Quantitative chemical mapping of plagioclase as a tool for the interpretation of volcanic stratigraphy: an example from Saint Kitts, Lesser Antilles

Establishing a quantitative link between magmatic processes occurring at depth and volcanic eruption dynamics is essential to forecast the future behaviour of volcanoes, and to correctly interpret monitoring signals at active centres. Chemical zoning in minerals, which captures successive events or states within a magmatic system, can be exploited for such a purpose. However, to develop a quantitative understanding of magmatic systems requires an unbiased, reproducible method for characterising zoned crystals. We use image segmentation on thin section scale chemical maps to segment textural zones in plagioclase phenocrysts. These zones are then correlated throughout a stratigraphic sequence from Saint Kitts (Lesser Antilles), composed of a basal pyroclastic flow deposit and a series of fall deposits. Both segmented phenocrysts and unsegmented matrix plagioclase are chemically decoupled from whole rock geochemical trends, with the latter showing a systematic temporal progression towards less chemically evolved magma (more anorthitic plagioclase). By working on a stratigraphic sequence, it is possible to track the chemical and textural complexity of segmented plagioclase in time, in this case on the order of millennia. In doing so, we find a relationship between the number of crystal populations, deposit thickness and time. Thicker deposits contain a larger number of crystal populations, alongside an overall reduction in this number towards the top of the deposit. Our approach provides quantitative textural parameters for volcanic and plutonic rocks, including the ability to measure the amount of crystal fracturing. In combination with mineral chemistry, these parameters can strengthen the link between petrology and volcanology, paving the way towards a deeper understanding of the magmatic processes controlling eruptive dynamics.

Forming an economic bentonite resource in a volcanic arc environment: Milos island, Greece

<p>Volcanoes in island arcs can undergo edifice evolution that includes submarine and subaerial volcanism, providing a dynamic environment of magmatic heat and volatiles that drives hydrothermal fluid flow with potential inputs from sea and/or meteoric water. This, in turn, can generate significant hydrothermal alteration that can result in economic deposits of industrial minerals. One example includes bentonite, a smectitic rock composed dominantly of montmorillonite.</p><p>Economically viable bentonite deposits are typically only 0.5 – 5 meters thick and<strong> </strong>although Wyoming-type bentonites comprise 70% of the world’s known deposits, they are commonly no thicker than 8 m. The island of Milos is Europe’s largest and actively mined calcium bentonite resource from volcanic piles exceeding 80 m thickness. Here, we use the Milos island example to understand how magmatism, volcanic edifice evolution and hydrothermal activity interact. We integrate field relationships of volcanic stratigraphy and alteration zones, with clay mineralogy (XRD), stable (S, O and H) isotope analysis and high precision geochronology (CA-ID-TIMS zircon U-Pb, and alunite Ar-Ar) to elucidate the timescales, thermal drivers and fluid components that lead to the development of a globally important bentonite resource.</p><p>A vertical transect through bentonite-altered volcanic stratigraphy indicates multiple magmatic pulses ca. 2.8 Ma with a submarine andesitic cryptodome and accompanying pepperitic hyaloclastite. Cumulative volcanic and sub-volcanic processes occurred over ca. 170 kyrs, resulting in a vertically and laterally extensive volcanic pile overlain by an episode of magmatic quiescence and brackish-water diatomaceous sediments. It is overlain by a silicic pyroclastic flow host to pervasive silica-alunite-kaolinite alteration. Stable isotopic analyses of bentonite indicate a hydrothermal origin at around 70°C with the fluid being sourced from sea and meteoric waters. The timing of formation is defined by a maximum duration of ca. 170 kyrs, with clear geological evidence that a significant period of alteration occurred within < 20 kyrs at ~ 2.64 Ma. Alunite sulfur isotope compositions reflect steaming ground activity that could be interpreted as the oxidised, shallower level counterpart to a boiling geothermal system linked to development of extensive bentonite. However, the timing of alunite can be clearly resolved to > 1.5 myrs after bentonite formation to ~ 1.0 Ma, supporting a later overprint origin due to relatively recent steam heating of groundwater after emergence of the submarine system.</p><p>This study identifies key parameters that have resulted in the formation of an economic-scale bentonite resource on the emergent island of Milos. We conclude that the hydrology needed to form a bentonite deposit is not constrained to the marine environment and can be connected to emergent parts of the volcanic edifice. High precision geochronology indicates bentonite development happens on volcanic timescales (10 to 100 kyrs). A cumulative volcanic and sub-volcanic pile coeval with the formation of bentonite suggests multiple magmatic episodes over narrow timeframes provide and sustain the thermal driver for significant bentonite development. After emergence and development of a groundwater system, the subsequent steam heating is deleterious to grade and results in the development of alunite-kaolinite overburden.</p>

