cone emplacement following inherited fractures in these preferential orientations. The scoria cones show a wide range of morphologies and morphometries that can be related to different syn- and post-eruptive processes. Cones with large craters and high crater width / basal width ratios show clear evidence of phreatomagmatic eruptions (within their pyroclastic successions), whereas horseshoe-type cones are related to cone breaching or coeval lava flow effusion, and more pristine (i.e., relatively younger) cones are generally steeper.

The volcanism of the Jurassic Chon Aike Silicic LIP influenced by Paleozoic inherited crustal structures in the northeastern Deseado Massif, Patagonia

An area of ∼40 km2 located in the northeastern sector of the Deseado Massif is detailed studied, in which Paleozoic metamorphic and Jurassic volcanic rocks crop out. Two genetically linked and compositionally different Jurassic volcanic events of the Chon Aike Silicic LIP are documented by geological mapping and lithofacies and petrographic analyses. The first event was only represented by andesitic lava flows assignable to the intermediate portion of the Bahía Laura Volcanic Complex (Bajo Pobre Formation). The second was represented by the acidic rocks of the Bahía Laura Volcanic Complex (Chon Aike and La Matilde formations) and it was constituted by low-grade and moderate grade ignimbrites, pyroclastic fall deposits (ash tuffs) and several lava bodies (rhyolitic lava domes and lava flows). Firstly, the effusive eruptions dominated during the intermediate magmatic eruptions, whereas the explosives mechanisms dominated during the first stage of the acidic magma extrusion, to end with effusive eruptions of previously degassed magma. According to the morphology of the NNW-elongated lava domes (feeder conduits plugs) and internal measurements carried out on these rhyolitic bodies, fissure conduits are interpreted, which were directly controlled by the Paleozoic inherited structures during the Jurassic extensional tectonic reactivation ascribed to the initial stages of the Gondwana breakup.

Late Holocene Malpaís de Zacapu (Michoacán, Mexico) andesitic lava flows: rheology and eruption properties based on LiDAR image

Few monogenetic eruptions that produced lava flows have occurred in historical times, limiting the observations of their impact on human settlements. However, rheological models based on morphological and petrological datasets can contribute to decipher the eruptive dynamics and durations of ancient eruptions. The Malpaís de Zacapu, a temporal-spatial monogenetic volcano cluster at the western margin of the Zacapu lacustrine basin (Michoacán, Mexico), offers a good opportunity to apply such models because of the availability of a high-resolution LiDAR topography from which detailed morphological data was extracted. The Malpaís de Zacapu comprises late Holocene lava flow fields emplaced in the last 3200 years by four different low magnitude volcanic eruptions: Infiernillo, Malpaís Las Víboras, Capaxtiro, and Malpaís Prieto. Jointly these eruptions produced thick andesitic block lava flows covering an area of 38.3 km2 with a volume of ~ 4.4 km3. The lava viscosities at eruption vents were estimated from petro-textural analyses and range between 103 and 106 Pa s, while the final flow apparent viscosities, obtained from dimensional analyses, vary from 108 to 1010 Pa s. We estimated the mean effusion rate and lava flow emplacement duration for each flow field. Results revealed that the more viscous flows, Malpaís Las Víboras and Malpaís Prieto, could have been emplaced in less than 3 years, while the more fluid Infiernillo probably took less than 1 year. In stark contrast, the morphologically different and more voluminous Capaxtiro flow field could have been emplaced in ~ 27 years. These findings can help to evaluate the impact that these eruptions had on adjacent pre-Hispanic populations, known to have inhabited the region since at least 3000 years ago.

Cooling history of the Biella pluton and implication for Oligocene to Miocene tectonics of the Sesia-Lanzo Zone, Austroalpine, Western Alps

ABSTRACT We present new emplacement constrains for the Biella pluton within the eclogitic Sesia-Lanzo Zone, western Austroalpine domain. This zone, with the autochthon andesitic cover, is separated from the Southalpine Ivrea Zone by the western Periadriatic Line. Thermobarometry indicates 710 and 750°C, and 0.9 and 2.3 kbar for the final pluton emplacement that accomplished at depths between 3 and 8.5 km. Soon after the emplacement, the pluton was affected by hydrothermal circulations at a progressively lower temperature. By 3D modelling, surface and volume distribution of emplacement pressure and temperature are retrieved, and isobaric surfaces are interpolated. The orientation of isobaric surfaces indicates a post-intrusive southeastward rotation of 69 ± 5° about an N 35° trending axis. These results match palaeomagnetic studies on andesites and are consistent with the pluton feeding part of the andesite lava flows. According to the updated pluton emplacement conditions and recent fission track data, we suggest that: (i) the crustal section above the pluton consisted mostly of the andesites; (ii) before 26 Ma the andesites experienced first a depositional and then a tectonic burial during rotation; (iii) between 24 and 19 Ma uplift by back-thrusting with on-going rotation started; (iv) the rotation lasted at least until 16–12 Ma.

