Thick Lava Flows Research Articles

Stratigraphy and structure of Chachahuén volcanic complex, southern Mendoza province, Argentina

The Chachahuén Volcanic Complex is an eroded Upper Miocene arc to back-arc volcanic system that was emplaced in the north-eastern edge of the Neuquén Basin, southern Mendoza province, Argentina. It was formed as the result of shallowing followed by steepening of the Nazca plate during Miocene-Pliocene, and is characterized by a volcanism which ranges from trachydacites to basalts. Thanks to erosion, the stratigraphy and the shallow plumbing systems of the volcano are well-exposed. A new evolution for the volcano stratigraphy is proposed, based on new field observations, satellite imagery, and incorporating the radiometric and petrological data of previous works. The stratigraphy is constrained around one main explosive event, recorded by a thick dacitic pyroclastic density current (PDC) deposit (the Corrales Ignimbrite) (>150m). The main units forming the Chachahuén Volcanic Complex are (1) the pre-dacite PDC deposits (trachydacitic to rhyolitic) Vizcachas Formation, (2) the thick Corrales Ignimbrite, (3) a post-dacite trachyandesitic PDC deposits (dominated by block and ash (BAF) deposits) and thick lava flows, and (4) thin mafic basaltic lava flows. The collapse of a large elliptical caldera occurred during the Trachyandesite phase. The main magma transport channels and feeders are sub-vertical dykes, which exhibit a radial distribution centred on the main caldera depression. Moreover, cryptodomes mainly preserved outside the rim of the caldera accommodated the transport and emplacement of the more silicic magma during the Trachyandesite phase.

The lithologic diversity of the Moon recorded in lunar meteorites Northwest Africa 7611 and 10480

AbstractNorthwest Africa (NWA) 7611/10480 are lunar regolith breccia meteorites, composed of mineral fragments and various clasts including mare basalts, volcanic glasses, gabbroic lithologies, and a diverse variety of highland materials (ferroan anorthosite, Mg‐suite, magnesian anorthosite, and alkali suite rocks) as well as different subvarieties of impact melt breccia. The Apollo two‐component mixing model calculation reveals that the NWA 7611 source region contains 58 wt% mare materials and 42 wt% highland components, but the estimated mare components in NWA 10480 have a higher abundance (66 wt%). The predominantly very low‐Ti (VLT) composition in both fine‐grained basaltic and coarse‐grained gabbroic lithologies indicates a provenance associated with a thick lava flow or a single magmatic system. The co‐occurrence of zoning patterns and fine‐scale exsolution lamellae in pyroxene debris supports a cryptomare deposit as the best candidate source. Phosphate Pb–Pb ages in matrix fragments, impact melt breccia, and basaltic clast indicate that the breccia NWA 7611 records geological events spanning approximately 4305–3769 Ma, which is consistent with the ages of ancient lunar VLT volcanism and the products of basin‐forming impacts on the lunar nearside. The youngest reset age at ~3.2 Ga is potentially related to the strong shock lithification process of breccia NWA 7611. Moreover, the similar petrology, texture, geochemistry, cosmic‐ray exposure data, and crystallization ages support that basaltic component in Yamato (Y)‐793274, and Queen Alexandra Range (QUE) 94281, NWA 4884, and NWA 7611 clan came from the same basalt flow.

Compositional analysis of Apollo 12 Granitic Breccia 12013: Insights into protoliths and formation

