Dating the onset of volcanism at the Rum Igneous Centre, NW Scotland
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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 40 Ar/ 39 Ar 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.Keywords:
Large igneous province
Geochemical and isotopic data provide insights into the origin and evolution of magmatism found at destructive plate margins. Tholeiitic magmas are dominant in the early stages of oceanic island-arc genesis and calc-alkalic magmas are most common in mature oceanic arcs and in continental arcs where they may range from basalt to rhyolite in composition, including voluminous intermediate (andesitic) rocks. Experiments suggest that calcalkalic mafic magmas are formed by melting of a hydrated mantle wedge and undergo low pressure fractional crystallization under near-H2O saturated conditions. Intermediate to felsic magmas are derived in a wide variety of ways, including the fractionation of a more mafic parent, mixing between mafic and felsic magmas (a process supported, in many cases, by field and textural evidence), crustal contamination, or partial melting of the crust. All these processes appear to take place, to some degree, in arc systems, although in any given arc system, one mechanism may predominate. Arc-related calc-alkalic and tholeiitic basalts typically show moderate degrees of light rare-earth-element (LREE) enrichment, and flat heavy rare-earth-element (HREE) profiles, indicating an origin in a shallow (spinel lherzolite) mantle. More evolved magmas exhibit Eu anomalies, consistent with low pressure plagioclase fractionation. Compared to within-plate settings, tholeiitic and calc-alkalic arc magmas have lower abundances in high-field-strength (HFS) elements, possibly because these elements are bound during the accessory phases in the mantle wedge, and are stable during partial melting. Compared to arc tholeiites, calc-alkalic magmas have higher abundances of incompatible large ion lithophile (LIL) elements reflecting enrichment in the mantle wedge source. This characteristic depletion in HFS, and enrichment in LIL, elements, in arc magmas is the basis for a variety of discrimination diagrams. These diagrams constrain processes operating in modern and ancient arc systems and include chondrite-normalized, MORB-normalized and mantle-normalized spidergrams, which are characterized by jagged patterns of trace-element abundances (in contrast to the relatively smooth patterns of within-plate suites). Some arc suites have depleted initial 143Nd/144Nd and lower initial 87Sr/86Sr than the bulk earth, and are similar to MORB. Other suites have enriched isotopic patterns consistent with the influence of subducted oceanic sediments on the composition of the magma. Samarium-Nd and Rb-Sr isotopic studies can be used to distinguish between felsic magmas derived from fractional crystallization of a more mafic parent (which would have similar values) and those derived from the melting of ancient crust. SOMMAIRE
Les donnees geochimiques et isotopiques fournissent des indications quant a l'origine et a l'evolution du magmatisme des marges de subduction des plaques tectoniques. Les magmas sont principalement tholeiitiques dans les premieres phases de formation des arcs insulaires oceaniques, alors qu'ils sont principalement calco-alcalins pendant les phases terminales des arcs insulaires oceaniques ainsi que dans les arcs insulaires continentaux, ou leur composition peut aller du basalte a la rhyolite, dont des volumes considerables de roches de composition inter-mediaire (andesitique). Des experiences permettent de penser que les magmas mafiques calco-alcalins sont formes par la fusion d'un biseau mantelique hydrate qui subit une cristallisation fractionnee a basse pression en des conditions de quasi-saturation en H2O. Les magmas de composition intermediaire a felsique resultent de mecanismes tres varies, dont le fractionnement d'une roche mere plus mafique, le melange de magmas felsiques et mafiques (mecanismes mis en evidence par des donnees de terrain et l'analyse texturale), la contamination crustale, ou la fusion partielle de la croute. Tous ces mecanismes semblent se produire, au moins partiellement, au sein d'arcs insulaires, mais l'un d'eux peut constituer le mecanisme predominant de quelque systeme d'arcs insulaires particulier. L'enrichissement modere typique des basaltes calco-alcalins et tholeiitiques d'arcs insulaires en elements legers des terres (LREE) rares ainsi que le profil plat de leur contenu en elements lourds des terres rares (HREE) sont des indicateurs d'une origine