50 research outputs found
Cambrianâearly Ordovician volcanism across the South Armorican and Occitan domains of the Variscan Belt in France: Continental break-up and rifting of the northern Gondwana margin
The Cambrianâlower Ordovician volcanic units of the South Armorican and Occitan domains are analysed in a tectonostratigraphic survey of the French Variscan Belt. The South Armorican lavas consist of continental tholeiites in middle CambrianâFurongian sequences related to continental break-up. A significant volcanic activity occurred in the Tremadocian, dominated by crustal melted rhyolitic lavas and initial rifting tholeiites. The Occitan lavas are distributed into five volcanic phases: (1) basal Cambrian rhyolites, (2) upper lower Cambrian Mg-rich tholeiites close to N-MORBs but crustal contaminated, (3) upper lowerâmiddle Cambrian continental tholeiites, (4) Tremadocian rhyolites, and (5) upper lower Ordovician initial rift tholeiites. A rifting event linked to asthenosphere upwelling took place in the late early Cambrian but did not evolve. It renewed in the Tremadocian with abundant crustal melting due to underplating of mixed asthenospheric and lithospheric magmas. This main tectono-magmatic continental rift is termed the âTremadocian Tectonic Beltâ underlined by a chain of rhyolitic volcanoes from Occitan and South Armorican domains to Central Iberia. It evolved with the setting of syn-rift coarse siliciclastic deposits overlain by post-rift deep water shales in a suite of sedimentary basins that forecasted the South ArmoricanâMedio-European Ocean as a part of the Palaeotethys Ocean.This research was funded by project CGL2013-48877-P from Spanish MINECO.Peer reviewe
Les laves du mont Bangou : une premiÚre manifestation volcanique éocÚne, à affinité transitionnelle, de la Ligne du Cameroun
Les basaltes Ă©ocĂšnes Ă affinitĂ© transitionnelle du plateau Bamoun, tĂ©moins dâun rĂ©servoir mantellique enrichi sous la ligne volcanique du Cameroun
Les pyroxÚnes et leurs inclusions, marqueurs privilégiés des nuées ardentes (Saint-Vincent, Antilles, 1979)
Pyroxenes of St. Vincent SoufriÚre volcano andesito-basaltic nuées ardentes (1979) show clearly three stages of crystallization. 1) Cr-rich diopsides (with chromites and Fo 89 olivines) crystallize early (P > 10 kbar) from primary basaltic magma. 2) Salites characterize a second high P H2O stage (P H2O = 5 kbar) and occur in cumulate with Al-magnetites, Fo 75 olivines, Ca-plagioclases and amphiboles. 3) Augites and orthopyroxenes appear in the last stage with Na-plagioclases. Primary melt inclusions do not occur in diopsides ; their compositions are andesito-basaltic in salites and andesito-dacitic in augites. The different crystallization-stages are displayed in some products of the eruption (cumulates, blocks and ash-matrix of nuées ardentes, domes).
The proposed volcanological interpretation is that of mechanical mixing of residual dacitic magma (in small amount) produced by fractional crystallization with reinjected andesitic magma. This mixing produces sudden variations of thermodynamic conditions, triggering nuées ardentes.Les pyroxÚnes des nuées ardentes andésito-basaltiques émises en 1979 par la SoufriÚre de Saint-Vincent (Antilles) montrent trois stades de cristallisation. 1) Les diopsides chromifÚres (accompagnés de chromites et d'olivines Fo 89) ont cristallisé précocement (P > 10 kbar) à partir d'un magma basaltique primaire. 2) Les salites qui caractérisent un second stade à forte P H2O (5 kbar), sont dans des cumulats à magnétites-Al, olivines Fo 75, plagioclases calciques et amphiboles. 3) Les augites et les orthopyroxÚnes apparaissent enfin avec les plagioclases sodiques. Les inclusions vitreuses primaires apparaissent dans les minéraux des deux derniers stades de cristallisation ; elles sont de composition andésito-basaltique dans les salites et andésito-dacitique dans les augites. Ces différentes associations sont observées dans certains des produits de l'éruption (cumulats, blocs et matrices cendreuses des nuées, dÎmes).
