Archaean volcanism and sedimentation near Meekatharra,Western Australia

Archaean volcanic rocks and volcanogenic sediments exposed in a regional syncline south of Meekatharra, Western Australia are described. Initial volcanic activity produced a suite of high-Mg basalts containing 10–19% MgO. Pillowed tholeiitic basalts overlie the high-Mg basalts. These lower units are thought to have been derived from a central fracture zone. The uppermost units consist of volcanogenic sediments interlayered with andesite and dacite flows which appear to have been derived from a marginal andesite pile to the east. The Archaean sequence has been tightly folded, cross-faulted and intruded by post-Archaean dykes.The central succession is predominantly submarine, although the marginal andesite pile may be in part subaerial. Whilst there are some similarities with the Marda complex to the southeast (Hallberg et al., 1976), the rocks near Meekatharra are more analogous to those associated with modern island arcs.     

The Warnie volcanic province: Jurassic intraplate volcanism in Central Australia

The Cooper and Eromanga Basins of South Australia and Queensland are the largest onshore hydrocarbon producing region in Australia. Igneous rocks have been documented infrequently within end of well reports over the past 34 years, with a late Triassic to Jurassic age determined from well data. However, the areal extent and nature of these basaltic rocks were largely unclear. Here, we integrate seismic, well, gravity, and magnetic data to clarify the extent and character of igneous rocks preserved within Eromanga Basin stratigraphy overlying the Nappamerri Trough of the Cooper Basin. We recognise mafic monogenetic volcanoes that extend into tabular basalt lava flows, igneous intrusions and, more locally, hydrothermally altered compound lava flows. The volcanic province covers ∼7500 km² and is proposed to have been active between ∼180–160 Ma. We term this Jurassic volcanic province the Warnie Volcanic Province (WVP) after the Warnie East 1 exploration well, drilled in 1985. The distribution of extrusive and intrusive igneous rocks is primarily controlled by basement structure, with extrusive and intrusive igneous rocks elongate in a NW-SE direction. Finally, we detail how the WVP fits into the record of Jurassic volcanism in eastern Australia. The WVP is interpreted as a product of extension and intraplate convective upwelling above the subducting Pacific Slab. The discovery of the WVP raises the possibility of other, yet unidentified, volcanic provinces worldwide.     

Archaean basic volcanism in the Eastern Goldfields Province,Yilgarn Block,Western Australia

Petrographic, petrological and geochemical studies have demonstrated the presence of three distinctive basic volcanic suites in the Eastern Goldfields Province, Yilgarn Block, Western Australia. These are termed the high magnesian series basalts (HMSB), the low magnesian series basalts (LMSB) and the siliceous high magnesian series basalts (SHMSB).The HMSB and SHMSB constitute differentiation series which contain both high MgO (9.5–14 wt.%) and low MgO (<9.5 wt.%) members. These suites are commonly characterized by igneous textures indicative of very rapid crystallization suggesting high eruption temperatures. This feature clearly distinguishes those low MgO members of HMSB which contain amphibole pseudomorphs after spherulitic-textured pyroxene from compositionally similar LMSB. The LMSB are generally characterized by an intergranular texture consisting of plagioclase laths and interstitial amphibole pseudomorphs after pyroxene grains. Variolitic-textured basalts are common and appear to be restricted to the SHMSB suite of basic volcanics.The HMSB and LMSB were derived from source mantle regions which were variably depleted in the incompatible elements. Archaean komatiites were derived from similarly depleted source regions and it is argued that the main petrogenetic difference between these three volcanic suites was the degree of partial melting from which they were derived. The depleted nature of the source regions may have been induced by earlier small degree (< 5%) partial melting events with subsequent extraction of a proportion of that melt. Variations in both the degree of such melting, and the proportions of melt removal, could induce considerable heterogeneity of incompatible elements in the Archaean upper mantle.Source mantle regions of the SHMSB were variably enriched in the incompatible elements and water and parental magmas of the SHMSB were derived from moderately hydrous conditions of partial melting.The relative proportions of each basalt suite varies considerably between the layered successions examined. For example, the basic volcanics overlying the komatiites at Kambalda are SHMSB, while the footwall volcanics consist predominantly of HMSB with subordinate LMSB. However, the Norseman succession, where no ultramafic volcanics are known to occur, is comprised mainly of LMSB with a smaller proportion of HMSB.     

