Volcanic Hiatus Research Articles

Messengers from the Magma Chambers: Petrostratigraphic Analysis of Plagioclase-Rich Flood Basalt Lavas in Turkana, Kenya

Abstract Continental large igneous provinces are among the largest magmatic events on Earth, characteristically manifesting at the surface as stacks of monotonous flood basalt lavas that form through a dynamic pattern of eruptive pulses separated by hiatuses. Such dynamic patterns may be difficult to detect from the composition of the lavas. Instead, lava crystal cargoes can be used to study how the eruption cycles inherent to continental large igneous provinces are moderated by episodic recharge and evacuation at multiple crustal levels. Here, we present a stratigraphically well-constrained series of 56 flood basalt flows from the Eocene Initial Phase of magmatic activity in East Africa. These flows and their crystal cargo permit temporal insight into the development of one of the youngest and best-preserved continental large igneous provinces. The stratiform mafic lavas exposed in northern Kenya consist of alternating aphyric and plagioclase-phyric lava packages consistent with periods of eruption punctuated by volcanic hiatus, where magmas stall and crystallize plagioclase at medium to shallow crustal levels. Plagioclase compositions (n = 541) exhibit little intra-crystal or intra-sample compositional diversity. Intra-crystal equilibrium calculations for Sr and Ti indicate internal chemical equilibrium, requiring storage at high temperatures over a prolonged time interval (10 000–100 000 years). Using a series of seven interlinked partial crystal fractionation models under equilibrium conditions, we replicate the observed stratigraphic patterns in plagioclase composition. We find that the balance between recharge, evacuation, and diffusive equilibration within a shallow magmatic system controls the composition of plagioclase in these flood basalts. We conclude that the shallow fractionation system modulates eruptive cycles and thus constitutes a critical component in studies of continental large igneous provinces.

Electron microprobe dating of monazites from rhyolites of the Veľká Stožka Massif (Muráň nappe, Western Carpathians) – implications for the Permian volcanic evolution in Internal Western Carpathians

New results of field research, as well as petrographical and geochronological studies are presented in the form of a geological map, tectonostratigraphic scheme and new magmatic age data of monazite crystallization from the rhyolite body located on SW slopes of the Veľká Stožka Massif, Dudlavka Valley (Muráň Plateau). The rhyolite bodies in the Veľká Stožka area are tectonically incorporated into the Lower Triassic formations at the base of the Muráň nappe. The electron microprobe dating of monazites yields magmatic / volcanic age 281 ± 4.5 Ma (Artinskian – Kungurian), which complements geochronological data from the same rhyolite body below the Veľká Stožka Massif, as well as other rhyolite bodies from Telgárt, Poniky and Tisovec, which, originally erupted in a Permian volcanically active fault zone (VAFZ) geodynamical setting. The comparison of geochronological data 281 ± 4.5 Ma and 269.5 ± 1.8 Ma from the studied locality suggest a geodynamic evolution, where not only one, but several separate volcanic eruption phases acted in the original domain of rhyolitic volcanism. The geochronological correlation of the rhyolite bodies in the Upper Hron Valley, active in time span 281–263 Ma, allowed to identify a volcanic hiatus between 280–271 Ma. Such eruptional pause correlates very well with other volcanic provinces such as Harnobis volcanogenic horizon of the North Veporic realm, the Petrova hora and Novoveská Huta Fm. of the Northern Gemericum, which erupted during hiatus of VAFZ and their activities have ended in during recovering rhyolite eruptions in VAFZ. Reactivation of rhyolitic volcanic activity in VAFZ 271–263 Ma is a result of paleostress reorganisation in Pangea lithosphere. The revealed paleovolcanic relationships highlight interconnection of paleotectonic and paleovolcanic events in the known paleotectonic units of Internal Western Carpathians during Permian.

Tracing magmatic genesis and evolution through single zircon crystals from successive supereruptions from the Socorro Caldera Complex, USA

Large volume rhyolitic ignimbrite volcanism is a significant contributor to the evolving crust. The introduction of high-silica material into the upper crust, differentiation within the middle crust, and partial melting in the lower crust contributes to geochemical and isotopic evolution of the crust. Developing accurate models for the genetic evolution of these events is dependent upon geochronology to determine rates of magmatic processes as model constraints. We present new zircon high-precision CA-ID-TIMS U-Pb geochronology and MC-ICPMS Hf isotope geochemistry for four ignimbrites from the nested caldera complex near Socorro, New Mexico (USA), within the Mogollon-Datil volcanic field. In agreement with past 40Ar-39Ar data, interpretations of new U-Pb data indicate eruptions from the Socorro caldera cluster were pulsed. These pulses were intermittently spaced, and a volcanic hiatus following the Hells Mesa Tuff at 33.442 ± 0.015 Ma was interrupted by four successive eruptions, beginning with the La Jencia Tuff at 29.158 ± 0.025 Ma and finishing with the South Canyon Tuff at 28.066 ± 0.021 Ma. Zircon age spectra became more protracted with each eruption, exhibiting age dispersions ranging from 0.347 Myr in the Hells Mesa Tuff to 4.502 Myr in the South Canyon Tuff. The increased dispersion is paralleled by an increase in the proportion of normally discordant grains, indicative of xenocryst incorporation. These protracted age spectra are not necessarily a function of thermal maturation in the middle to upper crust due to long-lived magma chambers. Rather, they are likely the result of increased melting of zircon-bearing lower crust due to deep thermal maturation from repeated juvenile magma injections based on the incorporation of zircon material at the melt source. In contrast, the Hf isotope record is volumetrically dominated by autocrystic zircon domains and becomes more radiogenic through time, recording juvenile replenishment of the lower crust during progressive melting. Together, these data record the protracted evolution of the lower crust sampled by ignimbrites, lend insight into that evolution, and emphasize the need for detailed interpretation of high-precision datasets to advance volcanic models.