L'interprétation proposée est celle d'un mélange mécanique d'un magma résiduel dacitique (en faible quantité) résultant de cette cristallisation fractionnée et d'un magma andésitique réinjecté. Ce mélange provoque de brusques variations des conditions thermodynamiques, responsables du déclenchement des nuées ardentes.Bardintzeff Jacques-Marie. Les pyroxÚnes et leurs inclusions, marqueurs privilégiés des nuées ardentes (Saint-Vincent, Antilles, 1979). In: Bulletin de Minéralogie, volume 107, 1, 1984. pp. 41-54
Les risques volcaniques directs ou différés : exemples du Pinatubo (Philippines, 1991) et de Rabaul (Papouasie - Nouvelle-Guinée, 1994) (Direct or indirect volcanic hazards : case-studies of Pinatubo (Philippines, 1991) and Rabaul (Papua - New Guinea, 1994)
Abstract. - Two recent volcanic eruptions (Pinatubo, Philippines, 1991 and Rabaul, Papua - New-Guinea, 1994) have examplified direct and indirect volcanic hazards. These could occur immediately, during the eruption (ash fall...) or after a more or less long time span (lahars), sometimes in a distant area (tidal wave). Moreover, human occupation increases the volcanic risk as well as meteoric / climatic factors (heavy rain, wind direction). All of these risks must be taken into account by scientific teams and public responsibles.RĂ©sumĂ©. - Deux Ă©ruptions rĂ©centes (Pinatubo, Philippines, 1991 et Rabaul, Papouasie - Nouvelle-GuinĂ©e, 1994) ont eu un impact humain important. Les risques volcaniques apparaissent multiples, immĂ©diats (chute de cendres...) ou diffĂ©rĂ©s dans le temps (lahars) ou dans l'espace (raz de marĂ©e). En outre des facteurs anthropogĂ©niques (occupations urbaine) mais aussi climatiques (fortes pluies, rĂ©gime des vents) interviennent. Tous doivent ĂȘtre pris en compte par les scientifiques et les autoritĂ©s.Bardintzeff Jacques Marie. Les risques volcaniques directs ou diffĂ©rĂ©s : exemples du Pinatubo (Philippines, 1991) et de Rabaul (Papouasie - Nouvelle-GuinĂ©e, 1994) (Direct or indirect volcanic hazards : case-studies of Pinatubo (Philippines, 1991) and Rabaul (Papua - New Guinea, 1994). In: Bulletin de l'Association de gĂ©ographes français, 76e annĂ©e, 1999-4 ( dĂ©cembre). L'homme et le volcanisme. Les eurorĂ©gions, sous la direction de GĂ©rard Mottet et Henri Rougier. pp. 360-370
La spécificité granulométrique des retombées volcaniques : l'exemple de la chute de ponces P1 à 1300 ans ap. J.-C. à la Montagne Pelée (Martinique)
Pumice falls, emitted during 1 300 A D plinian eruption (PI) at Mont Pelée (Martinique) have capped the whole volcano. This tephrochronological layer is studied in details.
- Pumice fall deposits are several meters thick near the vent, and still 50 cm thick, 10 km far from it.
- Grain-size medians are mostly between 4,8 and 13,9 mm ( â 2,25 and â 3,8 â) and sorting between 1,33 and 2,75 â
- Local variations of grain-size parameters are resulting from sorting by wind or by « dry » gliding.
- Grain-size parameters are typical of plinian pumice falls and contrasting with associated pumice flow, badly-sorted with a weak medianLes ponces émises lors de l'éruption plinienne P1 (1 300 ans ap. J.- C.) à la Montagne Pelée (Martinique) ont recouvert l'ensemble du volcan. Ce niveau, constituant un bon repÚre tephrochronologique a été étudié en détail.
- L'épaisseur des retombées (isopaques), de plusieurs mÚtres au niveau du cratÚre est encore de 50 cm à 10 km de celui-ci.
- Les mĂ©dianes sont pour la plupart comprises entre 4,8 et 13,9 mm (â2,25 et â 3,8â), les classements entre 1,33 et 2,75 â
- Les variations locales des paramÚtres granulométriques traduisent l'influence du tri par le vent ou par glissement « à sec ».
- Les paramÚtres granulométriques obtenus sont typiques des retombées pliniennes, bien différents de ceux des dépÎts de la nuée ponceuse associée, trÚs mal classée, avec une médiane faible.Miskovsky Jean Claude, Bardintzeff Jacques Marie. La spécificité granulométrique des retombées volcaniques : l'exemple de la chute de ponces P1 à 1300 ans ap. J.-C. à la Montagne Pelée (Martinique). In: Bulletin de l'Association française pour l'étude du quaternaire, vol. 22, n°4, 1985. pp. 249-256
Identité granulométrique de Tephra
Grain-size analyses (median and sorting) of ash-fraction (size less than 2 mm) of tephra emitted by Mont Pelee and some other andesitic volcanoes allow us to determine their original dynamism. Four types can be distinguished :
âą Pumice falls produce coarse and well sorted deposits, whereas associated pumice flows produce well-sorted too, but very finer deposits
⹠Nuées ardentes (Pelean and St Vincent types) produce fine deposits (median less than 0,5 mm).