Neogene–Quaternary Magmatism in Eastern Balkaria (North Caucasus, Russia): Evidence from the Isotope–Geochronological Data

An isotope-geochronological study for a number of young volcanic localities in the eastern part of Balkaria (North Caucasus, Russia), referred to the Neogene–Quaternary stage in the evolution of the Greater Caucasus by previous researchers, was carried out. The results of the K–Ar dating obtained have proved for the first time that volcanic activity was intense in the Late Cenozoic within the territory of modern Eastern Balkaria. It has been established that the young igneous rocks of the Pliocene (4.7–4.0 Ma) Tsana Formation with various compositions (basaltic trachyandesite, dacite, and granodiorite-porphyry) are the most common in the considered region. The area of presence of the rocks of the Tsana Formation was specified within the Russian territory of the North Caucasus. Based on the new isotopic data, the territory that is prospective for exploration of Au–As–Sb–Sn–Mo deposits often associated with young plutonic bodies of potentially ore-bearing Tsana Formation was localized. A new locus of Late Quaternary magmatism where the andesite lava flow was formed ~800–700 ka in the paleovalley of the Udursu River (near the foot of Mt. Cheget-Dzhora) was identified within the Greater Caucasus.

Rancho Seco monogenetic volcano (Michoacán, Mexico): Petrogenesis and lava flow emplacement based on LiDAR images

Given the high eruption recurrence in the Michoacán-Guanajuato volcanic field (MGVF) in central Mexico, the birth of a new monogenetic volcano can be expected in the future. It is important, therefore, to reconstruct the past eruptions of its many different volcanoes, including estimates of lava flow emplacement times and their rheological properties. These studies define the range of possible future eruptive scenarios and are necessary to evaluate potential risk. The Rancho Seco monogenetic volcano, located in the central part of the MGVF (19°37′03”N, 101°28′21”W), was radiocarbon-dated at ~27,845 years BP. Its eruption initiated with a violent-Strombolian phase that produced a scoria cone and was followed by the effusion of at least seven associated andesitic lava flows, reflecting drastic changes in the eruptive dynamics. Effusive activity probably involved decreases in the magma ascent and discharge rates linked to efficient degassing in an open system. Lava chemical composition suggest an origin of partial melting of a subduction-modified hydrated heterogenous mantle wedge and textural and mineralogical analysis indicates significant crystal fractionation and minor assimilation of granodioritic basement rocks. High-resolution LiDAR imagery was used to estimate lava flow viscosities and emplacement times by following a morphology-based methodology. Results indicate that lava flow viscosities ranged from 105 to 109 Pa·s and emplacement durations between 32 and 465 days for the flow units considered (F5 and F6). The entire eruption may have lasted from 2 to 6 years with a mean effusion rate of 4 to 15 m3/s. Our results are also pertinent to archaeologists studying the architectural remains of Angamuco, a large urban pre-Hispanic site built on Rancho Seco's lava flows.

The role of microporosity on the permeability of volcanic-hosted geothermal reservoirs: A case study from Los Humeros, Mexico

Permeability is a key parameter in the analysis of geothermal systems, usually controlled by secondary structures such as fractures, faults or stratigraphic contacts. Contrarily, primary porosity is considered to have a negligible effect on the permeability of those systems, as it is commonly observed that there is low connectivity between macropores (conventionally defined as larger than 1μm). However, we observe that at the micrometric scale this scenario is different, and connectivity is clearly identified. In this study, an updated innovative petrophysical method is developed to calculate microporosity with high precision and simulate permeability on andesite lava flow core-samples from a geothermal production well in Los Humeros Geothermal Field (LHGF), Mexico. The method is based on high-resolution X-ray microCT and numerical post-processing algorithms, producing porosity-permeability estimates, which are in good agreement with other independent petrophysical methods such as Helium porosimetry-permeameter. Our results indicate that macropores account for an average of 3.7 ± 0.9 % of total rock volume fraction, whereas microporosity is generally higher, with a volume fraction in the order of 6.2 ± 0.6 %. Furthermore, when removing microporosity from the pore network, the permeability will account for an average of 3.50 × 10−17 m2, which is only the 17.92 % of the total average permeability for all samples. On the other hand, when microporosity is added, there is an average increment in the permeability of 82.08 %. This means that more than 80 % of the permeability of the samples is due to microporosity, and that microporous space indeed plays a major role in the permeability of these rocks. These results reveal that microporosity plays an important role on the permeability of rocks from volcanic-hosted LHGF. This approach has not been previously fully considered, and in this study, we show that a detailed microporosity analysis is essential for further estimations of permeability of volcanic-hosted geothermal systems.