We report bulk rock composition and mineral chemistry of previously unstudied fragments and thin sections of lunar granitic breccia 12013. Instrumental Neutron Activation Analysis data on 25 fragments indicate that the 12013 breccia can be described as a three-component system comprising granitic, rare earth element (REE)-rich, and mafic components. The granitic component is low in FeO and REE but rich in K2O, Th, and associated incompatible elements. The REE-rich component has an elevated concentration of REE, moderate FeO, and low concentrations of those elements that are high in the granitic component, in other words, its composition is complementary to that of the granitic component. The mafic component is richest in FeO, and low in those elements that are concentrated in the granitic and REE-rich components. Petrographically, the REE-rich component is an impact melt breccia with a composition unique to Apollo 12 impact melt breccias. Trace-element concentrations in the REE-rich component indicate that its protolith is possibly monzogabbro, although its composition is significantly more magnesian than other known lunar monzogabbros. The mafic component is dominated by fine-grained pyroxene and plagioclase that appear to have recrystallized after impact. However, preserved lithic clasts with an igneous texture occur in the mafic component and are inferred to represent its protolith. The textures of the preserved mafic lithic fragments and lack of Mg-Fe zoning in pyroxene indicate that the protolith of the mafic component formed either as a crustal intrusion or at the bottom of a thick lava flow. The composition of the mafic component is unlike any previously studied mare basalt samples. Textures of this complex breccia suggest that the granitic breccia was first incorporated with the mafic component to form the “gray breccia”, and that the gray breccia was then incorporated with the REE-rich component while the components were still in a hot, plastic state. The complementary trace-element compositions, including the REE patterns, indicate that the petrogenesis of the granitic and REE-rich components are related. We infer that silicate-liquid immiscibility is likely not the formation mechanism for the 12013 components. Instead, bimodal volcanism owing to basaltic underplating may have led to the formation of the 12013 components.

Determining the thermal histories of Apollo 15 mare basalts using diffusion modelling in olivine

Mare basalts collected at the Apollo 15 landing site can be classified into two groups. Based on differing whole-rock major element chemistry, these groups are the quartz-normative basalt suite and the olivine-normative basalt suite. In this study we use modelling of Fe-Mg interdiffusion in zoned olivine crystals to investigate the magmatic environments in which the zonation was formed, be that within the lunar crust or during cooling within a surficial lava flow, helping to understand the thermal histories of the two basalt suites. Interdiffusion of Fe-Mg in olivine was modelled in 29 crystals in total, from six olivine-normative basalt thin sections and from three quartz-normative basalt thin sections. We used a dynamic diffusion model that includes terms for both crystal growth and intracrystalline diffusion during magma cooling. Calculated diffusion timescales range from 5 to 24 days for quartz-normative samples, and 6 to 91 days for olivine-normative samples. Similarities in diffusion timescales point to both suites experiencing similar thermal histories and eruptive processes. The diffusion timescales are short (between 5 and 91 days), and compositional zonation is dominated by crystal growth, which indicates that the diffusion most likely took place during cooling and solidification within lava flows at the lunar surface. We used a simple conductive cooling model to link our calculated diffusion timescales with possible lava flow thicknesses, and from this we estimate that Apollo 15 lava flows are a minimum of 3–6 m thick. This calculation is consistent with flow thickness estimates from photographs of lava flows exposed in the walls of Hadley Rille at the Apollo 15 landing site. Our study demonstrates that diffusion modelling is a valuable method of obtaining information about lunar magmatic environments recorded by individual crystals within mare basalt samples.

Lava Flow Hazard and Its Implication in Geopark Development for the Active Harrat Khaybar Intracontinental Monogenetic Volcanic Field, Saudi Arabia

Harrat Khaybar is an active monogenetic volcanic field in western Saudi Arabia that hosts spectacular monogenetic volcanoes and a Holocene volcanic cone with extensive lava fields. The volcanic region is a subject of intensive land use development, especially along tourism ventures, where the volcanic features are the key elements to utilize for increasing visitation rates to the region. The youngest eruption is suspected to be Holocene and occurred fewer than 5000 years ago based on the cross-cutting relationship between the youngest lava flows and archaeological sites. Lava flows are typical, from pāhoehoe to ‘a‘ā types with great diversity of transitional textural forms. Here, we recorded typical transitional lava flow surface textures from the youngest flows identified by digital-elevation-model-based terrain analysis, satellite imagery, and direct field observations. We performed lava flow simulations using the Q-LavHA plug-in within the QGIS environment. Lava flow simulations yielded satisfactory results if we applied eruptions along fissures, long simulation distances, and ~5 m lava flow thickness. In these simulations, the upper flow regimes were reconstructed well, but long individual lava flows were not possible to simulate, suggesting that morphological steps likely promoted lava ponding, inflation, and sudden deflation by releasing melts further along shallow syneruptive valley networks.

