Elevated arsenic concentrations (up to 19 μg L−1) occur in private groundwater wells in fractured bedrock aquifers close to the contact between Silurian greywackes of the Longford-Down Terrane, and the Palaeogene Slieve Gullion and Carlingford igneous complexes in NE Ireland. Palaeogene basaltic intrusions intersected in two drill cores were found to contain up to 80 mg kg−1 arsenic; concentrations that are elevated compared with the global averages for basalt. Fine grained (c. 5 μm) disseminated sulfarsenides, associated with cobalt and nickel were identified in the basalts as the primary sources of groundwater arsenic in this fractured bedrock aquifer system. Many of the high-As waters exhibit relatively high pH, arguing against a simple single-stage sulphide oxidation model for arsenic solubilisation. This observation coupled with the widespread presence of iron oxyhydroxide coatings on natural rock fractures intersected in the drill cores suggests a multi-stage As mobilisation process. This interpretation envisages sulfarsenide mineral oxidation, adsorption of dissolved As to iron oxyhydroxide mineral surfaces, and finally desorptive release of oxyanion As species from these surfaces as waters evolve to higher pH as a result of water-rock reactions.
The Tyrone Plutonic Group of Northern Ireland represents the upper portions of a tectonically dissected suprasubduction-zone ophiolite accreted to the composite Laurentian margin during the Middle Ordovician. Understanding its development and relationship to the Tyrone Central Inlier, an outboard fragment of relatively high-grade, peri-Laurentian continental crust, is essential for reconstructing the closure of the Iapetus Ocean. The Tyrone Plutonic Group is composed of tectonized layered, isotropic and pegmatitic gabbros, sheeted dolerite dykes and rare pillow lavas. New U–Pb zircon thermal ionization mass spectrometry geochronology has yielded an age of 483.68 ± 0.81 Ma from pegmatitic gabbro. Geochemical characteristics, Nd and Sr isotope systematics, and zircon inheritance indicate that the Tyrone Plutonic Group formed above a north-dipping subduction zone, by the propagation of a spreading centre into a microcontinental block. Synkinematic, calc-alkaline tonalitic to granitic material preserved in the contact zone between the Tyrone Plutonic Group and the Tyrone Central Inlier has produced pressure estimates of 2.3–4.0 ± 0.6 kbar and temperatures of 525–610 °C. Coeval arc–ophiolite accretion at c . 470 Ma may explain how sillimanite-grade metamorphic conditions were reached locally in the underlying Tyrone Central Inlier. Strong temporal, geochemical and lithological similarities exist to the Annieopsquotch Ophiolite Belt of Newfoundland. Supplementary materials: Petrographic photographs, whole-rock, isotopic and mineral geochemical data, and U–Th–Pb isotopic data are available at www.geolsoc.org.uk/SUP18646 .
<p>The Northern Ireland sector of the North Atlantic Large Igneous Province (NIAP) is the biggest onshore exposure of the British and Irish Igneous Province. The Antrim Lava Group is composed mostly of flood basalt sequences (Lower and Upper Basalt formations) with associated acid-basic central complexes, dyke swarms, plugs and sill complexes (Cooper 2004; Cooper & Johnston 2004; Cooper et al. 2012) that display unconformable and cross cutting field relationships. This study has for the first time generated a self-consistent and representative chronology using high-precision CA-ID-TIMS U-Pb zircon isotopic dating across the range of lithologies to provide a fuller picture of how the region was operating during the Paleogene.</p><p>Results indicate that punctuated magmatism within the north of Ireland lasted at least c. 5.5 Myrs from c.61.5 to 56 Ma. The 61.5 Ma age comes from magmatic zircons from a paleosoil immediately below the Lower Basalt Formation (LBF), and is believed to represent the onset of magmatism in the region. This age is matched by that of the Killala-Erne Dyke Swarm (c.61.5 to c.61 Ma) which is now considered to have fed melt to LBF lava flows. Dates of c.61 Ma for the Tardree and c.60.7 Ma for Slieve Gullion igneous complexes are the youngest of this grouping which together spans about 1 Myrs and may represent the igneous activity associated with a single pulse of the Icelandic Plume.</p><p>Following the initial activity there was a break in magmatism-volcanism which lead to the development of a thick weathering profile referred to as the Interbasaltic Formation. We then see the development of a regionally significant unconformity&#160;and deposition of the Coagh Conglomerate Member which includes clasts of Tardree Complex (or similar) rhyolite.&#160;This was followed by extrusion of the Causeway Tholeiite Member (CTM) and Upper Basalt Formation (UBF) across the region. A new age for the Portrush Sill Complex at c. 58.5 Ma provides a constraint on this episode of magmatism, however, it the combination of regional unconformity, outpouring of flood basalts and other magmatism that suggest a second pulse may be represented.</p><p>The Mourne Mountains Complex at c.56-56.5 Ma is not associated with flood basalts in Northern Ireland, however, in Scotland basalts of the Upper Skye Lava Formation are of similar age to the Mourne granites and together they might represent a third pulse of the plume in the region.</p><p>New U-Pb zircon geochronology has allowed for significant reinterpretation of the regional scale geology and stratigraphy of the Antrim Lava Group. Geochronological constraints define an early episode of igneous activity that is separated from the next by a prolonged period of weathering and the formation of a regionally significant unconformity. In summary three temporally discrete episodes of magmatism and tectonics with 1-2 Myr periodicity are observed that we believe resulted from a pulsing Icelandic Plume head.</p>
An intrusion of trachy-andesite, representative of a newly discovered suite of high-K–Ba–Sr, calc-alkaline minor intrusions (termed herein the Sperrin Mountains suite), hosted within the Grampian terrane in the north of Ireland, has been dated by U–Pb zircon at 426.69 ± 0.85 Ma (mid-Silurian; Wenlock–Ludlow boundary). Geochemistry reveals a close association with the Fanad, Ardara and Thorr plutons of the Donegal Batholith and the Argyll and Northern Highlands Suite of Scotland. The deep-seated Omagh Lineament appears to have limited eastward propagation of the Sperrin Mountains suite from beneath the main centre of granitic magmatism in Donegal. A Hf depleted mantle model age (T DM Hf) of c . 800 Ma for trachy-andesite zircons indicates partial melting from a source previously separated from the mantle. Whole-rock geochemistry of the suite is consistent with a model of partial melting, triggered by slab break-off, following thrusting of Ganderia–Avalonia under the Southern Uplands–Down–Longford accretionary prism (i.e. Laurentian margin). The new age constrains the timing of this event in the north of Ireland and is consistent with the petrogenesis of Late Caledonian high-K granites, appinites and minor intrusions across the Caledonides of northern Britain and Ireland. Supplementary material: Geochemical data are available at www.geolsoc.org.uk/SUP18600 .
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