The Geochemistry of Magnetite and Apatite from the El Laco Iron Oxide-Apatite Deposit, Chile: Implications for Ore Genesis

Abstract The textures of outcrop and near-surface exposures of the massive magnetite orebodies (>90 vol % magnetite) at the Plio-Pleistocene El Laco iron oxide-apatite (IOA) deposit in northern Chile are similar to basaltic lava flows and have compositions that overlap high- and low-temperature hydrothermal magnetite. Existing models—liquid immiscibility and complete metasomatic replacement of andesitic lava flows—attempt to explain the genesis of the orebodies by entirely igneous or entirely hydrothermal processes. Importantly, those models were developed by studying only near-surface and outcrop samples. Here, we present the results of a comprehensive study of samples from outcrop and drill core that require a new model for the evolution of the El Laco ore deposit. Backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to investigate the textural and compositional variability of magnetite and apatite from surface and drill core samples in order to obtain a holistic understanding of textures and compositions laterally and vertically through the orebodies. Magnetite was analyzed from 39 surface samples from five orebodies (Cristales Grandes, Rodados Negros, San Vicente Alto, Laco Norte, and Laco Sur) and 47 drill core samples from three orebodies (Laco Norte, Laco Sur, and Extensión Laco Sur). The geochemistry of apatite from eight surface samples from three orebodies (Cristales Grandes, Rodados Negros, and Laco Sur) was investigated. Minor and trace element compositions of magnetite in these samples are similar to magnetite from igneous rocks and magmatic-hydrothermal systems. Magnetite grains from deeper zones of the orebodies contain >1 wt % titanium, as well as ilmenite oxyexsolution lamellae and interstitial ilmenite. The ilmenite oxyexsolution lamellae, interstitial ilmenite, and igneous-like trace element concentrations in titanomagnetite from the deeper parts of the orebodies are consistent with original crystallization of titanomagnetite from silicate melt or high-temperature magmatic-hydrothermal fluid. The systematic decrease of trace element concentrations in magnetite from intermediate to shallow depths is consistent with progressive growth of magnetite from a cooling magmatic-hydrothermal fluid. Apatite grains from surface outcrops are F rich (typically >3 wt %) and have compositions that overlap igneous and magmatic-hydrothermal apatite. Magnetite and fluorapatite grains contain mineral inclusions (e.g., monazite and thorite) that evince syn- or postmineralization metasomatic alteration. Magnetite grains commonly meet at triple junctions, which preserve evidence for reequilibration of the ore minerals with hydrothermal fluid during or after mineralization. The data presented here are consistent with genesis of the El Laco orebodies via shallow emplacement and eruption of magnetite-bearing magmatic-hydrothermal fluid suspensions that were mobilized by decompression-induced collapse of the volcanic edifice. The ore-forming magnetite-fluid suspension would have rheological properties similar to basaltic lava flows, which explains the textures and presence of cavities and gas escape tubes in surface outcrops.

Triple Oxygen (δ18O, Δ17O), Hydrogen (δ2H), and Iron (δ56Fe) Stable Isotope Signatures Indicate a Silicate Magma Source and Magmatic-Hydrothermal Genesis for Magnetite Orebodies at El Laco, Chile