Orogenic magmatism along the transition Precordillera-Sierras Pampeanas, between 29° and 31° S, Argentina

New geochemical data and petrological analysis of Neogene volcanic units from the Pampean Flat Slab Segment (29°-31°SL) are presented in this contribution. Rocks were collected along a cross-section from the Western Sierras Pampeanas to the Eastern Precordillera. The outcrops delimit areas with a sub-parallel disposition to the orogenic front and main Andean regional structures. The study involves several volcanic rocks of the Formación Cerro Morado, sampled from the Huachi eruptive center and the Helvecia and Portezuelo de Los Burros mine districts, with thick lava flows and pyroclastic rocks. Lavas and breccias from several medium size outcrops near Huaco are also analyzed. To the NE, near the town of Guandacol, the Unidad Volcánica El Corral crops out. These rocks constitute the easternmost outcrops of the Precordillera in San Juan. We also studied rocks from the La Ciénaga volcano of the Sierra del Pocho Volcanic Field within the Sierras Pampeanas of Cordoba, which are the easternmost magmatic expression of the volcanic arc in this shallow subduction zone of the Central Andes.The new geochemical and isotopic data obtained from Formación Cerro Morado and lava flows belonging to the Unidad Volcánica El Corral, and its comparison with new geochemical data of the La Ciénaga volcano of the Sierra de Pocho show that these rocks result from the melting of two sources coming from two different types of crustal basements (Precordillera and Sierras Pampeanas). We found a magmatic association with similar geochemical features within the Formación Cerro Morado in Huachi region and Helvecia mine and Agua de Los Burros. It shows abundant hydrothermal fluids, which are often related to mineralization of economic interest.On the other hand, the rocks of the Formación Cerro Morado from Huaco, of the Unidad Volcánica El Corral and El Pocho Volcanic Field have a similar isotopic signature that allows us to infer the participation of the underlying Sierras Pampeanas Crust in their development. This feature leads us to interpret the continuity of the Sierras Pampeanas Basement Crust below the Western Precordillera.

Environmental risk assessment associated with acidic volcanics in Egypt

The present work aims to study gamma rays emitted by radionuclides such as 238U, 232Th and 40K from acidic Monqul volcanics. The studied volcanics are represented by a thick stratified lava flows interbanded with their pyroclastics. They are composed of thick lava flows of andesite and, to a lesser extent of basalt, and acidic volcanics including rhyolite and dacite. The average values of 238U, 232Th and 40K are (46 ± 24 Bq kg-1), (62 ± 11 Bq kg-1) and (1227 ± 318 Bq kg-1) in the rhyolite-dacite samples are greater than the worldwide average. The variation of radioactive bearing minerals observed inside granite faults produced the great amounts of radioactivity perceived in the samples. Calculating radiological risks is used to assess the public's radioactive risk from radionuclides revealed in the studied Rhyolite-dacite samples. The acceptable limit for excess lifetime cancer (ELCR) evaluations has been exceeded. As a result, Rhyolite-dacite are inappropriate for apply in building materials.