Temperature in volcanism

Direct or indirect temperature measurements of magmas erupted by volcanoes are still very scarse. All available thermal data have been measured in lava lakes and on volcanoes with limited explosive activity, i. e. the data refer to basic products. Explosive activity, which prevails for 80 % of the active volcanoes, prevents any measurement. The thermal data show that all basic magmas, whatever their origin and tectonic position may be, reach the surface within the same temperature interval. This seems to be related to the fact that all basic magmas are in equilibrium with some solid phases.The great improvement of physico-chemical knowledge of equilibrium conditions of artificial melts allows a more precise utilization of geologic thermometers. The plagioclase geothermometer can be used to evaluate the spreading rate of plates in oceanic rift zones.The heat of magmas offers an energy source of great importance for the future. The highest probability of finding high temperatures at relatively shallow depths for use as geothermal energy is obviously in the proximity of active volcanoes. Programs to extract energy directly from such shallow-seated magma sources have been taken into consideration in the Kamchatka peninsula/SU and on Hawaii/USA.

---

Zusammenfassung Direkte und indirekte Temperaturmessungen von geschmolzenen Förderprodukten der Vulkane sind sehr spärlich. Solche Messungen sind bislang nur bei Eruptionen basischer Magmen ausgeführt worden, da diese von Natur aus wenig oder gar nicht explosiv sind. Hinzu kommen Messungen der Temperatur in den seltenen Lava-Seen. Alle diese Messungen zeigen, daß die magmatischen Produkte verschiedener Herkunft die Oberfläche immer im gleichen Temperatur-Intervall erreichen. Diese Tatsache dürfte darin ihren Grund haben, daß alle basischen Magmen im Gleichgewicht mit festen Phasen sehr ähnlicher Zusammensetzung stehen. Die indirekten Bestimmungen der Temperatur mittels geologischer Thermometer, wie z. B. die Zusammensetzung der erstausgeschiedenen Plagioklase in Basalten, die längs der sich öffnenden mittelozeanischen Rücken zu Tage kommen, können nützliche Hinweise geben auf die Geschwindigkeit des Auseinanderdriftens der Platten.Die Anwesenheit von bedeutenden Mengen hochtemperierter Magmen, die einen Teil ihrer Wärme an die Oberfläche abgeben, hat neuerdings zu Überlegungen geführt, wie daraus geothermische Energie nutzbar zu machen ist. Solche geothermisch nutzbaren Gebiete gibt es z. B. in Kamtchatka/UdSSR und auf Hawaii/USA.

---

Riassunto Le misure dirette o indirette di temperatura dei materiali fusi emessi dai vulcani attivi sono molto scarse e si riferiscono solamente a eruzioni di magmi basici poco o punto esplosive o ai rari laghi di lava incandescente. Tutte le misure esistenti indicano comunque che prodotti magmatici di origine diversa arrivano in superficie nello stesso intervallo di temperature. Ciò può essere in relazione al fatto che tutti i magmi basici sono in equilibrio con fasi solide di composizione molto simile. Le misure indirette di temperatura eseguite con geotermometri, ad esempio la composizione del plagioclasio di cristallizzazione iniziale nei basalti delle zone di spreading oceanico, possono fornire utili indicazioni sulla velocità di allontanamento delle placche.La presenza di quantità importanti di magmi a temperatura elevata che dissipano parte del loro calore vicino alla superficie ha fatto recentemente prendere in considerazione la possibilità di sfruttamento di energia geotermica nelle aree di vulcani attivi sia negli USA che nell'URSS.

---

. , .. . . , . , , , . , .: , , . , , . (CCCP) (CACIII).

     

About deep-sea volcanism

Hyoclastites mainly result from underwater comminution of molten basalts initially explosively erupted out of the sea-floor and instantaneously pulverized by closely succeeding phreatic explosion (s).Many sea-mounts probably never were the alleged volcanic islands, later sea-level eroded into truncated cones and eventually drowned several km down, they are claimed to be. They are here considered as submarine polygenic volcanoes, the shape of which is congenital. Their building up probably started by accumulation of numberless flows of basalt, quietly poured out from a long-lived central vent; when this lava-volcano's crater, so progressively carried higher and higher, reached depths where explosive phenomena became possible because of lowered hydrostatic pressure, magmatic explosions occurred due to violent release of primitively dissolved (or combined) gases. Shattering of lava, 1) increases by several orders of magnitude lava's surface to volume ratios, so allowing huge quantities of super-heated steam to be engendered; 2) this super-heated steam trapped below the lava-lumps, as well as in their numberless holes, immediately explodes and comminutes the primary lavalumps; 3) so other super-heated steam is produced and further steam explosions are resumed in confined room until almost all the primitive heat content of the magma is transformed into kinetic energy and the lava is comminuted into glassy, ashy, hyaloclastites.This process also works above fissural eruptions. The difference is that fissural volcanoes, contrarily to large central ones, are usually monogenic (i. e. delivering one eruption only through the same vent instead of numberless ones for polygenic volcanoes). Linear effusive eruptions also produce quietly flowing basaltic flows but — because being monogenic — they cannot build up big, and eventually steep, reliefs as polygenic volcanoes do. When not poured over steep slopes where pillowlavas develop, submarine flows are characterized by 1) the lack of any scoriaceous, more or less thick, upper part (or jacket), and 2) a regular pavingstone-like surface, each polygon of which being the upper face of short prisms similar to ordinary columnar prismation, but one or two orders of magnitude shorter. As for central volcanoes, explosive activity along submarine fissures produces huge quantities of hyaloclastites, but these cannot be heaped up into steep ridges, as happens for subglacial eruptions, because sea-currents spread them far and wide.