Volcanology, geochemistry and geodynamic setting of the Neoarchean Sunrise volcanogenic massive sulfide deposit, Beaulieu River volcanic belt, Slave craton, Northwest Territories, Canada

The Sunrise volcanogenic massive sulfide (VMS) deposit is a banded, polymetallic Zn-Pb-Cu-Ag sulfide lens hosted by felsic volcanic rocks of the Neoarchean Beaulieu River volcanic belt in the western Slave craton in the Sunset Lake area of the Northwest Territories, Canada. With historic indicated resources of 1.52 Mt at 5.99% Zn, 2.39% Pb, 0.08% Cu, 262 g/t Ag, and 0.67 g/t Au—it is the largest known VMS deposit in the Northwest Territories.The footwall stratigraphy is interpreted to be the product of subaqueous, largely effusive, bi-modal volcanism with periods of pyroclastic eruptions and concomitant subsidence. It consists of basalt to basaltic-andesite lavas with minor associated volcaniclastic rocks, rhyolite lobe-hyaloclastite flows and rhyolite domes, and felsic volcaniclastic rocks. Though hosted within felsic volcaniclastic lithofacies, the mineralization is spatially associated with the thickest portions of a quartz-feldspar porphyritic lobe-hyaloclastite flow and rhyolite dome. Felsic tuff to lapillistone units, characterized by fine ash, pumice and broken crystal clasts indicate derivation from pyroclastic volcanism and deposition by eruption-fed density currents. Felsic tuff breccia units are characterized by block-sized fragments of quartz-feldspar porphyritic rhyolite and were deposited during mass wasting events along fault scarps or by the oversteepening of domes. Massive to stringer-style sulfide mineralization is dominantly within felsic (seldom heterolithic) tuff to tuff breccia units, but laterally discontinuous argillite units occur at the top of the footwall and commonly contain stringer to semi-massive sulfide mineralization, indicating a hiatus in volcanism before the onset of basaltic andesite lavas that form the hanging wall to the deposit.Volcanic stratigraphy and architecture, and the occurrence of a volcanogenic massive sulfide deposit, combined with major and trace element geochemistry and Nd, Hf, and Pb isotopic data indicate the volcanic rocks and the Sunrise deposit formed on rifted Mesoarchean crust.

Conceptualizing a multi‐layered shingle aquifer model based on volcanic stratigraphy and water inflow to lava caves in Jeju Island, Korea

AbstractVolcanic aquifers supply a substantial portion of water resources in many parts of the world, including islands, and their productivity depends strongly on volcanic stratigraphy, which exhibits considerable heterogeneity. We investigated water inflow to lava tube caves formed from numerous basaltic lava flows in the northeastern coastal area of Jeju Island after storm events and monitored relative inflow rates monthly over 1 year to characterize groundwater flow processes in the upper parts of volcanic aquifers, and to evaluate the applicability of the previous hydrogeological models proposed for the island. Considerable water inflow arose shortly after storms from exposed palaeosol layers on the walls of the caves. The monthly monitoring results showed that wall inflow associated with these palaeosol layers is substantial. In both cases, discharge from ceiling drips was much less and more temporally variable compared to wall inflow discharge. Water flowing into the caves was rapidly drained through the floor at all monitoring sites. The lateral extent of the palaeosol layers was identified using drill core logs near the cave and outcrops in the coastal area. Based on these results, we inferred that multiple perched aquifers are formed by low‐permeability palaeosol layers between lava flows, which are connected by vertical flows at discontinuities in the palaeosol layer, eventually reaching the basal aquifer. This study revealed the water inflow processes observed in lava tube caves constrained by palaeosol layers, and established a hydrogeological conceptual model incorporating multiple perched aquifers in both coastal and mountainous areas associated with extensive palaeosol layers formed during volcanic hiatuses. This finding would help elucidate recharge, groundwater flow, and contaminant transport processes in many volcanic aquifers that are not adequately represented by the previous models, and contribute to better management of groundwater in those areas.