Abstract The Plio-Pleistocene El Laco iron oxide-apatite (IOA) orebodies in northern Chile are some of the most enigmatic mineral deposits on Earth, interpreted to have formed as lava flows or by hydrothermal replacement, two radically different processes. Field observations provide some support for both processes, but ultimately fail to explain all observations. Previously proposed genetic models based on observations and study of outcrop samples include (1) magnetite crystallization from an erupting immiscible Fe- and P-rich (Si-poor) melt and (2) metasomatic replacement of andesitic lava flows by a hypogene hydrothermal fluid. A more recent investigation of outcrop and drill core samples at El Laco generated data that were used to develop a new genetic model that invokes shallow emplacement and surface venting of a magnetite-bearing magmatic-hydrothermal fluid suspension. This fluid, with rheological properties similar to basaltic lava, would have been mobilized by decompression-induced collapse of the volcanic edifice. In this study, we report oxygen, including 17O, hydrogen, and iron stable isotope ratios in magnetite and bulk iron oxide (magnetite with minor secondary hematite and minor goethite) from five of seven orebodies around the El Laco volcano, excluding San Vicente Bajo and the minor Laquito deposits. Calculated values of δ18O, Δ17O, δD, and δ56Fe fingerprint the source of the ore-forming fluid(s): Δ17Osample = δ17Osample – δ18Osample * 0.5305. Magnetite and bulk iron oxide (magnetite variably altered to goethite and hematite) from Laco Sur, Cristales Grandes, and San Vicente Alto yield δ18O values that range from 4.3 to 4.5‰ (n = 5), 3.0 to 3.9‰ (n = 5), and –8.5 to –0.5‰ (n = 5), respectively. Magnetite samples from Rodados Negros are the least altered samples and were also analyzed for 17O as well as conventional 16O and 18O, yielding calculated δ18O values that range from 2.6 to 3.8‰ (n = 9) and Δ17O values that range from –0.13 to –0.07‰ (n = 5). Bulk iron oxide from Laco Norte yielded δ18O values that range from –10.2 to +4.5‰ (avg = 0.8‰, n = 18). The δ2H values of magnetite and bulk iron oxide from all five orebodies range from –192.8 to –79.9‰ (n = 28); hydrogen is present in fluid inclusions in magnetite and iron oxide, and in minor goethite. Values of δ56Fe for magnetite and bulk iron oxide from all five orebodies range from 0.04 to 0.70‰ (avg = 0.29‰, σ = 0.15‰, n = 26). The iron and oxygen isotope data are consistent with a silicate magma source for iron and oxygen in magnetite from all sampled El Laco orebodies. Oxygen (δ18O Δ +4.4 to –10.2‰) and hydrogen (δ2H ≃ –79.9 to –192.8‰) stable isotope data for bulk iron oxide samples that contain minor goethite from Laco Norte and San Vicente Alto reveal that magnetite has been variably altered to meteoric values, consistent with goethite in equilibrium with local δ18O and δ2H meteoric values of ≃ –15.4 and –211‰, respectively. The H2O contents of iron oxide samples from Laco Norte and San Vicente Alto systematically increase with increasing abundance of goethite and decreasing values of δ18O and δ2H. The values of δ2H (≃ –88 to –140‰) and δ18O (3.0–4.5‰) for magnetite samples from Cristales Grandes, Laco Sur, and Rodados Negros are consistent with growth of magnetite from a degassing silicate melt and/or a boiling magmatic-hydrothermal fluid; the latter is also consistent with δ18O values for quartz, and salinities and homogenization temperatures for fluid inclusions trapped in apatite and clinopyroxene coeval with magnetite. The sum of the data unequivocally fingerprint a silicate magma as the source of the ore fluids responsible for mineralization at El Laco and are consistent with a model that explains mineralization as the synergistic result of common magmatic and magmatic-hydrothermal processes during the evolution of a caldera-related explosive volcanic system.

Jurassic volcanism of the Chon Aike Silicic LIP in the northeastern Deseado Massif

Two compositionally different volcanic cycles are documented by geological mapping, lithofacies and petrographic analyses of Jurassic volcanic rocks in the northeastern sector of the Deseado Massif, constituting the western counterpart of the Karoo-Ferrar LIPs in Patagonia. The first cycle is of intermediate composition and is represented by andesitic lava flows, phenoandesitic moderate-grade ignimbrites and volcaniclastic deposits. These rocks constitute a previously unknown large outcrop (~100 km2) of the Bajo Pobre Formation. The second cycle (Chon Aike Formation) is acidic in composition and constituted by several pyroclastic deposits (ignimbrites and co-ignimbrite lag breccias), lavic bodies (rhyolitic lava domes, dykes, and coulees) and rhyolitic pyroclastic dykes. Structural measurements collected in the studied area suggest an extensional tectonic control on the emplacement of, at least, the Chon Aike Formation rocks. We document two major NE-striking (SE-dipping) normal faults, which constitute direct field evidence of the extensional tectonic control on the Jurassic Chon Aike Silicic LIP in the Deseado Massif at the time when southern Gondwana broke up.

Late Pleistocene eruptive recurrence in the post-collisional Mt. Hasan stratovolcanic complex (Central Anatolia) revealed by zircon double-dating

Mt. Hasan (3262 m) is the second highest stratovolcano in Central Anatolia, Turkey, and is considered active based on a ca. 9 ka explosive summit eruption that was arguably depicted in a Neolithic mural at the nearby Çatalhöyük archaeological site. Besides evidence for Holocene activity of Mt. Hasan, its eruptive chronostratigraphy and hazard potential remain poorly constrained. Here, we apply (U–Th)/He thermochronology in combination with U–Th–Pb crystallization ages (zircon double-dating), including a novel population-based approach to disequilibrium correction of (U–Th)/He data, in an attempt to determine the eruption ages of strategically sampled lavas and pyroclastic deposits and to refine the Late Pleistocene chronostratigraphy of Mt. Hasan.During the Late Pleistocene, Mt. Hasan erupted multiple interfingering andesitic lava flows along with block-and-ash-flows. Seven of these lava flows, typically the stratigraphically youngest in a particular segment around the two central vents of the volcano, yielded Late Pleistocene eruption ages between 91.9 ± 3.9 and 18.1 ± 2.4 ka (uncertainties stated at 1σ). An even younger block-and-ash-flow was deposited on the western flank of the volcano at 13.5 ± 1.5 ka, which together with a ca. 9 ka small-volume explosive summit eruption define the youngest activity at the main edifice of Mt. Hasan. Collectively, these data indicate a Late Pleistocene recurrence of at least one eruptive phase every ca. 5–15 ka. New volume estimates for the Quaternary edifice of ~130–180 km3 translate into an integrated long-term eruptive magma flux of ~0.3 km3/ka, which is just slightly higher than Late Pleistocene flux estimates based on lava flow geochronology and volumes determined by analysis of digital elevation models. This long-term eruptive flux estimate is similar to those of stratovolcanoes in continental arcs, indicating that eruptive productivity in post-collisional settings can match that of active continental margins. All Mt. Hasan eruptions, including the Holocene event, sampled zircon which crystallized coeval with earlier eruptive phases, indicating longevity of the magma plumbing system. Along with ongoing fumarole activity and evidence for mid-crustal low seismic velocity zones, this suggests that a subvolcanic magma reservoir remains active to the present day.