---

Zusmmenfassung Die Hyaloclastiten entstehen hauptsächlich durch submarines Zerspratzen von Lava, die bei vulkanischen Explosionen im Meer ausgeworfen wurde.Zahlreiche sea-monts waren wahrscheinlich niemals vulkanische Inseln, die später abgestumpft und überschwemmt wurden, wie es allgemein angenommen wird. Wir sind überzeugt, daß ein großer Teil der Vulkane sich unter Wasser gebildet hat aus Laven, die aus einem langlebigen Zufuhrkanal gefördert wurden und die allmählich nach oben wuchsen.Die Bildungsart der Hyaloclastiten, die hier beschrieben wird, erklärt die Tafelformen und die aus Palagonit bestehenden zackigen Berggrate, die Islands Unter-Eis-Vulkanismus kennzeichnen.

---

Résumé Les hyaloclastites (palagonites) sont formées essentiellement par la fragmentation en milieu aqueux des lambeaux de lave lancés par explosions volcaniques sous-marines (ou sous-lacustres ou sous-glaciaires). Cette fragmentation résulte de l'explosion de la vapeur prisonnière dans et sous les dits lambeaux.Beaucoup de guyots (sea-mounts) n'ont probablement jamais été, comme on le croit généralement, des îles volcaniques ultérieurement tronquées par érosion et englouties. Nous sommes convaincus qu'une forte proportion de guyots sont des volcans sous-marins, faits de coulées interstratifiées avec des hyaloclastites, et que leur forme tronconique est congénitale.Le processus de formation des hyaloclastites que nous décrivons rend compte également des montagnes tabulaires et des crêtes dentelées, constituées de palagonites, caractéristiques du volcanisme sous-glaciaire d'Islande.Les coulées subaquatiques subhorizontales offrent une surface polygonale de « basaltes en pravés ».

---

, , , .- -monts, , , , , . , , , . - , . - , Guyots , , , , .

     

Geodynamic control of rift volcanism

Development of pre-rift domal uplifts and volcanism in East Africa is related to mantle plume activity during two (possibly three) stable periods in the plate motion: before 80 myr, at 60-35 myr, and at 20-14 myr. Volume and composition of volcanics depend on the length of the stable period. Subalkaline basalts appeared only during long stable periods, while during short stable periods only alkaline magmas were generated. The general tendency of evolution in the pre-rift volcanic areals is from alkaline to transitional and then again to alkaline basalts.Formation of rift fractures initiates magmatism independent of mantle plume activity. Volume and composition of volcanics are controlled by opening rates. At rates less than 1 mm/year there is no volcanism, or only small volumes of ultra-alkaline basic lavas are erupted. At rates 1–10 mm/year alkaline to mildly alkaline basalts appear accompanied by large volumes of peralkaline silicics, and rates of 1–2 cm/year correlate with large volumes of transitional basalts with subordinate peralkaline silicics.

---

Zusammenfassung Die Entwicklung der Präriftauswölbung und des Vulkanismus in Ostafrika ist eng verbunden mit einer Mantel-Plume-Aktivität während zwei (möglicherweise drei) stabiler Perioden in der Plattenbewegung: vor 80 Mio Jahren, zwischen 60 und 35 Mio Jahren und zwischen 20–14 Mio Jahren. Volumen und Zusammensetzung der Vulkanite hängen von der Länge der stabilen Periode ab. Subalkali-Basalte erscheinen nur während längerer stabiler Perioden, während in kürzeren stabilen Perioden nur Alkalimagmen entstehen. Die generelle Tendenz der Entwicklung der Prärift-Vulkanite verläuft von alkalisch zu einem übergangstyp und dann wieder zurück zu Alkali-Basalten.Die Bildung von Riftbrüchen bewirkt einen Magmatismus, der unabhängig von der Mantel-Plume-Aktivität abläuft. Das Volumen und die Zusammensetzung der Vulkanite wird von der öffnungsrate der Riftzone kontrolliert. Bei Raten kleiner als 1 mm pro Jahr gibt es keinen Vulkanismus, oder es werden nur kleinere Mengen ultrabsischer Laven gefördert. Bei Raten von 1–10 mm pro Jahr erscheinen Alkali- bis Mittelalkalibasalte begleitet von großen Mengen peralkalischer Magmen. Bei Raten von 1–2 cm pro Jahr erscheinen große Mengen von übergangsbasalten mit untergeordnet peralkalischen Magmen.