mantelique peu profonde (iherzolithe a spinelle). Les magmas plus evolues affichent des anomalies en Eu, ce qui concorde avec un fraction-nement a basse temperature des plagioclases. Compares a ceux des contextes intra-plaques, les magmas tholeiitiques et calco-alcalins d'arcs insulaires affichent des contenus moindres en elements a grande intensite de champ, peut-etre parce que ces elements sont lies pendant les phases accessoires dans le biseau mantelique, et sont stables pendant la phase de fusion partielle. Compares aux tholeiites d'arcs insulaires, les magmas calco-alcalins ont des contenus plus eleves en elements lithophiles a grands champs ioniques incompatibles, ce qui est le reflet d'un enrichissement au sein du biseau mantelique source. Cet appauvrissement caracteristique en elements a grande intensite de champ (HFS) et cet enrichissement en elements lithophiles a grands champs ioniques (LIL) des magmas d'arcs insulaires forment la base d'une variete de diagrammes de discrimination. Ces diagrammes permettent de preciser les processus en jeu des systemes d'arcs insulaires modernes et anciens et incluent des diagrammes radiaux normalises pour les chondrites, pour les basaltes de dorsales oceaniques (MORB) et pour le manteau, lesquels son caracterises par des profils anguleux irreguliers des courbes de contenus en elements traces (en contraste avec les profils relativement reguliers des suites intra-cratoniques). Certaines suites d'arcs insulaires montrent des ratios initiaux 143Nd/144Nd appauvris et 87Sr/86Sr inferieurs a celui de la valeur planetaire actuelle, et qui sont semblables a celui des basaltes de dorsales oceaniques. D'autres suites ont des profils isotopiques enrichis, ce qui correspond a une influence de sediments oceaniques subductes sur la composition du magma. Les etudes samariumneodymium et rubidium-strontium peuvent etre utilisees pour differencier entre les magmas felsiques issus d'une cristallisation fractionnee d'une roche mere plus mafique (qui montrerait des valeurs similaires) et ceux provenant de la fusion d'une croute ancienne.
Igneous petrology
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Abstract: New Re–Os molybdenite geochronology is presented from the Ballachulish Igneous Complex (433.5 ± 1.8 Ma) and the Kilmelford Igneous Complex (425.8 ± 1.7 Ma) of the SW Scottish Highlands. The sulphide mineralization in the Ballachulish Igneous Complex is related to the latest phase of granite magmatism, and therefore the new Re–Os age provides a minimum crystallization age, c . 3.7 Ma earlier than previous crystallization age estimates. This Re–Os age overlaps U–Pb ages obtained from subduction-related granitic and appinitic magmatism north of the Great Glen Fault, and an origin related to active subduction rather than slab breakoff is proposed for the Ballachulish Igneous Complex. Molybdenite mineralization in the Kilmelford Igneous Complex is spatially and genetically associated with porphyry Cu mineralization, which is consistent with the rapid ascent of volatile-rich magma during early rebound following the breakoff of subducted oceanic lithosphere. The Kilmelford Igneous Complex and the coeval Lorn Lava Pile may represent the earliest of the igneous bodies predicted by slab breakoff, indicating that slab breakoff occurred at c . 426 Ma.
Molybdenite
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Large igneous provinces are usually emplaced in one short pulse of ~1-5 million years, or several punctuated, ~1-5 million year pulses.Here, our new plagioclase 40 Ar/ 39 Ar plateau ages for the main construct of the Kerguelen large igneous province-the Southern and Central Kerguelen Plateau, Elan Bank and Broken Ridge -show continuous volcanic activity from range from 122.2 ± 2.6 Ma to 89.9 ± 1.0 Ma and more specifically from ~122 to ~90 Ma for the Southern Kerguelen Plateau, from ~111 to ~106 Ma for Elan Bank, from ~109 to ~93 Ma for the Central Kerguelen Plateau, and from ~99 to ~98 Ma for Broken Ridge, i.e. a long lifespan of > 32 million years.This suggests that the Kerguelen large igneous province records a previously undocumented emplacement tempo for large igneous provinces.Distinct from short-lived and multiple-pulsed large igneous provinces, we propose that Kerguelen is a new type of large igneous province that formed due to long-term plume-ridge interaction and jump(s) of the spreading ridge towards the plume.Such a process allows for transport of magma products away from the eruption centre, thus creating space for the magma to continuously rise, and results in longlived, continuous magmatic activity.