A reinterpretation of the Snow Lake gold camp, Trans-Hudson Orogen, Canada: The use of cleavages as markers to correlate structures across deformed terranes

The Snow Lake gold camp is located within amphibolite facies volcanic rocks of the ca. 1.88 – 1.87 Ga Flin Flon-Glennie Complex (FFGC) in the Trans-Hudson Orogen, Manitoba. During thrusting and collision with the Archean Sask craton, volcanic rocks were interleaved with turbidites of the ca. 1.855 - 1.84 Ga Burntwood Group and sandstone and conglomerate of the ca. 1.845 - 1.835 Ga Missi Group. The main cleavage in the turbidites was previously attributed to thrusting and used as a marker for correlating structures across the camp. A re-examination of this cleavage suggests that it overprints the thrust faults and formed during later collision between the FFGC and the Archean Superior craton. This has important implications as it further suggests that (1) previously unrecognized, early brittle thrust faults repeat volcanic stratigraphy and may have created the boundary conditions that enabled the formation of ductile thrust faults, fold nappes, and mega sheath folds; (2) shear sense indicators along ductile thrust faults formed during their reactivation as sinistral shear zones rather than during thrusting; and (3) peak metamorphic conditions were caused by thrusting and stacking during collision with the Sask craton but were attained later during collision with the Superior craton due to the time lag between orogenesis and the re-equilibration of regional isotherms. Results from this study may be applicable to other complexly deformed terranes where the dominant regional cleavage differs in expression in mixed volcanic and sedimentary successions and has been used as a marker for correlating structures.

Linking surface and subsurface volcanic stratigraphy in the Turkana Depression of the East African Rift system

The Northern Kenya Rift is an important natural laboratory for understanding continental rifting processes. However, much of the current understanding of its geological evolution is based on surface outcrops within footwall highs due to a lack of subsurface geological constraints. In this paper, we present an investigation of the Cenozoic stratigraphy and volcano-tectonic relationship of the volcanic sequences within the Turkana Depression (namely the North Lokichar, North Kerio and Turkana Basins). We integrate regional seismic reflection data collected as part of ongoing petroleum exploration in the area with lithological and biostratigraphic data from new wells that were drilled in 2014 and 2015 (Epir-1 and Emesek-1). This has allowed linking and extrapolation of the detailed stratigraphy of the paleontologically important Lothagam site to the volcanic sequences within the Napedet Hills, North Lokichar, North Kerio and Turkana Basins. The site of the Plio-Pleistocene-age Turkana Fault, which separates the North Lokichar Basin from the Turkana and North Kerio Basins, appears previously to have acted as a focus of Middle Miocene volcanism c. 5 Ma prior to the main period of movement on the fault. Our study highlights how subsurface and outcrop information can be combined to give a more in-depth knowledge of the magmatic history within rift basins.

New Cryogenian, Neoproterozoic, and middle Paleozoic U–Pb zircon ages from the Caledonia terrane, southern New Brunswick, Canada: better constrained but more complex volcanic stratigraphy

New U–Pb zircon ages from volcanic, plutonic, and sedimentary units in the Avalonian Caledonia terrane of southern New Brunswick provide better timing constraints in this geologically complex area. Previous ca. 620 Ma ages from the Broad River Group are now corroborated by additional dates from felsic tuff in the Gordon Falls Formation and rhyolite in the former Fairfield (now East Branch Black River) Formation of 620 ± 5 Ma and 622 ± 1.9 Ma, respectively. Combined with ages ranging from ca. 625 Ma to 615 Ma from crosscutting plutons, the data suggest that the minimum age of the Broad River Group is about 615 Ma. A quartzfeldspar porphyry dyke in mafic volcanic rocks of the previously undated Long Beach Formation yielded an igneous crystallization age of 685 ± 10 Ma, the oldest unit yet dated in the Caledonia terrane but similar in age to porphyry in the Stirling belt in the Avalonian Mira terrane of Nova Scotia. The age of the Coldbrook Group was constrained previously by U–Pb (zircon) ages of volcanic rocks between 560 and 550 Ma as well as by similar ages from comagmatic plutons. Five additional samples from both volcanic and plutonic units lie in the same range of 560–550 Ma, including errors, demonstrating that the Coldbrook Group and related plutons formed in less than 10 million years. Such a large volume of mainly felsic magma erupted and emplaced in a short time span suggests a “supereruption/supervolcano” environment such as the late Cenozoic southwestern USA but not yet recognized at ca. 560–550 Ma elsewhere in Avalonia. Two units yielded Paleozoic ages: felsite of the Bloomsbury Mountain Formation with a zircon population at 427 ± 9 Ma, indicating a Silurian maximum emplacement age, and dacite of the Grassy Lake Formation with several zircon grains at 382.8 ± 8.3 Ma, indicating a maximum age of middle Devonian, the first rocks of this age to be identified in the Caledonia terrane.

