| 1. |
- Meyer, R., et al.
(författare)
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Trace element and isotope constraints on crustal anatexis byupwelling mantle melts in the North Atlantic Igneous Province: anexample from the Isle of Rum, NW Scotland
- 2009
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Ingår i: Geological Magazine. - 0016-7568 .- 1469-5081. ; 146:3, s. 382-399
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Tidskriftsartikel (refereegranskat)abstract
- Sr and Nd isotope ratios, together with lithophile trace elements, have been measured in arepresentative set of igneous rocks and Lewisian gneisses from the Isle of Rum in order to unravel thepetrogenesis of the felsic rocks that erupted in the early stages of Palaeogene magmatism in the NorthAtlantic Igneous Province (NAIP). The Rum rhyodacites appear to be the products of large amountsof melting of Lewisian amphibolite gneiss. The Sr and Nd isotopic composition of the magmas canbe explained without invoking an additional granulitic crustal component. Concentrations of the traceelement Cs in the rhyodacites strongly suggests that the gneiss parent rock had experienced Cs and Rbloss prior to Palaeogene times, possibly during a Caledonian event. This depletion caused heterogeneitywith respect to 87Sr/86Sr in the crustal source of silicic melts. Other igneous rock types on Rum (dacites,early gabbros) are mixtures of crustalmelts and and primarymantle melts. Forward Rare Earth Elementmodelling shows that late stage picritic melts on Rum are close analogues for the parent melts of theRum Layered Suite, and for the mantle melts that caused crustal anatexis of the Lewisian gneiss.These primary mantle melts have close affinities to Mid-Oceanic Ridge Basalts (MORB), whose traceelement content varies from slightly depleted to slightly enriched. Crustal anatexis is a common processin the rift-to-drift evolution during continental break-up and the formation of Volcanic Rifted Marginssystems. The ‘early felsic–later mafic’ volcanic rock associations from Rum are compared to similarassociations recovered from the now-drowned seaward-dipping wedges on the shelf of SE Greenlandand on the Vøring Plateau (Norwegian Sea). These three regions show geochemical differences thatresult from variations in the regional crustal composition and the depth at which crustal anatexis took place.
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| 2. |
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| 3. |
- Carracedo, J.C., et al.
(författare)
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The NE Rift of Tenerife: towards a model on the origin and evolution of ocean island rifts
- 2009
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Ingår i: Estudios Geologicos. - : Editorial CSIC. - 1988-3250 .- 0367-0449. ; 65:1, s. 5-47
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Tidskriftsartikel (refereegranskat)abstract
- The NE Rift of Tenerife is an excellent example of a persistent, recurrent rift, providing important evidence of the origin and dynamics of these major volcanic features. The rift developed in three successive, intense and relatively short eruptive stages (a few hundred ka), separated by longer periods of quiescence or reduced activity: A Miocene stage (7266 +/- 156 ka), apparently extending the central Miocene shield of Tenerife towards the Anaga massif; an Upper Pliocene stage (2710 +/- 58 ka) and the latest stage, with the main eruptive phase in the Pleistocene. Detailed geological (GIS) mapping, geomagnetic reversal mapping and stratigraphic correlation, and radioisotopic (K/Ar) dating of volcanic formations allowed the reconstruction of the latest period of rift activity. In the early phases of this stage the majority of the eruptions grouped tightly along the axis of the rift and show reverse polarity (corresponding to the Matuyama chron). Dykes are of normal and reverse polarities. In the final phase of activity, eruptions are more disperse and lavas and dykes are consistently of normal polarity (Brunhes chron). Volcanic units of normal polarity crossed by dykes of normal and reverse polarities yield ages apparently compatible with normal subchrons (M-B Precursor and Jaramillo) in the Upper Matuyama chron. Three lateral collapses successively mass-wasted the rift: The Micheque collapse, completely concealed by subsequent nested volcanism, and the Guimar and La Orotava collapses, that are only partially filled. Time occurrence of collapses in the NE rift apparently coincides with glacial stages, suggesting that giant landslides may be finally triggered by sea level changes during glaciations. Pre-collapse and nested volcanism is predominantly basaltic, except in the Micheque collapse, where magmas evolved towards intermediate and felsic (trachytic) compositions. Rifts in the Canary Islands are long-lasting, recurrent features, probably related to primordial, plume-related fractures acting throughout the entire growth of the islands. Basaltic volcanism forms the bulk of the islands and rift zones. However, collapses of the flanks of the rifts disrupt their established fissural feeding system, frequently favouring magma accumulation and residence at shallow emplacements, leading to differentiation of magmas, and intermediate to felsic nested eruptions. Rifts and their collapse may therefore act as an important factor in providing petrological variability to oceanic volcanoes. Conversely, the possibility exists that the presence of important felsic volcanism may indicate lateral collapses in oceanic shields and ridge-like volcanoes, even if they are concealed by post-collapse volcanism or partially mass-wasted by erosion.
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| 4. |
- Longpré, Marc-Antoine, et al.