Lower Jurassic felsic diatreme volcanism recognized in central Patagonia as evidence of along-strike rift segmentation

The early Jurassic volcanism of Central Patagonia covers an extensive area of 50,000 km2 where the volcanic deposits occur as isolated systems. The volcanic records form an elongated belt in which the composition, depositional and genetic features, show several differences along with its distribution. The Cañadón Chileno Complex (CCHC), located in the Río Negro province, provides the opportunity to evaluate and improve the knowledge about the Lower Jurassic volcanic stratigraphy and the Lower Jurassic regional setting of Central Patagonia.Based on the field and laboratory data developed in the present work, 22 lithofacies were recognized and grouped into eight facies associations. The continental sedimentary environments include alluvial fan deposits (FA 1), ephemeral deposits (FA 2), braided fluvial deposits (FA 3), alluvial plains, or over banks (FA 4), and lacustrine deposits (FA 9). On the other hand, the pyroclastic facies were separated according to the pyroclastic currents involved during the deposition; into fall-out, dilute, and density currents (FA 5, FA 6, and FA 7). Effusive andesitic feeders and lava-flows (FA 8) were also recognized.The stratigraphic data obtained in the present work allow proposing the existence of two felsic diatreme volcanoes in the CCHC records - described here as Southern and Northern Zone-, developed over a local subsided area, represented by an asymmetrical basin.Seven units were recognized and described in the CCHC (Units a, b, c, d, e, f, and g: unit (a) represents an initial stage of continental sedimentation recorded throughout the entire Complex. Unit (b) represents the upper felsic diatreme facies, in Southern Zone of the Complex, where debris flows are interbedded with massive lapilli-tuff deposits. Unit (c) consists of local andesitic lavas flows and feeder dikes, and the unit (d) consists of welded lapilli tuff deposits interpreted as the growth of the volcanic system. The Northern Zone evolves similarly with the felsic intra-diatreme deposits of unit (e). The unit (f) consisting of an effusive stage represented by extended andesitic lava flows. Finally, a deep lacustrine system is installed (unit g) that includes shallow and deep facies, with intercalations of local pyroclastic deposits.Also, new geochronological data (U-Pb zircon age of 188 ± 3 Ma) was determined to confirm and reinforce the correlation criteria between the different volcanic areas in Central Patagonia, indicating that the volcanism described here is synchronous with the Northern silica-rich calderas of the Garamilla Formation as well as the large andesitic volcanoes of the Lonco Trapial Formation located southward to the CCHC. The regional volcanism changes described, in the present work, are connected with the N-S rift segment limited by E-W regional transfer fault systems developed in continuity to the strike-slip structures of the La Esperanza area.

Oxidation during magma mixing recorded by symplectites at Kusatsu\u2013Shirane Volcano, Central Japan

Kusatsu–Shirane Volcano is an active Quaternary andesitic-to-dacitic volcano located in the Central Japan Arc. We conducted a detailed petrological investigation of orthopyroxene (opx)–magnetite (mt) symplectites associated with olivine in the Sessho lava, an andesitic lava flow from Kusatsu–Shirane. We concluded that the symplectites are pseudomorphs after olivine and were formed through the breakdown of olivine in a mafic magma as a result of oxidation during mixing with a felsic magma. Various olivines and opx–mt symplectites that show different stages of the progressive breakdown reaction of olivine coexist in a single lava flow. We suggest that basaltic recharge into the magma reservoir beneath Kusatsu–Shirane occurred repeatedly, leading to a hybrid andesite magma with different types of olivine and symplectite being erupted at Kusatsu–Shirane Volcano.