Volcanic reconstruction and geochemistry of the Powderhouse formation in the Paleoproterozoic VMS-hosting Chisel sequence, Snow Lake, Manitoba, Canada

The Powderhouse formation of the Paleoproterozoic Snow Lake arc assemblage comprises the stratigraphic footwall to six volcanogenic massive sulfide (VMS) deposits at Snow Lake, Manitoba, Canada. It is interpreted to be a product of voluminous pyroclastic eruptions and concomitant subsidence followed by a period of relative volcanic quiescence that was dominated by suspension sedimentation, the reworking of previously deposited pyroclastic units by debris flows and bottom currents, and localized emplacement of rhyolite domes. The rhyolite domes are spatially associated with the Chisel, Chisel North, Lost, Ghost, Photo, and Lalor deposits. The Chisel, Lalor, and Lost members compose the Powderhouse formation and are subdivided into 13 lithologically distinct lithofacies, which allows, for the first time, correlation of stratigraphy between the South Chisel basin and Lalor areas, critical in predicting the location of largely stratiform VMS deposits. The Chisel and Lalor members contain lithofacies and bedforms that are characteristic of emplacement by subaqueous pyroclastic mass flows and concomitant subsidence. The Chisel member also contains coarse volcaniclastic breccias emplaced by mass debris flows derived from movement along fault scarps following early pyroclastic eruptions, and during continued subsidence. The Lost member consists of lithofacies deposited by mass flows generated from faults scraps during continued subsidence, but also contains lithofacies reworked by bottom currents, those deposited by suspension sedimentation, and, locally, coherent rhyolite. The Lost member represents a time stratigraphic interval, the “ore interval”, that marks contemporaneous rhyolite dome emplacement, VMS formation, and a hiatus in explosive volcanism.

Transient mantle cooling linked to regional volcanic shut-down and early rifting in the North Atlantic Igneous Province

The Paleocene to Early Eocene Faroe Islands Basalt Group (FIBG) comprises a c. 6.5-km-thick lava flow–dominated sequence located within the centre of the North Atlantic Igneous Province (NAIP). The currently defined pre-breakup and syn-breakup sequences of the FIBG are separated by a significant volcanic hiatus, during which time the coal-bearing Prestfjall Formation was deposited. This major volcanic hiatus is identified across large parts of the NAIP and was preceded on the Faroe Islands by a reduction in eruption rate evidenced by an increased number and thickness of inter-lava sedimentary beds between the simple lava flows of the pre-breakup Beinisvørd Formation. High tempo eruptions resumed after this hiatus with the development of the compound lava flow fields of the Malinstindur Formation which reveal limited evidence for inter-lava breaks. In order to investigate this key transition, flow by flow geochemical sampling of a composite c.1.1-km-thick lava flow sequence spanning this transition were collected and analysed. Three chemically distinct groups are defined based on rare earth elements (REEs) and incompatible trace element signatures. Two high-Ti groups (TiO2 > 2 wt%), B2 and B3, dominate the sampled Beinisvørd Formation and display light REE-enriched signatures (La/YbN c. 2.9–5.9) and evidence for garnet in the source melting region (Dy/YbN c. 1.5–1.6). At the very top of the Beinisvørd Formation, a distinct group of lava flows, B1, displaying lower TiO2 for a given MgO wt% (TiO2 c. 1–2 wt%), weakly light REE-enriched profiles (La/YbN c. 1.7–2.4) and a spinel-dominated mantle melting signature (Dy/YbN c. 1.1–1.2) is identified. Sr, Nd and Pb isotopic signatures for the three groups overlap, revealing limited evidence of crustal contamination, and therefore supporting a mantle melting origin for inter-group variations, rather than source composition or contamination. The group B1 lava flows form a unique stratigraphic occurrence on the islands and provide clear evidence for both a reduction in the initial pressure of melting, alongside an increase in the overall degree of partial melting relative to groups B2 and B3. Increased partial melting is interpreted as evidence for the early onset of rifting and lithospheric thinning to the north of the Faroe Islands. The accompanying reduction in initial pressure of melting provides the first petrological evidence that a transient reduction in mantle temperature leads to the province-wide volcanic hiatus. Our study demonstrates an intimate linkage between rifting history and fluctuations in mantle temperature highlighting that any over-arching model for the evolution of the NAIP must take both into equal account.

Palaeogeographical evolution of the Rattray Volcanic Province, Central North Sea

The Rattray Volcanics Member, at the triple junction of the North Sea rift, is here subdivided into two informal sub-members based on analysis of core, wireline and seismic data. The Lower and Upper Rattray Volcanics were emplaced during two distinct phases of volcanism separated by a sustained volcanic hiatus. The presence of hyaloclastite and abundant freshwater algae at the base of the Lower Rattray indicates that large lakes were present in the area prior to the volcanism, possibly indicating that collapse of the regional Jurassic Central North Sea dome began prior to volcanism. Pulsed subsidence probably occurred through the duration of the volcanism with lacustrine conditions becoming re-established during the mid-volcanic hiatus. Sediments were deposited across the Rattray Volcanic Province in fluvial systems and floodplain coal swamps after the final cessation of volcanism, with later marine transgression leading to drowning of the area in the Callovian to Oxfordian. In terms of hydrocarbon prospectivity, no evidence is currently found to confirm the presence of an intra-basaltic play analogous to the Rosebank Field in the Faroe–Shetland Basin, although post-volcanic Pentland Formation sedimentary sequences in the Fisher Bank Basin area indicate the possibility of supra-basaltic prospectivity in the triple junction. Supplementary material: A table containing the complete palynological record is available at https://doi.org/10.6084/m9.figshare.c.4857195