⹠Avalanche nuées (Merapi type) give coarser deposits (median greater than 0,5 mm)
âą Surge derived tephra are varied but always well sortedL'Ă©tude granulomĂ©trique (mĂ©diane et classement) de la fraction cendreuse (infĂ©rieure Ă 2 mm) des tephra volcaniques (Ă©mis par la Montagne PelĂ©e et d'autres volcans andĂ©sitiques) permet de distinguer leurs diffĂ©rents dynamismes gĂ©nĂ©rateurs. Ainsi quatre grands types peuvent ĂȘtre sĂ©parĂ©s :
⹠Les retombées ponceuses donnent des dépÎts grossiers et bien classés, alors que ceux des coulées de ponce associées sont également bien classés mais beaucoup plus fins.
⹠Les produits des nuées ardentes s. s. (type Pelée et St Vincent) sont relativement fins (médiane inférieure à 0,5 mm)
⹠Les nuées d'avalanches (type Merapi) laissent des dépÎts plus grossiers (médiane supérieure à 0,5 mm) que les précédents.
⹠Les déferlantes (ou « surges ») génÚrent des tephra particuliers, toujours bien classés.Bardintzeff Jacques Marie, Miskovsky Jean Claude. Identité granulométrique de Tephra . In: Bulletin de l'Association française pour l'étude du quaternaire, vol. 21, n°4, 1984. pp. 221-231
The mineralogical and petrological constraints of the Cretaceous Kermanshah ophiolitic complex in Nourabad and Dinavar regions in western Iran
International audienceAs a part of the Kermanshah ophiolite in western Iran, the Cretaceous Nourabad-Dinavar ophiolitic complex is a remnantof the Neo-Tethys oceanic lithosphere and represents transitional mantle-crust and upper crust units in the Nourabad andDinavar regions, respectively. All the units were affected by the two metamorphic regimes of static metamorphism anddynamic metamorphism. The whole-rock chemical data of the basic samples (i.e. gabbros, basalts, and dykes) show thatthey are related to the island-arc regime. The main reasons for this conclusion are as follows: their affinity with the calc-alkaline series, LREE enrichment, and subduction-related proxies such as the negative anomalies of Nb, Ta, Zr, and Hf andthe positive anomaly of Th. On the other hand, the mineral chemistry analysis confirms that the studied ophiolitic complexis a MORB-type ophiolite emplaced in the supra-subduction zone. This is supported by mineralogical evidence including thecompositional dependence of olivines (fo 90-91 ) on the spinel peridotite mantle facies, spinel minerals (Al-chromite and Mg/Cr-bearing hercynite), and Mg-rich orthopyroxenes (enstatite) in the harzburgites. The geochemical modeling implies thatthis complex evolved through the following successive magmatic steps: 1) the partial melting of a mixed NMORB-EMORB(50:50) source producing spinel harzburgite residues; 2) the fractional crystallization of the basic partial melts during theirascent to the surface and the formation of gabbro bodies; 3) the assimilation and fractional crystallization process as theNMORB components re-enter the chamber and produce basic pillow lavas, lava flows, and some fine-grained gabbro bod-ies (i.e. dykes). Accordingly, it can be interpreted that the emplacement history of the studied ophiolite succession has twostages: 1) an obduction stage in the Campanian; 2) an exhumation stage in the post-Cretaceous
Variscan lamprophyres of the South Armorican Domain and comparison with lamprophyres of the Western European Variscan belt
International audienceLate to post-orogenic lamprophyres of the European Variscides attest variable compositions of the mantle beneath the structural zones of the belt. These compositions resulted from different contributions of mantle components involving geotectonic processes during the orogeny, such as oceanic subduction of mafic crust and sediments, continental subduction, collision with mantle input, and delamination of overriding plates. For documenting these processes, we have surveyed three sites of lamprophyre intrusions in the Vendean part of the South Armorican tectonic Zone with spessartite sills and minette dykes, and a fourth site in the West-Armorican kersantite swarm. The age of spessartite is estimated between 320 and 315 Ma on the base of structural relationships with the dated neighbouring granite. Dykes of minette share similar intrusive setting along the post-orogenic NW-SE dextral shear zones. One dyke is dated at 286.2 ± 6.6 Ma (Early Permian) by K/Ar method. The Western Brittany kersantite swarm is Middle to Late Carboniferous in age. All these rocks display common mineral and chemical compositions of lamprophyres. A review of the Variscan European lamprophyres is conducted in order to document their geochemical fingerprints compared with those of the studied samples