The integrated history of repeated caldera formation and infill at the Okataina Volcanic Centre: Insights from 3D gravity and magnetic models

Multistage collapse caldera create complex geological structures that are often buried by kilometers of pyroclastic infill and late stage lava domes, making study of their origins difficult from outcrop alone. Here we present new gravity and aeromagnetic data compilations derived from terrestrial, lake and airborne surveys to investigate the buried internal structure of the Okataina Volcanic Centre (OVC) and interpret its stages of development. Magnetic highs (1300 nT) are caused by a combination of thick lava flows and domes that infill the collapse structures, and a deeper feeder structure/dyke complex that extends below the basement to at least 6 km depth. Magnetic lows caused by hydrothermal alteration are associated with topographically low areas near the topographic and structural collapse margins suggesting fluid circulation within the caldera exploits both deep and shallow structures. Several large rhyolite lava flows contain low magnetisation rims reflecting the glassy outer carapace and crystalline core.The gravity data show a − 62 mGal residual gravity low associated with the OVC, with the steepest gradients occurring inside the topographic margins. The gradient of the gravity low is stepped towards its lowest point near the outlet of Lake Tarawera. Each step is interpreted as relating to the buried structural collapse margins of the Utu, Matahina, Rotoiti and possibly Kawerau, catastrophic caldera forming eruptions, creating an overlapping and nested caldera structure, however there is no evidence for an OVC encircling ring structure. Buried caldera margins associated with oldest Utu eruption may play a role in the location of the youngest Tarawera eruptions. We propose smaller amplitude gravity lows that extend outside the topographic margins of the caldera are related to lateral magma migration to the south-west, exploiting regional tectonic stress regimes, towards eruption vents within the caldera. 3D gravity inversion, including models numerically constrained by a 3D magnetotelluric model, suggest a caldera depth of 5000 ± 500 m that accumulated over multiple collapse episodes along with rifting induced subsidence in the past 550 ka.Our multiphysics approach to investigating caldera structure has revealed buried structural margins and their relationships to eruptive vents and hydrothermal features at the OVC. Detailed geophysical models of caldera structure are rare globally, and our findings are consistent with analogue and numerical models of caldera formation that accompanies the most violent volcanic eruptions on Earth.

The Los Angeles martian diabase: Phosphate U-Th-Pb geochronology and mantle source constraints

In situ laser ablation inductively coupled mass spectrometry (LA ICP-MS) is used to determine the U-Th-Pb age of the phosphates ferromerrillite and apatite in the Los Angeles shergottitic meteorite. The initial 207Pb/206Pb was refined by analyzing K-rich diaplectic glass. LA ICP-MS mapping was used to document zones of elevated U and Th content and to establish textural controls on isotope ages. By critically assessing dispersion in the U-Th-Pb dataset due to Pb-diffusion in phosphates during high-temperature shock metamorphism, and as a result of subsequent terrestrial contamination, we obtain a best-estimate U-Pb age of 169 ± 5 Ma anchored at an initial 207Pb/206Pb of 0.98390 ± 0.00018. This is statistically indistinguishable from a joint-isochron age of 179 ± 6 with initial 208Pb/206Pb of 2.5151 ± 0.0028. These results complement previously determined Rb-Sr and Sm-Nd isotope ages and provide independent evidence for LA having crystallized as a medium-grained basic rock from a thick lava flow or high-level intrusion in the late Amazonian at ∼170 Ma. In the context of martian mantle evolution, the initial common-Pb values suggest that Los Angeles originated from a source (µ2 ∼ 3.2) that is similar to enriched members of the shergottite meteorite clan. The U-Th-Pb systematics of both ferromerrillite and apatite were locally affected by diffusive Pb-loss in thin U-enriched marginal domains and more profoundly in shock-induced melt pockets where temperatures briefly exceeded 2000 °C. The results reveal: (1) how precise U-Pb ages can be attained from phosphates; (2) the importance of microtextural contextualization of isotope data; (3) that the timescales of cooling from shock conditions were sufficient to promote local diffusive re-equilibration of Pb over 10 s of microns; and (4) that LA ICP-MS mapping can be used to locate domains with the highest U/Pb and Th/Pb, which increases precision on lower intercept ages and isochron regression lines.