High-resolution K-Ar dating of a complex magmatic system: The example of Basse-Terre Island (French West Indies)

For the first time in the Lesser Antilles volcanic arc, we combine for one island, 123K-Ar ages, with 133 major and trace elements geochemical analyses, in order to better constrain the volcanic history of Basse-Terre Island. In this study, nine new ages have been obtained from the southern part of the island, and complemented with eighty-three new major and trace element analyses of samples collected across the island. The southern part of Basse-Terre Island has been the loci of volcanic activity since the last 200kyr. It is characterized by the construction of the Grande-Découverte Volcanic Complex (GDVC) composed by the Grande-Découverte – Soufrière (GDS) and the Trois-Rivières Madeleine Field (TRMF). After the onset of construction at least at 205±28ka, the GDVC displays strikingly continuous activity between 140±13 and 56±3ka, followed by a 30kyr volcanic hiatus, which is coeval with the hiatus also observed for the TRMF activity. Two new ages of 125±14 and 140±13ka obtained on a lava flow from the Grande-Découverte caldera wall suggest the presence of a depression, resulting of a major flank collapse and/or explosive event, before 140ka. Finally, a new age of 9±6ka, obtained from outcrops exposed on the edge of the Class River, in the north of the GDVC, allows us to calculate channel incision rates between 11 and 56mm/yr. These values are consistent with incision rates determined on other volcanic islands with similar climates.In a broad sense, the petrology and geochemistry of Basse-Terre Island rocks appear fairly homogeneous, with mainly andesite and basaltic-andesite rocks and typical features of volcanic-arc lavas. Nevertheless, in detail, various magmatic processes can be discerned. Most variations are principally controlled by crystal-melt fractionation-accumulation, but major and trace elements also highlight episodic magmatic recharge, involving magma mixing. There are also indications for assimilation of crustal rocks with continental affinity, as well as mantle input of slab-derived fluids. Trace element ratios suggest the presence of at least two different magmatic sources characterized by different partial melting rates and different continental contributions for Basse-Terre Island. Different massifs show a bimodal behavior, with the Basal Complex, the Axial Chain (Piton de Bouillante and Southern Axial Chain), the Monts-Caraïbes volcanoes and the Sans-Toucher volcano in the first group, and the Septentrional Chain, and the Grande-Découverte Volcanic Complex (GDS and TRMF) in the second. Given the unique amount of time-constrained geochemical data, this study provides a complete and detailed investigation of volcanic evolution in the central part of the Lesser Antilles active arc.

The zircon record of high-pressure metasedimentary rocks of New Caledonia: Implications for regional tectonics of the south-west Pacific

We report the U-Pb age, and trace element and hafnium isotope composition of zircons recovered from clastic metasedimentary rocks that span a range of metamorphic grades (prehnite-pumpellyite to eclogite facies) across the high-pressure metamorphic belt of northern New Caledonia. We use these data to evaluate the sedimentary source and environment of formation of these rocks, as well as their respective metamorphic evolution.Metasediments from the low-grade Koumac and blueschist-facies Diahot sequences contain zircon with age populations of 1800–1500Ma, 700–500Ma, 300–250Ma and 140–80Ma. These grains have highly variable Hf isotope compositions (εHf −36 to +12), and some have thin metamorphic rim zones dated to ~38Ma. Zircon grains from the high-grade metamorphic section (Pouebo Eclogite Mélange) are mostly Upper Cretaceous and lowermost Eocene in age, with ~40Ma metamorphic overgrowth zones. The Upper Cretaceous zircons from across the belt have similar isotopic compositions, whereas the lowermost Eocene zircons have relatively high εHf of +6 to +13. The zircon record of the eclogites often lacks the diverse old detrital signature that is prevalent in the blueschist-facies rocks; rather the age populations of the eclogite grade zircons reflect derivation from nearby volcanic sources.We suggest that the bulk of Koumac and Diahot sequences represent the erosional products of the Norfolk Ridge that were transported eastwards and accumulated into the East New Caledonia Basin. Paleozoic and Precambrian zircon grains were originally sourced from Paleozoic orogenic belts of the eastern margin of Australia, and possibly from now-submerged ridges of continental character. We suggest that the Mesozoic zircon grains derived from volcanic arc activity on the eastern margin of Gondwana. Extension led to a reduced continental sediment input into the East New Caledonia Basin during the Cretaceous, which explains the limited amounts of ancient detritus in the mid-Cretaceous to Eocene sedimentary protoliths of the Pouebo Eclogite Mélange. The combined Hf isotope and age data for our zircon record point to the existence of a west-dipping subduction zone active throughout the Late Triassic to the Late Cretaceous, possibly with some short-lived periods of volcanic hiatus. In the early Eocene, a new east-dipping subduction formed following a major change in plate kinematics, producing younger metavolcanic zircons and eventually leading to the metamorphism of sedimentary units in the middle Eocene.