The eruptive chronology of the Yucamane-Calientes compound volcano: A potentially active edifice of the Central Andes (southern Peru)

We reconstruct the eruptive chronology of the Yucamane–Calientes compound volcano in southern Peru based on extensive fieldwork and a large dataset of geochronological (K-Ar, 40Ar/39Ar, U-Pb, and 14C) and geochemical (major and trace element) analyses. This compound volcano is composed of two edifices that have experienced discontinuous volcanic activity from the middle Pleistocene to the Holocene. The Calientes volcano has been constructed in four successive stages: Calientes I is composed of andesitic lava flows dated at ~500 ka. Subsequently, the Callazas ignimbrite (Calientes II stage) was emplaced ~160–190 ka, followed by the main cone-building stage (Calientes III) at ~130–100 ka. Finally, the Holocene Calientes domes were emplaced and represent the last eruptive products of this edifice. The Yucamane volcano has been constructed in three stages: Yucamane I consists of a succession of andesitic lava flows exposed at the base of the volcano that are older than 40 ka. Yucamane II (~36–30 ka) comprises a thick sequence of block-and-ash-flow deposits that represents an episode of dome growth predating the younger Yucamane cone (Yucamane III) built after 20–25 ka. During the Holocene, Yucamane vulcanian to sub-Plinian activity has emplaced tephra-fall and pyroclastic-density-current deposits. The most recent explosive eruptions occurred ca. 3000 BP and emplaced a tephra-fall and pumice-flow deposits. Most samples from Calientes volcano are andesites and dacites (60.1–67.7 wt% SiO2), while rocks from Yucamane volcano are basaltic-andesites to dacites (53.4–66.9 wt% SiO2). The rocks have a mineral assemblage of plagioclase, amphibole, biotite, orthopyroxene, clinopyroxene, olivine, and Fe-Ti oxides. The analyzed samples are categorized within a high-K, calc-alkaline series. Calientes volcano erupted mostly andesitic magmas, but its history is punctuated by rare eruptions of silica-rich magmas. In contrast, Yucamane volcano follows a different trend characterized by a gradual decrease in silica content through post-glacial time, from the large (VEI 3) sub-Plinian andesitic eruption of ~3 ka to moderate (VEI ≤ 2) vulcanian eruptions of basaltic-andesitic. On the basis of such recurrent and recent (Holocene), low-to-moderate explosive activity, Yucamane must be considered an active and potentially threatening volcano, which may affect the province of Candarave with ~8000 inhabitants.

14C and 40Ar/39Ar radiometric dating and geologic setting of young lavas of Rancho Seco and Mazcuta volcanoes hosting archaeological sites at the margins of the Pátzcuaro and Zacapu lake basins (central Michoacán, Mexico)

Geologic mapping of two neighbouring areas (totalling ∼770km2) centred around the Rancho Seco and Mazcuta monogenetic scoria cones and situated between the Pátzcuaro and Zacapu lake basins in the Michoacán-Guanajuato Volcanic Field (central portion of the Trans-Mexican Volcanic Belt), allowed the recognition of 62 Quaternary monogenetic volcanic structures, which include cinder cones with associated lava flows, domes, and small-to-medium sized shields. 40Ar/39Ar and 14C radiometric dating, petrography, and whole-rock chemical analyses of volcanic products provide a stratigraphic framework and insight into different eruptive styles, and erupted volumes (∼45km3 of calc-alkaline magma). Pliocene to Holocene monogenetic activity has been frequent in this region, forming structures, which are generally aligned along an ENE-WSW direction associated to the Morelia-Cuitzeo-Acambay normal fault system.The young Rancho Seco and Mazcuta volcanoes are of particular interest because their distal lava flows host the pre-Hispanic archaeological urban centres of Angamuco and Cortijo Viejo, respectively. The Rancho Seco scoria cone was radiocarbon-dated at 27,845+445/−425yr BP and emitted at least six andesitic lava flows which were emplaced toward the SW (Lake Pátzcuaro) covering an area of 21.3km2 with a volume of ∼0.64km3. The Mazcuta scoria cone dated at 7970±135yr BP produced five basaltic andesite-andesite lava flows toward the NW (Lake Zacapu), where they cover an area of 28.85km2 with a volume of ∼0.57km3. The recently studied archaeological site of Angamuco became an important urban centre during the rise of the Tarascan Empire in the Post-Classic period (AD 900–1521), while the Cortijo Viejo site remains largely unexplored, but its occupation might fall within the same period. Although the young lava flows on which these sites are built seem inhospitable and barren with a rugged topography and rocky substrate unfit for agriculture, early inhabitants adapted and modified its surface according to their needs. Hence, these archaeological sites exemplify how a seemingly hostile landscape was ingeniously modified to establish population centres with patterns similar to those observed on lava flows elsewhere.