---

Résumé Le développement des soulèvements en dÔmes prériftaux et du volcanisme de l'Est Africain est rapporté à une activité du manteau durant deux (peut-Être trois) périodes stables dans le mouvement de la plaque: avant 80 M, à 60-35 m., et à 20-14 M. Le volume et la composition des volcanites dépendent de la durée de la période stable. Les basaltes subalcalins apparaissent seulement au cours de longues périodes stables, tandis que durant de courtes périodes stables seuls sont engendrés des magmas alcalins. La tendance générale de l'évolution dans les régions volcaniques pré-rift est alcaline à transitionnelle et, ensuite, de nouveau alcaline.La formation des fractures du rift marque le début du magmatisme indépendant de l'activité du manteau. Le volume et la composition des volcanies sont controlés par la vitesse d'ouverture. A des vitesses inférieures à 1 mm/an, il n'y a pas de volcanisme ou sontémises seulement de faibles quantités de volcanites. A des vitesses de 1–10 mm/an, des basaltes alcalins à moyennement alcalins apparaissent, accompagnés par de grands volumes de volcanites peralcalines, et, à des vitesses de 1–2 cm/an, correspondent de grands volumes de basaltes de transition avec des volcanites peralcalines.

---

( ) : 80 .., 60–35 .. 20–14 .. . . , . ( ) , , . , . . 1 / . 1–10 / , . 1–2 / . — .

     

Famines and cataclysmic volcanism

The gases released by some large volcanic eruptions in history (e.g. Santorini in the seventeenth century BC) have led to famine. Similar events are likely in the future but could be made worse by the huge quantities of material already in the atmosphere as a result of industrial and domestic Processes.     

Early Permian strike-slip basin formation and felsic volcanism in the Manning Group,southern New England Orogen,eastern Australia

The Manning Group is characterised by rapidly filled strike-slip basins that developed during the early Permian along the Peel--Manning Fault System in the southern New England Orogen. Typically, the Manning Group has been difficult to date owing to the lack of fossiliferous units or igneous rocks. Thus, the timing of transition from an accretionary convergent margin in the late Carboniferous to dominantly strike-slip tectonic regimes that involved development and emplacement of the Great Serpentinite Belt (Weraerai terrane) is not well constrained. One exception are rhyolites of the Ramleh Volcanics that were erupted into the Echo Hills Formation. These developed along the dextral Monkey Creek Fault splay east of the Peel--Manning Fault System. Zircons extracted from the Ramleh Volcanics yield a U–Pb (SHRIMP) age of 295.6?±?4.6?Ma that constrains the minimum age of deposition in this basin to earliest Permian. Whole-rock geochemistry indicates these are peraluminous felsic melts enriched in LREE and incompatible elements with strong depletions in U, Nb, Sr and Ti. These are similar in age and composition to the nearby S-type Bundarra and Hillgrove plutonic supersuites. We suggest that extensive movement along the east-dipping Peel--Manning Fault System was responsible, not only for strike-slip basin development at the surface (Manning Group), but was also the locus for crustal melting that was responsible for generating S-type felsic melts that utilised hanging-wall fault splays as conduits to the surface or to coalesce in the crust as batholiths exclusively to the east of the Peel--Manning Fault System.     

On the origin of orogenic volcanism

It is widely believed that the explanation of orogenic volcanism lies in the shear heating and subsequent partial melting of the crustal fraction of subducted lithosphere. Careful examination of simple models of the shear-heating process indicates, however, that this process cannot occur in a system with constant strain-rate boundary conditions, for reasonable values of the strain rate and material parameters. Thermal runaway cannot occur because viscous dissipation process is limited by the effect of temperature on viscosity, although shear-heating instability may develop during non-steady-state shear at stress or strain-rate values above certain critical values. Examination of subduction history models indicates that supercritical shear stresses constant over some tens of kilometers may be found in subduction shear zones. Since instability may develop if such conditions are present over even a 1–10 km range, it is still possible that non-steady-state process could explain the origin of orogenic volcanism.