The geochemistry of the 2.75 Ga-old Helen Iron Formation, Wawa, Ontario – Insights into iron formation deposition from carbon isotopes and rare earth elements

A high resolution stratigraphically controlled rare earth element and carbon isotope study of the 2.75 Ga old Algoma-type Helen Iron Formation has provided new insights into the chemistry of late Archean seawater. The Helen Iron Formation passes from a sediment dominated by rhyolitic detrital material in the first 20m of the section into a purely chemical sediment reflecting the chemistry of the seawater from which it precipitated. An overprint of a hydrothermal signature, seen in the development of a positive europium anomaly, increasing in magnitude upsection is in good accordance with field observations indicating a deepening of the Helen basin through time. The lack of a cerium anomaly along with the predominance of Fe2+ over Fe3+ indicates that oxygen played a minor role in the deposition of the Helen Iron Formation.Carbon isotope data are unusual for iron formation as they cluster around 0‰ (VPDB) and indicate direct precipitation from seawater. However, highly negative carbon isotope excursions suggest two episodes of dissimilatory iron reduction in sediments rich in organic matter, the negative carbon isotope signatures being inherited from the oxidation of this organic matter. These excursions have high magnitudes of 3 and 10.5‰, respectively.These results indicate that the Helen Iron Formation was deposited in a restricted basin with high input of Fe2+ from distal hydrothermal sources and high levels of CO2, explained by submarine volcanic activity in the Helen Basin. We suggest that CO2 and Fe oversaturation led to the direct precipitation of siderite and minor ankerite and that the negative carbon isotope excursions observed are the result of the oxidation of organic matter previously produced by bacterial dissimilatory iron reduction. The Helen Iron Formation thus is a direct precipitate from seawater in a basin increasing in water depth from moderately deep to deep water and presents a significant local hiatus in submarine volcanism.

Frontier exploration and the North Atlantic Igneous Province: new insights from a 2.6 km offshore volcanic sequence in the NE Faroe–Shetland Basin

The Lagavulin exploration well 217/15-1Z penetrated a c . 2.6 km thick volcanic sequence dominated by extrusive basaltic rocks spanning the Paleocene–Eocene boundary in the NE Faroe–Shetland Basin. The well comprises one of the thickest drilled sequences through the North Atlantic Igneous Province. Integrated analysis of drill cuttings and wireline-log data reveals key volcanic lithofacies: (1) tabular lava flows; (2) compound lava flows; (3) hyaloclastite; (4) volcaniclastic rocks. The volcanic facies reveal two major sub-aqueous to subaerial sequences consistent with lava delta progradation. These sequences are separated by a volcanic hiatus represented by extensive reddened soils, which preceded the re-submergence of the area. Emergence followed by submergence of the first lava delta is interpreted to record an intra-T40 transient uplift event near the Paleocene–Eocene boundary. Basalts from the lower c . 1.3 km have low TiO 2 (<1.5 wt%) and low Zr/Y (2–3), with olivine-phyric picrites towards the base (Mg# 70–82; olivine Fo 85–91 ). The hiatus correlates precisely with a change to high-TiO 2 (2.5–3.2 wt%), high-Zr/Y (>4) compositions, which dominate the upper sequence. The associated change in lava geochemistry, transient uplift and volcanic hiatus appears consistent with a transient pulse of hot buoyant plume material passing beneath the area. Supplementary material: All raw geochemical data and supplementary analyses are available at http://www.geolsoc.org.uk/SUP18888 .

Extrinsic forcing of plant ecosystems in a large igneous province: The Columbia River flood basalt province, Washington State, USA

Volcanism associated with large igneous provinces (LIPs) has been implicated in both global climate and environmental change. To determine the impact of LIP volcanism on plant ecology we have examined plant community succession in sedimentary interbeds of the Columbia River flood basalt province (CRBP; northwest United States). Interbasaltic vegetation is characterized by primary succession communities that inhabit fresh lava surfaces until terminated by the next eruptive event, and it is assumed that longer volcanic hiatuses should lead to more mature plant communities. This expected succession trajectory is contradicted by palynological data that show that seral succession declines during the phase of waning CRPB volcanism and prolonged interbed intervals. Frequent volcanic activity and increased deposition of Snake River Plain hotspot ashes during this phase resulted in ecological disturbance of intralava field vegetation. Together with geochemical proxies from interbed sediments, this suggests that CRBP flora was largely driven by extrinsic forcing, and implies that LIP volcanism of similar scale and magnitude to that of the CRBP had a limited environmental impact. This study supports the theory that past biotic extinctions were triggered by numerous factors rather than a single geological event.