Large igneous province
Mantle plume
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The Istanbul Zone (NW Turkey) forms the eastward extension of Avalonia and was subjected to deformation, uplift and erosion for a time period of 40–50 Ma following the collision with the Sakarya Zone during Early to Late Carboniferous. This paper deals with the petrology and age of the volumetrically minor basic and acidic volcanism at the lowermost horizons of Middle Permian continental red beds, which are overlain by Lower Triassic marine sedimentary rocks in the Kocaeli Peninsula. The volcanic activity is represented mainly by amygdaloidal basalt, rhyolite and minor trachydacite. The amygdaloidal basalt was derived from near-primary middle-K calc-alkaline mantle melts with negligible crystal fractionation. On the other hand, the rhyolite and trachydacite compositionally resemble A2-type rhyolites and underwent low-pressure crystal fractionation as indicated by the presence of a significant Eu anomaly. Initial ɛNd values of amygdaloidal basalt range from 0.0 to 1.5 and those of rhyolite-trachydacite are between −0.4 and −3.4. Amygdaloidal basalt and rhyolite-trachydacite are not directly related to each other by crystal fractionation. Amygdaloidal basalt probably represents the product of the near-primary mantle melts from low-degree melting of a spinel peridotitic source, and the rhyolite-trachydacite originated from highly-fractionated products of basic magmas that are slightly more alkaline than amygdaloidal basalt. However, basic and intermediate products of alkaline basic magmas are unknown in this region to date. U-Pb dating of zircons from a rhyolite sample yielded an igneous crystallization age of 261 ± 3 Ma (2σ), suggesting that the date of deposition of the continental red beds goes back to the latest Middle Permian. Based on the transgressive nature of the Permian-Triassic sequence that starts from the Middle Permian continental red beds and grades into Lower Triassic marine deposits, we suggest that the volcanism likely occurred in an extensional setting. This extension was concurrent with the northward subduction of the Palaeo-Tethys beneath the Sakarya and Istanbul zones after the Variscan orogeny. Therefore, the latest Middle to Late Permian volcanism might have occurred during the initial stage of a back-arc extensional setting
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Petrologic and geochemical studies of basaltic rocks in the Yucca Mountain region are currently focused on understanding the evolution of volcanism in the Crater Flat volcanic field and the mechanisms of polycyclic volcanism at the Lathrop Wells volcanic center, the youngest center in the Crater Flat volcanic field. Geochemical and petrologic data indicate that the magma chambers which supplied the volcanic centers in Crater Flat became situated at greater crustal depths as the field evolved. Deep magma chambers may be related to a waning magma flux that was unable to sustain upper crystal magma conduits and chambers. Geochemical data from the Lathrop Wells volcanic center indicate that eruptive units identified from field and geomorphic relationships are geochemically distinct. The geochemical variations cannot be explained by fractional crystallization of a single magma batch, indicating that several magma batches were involved in the formation of the Lathrop Wells center. Considering the low magma flux in the Yucca Mountain region in the Quaternary, the probability of several magma batches erupting essentially simultaneously at Lathrop Wells in considered remote. It is more likely that the Lathrop Wells center was formed by a series of eruptions that took place over many thousands of years. Themore » geochemical data from Lathrop Wells is consistent with the concept of a complex, polycyclic volcano, which was originally proposed based on geomorphic and soil-development data.« less
Volcanology
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