The interactions of volcanism and clastic sedimentation in rift basins: Insights from the Palaeogene‐Neogene Shaleitian uplift and surrounding sub‐basins, Bohai Bay Basin, China

AbstractAlthough volcanism is an important process in the evolution of rift basins, current tectono‐sedimentary models largely neglect its impact on sediment supply, transport pathways, and depositional systems. In this paper, we integrate core, well logs, and 3D seismic data from the Palaeogene‐Neogene Shaleitian (SLT) uplift and surrounding sub‐basins, Bohai Bay Basin, China, to investigate the sedimentology and geomorphology of a volcanic rift basin. Results of this study show that the spatial distribution of extrusive centres was strongly controlled by basement‐involved intra‐basin normal faults. During the early part of the syn‐rift stage, the SLT uplift supplied sediments to transverse fan deltas and braided‐river deltas that fringed the adjacent syn‐rift depocentres. Volcanic deposits mainly occurred as relatively thin lava flow and pyroclastic facies that partially filled fault‐controlled topographic lows, reducing topographic rugosity, and enhanced breaching of basement highs between syn‐rift depocentres. Integration of drainage to the syn‐rift depocentres and development of through‐flowing axial depositional systems was enhanced. During the later part of syn‐rift and in early post‐rift stages, the SLT uplift was progressively inundated, reducing sediment supply to the fringing transverse depositional systems. In contrast, axial braided‐river deltas became the main depositional systems, sourced by large hinterland drainage from the Yanshan fold‐belt to the northwest. Volcanism in the late syn‐rift and early post‐rift occurs as thick lava flow and pyroclastic facies that infill rift topographic lows and locally blocked axial fluvial systems creating isolated lakes. Within hanging wall depocentres, volcanic topographic highs split and diverted axial fluvial and deltaic systems. Furthermore, volcanism supplied large volumes of volcanic sediment to the rift resulting in increased sedimentation rates, and the development of unstable subaerial and subaqueous slopes and deposits, increasing the occurrence of landslides. Based on the observations of this study we update tectono‐sedimentary models for rift basins to include volcanism.

Some Lava Flows May Not Have Been as Thick as They Appear

AbstractIndividual lava flows in flood basalt provinces are composed of sheet pāhoehoe lobes and the 10–100 m thick lobes are thought to form by inflation. Quantifying the emplacement history of these lobes can help infer the magnitude and temporal dynamics of prehistoric eruptions. Here we use a phase‐field model to describe solidification and remelting of sequentially emplaced lava lobes to explore additional processes that may lead to thick flows and lobes. We calibrate parameters using field measurements at Makaopuhi lava lake. We vary the lobe thicknesses and the time interval between eruptions to study the interplay between these factors and their impact on the thermal evolution of flows. Our analysis shows that if the time between emplacements is sufficiently short, remelting may merge sequentially emplaced lobes—making lava flows appear thicker than they actually were—which suggests that fused lobes could be another mechanism that creates apparently thick lava flows.

Stratigraphic reconstruction of the Víti breccia at Krafla volcano (Iceland): insights into pre-eruptive conditions priming explosive eruptions in geothermal areas