The control of preexisting faults on the distribution, morphology, and volume of monogenetic volcanism in the Michoacán-Guanajuato Volcanic Field

Interactions between volcanic and tectonic processes affect the distribution, morphology, and volume of eruptive products in space and time. The Queréndaro area in the eastern Michoacán-Guanajuato Volcanic Field affords an exceptional opportunity to understand these relationships. Here, a Pleistocene lava plateau and 20 monogenetic volcanoes are vented from an active ENE-striking segment of the Morelia-Acambay fault system. Thirteen scoria cones are aligned along this structure, vented from an extensional gap in between two rotated hanging wall blocks of a listric fault. A new geological map, volcanic stratigraphy, and 40Ar/39Ar dating indicate that this lava plateau and volcanic cluster were emplaced from 0.81 to 0.25 Ma by 11 intermittent eruptive epochs separated by ca. 0.05 Ma, emplacing a total magma volume of 5 km3. Petrography and chemistry of rocks suggest that all volcanic structures were fed by three different magma batches but vented from independent feeder dikes. Our results indicate that preexisting faults exert a strong influence on volcanic spatial and temporal distribution, volcanic morphology, magma volume, and eruptive dynamics in this area. ENE-breached and ENE-elongated scoria cones indicate parallel subsurface fissure and feeder dikes. Additionally, points of maximum fault dilation at depth related to a transtensive state of stress coincide with less fragmented deposits and larger magma volumes. Furthermore, this study raises important questions on the geodynamics of volcano-tectonic interactions possible in similar monogenetic volcanic alignments worldwide.

Forming an economic industrial mineral resource in a volcanic arc environment: timescales, fluids and thermal drivers of Europe’s largest bentonite resource

<p>Volcanoes in island arcs can undergo edifice evolution that includes submarine and subaerial volcanism. This provides a dynamic environment of magmatic heat and volatiles that drives hydrothermal fluid flow with potential inputs from sea and/or meteoric waters. This, in turn, can generate significant hydrothermal alteration that can result in economic deposits of industrial minerals such as bentonite and kaolinite. The island of Milos is Europe’s largest and actively mined calcium bentonite resource, with production capacities exceeding 400,000 tons per year. Here, we use the Milos island example to understand how magmatism, volcanic edifice evolution and hydrothermal activity interact to generate important bentonite mineralisation. We integrate field relationships of volcanic stratigraphy and alteration zones, with clay mineralogy (XRD), stable (S, O and H) isotope analysis and high precision geochronology (CA-ID-TIMS zircon U-Pb, and alunite Ar-Ar) to elucidate the timescales, thermal drivers and fluid components that lead to the development of a globally important bentonite resource.</p><p>A vertical transect through bentonite-altered volcanic stratigraphy indicates multiple magmatic pulses initiated at ca. 2.8 Ma with a submarine andesitic cryptodome and accompanying hyaloclastite carapace that display quenched and peperitic contacts. Cumulative volcanic and sub-volcanic processes occurred over ca. 170 kyrs, resulting in a volcanic pile exceeding 80 m. This period included an episode of magmatic quiescence and diatomite formation in a shallow submarine environment and is overlain by a silicic pyroclastic flow. In this upper unit, a pervasive alunite-kaolinite alteration assemblage was developed. Stable isotopic analyses of bentonite (> 85% montmorillonite) indicate a hydrothermal origin at around 125°C with the fluid being sourced from sea and meteoric waters. The timing of formation is defined by a maximum duration of ca. 170 kyrs, with clear geological evidence that a significant period of alteration occurred within <20 kyrs at ca. 2.64 Ma. Sulfur isotope analysis on alunite indicates a steaming ground origin that could be interpreted as the oxidised, shallower level counterpart to a boiling geothermal system linked to development of extensive bentonite. However, the timing of alunite can be clearly resolved to > 1 Ma after bentonite formation to 1.2 Ma, supporting a later overprint origin due to relatively recent steam heating of groundwater after emergence.</p><p>This study identifies new key parameters that have resulted in the formation of an economic-scale bentonite resource on the emergent island of Milos. In addition to the requisite appropriate protolith, we conclude that in an emergent volcanic arc setting the hydrology needed to form a bentonite deposit is not constrained to the marine environment and can be connected to emergent parts of the volcanic edifice. High precision geochronology indicates bentonite development happens on volcanic timescales (10 to 100 kyrs). A cumulative volcanic and sub-volcanic pile coeval with the formation of bentonite suggests multiple magmatic episodes over narrow timeframes provide and sustain the thermal driver for significant bentonite development. Once the volcanic edifice has completely emerged and developed a groundwater system, the steam heating of groundwater is deleterious to grade and results in the development of alunite-kaolinite overburden.</p>