El Estribo Volcanic Complex: Evolution from a shield volcano to a cinder cone, Pátzcuaro Lake, Michoacán, México

El Estribo Volcanic Complex (EVC) is located in the northern part of the Michoacán–Guanajuato Volcanic Field within the Trans-Mexican Volcanic Belt (TMVB). El Estribo is located at the southern edge of the E-W Pátzcuaro fault that belongs to the Pátzcuaro-Jarácuaro graben, a western extension of the E-W Morelia–Acambay fault system. Stratigraphy, geochronology, chemistry, and mineral assemblages suggest that the volcanic complex was constructed in two periods separated by a ~100ka volcanic hiatus: a) emission of lava flows that constructed a shield volcano between 126ka, and b) mixed phreatomagmatic to Strombolian activity that formed a cinder cone ~28ka. The magmas that fed these monogenetic volcanoes were able to use the same feeding system. The cinder cone itself was constructed by Strombolian fallouts and remobilized scoria beds, followed by an erosion period, and by a mixed phreatomagmatic to magmatic phase (Strombolian fallouts ending with lava flows). Soft-sedimentary deformation of beds and impact sags, cross-bedding, as well as pitting and hydrothermal cracks found in particles support the phreatomagmatic phase. The erupted magmas through time ejected basaltic andesitic lava flows (56.21–58.88% SiO2) that built the shield volcano and then basaltic andesitic scoria (57.65–59.05% SiO2) that constructed the cinder cone. Although they used the same feeding system, the geochemical data and the mineral chemistry of the magmas indicate that the shield volcano and the cinder cone were fed by different magma batches erupted thousands of years apart. Therefore, the location of El Estribo Volcanic Complex along an E-W fault that has generated two sector collapses of the shield volcano to the north may be directly linked to this complex redistribution of the magmatic paths to the surface. Our findings show that magmatic feeding systems within monogenetic volcanic fields could be long lived, questioning the classic view of the monogenetic nature of their volcanoes and yielding information about the potential volcanic risk of these settings, usually considered risk-free.

Precise U-Pb Geochronology of the Matagami Mining Camp, Abitibi Greenstone Belt, Quebec: Stratigraphic Constraints and Implications for Volcanogenic Massive Sulfide Exploration

The Matagami mining camp in the northern Abitibi greenstone belt of Canada contains 19 known Archean volcanogenic massive sulfide (VMS) deposits, 11 of which have collectively produced 46.5 Mt of zinc-rich ore to date. The VMS deposits occur in three NW-SE– to WNW-ESE–oriented trends called the north flank, the south flank, and the west camp, which are composed of a felsic to mafic volcanic sequence cut by mafic to intermediate, synvolcanic sills and dikes. In order to clarify stratigraphic relationships between the south flank and the west camp, and to constrain the temporal evolution of volcanic activity, six new high-precision U-Pb zircon ages have been obtained. These data show that the total duration of felsic volcanism in the south flank was no more than 2.5 m.y., with the rhyolites extruded in the following order: Watson Rhyolite (2725.9 ± 0.8 Ma), Bracemac Rhyolite (2725.8 ± 0.7 Ma), Dumagami Rhyolite at the Perseverance Mine (2725.4 ± 0.7 Ma), and Dumagami Rhyolite in the Orchan West VMS deposit area (2724.9 ± 0.7 Ma). A hiatus in effusive volcanism is represented by the Key Tuffite, an important marker horizon in the camp. The hiatus probably lasted on the order of 0.5 m.y. Significantly, the rhyolite from the footwall of the Caber VMS deposit in the west camp has an age of 2725.9 ± 1.2 Ma, identical to that of the Watson Rhyolite on the south flank.

The origin of a large (> 3 km) maar volcano by coalescence of multiple shallow craters: Lake Purrumbete maar, southeastern Australia

Lake Purrumbete maar is located in the intraplate, monogenetic Newer Volcanics Province in southeastern Australia. The extremely large crater of 3000m in diameter formed on an intersection of two fault lines and comprises at least three coalesced vents. The evolution of these vents is controlled by the interaction of the tectonic setting and the properties of both hard and soft rock aquifers. Lithics in the maar deposits originate from country rock formations less than 300m deep, indicating that the large size of the crater cannot only be the result of the downwards migration of the explosion foci in a single vent. Vertical crater walls and primary inward dipping beds evidence that the original size of the crater has been largely preserved. Detailed mapping of the facies distributions, the direction of transport of base surges and pyroclastic flows, and the distribution of ballistic block fields, form the basis for the reconstruction of the complex eruption history,which is characterised by alternations of the eruption style between relatively dry and wet phreatomagmatic conditions, and migration of the vent location along tectonic structures. Three temporally separated eruption phases are recognised, each starting at the same crater located directly at the intersection of two local fault lines. Activity then moved quickly to different locations. A significant volcanic hiatus between two of the three phases shows that the magmatic system was reactivated. The enlargement of especially the main crater by both lateral and vertical growth led to the interception of the individual craters and the formation of the large circular crater. Lake Purrumbete maar is an excellent example of how complicated the evolution of large, seemingly simple, circular maar volcanoes can be, and raises the question if these systems are actually monogenetic.