Krafla central volcano in Iceland has experienced numerous basaltic fissure eruptions through its history, the most recent examples being the Mývatn (1724‒1729) and Krafla Fires (1975–1984). The Mývatn Fires opened with a steam-driven eruption that produced the Víti crater. A magmatic intrusion has been inferred as the trigger perturbing the geothermal field hosting Víti, but the cause(s) of the explosive response remain uncertain. Here, we present a detailed stratigraphic reconstruction of the breccia erupted from Víti crater, characterize the lithologies involved in the explosions, reconstruct the pre-eruptive setting, fingerprint the eruption trigger and source depth, and reveal the eruption mechanisms. Our results suggest that the Víti eruption can be classified as a magmatic-hydrothermal type and that it was a complex event with three eruption phases. The injection of rhyolite below a pre-existing convecting hydrothermal system likely triggered the Víti eruption. Heating and pressurization of shallow geothermal fluid initiated disruption of a scoria cone “cap” via an initial series of small explosions involving a pre-existing altered weak zone, with ejection of fragments from at least 60-m depth. This event was superseded by larger, broader, and dominantly shallow explosions (~ 200 m depth) driven by decompression of hydrothermal fluids within highly porous, poorly compacted tuffaceous hyaloclastite. This second phase was triggered when pressurized fluids broke through the scoria cone complex “cap”. At the same time, deep-rooted explosions (~ 1-km depth) began to feed the eruption with large inputs of fragmented rhyolitic juvenile and host rock from a deeper zone. Shallow explosions enlarging the crater dominated the final phase. Our results indicate that at Krafla, as in similar geological contexts, shallow and thin hyaloclastite sequences hosting hot geothermal fluids and capped by low-permeability lithologies (e.g. altered scoria cone complex and/or massive, thick lava flow sequence) are susceptible to explosive failure in the case of shallow magmatic intrusion(s).

Planet Mercury: Volcanism in a theatre of global contraction, with examples from the Hokusai quadrangle

Mercury's geological history has been dominated by global contraction caused by secular cooling of the planet's interior. This cooling has had a profound effect on the expression of the planet's volcanism and tectonism, and the expressions of these two surface evolutionary processes are deeply intertwined. Here, we use case studies from the Hokusai quadrangle of Mercury to gain insight into the interplay between Mercury's volcanism and tectonism, which we review throughout this paper. We perform the first crater size–frequency analysis of the southernmost extent of Borealis Planitia, Mercury's largest expanse of volcanic plains, and find that it formed ~3.8–3.7 Ga. We discuss the importance of “intermediate plains”, a widespread unit in the Hokusai quadrangle, as the manifestation of relatively low-volume effusions with an uncertain stratigraphic relationship with Borealis Planitia. Finally, we detail the formation of the Suge Facula pitted ground during the geological history of Rachmaninoff crater, and hypothesise that such textures probably formed more widely on Mercury but have often either been buried by thick lava flows or otherwise obscured. Unanswered questions in this work can be used to drive the next phase of Mercury exploration and research with the arrival of the BepiColombo mission.

Geological evolution of the Guallatiri volcano, Arica y Parinacota Region, northern Chile

Guallatiri volcano is an active stratovolcano located in the Arica y Parinacota Region of northern Chile. It belongs to the Central Volcanic Zone (CVZ) of the Andean range and is the youngest and southernmost volcano of the Nevados de Quimsachata volcanic chain. Guallatiri is an ice-capped volcano considered the third-highest risk center in northern Chile with a current activity characterized by two fumarolic fields with strong and persistent gas emissions. This work presents the first detailed geological mapping of the volcano based on stratigraphic, petrographic, morphological, and geochemical data. The volcano has evolved into seven units, whit the initial stages characterized by the widespread and effusive emission of several thick lava flows around the volcanic edifice. The later evolution units involved the extrusion of lava domes at the summit, with a Holocene plinian eruption accompanied by pyroclastic density currents, tephra fallout, and lahars. Furthermore, two parasitic Holocene domes, Tinto and South domes were nested on the volcano's southwestern flank. High eruptive rates (0.36 km3/ka) indicate that Guallatiri volcano has been one of the most active volcanoes during the Late Pleistocene-Holocene. The volcanic products from Guallatiri mainly correspond to trachyandesites, trachydacites, and dacites rich in amphibole and biotite, with minor clinopyroxene and olivine. They belong to the High K2O Calc-Alkaline Series, similar to other active volcanoes of the region. The construction of detailed geological maps of active volcanoes is used as the basis for the generation of hazard and risk maps, which enable the mitigation of associated volcanic risks. In the case of Guallatiri, the occurrence of lahars, pyroclastic flows, and tephra fallout are considered the main volcanic hazards, which could affect the villages of Guallatire and Ancuta, in addition to small hamlets in Bolivia.