(författare)
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Volcanic and geochemical evolution of the Teno massif, Tenerife, Canary Islands : Some repercussions of giant landslides on ocean island magmatism
- 2009
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Ingår i: Geochemistry Geophysics Geosystems. - 1525-2027. ; 10:12, s. 31-
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Tidskriftsartikel (refereegranskat)abstract
- Large-scale, catastrophic mass wasting is a major process contributing to the dismantling of oceanic intraplate volcanoes. Recent studies, however, have highlighted a possible feedback relationship between flank collapse, or incipient instability, and subsequent episodes of structural rearrangement and/or renewed volcano growth. The Teno massif, located in northwestern Tenerife (Canary Islands), is a deeply eroded Miocene shield volcano that was built in four major eruptive phases punctuated by two lateral collapses, each removing >20–25 km3 of the volcano's north flank. In this paper, we use detailed field observations and petrological and geochemical data to evaluate possible links between large-scale landslides and subsequent volcanism/magmatism during Teno's evolution. Inspection of key stratigraphic sequences reveals that steep angular unconformities, relics of paleolandslide scars, are marked by polymict breccias. Near their base, these deposits typically include abundant juvenile pyroclastic material, otherwise scarce in the region. While some of Teno's most evolved, low-density magmas were produced just before flank collapses, early postlandslide lava sequences are characterized by anomalously high proportions of dense ankaramite flows, extremely rich in clinopyroxene and olivine crystals. A detailed sampling profile shows transitions from low-Mg # lavas relatively rich in SiO2 to lavas with low silica content and comparatively high Mg # after both landslides. Long-term variations in Zr/Nb, normative nepheline, and La/Lu are coupled but do not show a systematic correlation with stratigraphic boundaries. We propose that whereas loading of the growing precollapse volcano promoted magma stagnation and differentiation, the successive giant landslides modified the shallow volcano-tectonic stress field at Teno, resulting in widespread pyroclastic eruptions and shallow magma reservoir drainage. This rapid unloading of several tens of km3 of near-surface rocks appears to have upset magma differentiation processes, while facilitating the remobilization and tapping of denser ankaramite magmas that were stored in the uppermost mantle. Degrees of mantle melting coincidently reached a maximum in the short time interval between the two landslides and declined shortly after, probably reflecting intrinsic plume processes rather than a collapse-induced influence on mantle melting. Our study of Teno volcano bears implications for other oceanic volcanoes where short-term compositional variations may also directly relate to major flank collapse events.
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| 5. |
- Manconi, Andrea, et al.
(författare)
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The effects of flank collapses on volcano plumbing systems
- 2009
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 37:12, s. 1099-1102
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Tidskriftsartikel (refereegranskat)abstract
- The growth of large volcanoes is commonly interrupted by episodes of flank collapse that may be accompanied by catastrophic debris avalanches, explosive eruptions, and tsunamis. El Hierro, the youngest island of the Canary Archipelago, has been repeatedly affected by such mass-wasting events in the last 1 Ma. Our field observations and petrological data suggest that the largest and most recent of these flank collapses—the El Golfo landslide—likely influenced the magma plumbing system of the island, leading to the eruption of higher proportions of denser and less evolved magmas. The results of our numerical simulations indicate that the El Golfo landslide generated pressure changes exceeding 1 MPa down to upper-mantle depths, with local amplification in the surroundings and within the modeled magma plumbing system. Stress perturbations of that order might drastically alter feeding system processes, such as degassing, transport, differentiation, and mixing of magma batches.
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| 6. |
- Meade, F. C., et al.
(författare)
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Magma Ascent along a Major Terrane Boundary: Crustal Contamination and Magma Mixing at the Drumadoon Intrusive Complex, Isle of Arran, Scotland
- 2009
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Ingår i: Journal of Petrology. - : Oxford University Press (OUP). - 0022-3530 .- 1460-2415. ; 50:12, s. 2345-2374
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Tidskriftsartikel (refereegranskat)abstract
- The composite intrusions of Drumadoon and An Cumhann crop out on the SE coast of the Isle of Arran, Scotland and form part of the larger British and Irish Palaeogene Igneous Province, a subset of the North Atlantic Igneous Province. The intrusions (shallow-level dykes and sills) comprise a central quartz-feldspar-phyric rhyolite flanked by xenocryst-bearing basaltic andesite, with an intermediate zone of dark quartz-feldspar-phyric dacite. New geochemical data provide information on the evolution of the component magmas and their relationships with each other, as well as their interaction with the crust through which they travelled. During shallow-crustal emplacement, the end-member magmas mixed. Isotopic evidence shows that both magmas were contaminated by the crust prior to mixing; the basaltic andesite magma preserves some evidence of contamination within the lower crust, whereas the rhyolite mainly records upper-crustal contamination. The Highland Boundary Fault divides Arran into two distinct terranes, the Neoproterozoic to Early Palaeozoic Grampian Terrane to the north and the Palaeozoic Midland Valley Terrane to the south. The Drumadoon Complex lies within the Midland Valley Terrane but its isotopic signatures indicate almost exclusive involvement of Grampian Terrane crust. Therefore, although the magmas originated at depth on the northern side of the Highland Boundary Fault, they have crossed this boundary during their evolution, probably just prior to emplacement.