---

Zusammenfassung Schererwärmung bis zum partiellen Aufschmelzen der subduzierten Kruste wird vielfach als Ursache des orogenen Vulkanismus genannt. Eine sorgfältige Betrachtung eines einfachen Schererwämungsmodells führt zu folgenden Resultaten: Schererwärmung unter Vorgabe konstanter Dehnungsrate in der Scherzone führt zu allmählicher Erwärmung, wobei die Temperatur unter der Solidusgrenze bleibt, sofern man vernünftige Werte der Modellparameter wählt. Die Temperaturerhöhung bleibt in diesem Fall beschränkt durch die Rückkopplung von Temperatur und Viskosität. Schererwärmungsinstabilitäten, die zu partiellem Aufschmelzen führen, sind möglich, sofern bei nicht-stationärer Schererwärmung die Scherspannung oder die Dehnungsrate gewisse kritische Werte übersteigen. Es wird gezeigt, daß die Annahme superkritischer und konstanter Scherspannung entlang von 1–10 km langen Wegstücken der subduzierten Kruste mit Beobachtungen und Modellrechnungen verträglich ist. In diesem Fall können Scherinstabilitäten in einer Tiefe von 100 km bis 200 km in der subduzierten Kruste auftreten.

---

Résumé On croit couramment que l'explication du volcanisme orogénique se trouve dans l'échauffement causé pai les tensions d'arrachement et dans la fusion partielle consécutive de la fraction crustale de la lithosphere en voie de subduction. Une considération soigneuse d'un modèle simple d'échauffement par tensions conduit aux résultats suivants: un échauffement sous de telles tensions et dans des conditions avantageuses d'un degré d'extension constant dans la zone de cisaillement conduit à un échauffement par broyage où la température reste maintenue en-dessous de la limite du solidus pour autant qu'on choisisse des valeurs raisonables pour les paramètres du modèle. L'élévation de température reste dans ce cas limitée par le couple température-viscosité. Les instabilités par échauffement sous tension qui conduisent à la mise en fusion partielle, sont possibles pour autant que, sous un échauffement non stationnaire, le cisaillement ou le degré d'extension ne dépasse pas certaines valeurs critiques. On montre que l'adoption d'une tension de cisaillement supercritique et constante le long de tronçons de 1–10 km de la croûte en voie de subduction est compatible avec les observations et avec les calculs déduits du modèle. Dans ce cas des instabilités sous l'effet du cisaillement peuvent se produire dans la croûte en voie de subduction à une profondeur de 100 à 200 km.

---

, , . , , , , . , , , . , . 1 10 , .. .

     

Some problems of volcanism in Iceland

Until two decades or so ago the petrography of Iceland seemed to present a rather simple picture. About 99 per cent of the rocks were regarded as basaltic, 1 per cent as rhyolitic and rocks of intermediate composition as nearly non-existent. Recent research has changed this picture. Central volcanoes producing acid and intermediate lava and tephra have played a considerable role both in the Tertiary and Quaternary. Contrary to the lava massproducing basalt volcanoes, fed by a deep-seated and rather uniform basalt magma source, the differentiated central volcanoes are normally fed by separated magma chambers at smaller depth, where differentiation takes place between the eruptions, resulting in intermediate and acid products which bridge the immiscibility gap between the basalts and the rhyolites. Eight to ten per cent of the postglacial lavas and tephras and about 25 per cent of the historical ones are acid or intermediate.Opinions still differ as to whether the existence of these rocks can be explained solely as the result of gravitational differentiation of parent basalt magma or has to be explained partly by a remelting of a continental layer beneath the plateau-basalts. There are, however, some rather strong geological evidences in favour of the existence of such a layer and they seem at the moment not to be contradicted by geophysical research.The Surtsey eruption has demonstrated the close relation between the craterrows, shieldvolcanoes and tablemountains and convincingly shown how greatly external circumstances may influence volcanic activity and the shape of a volcano. The eruption focused interest on the neovolcanic zone of Iceland as a supra marine part of the Mid-Atlantic rift-ridge zone, where volcanic activity has taken place under similar conditions as along its submarine parts, viz. under water (glacial meltwater) when the Pleistocene ice-sheets blanketed the volcanic activity. Similar morphological types of volcanoes can therefore be expected in both cases, viz. steepsided mounds and long steepsided ridges.