Thickness of Lava Flows Within the Northern Smooth Plains on Mercury as Estimated by Partially Buried Craters

AbstractPartially and completely buried impact craters are widely distributed over the northern smooth plains of Mercury. Using imagery data acquired from the MESSENGER mission, we compiled a database of 257 partially buried and 765 completely buried craters on Mercury. The elevation profiles of fresh craters were constructed, and by inputting these into a topographic degradation model, we estimated the lava flow thicknesses around partially buried craters. Along the edges of the northern smooth plains, lava flow thicknesses were inverted to be 23–536 m with a median of 228 m. Lava flow thicknesses on Mercury are twice that on the Moon, possibly due to a denser crust on Mercury that favors the ascent of magmas or to higher production rates of magmas. The topographic diffusivity used for crater degradation on Mercury is 3 times that for lunar craters, which is likely a result of a higher cratering rate on Mercury.

New Constraints on the Young Lava Flow Profile in the Northern Mare Imbrium

AbstractPrevious studies showed that the Eratosthenian mare (Em) unit in the northern Mare Imbrium, which is the location of the Chang'e‐3 (CE3) landing site, was simply formed by one period of thick lava flow. However, the CE3 Yutu rover equipped with lunar penetrating radar (LPR) recognized multilayered Em basalts based on the Channel 1 data reprocessed in this paper, indicating that three subdivided thin Eratosthenian lava flows should exist. The spatial variation and thickness distribution of each layer are further estimated based on the location of interfaces recognized from the shallow radar profile. Finally, we build a three‐dimensional stratigraphic model of young mare basalts with filling directions beneath the Chang'e‐3 landing site. The result infers that these young lava flows in the northern Mare Imbrium probably erupted intermittently from the same source.

Facies variations and permeability of ignimbrites in active geothermal systems; case study of the Xáltipan ignimbrite at Los Humeros Volcanic Complex

Ignimbrites’ facies are key for interpreting the dynamics of explosive eruptions. Furthermore, they provide evidence of the physicochemical processes that acted during and after their deposition. Nevertheless, ignimbrites commonly present complex facies associations that require an integrative analytical methodology over a robust stratigraphic framework for their proper characterization. This work presents a facies characterization of the caldera-forming Xáltipan ignimbrite at Los Humeros Volcanic Complex, by integrating a combination of petrophysical and geochemical analyses aimed at its outflows (exposed) and intracaldera (buried) deposits. Based on the petrophysical properties, a three-facies association is identified controlled by welding and secondary mineralization effects: (1) The basal facies, characterized in the outflows by highly welded deposits (ranks V-VI), with low porosity (~4–8%), and practically impermeable conditions (<0.01–0.05 mD). In the intracaldera deposits, this facies shows pervasive geothermal-related propylitic mineralization that can reduce porosity up to ca. 80%. (2) A middle facies, with moderately welded deposits (ranks III-IV) and porous conditions consisting of a combination of intershard, vesicular, and fracture spaces, partially filled by secondary minerals, which reduce the porosity (6–11.5% in the outflows; 16–28% for the intracaldera), resulting in semi-permeable conditions (6.11–46.38 mD). (3) The upper facies are defined by being not to poorly welded deposits (ranks I-II), and characterized by being highly porous (dominated by intershard spaces) and permeable (310–1557 mD), and barely affected by secondary mineralization. The geochemical results (major and trace element) indicate that the Xáltipan ignimbrite shows a vertical compositional zonation, from rhyolitic (70.9–76.2 wt% SiO2) in the lower flow unit to rhyolitic-trachydacitic (67–70.7 wt% SiO2) composition in its upper flow unit. This last unit shows a geochemical affinity with the subsequent pyroclastic deposits. Geochemical analyses of intercalated lavas from well logs also provided evidence of unprecedented breccias formed during the caldera collapse, discarding the existence of previously assumed thick lava flows. Facies correlation, among other well logs, shows that the facies distribution is directly controlled by the ignimbrite thickness. Where the ignimbrite is > 400 m thick, the entire facies association is observed, while when the thickness is < 400 m, the upper and middle facies occurs preferentially. This facies analysis may contribute to the overall model of Los Humeros subsurface stratigraphy, by defining the Xáltipan ignimbrite distribution and its porosity-permeability-based zones, which are of particular interest for geothermal purposes. Finally, this work provides the first example of a caldera-forming ignimbrite that presents analog facies associations between its outflows and intracaldera deposits that may be applied for other ignimbrites worldwide.