Geochemical and paleomagnetic variations in basalts from the Wendell Regional Aquifer Systems Analysis (RASA) drill core: Evidence for magma recharge and assimilation–fractional crystallization from the central Snake River Plain, Idaho

The temporal and magmatic evolution of central Snake River Plain (SRP; Idaho, USA) olivine tholeiites erupted within the past 4 m.y. is evaluated here. This investigation correlates and merges both geochemical and paleomagnetic measurements to constrain the volcanic history recovered from the 340 m Regional Aquifer Systems Analysis (RASA) test well located near Wendell, Idaho. Only a handful of studies have accomplished this task of shedding light on the chemical stratigraphy of the SRP and the petrogenesis of basalts with depth, and therefore through time. Paleomagnetic relationships suggest that time breaks between individual lava fl ows represent a few years to decades, time breaks between fl ow groups represent at least a couple of hundred years or possibly much longer, while significant hiatuses in volcanism, revealed by thick sediment packages or polarity reversals (both are evidenced in this well), are inferred to last thousands to tens of thousands of years. Major element geochemistry from 52 basaltic lava fl ows demonstrates near primitive compositions (i.e., ~10 wt% MgO) and tholeiitic iron enrichment trends, similar to lavas from the eastern SRP. Trace element concentrations are similar to those of ocean island basalts, with enriched Ba and Pb, and light rare earth element (REE)/heavy REE ratios similar to those of many Neogene volcanics of the western Cordillera. When combined, we identify a total of 11 fl ow groups, which we also classify as fractionation or recharge on the basis of decreasing or increasing MgO weight percent with depth. Taking into consideration these trends, we review the potential recharge, fractionation, and assimilation processes that characterize much of SRP olivine tholeiite, and conclude that assimilation, in combination with fractional crystallization, is the dominant petrogenesis for the basalts in the central SRP. Although fractionation of Wendell parent magmas was accompanied by assimilation of crustal material, this could not have been assimilation of ancient cratonic crust. The geochemical cycles observed in this well are inferred to represent fractionation and recharge of basaltic magma from a series of sill-like layered mafi c intrusions located in the middle crust, similar to what has been proposed for the processes that control the eruptive history of basalts in the eastern SRP.

Episodic eruptions of the Late Mesozoic volcanic sequences in southeastern Zhejiang, SE China: Petrogenesis and implications for the geodynamics of paleo-Pacific subduction

Zircon U–Pb and Hf isotope compositions have been systematically measured to constrain the timing and petrogenesis of the Late Mesozoic volcanic sequences in southeastern Zhejiang, SE China. The Early Yanshanian (Jurassic) volcanism took place at 177Ma and formed scattered volcanic rocks in the region. In contrast, the large-scale Late Yanshanian (Cretaceous) volcanism, with multiple eruptive stages between 140 and 88Ma, formed the widespread lower and upper volcanic series. Two volcanic hiatuses were clearly identified at 120–110Ma between the lower and upper volcanic series and at 128–122Ma between the two subdivided cycles within the lower series. In addition, the magnitude of the two volcanic cycles in the lower series is significantly different. The cycle I of the lower series (140–128Ma) occurred in the largest scale, which is much larger than the cycle II (122–120Ma) as well as the whole upper series (110–88Ma). Petrogenetically, the widespread Late Mesozoic volcanic rocks were mainly derived from the partial melting of the Paleoproterozoic crustal basement materials, with obvious addition of juvenile materials after 122Ma. The analyzed zircons of the Early Yanshanian volcanic rocks show crustal model ages (TDM2) mainly of 2.00–2.16Ga, and weighted mean εHf(t) value of −13.4. The εHf(t) and TDM2 values are ca. −10 and 1.67–2.00Ga for zircons from the cycle I volcanic rocks, −11.4 to −1.4 and 1.27–1.90Ga for those from the cycle II volcanic rocks, −9.3 to −2.7 and 1.33–1.78Ga for those from the acidic rocks of the upper series, respectively. The characteristics of the Late Mesozoic volcanic sequences indicate the varying influence of the subduction of the paleo-Pacific plate. Our study supports that the subduction style changed from low-angle to intermediate-angle forward subduction at the first hiatus of 128–122Ma and then dramatically changed to rapid high-angle rollback subduction at the second hiatus of 120–110Ma.