---

Zusammenfassung Bis vor ungefähr zwei Jahrzehnten schien die Petrographie Islands ein verhältnismäßig einfaches Bild zu bieten. Etwa 99% des Gesteins wurde für basaltisch, 1% für rhyolithisch, und Gesteine von intermediärer Zusammensetzung für fast nicht existent gehalten. Die jüngste Forschung hat dieses Bild gewandelt. Zentralvulkane, die saure und intermediäre Lava und Tephra ausstoßen, haben im Tertiär wie auch im Quartär eine erhebliche Rolle gespielt. Im Gegensatz zu den Basaltvulkanen, welche von einer tiefgelegenen und ziemlich einheitlichen Basaltmagmaquelle gespeist werden und Lava in Massen ausstoßen, werden die differenzierten Zentralvulkane normalerweise von getrennten Magmakammern in geringerer Tiefe gespeist, wo eine Differentiation zwischen den Eruptionen stattfindet. Es entstehen intermediäre und saure Materialien, die die Immiszibilitätslücke zwischen den Basalten und den Rhyolithen überbrücken. 8–10% der postglazialen Laven und Tephren und etwa 25% der historischen sind sauer oder intermediär.Die Meinungen gehen noch auseinander hinsichtlich der Frage, ob das Vorkommen dieses Gesteins allein als das Ergebnis einer gravitativen Differentiation des basaltischen Stamm-Magmas erklärt werden kann, oder ob man es teilweise mit einem Wiedereinschmelzen einer Sialschicht unterhalb der Tafelbasalte erklären muß. Es gibt für die Existenz einer solchen Schicht einige ziemlich überzeugende geologische Zeugnisse, und es scheint so, als ob diesen gegenwärtig durch die geophysikalische Forschung nicht widersprochen würde.Die Surtsey-Eruption hat die enge Beziehung zwischen den Kraterreihen, den Schildvulkanen und den Tafelbergen aufgezeigt und überzeugend dargelegt, wie stark äußere Umstände die Vulkantätigkeit und die Form eines Vulkans beeinflussen können. Diese Eruption konzentrierte das Interesse auf die neovulkanische Zone Islands als eines supramarinen Teils des mittelatlantischen Rückens, wo die Vulkantätigkeit während der Glazialperioden der quartären Eiszeit unter ähnlichen Bedingungen stattgefunden hat, wie entlang ihrer submarinen Teile, d. h. unter Wasser (glazialem Schmelzwasser). Man darf deshalb in beiden Fällen ähnliche morphologische Vulkantypen erwarten, d. h. steilwandige Hügel und lange steilwandige Rücken.

---

Résumé Vingt ans plus tôt la pétrographie de l'Islande paraissaît assez simple. Les recherches récentes ont changé cette conception. Des volcans centraux ont joué un rôle considérable au Tertiaire et au Quaternaire. Les volcans basaltiques sont nourris par des sources profondes et assez homogénes de magma basaltique. Par contre les volcans centraux sont nourris par des chambres magmatiques moins profondes ou il y a une différenciation des éruptions: il y a des matériaux intermédiaires et acides. 8–10% des laves et tephras postglaciales et 25% des laves et tephras historiques sont acides ou intermédiaires.Les opinions diffèrent encore dans la question si l'existence de ces roches est le résultat d'une différenciation du magma basaltique original ou s'il s'agit d'une refonte d'une couche continentale au-dessous des plateaux basaltiques.L'éruption de Surtsey a démontré la relation étroite entre les chaînes de cratères, les volcans en bouclier et les guyots. Elle montre combien des circonstances extérieures ont influencé l'activité volcanique et la forme d'un volcan Cette éruption attircut l'intérêt vers la zone néo volcanique de l'Islande. C'est la partie supramarine de la faille-dorsale-zone mi-atlantique ou l'activité volcanique a en lieu sous des conditions pareilles à celles de la partie submarine, c'est à dire sous l'eau (eau glaciale). Dans les deux cas on pent supposer de pareils types morphologiques de volcans.

---

. , 99% 1% . , , . . . .

     

Proterozoic volcanism in the Jack Hills Belt,Western Australia: Some implications and consequences for the World's oldest zircon population