Dextral, normal, and sinistral faulting across the eastern California shear zone–Mina deflection transition, California-Nevada, USA

AbstractStrike-slip faults commonly include extensional and contractional bends and stepovers, whereas rotational stepovers are less common. The Volcanic Tableland, Black Mountain, and River Spring areas (California and Nevada, USA) (hereafter referred to as the VBR region) straddle the transition from the dominantly NW-striking dextral faults that define the northwestern part of the eastern California shear zone into a rotational stepover characterized by dominantly NE-striking sinistral faults that define the southwestern Mina deflection. New detailed geologic mapping, structural studies, and 40Ar/39Ar geochronology across the VBR region allow us to calculate Pliocene to Pleistocene fault slip rates and test predictions for the kinematics of fault slip transfer into this rotational stepover. In the VBR, Mesozoic basement is nonconformably overlain by a Miocene sequence of rhyolite, dacite, and andesite volcanic rocks that yield 40Ar/39Ar ages between 22.878 ± 0.051 Ma and 11.399 ± 0.041 Ma. Miocene rocks are unconformably overlain by an extensive sequence of Pliocene basalt and andesite lava flows and cinder cones that yield 40Ar/39Ar ages between 3.606 ± 0.060 Ma and 2.996 ± 0.027 Ma. The Pliocene sequence is, in turn, unconformably overlain by Quaternary tuffs and sedimentary rocks. This sequence of rocks is cut by NS- to NW-striking normal faults across the Volcanic Tableland that transition northward into NS-striking normal faults across the Black Mountain area and that, in turn, transition northward into NW-striking dextral and NE-striking sinistral faults in the River Spring area. A range of geologic markers were used to measure offset across the faults in the VBR, and combined with the age of the markers, yield minimum ∼EW-extension rates of ∼0.5 mm/yr across the Volcanic Tableland and Black Mountain regions, and minimum NW-dextral slip and NE-sinistral slip rates of ∼0.7 and ∼0.3 mm/yr, respectively, across the River Spring region. In the River Spring area, our preferred minimum dextral slip and sinistral slip rates are 0.8–0.9 mm/yr and 0.7–0.9 mm/yr, respectively. We propose three kinematic fault slip models, two irrotational and one rotational, whereby the VBR region transfers a portion of dextral Owens Valley fault slip northwestward into the Mina deflection. In irrotational model 1, Owens Valley fault slip is partitioned into two components, one northeastward onto the White Mountain fault zone and one northwestward into the Volcanic Tableland. Slip from the two zones is then transferred northward into the southwestern Mina deflection. In irrotational model 2, Owens Valley fault slip is partitioned into three components, with the third component partitioned west-northwest onto the Sierra Nevada frontal fault zone. In the rotational model, predicted sinistral slip rates across the southwestern Mina deflection are at least 115% greater than our observed minimum slip rates, implying our minimum observed rates underestimate true sinistral slip rates. A comparison of summed geologic fault slip rates, parallel to motion of the Sierra Nevada block relative to the central Great Basin, from the Sierra Nevada northeastward across the VBR region and into western Nevada are the same as geodetic rates, if our assumptions about the geologic slip rate across the dextral White Mountain fault zone is correct.

Post-emplacement dynamics of andesitic lava flows at Volcán de Colima, Mexico, revealed by radar and optical remote sensing data

We used optical and radar remote sensing datasets to map, estimate the volume, and measure the surface displacements of lava flows emplaced on the flanks of Volcán de Colima, Mexico by extrusion of lava dome material from the end of 2014 to early 2016. Our main result is that the flow motion of the lava contributes significantly to the recorded displacements several months after its emplacement. First, we mapped the deposits and estimated their volumes using two Digital Elevation Models (DEM), one derived from radar data acquired before the peak of activity and one derived from optical images acquired just after this peak of activity. Coherence information derived from the radar dataset added some temporal constraints on the timing of emplacement of various deposits. We thus estimated a mean extrusion rate of 1–2 m3 s−1 between November 2014 and February 2015. We then used a new approach to reconstruct the 3D displacement field, taking advantage of images acquired by the same satellite, on both ascending and descending tracks, and using a physical a priori on the direction of horizontal displacements. Our results show that about 2 cm yr−1 of horizontal motion is still recorded a few months after the emplacement on the SW lava flow, which is the only one covered by the two-acquisition geometries. In order to differentiate the potential causes of the observed displacements, we modeled the thermal contraction of the lava flow using a finite element numerical method. Removing the contribution of thermoelastic contraction from the measured displacements enable to infer both the viscoelastic loading and flow motion effects from the residuals. Results show that, thermal contraction, flow motion and viscoelastic loading contribute significantly to the displacements recorded.