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| 7. |
- Nicoll, Graeme R., et al.
(författare)
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Early mafic magmatism and crustal anatexis on the Isle of Rum : evidence from the Am Mam intrusion breccia
- 2009
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Ingår i: Geological Magazine. - 0016-7568 .- 1469-5081. ; 146:3, s. 368-381
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Tidskriftsartikel (refereegranskat)abstract
- The Rum Igneous Centre comprises two early marginal felsic complexes (the Northern Marginal Zone and the Southern Mountains Zone), along with the later central ultrabasic-basic layered intrusions. These marginal complexes represent the remnants of near-surface to eruptive felsic magmatism associated with caldera collapse, examples of which are rare in the North Atlantic Igneous Province. Rock units include intra-caldera collapse breccias, rhyolitic ignimbrite deposits and shallow-level felsic intrusions, as well the enigmatic 'Am Mam intrusion breccia'. The latter comprises a dacitic matrix enclosing lobate basaltic inclusions (similar to 1-15 cm) and a variety of clasts, ranging from millimetres to tens of metres in diameter. These clasts comprise Lewisian gneiss, Torridonian sandstone and coarse gabbro. Detailed re-mapping of the Am Mam intrusion breccia has shown its timing of emplacement as syn-caldera, rather than pre-caldera as previously thought. Textural analysis of entrained clasts and adjacent, uplifted country rocks has revealed their thermal metamorphism by early mafic intrusions at greater depth than their present structural position. These findings provide a window into the evolution of the early mafic magmas responsible for driving felsic magmatism on Rum. Our data help constrain some of the physical parameters of this early magma-crust interaction and place it within the geochemical evolution of the Rum Centre.
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| 8. |
- Byrne, Paul K., et al.
(författare)
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The geometry of volcano flank terraces on Mars
- 2009
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 281:1-2, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- Flank terraces are subtle, expansive structures on the slopes of many large Martian shield volcanoes. Several terrace formation hypotheses - including self-loading, lithospheric flexure, magma chamber tumescence, volcano spreading, and shallow gravitational slumping - have been suggested. Terraces are not readily visible on photogeological data; consequently, terrace geometry has not yet been comprehensively described. Terrace provenance, therefore, is poorly understood. We used three-dimensional Mars Orbiter Laser Altimeter (MOLA) data to characterise the geometry of these elusive structures, with a view to understanding better the role that flank terraces play in the tectonic evolution of volcanoes on Mars. Terraces have a broad, convex-upward profile in section, and a systematic "fish scale" imbricate stacking pattern in plan. They are visible at all elevations, on at least nine disparate Martian volcanoes. Terrace-like features also occur on three shield volcanoes on Earth, an observation not recorded before. Analysis of a suite of morphometric parameters for flank terraces showed that they are scale-invariant. with similar proportions to thrust faults on Earth. We compared predicted formation geometries to our terrace observations, and found that only lithospheric flexure can fully account for the morphology, distribution, and timing of terraces. As a volcano flexes into the lithosphere beneath it, its upper surface will experience a net reduction in area, resulting in the formation of outward verging thrusts. We conclude, therefore, that flank terraces are fundamental volcanotectonic structures, that they are the surface expressions of thrust faults, probably formed by lithospheric flexure. and that they are not restricted to Mars.
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| 9. |
- Troll, Valentin, et al.
(författare)
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Dating the onset of volcanism at the Rum Igneous Centre, NW Scotland
- 2008
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 165, s. 651-659
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Tidskriftsartikel (refereegranskat)abstract
- Major volcanic activity on the Isle of Rum commenced with the eruption of thick (>100 m) intra-caldera rhyodacite ash-flow sheets fed from steep-sided feeder conduits in the proximity of the Main Ring Fault. Twenty plagioclase phenocrysts of the rhyodacite were analysed using single crystal 40Ar/39Ar laser dating, yielding a mean apparent age of 60.83 ± 0.27 Ma (MSWD=3.65). On an age v. probability plot the feldspars do not, however, show a simple Gaussian distribution, but a major peak at 60.33 ± 0.21 Ma and two smaller shoulders at c. 61.4 Ma and 63 Ma. These older ages are interpreted to represent recycled and largely re-equilibrated feldspars. The age peak at 60.33 ± 0.21 Ma is interpreted to represent the intrusion and eruption age of the rhyodacites. This new age constraint overlaps with that for the ultrabasic intrusion, implying that the latter was already forming at depth and supplying necessary heat during the early felsic activity phase, and quickly thereafter migrated upwards to shallow structural levels and intruded the volcano's earlier deposits. Combined with previously published ages, these new age data highlight an extremely rapid succession of events at the Rum centre, the whole sequence occurring in potentially <500 ka.
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