Rare felsic volcanic rocks of dacitic to rhyolitic composition occur in the central part of the Jack Hills metasedimentary belt in the Narryer Terrane of Western Australia, interleaved with clastic sedimentary rocks and amphibolite. Representative samples of the four identified felsic volcanic units reveal a similar complex pattern of zircon age distribution, with all samples containing zircon populations at ∼3.3–3.4, ∼3.0–3.1, ∼2.6 and ∼1.8–1.9 Ga. The ∼3.3–3.4 Ga zircons show well-developed oscillatory zoning in cathodoluminescence (CL) images and are interpreted as inherited igneous zircon derived from granitic precursors, similar to the ∼3.3 Ga trondhjemitic granitoids currently exposed along the northern and southern margins of the belt. The ∼3.0–3.1 Ga zircons also reveal well-developed oscillatory zoning in CL and are most likely derived from granitoid and/or volcanic rocks of this age, as recorded in the Murchison domain to the south and possibly also present in the Narryer Terrane. The ∼2.6 Ga population matches the age of nearby late Archean granitoids intruding the Jack Hills belt and their oscillatory zoning and U–Th chemistry is consistent with their origin from such a source. The youngest discrete group of zircon grains, with ages ranging from ∼1970 to ∼1775 Ma, show strong oscillatory zoning and average Th/U ratios of 0.76, features consistent with an igneous origin. These younger zircons are therefore interpreted as defining the age of crystallisation of the volcanic rocks. These results establish that the Jack Hills metasedimentary belt contains significant post-Archean components. Taken together with similar results obtained from zircon occurring as detrital grains in clastic sedimentary rocks at Jack Hills, these results overturn the generally-accepted view that the belt is entirely Archean in age and that sedimentation was completed around 3.0 Ga ago. Instead, there is a distinct possibility that much of the material currently exposed in the Jack Hills belt formed in the Proterozoic. A further implication of this study is that the metamorphism affecting these rocks also occurred in the Proterozoic and consequently the rocks should not be considered as forming an Archean greenstone or metasedimentary belt. The paucity of zircons >4 Ga in the known Proterozoic sedimentary rocks and their total absence in the felsic volcanic rocks suggests that such ancient source rocks were no longer present in the area.     

High-K volcanism in the Afyon region, western Turkey: from Si-oversaturated to Si-undersaturated volcanism

Volcanic rocks of the Afyon province (eastern part of western Anatolia) make up a multistage potassic and ultrapotassic alkaline series dated from 14 to 12 Ma. The early-stage Si-oversaturated volcanic rocks around the Afyon city and further southward are trachyandesitic volcanic activity (14.23 ± 0.09 Ma). Late-stage Si-undersaturated volcanism in the southernmost part of the Afyon volcanic province took place in three episodes inferred from their stratigraphic relationships and ages. Melilite–leucitites (11.50 ± 0.03 Ma), spotted trachyandesites, tephryphonolites and lamproites (11.91 ± 0.13 Ma) formed in the first episode; trachyandesites in the second episode and finally phonotephrites, phonolite, basaltic trachyandesites and nosean-bearing trachyandesites during the last episode. The parameter Q [normative q-(ne + lc + kls + ol)] of western Anatolia volcanism clearly decreased southward with time becoming zero in the time interval 10–15 Ma. The magmatism experienced a sudden change in the extent of Si saturation after 14 Ma, during late-stage volcanic activity of Afyon volcanic province at around 12 Ma, though there was some coexistence of Si-oversaturated and Si-undersaturated magmas during the whole life of Afyon volcanic province.     

Periodicity and driving forces of volcanism

The volume and style of volcanism change periodically, with cycles of three main scales, which have different causes and effects. Short cycles of volcanic activity last from tens to thousands of years and are associated with periodic accumulation of magma in shallow chambers and its subsequent eruptions. The eruptions either have internal causes or are triggered externally by variations in solar activity, tidal friction, and Earth’s rotation speed. Medium-scale cycles, hundreds of thousands to millions of years long, are due to changes in spreading and subduction rates. Long cycles (30–120 Ma) are related to ascent of mantle plumes, which take away material and heat from the core-mantle boundary and change the convection rate. These appear to be the major controls of the average periodicity. Acceleration of asthenospheric convection caused by periodic plume activity pulses can change spreading rates and, correspondingly, the relative positions of moving plates. The medium-scale periodicity of volcanism is illustrated by the examples of Kamchatka and Japan, where the intensity of subduction magmatism changes periodically in response to the opening of back-arc basins (Shikoku, Sea of Japan, and South Kurile basin).     

The recent explosive volcanism at Pantelleria

Many of the least eighty eruptions of the six silicic eruptive cycles in the last 50 ka at Pantelleria started out with explosive phases and ended with lava effusion. The pyroclastic rocks are rhyolitic in composition while the later-erupted lavas are less evolved up to trachyte probably due to the presence of a shallow and compositionally zoned magma chamber tapped at increasing depth during the eruptions. The products of twenty explosive eruptions have been recognized. Most of them are represented by fallout deposits, the only exceptions being the Green Tuff and the Fossa Carbonara Tuff. Hydromagmatism is very rare though the magmas interacted with sea water. It is supposed that interaction was not efficient because the magmas were not vesiculated and disrupted. Horizons showing variable degrees of welding up to vitrophyres, have often been identified in the air-fall tuffs. Most of the eruptions were characterized by low eruptive columns and were likely to have involved a strombolian-type mechanism.