The Central Atlantic Magmatic Province (CAMP) in Morocco

AbstractThe Central Atlantic Magmatic Province (CAMP) is a large igneous province (LIP) composed of basic dykes, sills, layered intrusions and lava flows emplaced before Pangea break-up and currently distributed on the four continents surrounding the Atlantic Ocean. One of the oldest, best preserved and most complete sub-provinces of the CAMP is located in Morocco. Geochemical, geochronologic, petrographic and magnetostratigraphic data obtained in previous studies allowed identification of four strato-chemical magmatic units, i.e. the Lower, Intermediate, Upper and Recurrent units. For this study, we completed a detailed sampling of the CAMP in Morocco, from the Anti Atlas in the south to the Meseta in the north. We provide a complete mineralogical, petrologic (major and trace elements on whole-rocks and minerals), geochronologic (40Ar/39Ar and U–Pb ages) and geochemical set of data (including Sr–Nd–Pb–Os isotope systematics) for basaltic and basaltic–andesitic lava flow piles and for their presumed feeder dykes and sills. Combined with field observations, these data suggest a very rapid (<0·3 Ma) emplacement of over 95% of the preserved magmatic rocks. In particular, new and previously published data for the Lower to Upper unit samples yielded indistinguishable 40Ar/39Ar (mean age = 201·2 ± 0·8 Ma) and U–Pb ages (201·57 ± 0·04 Ma), suggesting emplacement coincident with the main phase of the end-Triassic biotic turnover (c.201·5 to 201·3 Ma). Eruptions are suggested to have been pulsed with rates in excess of 10 km3/year during five main volcanic pulses, each pulse possibly lasting only a few centuries. Such high eruption rates reinforce the likelihood that CAMP magmatism triggered the end-Triassic climate change and mass extinction. Only the Recurrent unit may have been younger but by no more than 1 Ma. Whole-rock and mineral geochemistry constrain the petrogenesis of the CAMP basalts. The Moroccan magmas evolved in mid-crustal reservoirs (7–20 km deep) where most of the differentiation occurred. However, a previous stage of crystallization probably occurred at even greater depths. The four units cannot be linked by closed-system fractional crystallization processes, but require distinct parental magmas and/or distinct crustal assimilation processes. EC-AFC modeling shows that limited crustal assimilation (maximum c.5–8% assimilation of e.g. Eburnean or Pan-African granites) could explain some, but not all the observed geochemical variations. Intermediate unit magmas are apparently the most contaminated and may have been derived from parental magmas similar to the Upper basalts (as attested by indistinguishable trace element contents in the augites analysed for these units). Chemical differences between Central High Atlas and Middle Atlas samples in the Intermediate unit could be explained by distinct crustal contaminants (lower crustal rocks or Pan-African granites for the former and Eburnean granites for the latter). The CAMP units in Morocco are likely derived from 5–10% melting of enriched peridotite sources. The differences observed in REE ratios for the four units are attributed to variations in both source mineralogy and melting degree. In particular, the Lower basalts require a garnet peridotite source, while the Upper basalts were probably formed from a shallower melting region straddling the garnet–spinel transition. Recurrent basalts instead are relatively shallow-level melts generated mainly from spinel peridotites. Sr–Nd–Pb–Os isotopic ratios in the CAMP units from Morocco are similar to those of other CAMP sub-provinces and suggest a significant enrichment of the mantle-source regions by subducted crustal components. The enriched signature is attributed to involvement of about 5–10% recycled crustal materials introduced into an ambient depleted or PREMA-type mantle, while involvement of mantle-plume components like those sampled by present-day Central Atlantic Ocean Island Basalts (OIB, e.g. Cape Verde and Canary Islands) is not supported by the observed compositions. Only Recurrent basalts may possibly reflect a Central Atlantic plume-like signature similar to the Common or FOZO components.

Magmatic differentiation at La Poruña scoria cone, Central Andes, northern Chile: Evidence for assimilation during turbulent ascent processes, and genetic links with mafic eruptions at adjacent San Pedro volcano

La Poruña (21°53′S; 68°30′W) is a 140 m high scoria cone composed of pyroclastic material and an extensive basaltic-andesite to andesite lava flow that is up to 8 km in length. Automated mineralogical analysis describes a suite of porphyritic mafic samples, comprising olivine- and pyroxene-bearing rocks. Well-defined major element compositional trends, as well as trace and rare earth element characteristics (e.g. Sr/Y < 47; Sm/Yb < 4), likely reflect magmatic differentiation at middle-upper crustal levels. Additionally, magma mixing and assimilation and fractional crystallization processes act on these La Poruña magmas within the thickened continental crust, which is typical in Andean volcanic systems. A remarkable compositional feature is the unusual reversed isotopic behaviour of increasing silica with decreasing 87Sr/86Sr compositions. In a process of crustal assimilation during turbulent magma ascent (ATA), the least differentiated rocks are the most contaminated ones since the turbulent hottest magmas effectively assimilate the crustal material. We relate the inverse Sr isotope trend to latter magmatic evolution involving ATA at shallow crustal levels prior to eruption, therefore differing from the broadly accepted Central Andean magmatic model. The older volcanics (>96 ka) from San Pedro volcano exhibit similar isotopic characteristics, therefore evidence of similar magmatic processes. This new dataset clearly defines magma compositional changes during the La Poruña eruption (ca. 100 ka), revealing an increase in crustal contamination at shallow crustal levels for the younger San Pedro lavas (<96 ka), likely controlled by increasing amounts of deep-sourced basaltic input over time.