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.

Rock slope kinematic instability controlled by large-scale variation of basalt column orientation

Field observations of columnar-jointed basalt lava flows at Organ Pipes National Park near Melbourne, Australia, show varying columnar cooling joint orientations. The valley-confined basalt lava flows have columnar joints forming a downward spreading fan as the joints adopt orientations perpendicular to the margins of the flow. The variation in the pattern of columnar joint orientations controls local slope failure mechanisms, including toppling and planar sliding. Manually collected field data and terrestrial laser scanning data for joint faces have been used to assess the kinematic stability of the rock slopes. The terrestrial laser scanning data obtained includes a greater number of joint face orientations than manually collected data, including data for otherwise inaccessible locations. However, the bulk data collected by TLS requires careful interpretation at sub-sites within the field of observation to enable variation in column orientation and the associated slope stability mechanisms, to be recognised. The collection of larger datasets by remote methods has the risk of smoothing out local variations. The data in this case study demonstrates that the commonly assumed simple parallel pattern of columnar joints cannot be assumed, even in large, thick lava flows such as the Organ Pipes National Park.

Restoring original composition of hydrothermally altered Archean metavolcanic rocks of the Carajás Mineral Province (Brazil): Geodynamic implications for the transition from lid to mobile tectonics

One of the most debated issues about the Carajás Mineral Province in Brazil is the tectonic setting that prevailed at the time of the major Neoarchean (ca. 2.7 Ga) volcanic event, with views ranging from subduction- to rift-related. During this event, thick lava flows of mainly basaltic and basaltic-andesitic composition extruded at the Earth's surface, cooled and underwent significant alteration by hydrothermal fluids rising through deep-seated crustal faults. Archean settings have been inferred for hydrothermally altered volcanic rocks using as a proxy their whole-rock compositions in discrimination diagrams for modern geotectonics. Despite its debatable application to Archean volcanic sites, the same working approach is used in reconsidering the geodynamic view about the Carajás volcanic event. We use an artificial neural networks-based method to perform a full mass balance calculation on a large compiled database of metavolcanic rocks from Carajás. Predicted precursors reproduce a linear and limited FeO*/MgO increase with SiO2 typical of modern calc-alkaline arc environment. Moreover, the identified least-altered rocks show that they may petrogenetically involve subducted water-bearing sediments. But contrary to what is expected for such a volcanic arc environment, amphibole and magnetite fractionations have not been observed in the precursors. Discrimination diagrams using incompatible trace elements were unable to rule out the subduction hypothesis so other geological and geochemical evidence from the granitoid record of Carajás was taken into consideration. In combination with the Archean granitoid evolution of CMP, we conclude that crustal contamination associated with continental rifting appears to be the most likely process for inducing the calc-alkaline affinity of the Carajás Neoarchean metavolcanic rocks. This calc-alkaline affinity in least-altered volcanic rocks appears to be best reproduced by petrogenetic modelling involving wall-rock assimilation and melting of the Carajás basement (TTG and other K-rich granitoids) by parental magmas derived from depleted mantle source, prior to their fractional crystallization. This volcanic event was coeval with the intrusion of A-type syntectonic granitoids at the transition from a stagnant lid to mobile plate tectonic regime associated with localized translithospheric transpressional rift.