---

Zusammenfassung Die schwächsten der 80 Ausbrüche der sechs sauren Eruptionszyklen bei Pantelleria wurden innerhalb der letzten 50000 Jahre eingeleitet durch explosive Phasen und endeten mit gleichmäßigem Lavaausfluß. Die Zusammensetzung der Pyroklastika ist rhyolitisch, die der späteren Laven eher trachytisch. Die Ursache für diesen unterschiedlichen Chemismus ist wahrscheinlich eine vertikal in der Zusammensetzung zonierte Magmenkammer in geringer Tiefe, die während der Ausbrüche aus immer tiefer gelegenen Bereichen angezapft wurde. Von 20 explosiven Ausbrüchen konnten charakteristische Auswurfprodukte gefunden werden. Dabei handelt es sich meist um Schlacken- und Lavaauswurf, als Ausnahmen existieren der »Green Tuff« und der »Fossa Cabonara Tuff«. Hydromagmatische Vorgänge sind äußerst selten, obwohl das Magma mit Meerwasser in Kontakt gekommen sein muß. Die Annahme, daß keine Reaktion zwischen Magma und Meerwasser stattgefunden hat, beruht auf der blasenfreien und nicht zerrissenen Erhaltung. Innerhalb der Tuffite gibt es mehrere Horizonte unterschiedlicher Verschweißungsstufen bis hin zu Vitrophyren. Die meisten der Ausbrüche hatten niedrige Eruptionssäulen und beinhalteten strombolianische Tätigkeit.

---

Résumé Parmi les 80 éruptions des 6 cycles d'éruptions acides qui ont affecté Pantelleria au cours des 50.000 dernières années, les plus faibles ont commencé par une phase explosive et se sont achevées par des effusions de laves. La composition des roches pyroclastiques est rhyolitique tandis que celle des laves ultérieures, moins évoluées, est trachytique. Ce fait est dû probablement à la présence d'une chambre magmatique peu profonde à zonage compositionnel, dont des parties de plus en plus profondes ont alimenté les phases successives des éruptions. Les produits de 20 éruptions explosives ont été identifiés. La plupart d'entre eux sont des dépôts de retombée directe, avec comme seules exceptions le «Green Tuff» et le «Fossa Carbonara Tuff». Les manifestations hydromagmatiques sont très rares, en dépit du fait que le magma a dû être en contact avec l'eau de mer. On suppose que leur interaction a été réduite parce que le magma n'était ni vésiculaire, ni dilacéré. Dans les dépôts formés par retombées aériennes, on a identifié à de multiples reprises des horizons de tufs soudés à des degrés divers, jusqu'à celui des vitrophyres. La plupart des éruptions ont présenté des colonnes éruptives peu élevées et ont probablement développé un mécanisme strombolien.

---

50000 80 , 6- , . , . , , : , , - . 20 . . . ; »Fossa Cabonara Tuff«. , . , , , . »« . .

     

Wenlock volcanism in the Welsh Basin

Within the Welsh Basin, proximal volcanism of Wenlock age is virtually unknown. However, two vitroclastic tuffs and a turbiditic tuffaceous mudstone have been discovered lying along the eastern side of the basin. Their sources have not been located, but the tuffaceous mudstone and the older tuff are both of riccartonensis Zone age and may be the products of the same event. The younger tuff is of early ludensis Zone age. All three occur in association with bentonites and represent, or were involved in, sedimentary instability. © 1998 John Wiley & Sons, Ltd.     

Seismicity and volcanism of Jan Mayen Island

The small, arctic Jan Mayen Island, site of the World's northernmost active volcano, Beerenberg, is part of the mid-Atlantic ridge system and located along the Jan Mayen Fracture Zone (JMFZ). Recent data from the local seismic network, and fault plane solutions from the global network, indicate that the island is located at a ridge-crest intersection, which might explain the origin of the island and the associated volcanism. Moreover, the new data suggest a series of offset segments of the Mohn's Ridge, overlapping in an en echelon pattern. In January 1985, a flank eruption was for the first time observed with the local seismic network. Volcanic tremors and low-frequency events were observed on 5 January at 2230 h and 10 hours later the first large earthquake occurred. No visual confirmation of the eruption was made until 6 January at 1630 h. The seismic observations rule out the possibility that the large earthquake caused the eruption; it seems more likely that the changes in local stress conditions triggered the earthquake and that the eruption started before the first large earthquake. Recent observations show that the local network provides an efficient tool for monitoring and warning of volcanic activity. However, since there was no change in the local seismicity in the days or months before the 1985 eruption, it seems to be difficult to make long-term predictions of Beerenberg flank eruptions without using other techniques such as observations of tilt.