Miocene magmatism in the Bodie Hills volcanic field, California and Nevada: A long-lived eruptive center in the southern segment of the ancestral Cascades arc

The Middle to Late Miocene Bodie Hills volcanic field is a >700 km 2 , long-lived (∼9 Ma) but episodic eruptive center in the southern segment of the ancestral Cascades arc north of Mono Lake (California, U.S.). It consists of ∼20 major eruptive units, including 4 trachyandesite stratovolcanoes emplaced along the margins of the field, and numerous, more centrally located silicic trachyandesite to rhyolite flow dome complexes. Bodie Hills volcanism was episodic with two peak periods of eruptive activity: an early period ca. 14.7–12.9 Ma that mostly formed trachyandesite stratovolcanoes and a later period between ca. 9.2 and 8.0 Ma dominated by large trachyandesite-dacite dome fields. A final period of small silicic dome emplacement occurred ca. 6 Ma. Aeromagnetic and gravity data suggest that many of the Miocene volcanoes have shallow plutonic roots that extend to depths ≥1–2 km below the surface, and much of the Bodie Hills may be underlain by low-density plutons presumably related to Miocene volcanism. Compositions of Bodie Hills volcanic rocks vary from ∼50 to 78 wt% SiO 2 , although rocks with 2 are rare. They form a high-K calc-alkaline series with pronounced negative Ti-P-Nb-Ta anomalies and high Ba/Nb, Ba/Ta, and La/Nb typical of subduction-related continental margin arcs. Most Bodie Hills rocks are porphyritic, commonly containing 15–35 vol% phenocrysts of plagioclase, pyroxene, and hornblende ± biotite. The oldest eruptive units have the most mafic compositions, but volcanic rocks oscillated between mafic and intermediate to felsic compositions through time. Following a 2 Ma hiatus in volcanism, postsubduction rocks of the ca. 3.6–0.1 Ma, bimodal, high-K Aurora volcanic field erupted unconformably onto rocks of the Miocene Bodie Hills volcanic field. At the latitude of the Bodie Hills, subduction of the Farallon plate is inferred to have ended ca. 10 Ma, evolving to a transform plate margin. However, volcanism in the region continued until 8 Ma without an apparent change in rock composition or style of eruption. Equidimensional, polygenetic volcanoes and the absence of dike swarms suggest a low differential horizontal stress regime throughout the lifespan of the Bodie Hills volcanic field. However, kinematic data for veins and faults in mining districts suggest a change in the stress field from transtensional to extensional approximately coincident with the inferred cessation of subduction. Numerous hydrothermal systems were operative in the Bodie Hills during the Miocene. Several large systems caused alteration of volcaniclastic rocks in areas as large as 30 km 2 , but these altered rocks are mostly devoid of economic mineral concentrations. More structurally focused hydrothermal systems formed large epithermal Au-Ag vein deposits in the Bodie and Aurora mining districts. Economically important hydrothermal systems are temporally related to intermediate to silicic composition domes. Rock types, major and trace element compositions, petrographic characteristics, and volcanic features of the Bodie Hills volcanic field are similar to those of other large Miocene volcanic fields in the southern segment of the ancestral Cascade arc. Relative to other parts of the ancestral arc, especially north of Lake Tahoe in northeastern California, the scarcity of mafic rocks, relatively K-rich calc-alkaline compositions, and abundance of composite dome fields in the Bodie Hills may reflect thicker crust beneath the southern ancestral arc segment. Thicker crust may have inhibited direct ascent and eruption of mafic, mantle-derived magma, instead stalling its ascent in the lower or middle crust, thereby promoting differentiation to silicic compositions and development of porphyritic textures characteristic of the southern ancestral arc segment.

Nature of and controls on volcanism in the ca. 2.32–2.06 Ga Pretoria Group, Transvaal Supergroup, Kaapvaal Craton, South Africa

The time of deposition of the Pretoria Group between 2.32 and 2.06Ga on South Africa's Kaapvaal Craton was characterized by the first major increase in atmospheric oxygen. It was accompanied by the extrusion of significant thicknesses of volcanic deposits, namely the Bushy Bend lavas of the Timeball Hill Formation, the Hekpoort Formation and the Machadodorp Volcanic Member of the Silverton Formation, marking the three major volcanic events within the Transvaal Supergroup which are thought to be precursors to the succeeding Bushveld Complex magmatism. The Bushy Bend Lava Member of the Timeball Hill Formation is characterized by subaqueous basaltic–andesitic fissure eruptions with fumarolic activity and probably minor subaerial explosive eruptions in the hinterland of the Transvaal Supergroup basin. The subaerial volcanism of the Hekpoort Formation appears to have been dominated by fissure eruptions with a preponderance of lava flows and locally important pyroclastic material. Intermittent hiatuses in volcanism were marked by local lacustrine shale deposition. Finally, the Machadodorp Lava Member can be interpreted as the deposits of several seamounts aligned along a fissure, probably within an extensional environment.The geodynamic control inferred for the evolution of the Pretoria Group basin encompasses two cycles of prerift uplift, subsequent mechanical rifting and long lived thermal subsidence. The limited extent of the Bushy Bend lavas in the south of the Pretoria Group depository attests to the likelihood that volcanism accompanying the first rifting event was short-lived. In contrast, the second rifting cycle postulated for the Pretoria Group basin has a strong association with widespread and large scale volcanism of the Hekpoort–Ongeluk flood basalts, which may have been plume-related. The scale of the second cycle volcanism and its importance in influencing the upper part of the Pretoria Group basin-fill is underlined by the Machadodorp volcanism, probably related to hot spot volcanism as the Ongeluk–Hekpoort plume waned.