Geodynamic setting of recent volcanism in North Eurasia

A GIS layout of the map of recent volcanism in North Eurasia is used to estimate the geodynamic setting of this volcanism. The fields of recent volcanic activity surround the Russian and Siberian platforms—the largest ancient tectonic blocks of Eurasia—from the arctic part of North Eurasia to the Russian Northeast and Far East and then via Central Asia to the Caucasus and West Europe. Asymmetry in the spatial distribution of recent volcanics of North Eurasia is emphasized by compositional variations and corresponding geodynamic settings. Recent volcanic rocks in the arctic part of North Eurasia comprise the within-plate alkaline and subalkaline basic rocks on the islands of the Arctic Ocean and tholeiitic basalts of the mid-ocean Gakkel Ridge. The southern, eastern, and western volcanic fields are characterized by a combination of within-plate alkaline and subalkaline basic rocks, including carbonatites in Afghanistan, and island-arc or collision basalt-andesite-rhyolite associations. The spatial distribution of recent volcanism is controlled by the thermal state of the mantle beneath North Eurasia. The enormous mass of the oceanic lithosphere was subducted during the formation of the Pangea supercontinent primarily beneath Eurasia (cold superplume) and cooled its mantle, having retained the North Pangea supercontinent almost unchanged for 200 Ma. Volcanic activity was related to the development of various shallow-seated geodynamic settings and deep-seated within-plate processes. Within-plate volcanism in eastern and southern North Eurasia is controlled, as a rule, by upper mantle plumes, which appeared in zones of convergence of lithospheric plates in connection with ascending hot flows compensating submergence of cold lithospheric slabs. After the breakdown of Pangea, which affected the northern hemisphere of the Earth insignificantly, marine basins with oceanic crust started to form in the Cretaceous and Cenozoic in response to the subsequent breakdown of the supercontinent in the northern hemisphere. In our opinion, the young Arctic Ocean that arose before the growth of the Gakkel Ridge and, probably, the oceanic portion of the Amerasia Basin should be regarded as a typical intracontinental basin within the supercontinent [48]. Most likely, this basin was formed under the effect of mantle plumes in the course of their propagation (expansion, after Yu.M. Pushcharovsky) to the north of the Central Atlantic, including an inferred plume of the North Pole (HALIP).     

Origin of felsic volcanism in the Izu arc intra-arc rift

An intra-arc rift (IAR) is developed behind the volcanic front in the Izu arc, Japan. Bimodal volcanism, represented by basalt and rhyolite lavas and hydrothermal activity, is active in the IAR. The constituent minerals in the rhyolite lavas are mainly plagioclase and quartz, whereas mafic minerals are rare and are mainly orthopyroxene without any hydrous minerals such as amphibole and biotite. Both the phenocryst and groundmass minerals have felsic affinities with a narrow compositional range. The petrological and bulk chemical characteristics are similar to those of melts from some partial melting experiments that also yield dry rhyolite melts. The hydrous mineral-free narrow mineral compositions and low-Al₂O₃ affinities of the IAR rhyolites are produced from basaltic middle crust under anhydrous low-temperature melting conditions. The IAR basalt lavas display prominent across-arc variation, with depleted elemental compositions in the volcanic front side and enriched compositions in the rear-arc side. The across-arc variation reflects gradual change in the slab-derived components, as demonstrated by decreasing Ba/Zr and Th/Zr values to the rear-arc side. Rhyolite lavas exhibit different across-arc variations in either the fluid-mobile elements or the immobile elements, such as Nb/Zr, La/Yb, and chondrite-normalized rare earth element patterns, reflecting that the felsic magmas had different source. The preexisting arc crust formed during an earlier stage of arc evolution, most probably during the Oligocene prior to spreading of the Shikoku back-arc basin. The lack of systematic across-arc variation in the IAR rhyolites and their dry/shallow crustal melting origin combines to suggest re-melting of preexisting Oligocene middle crust by heat from the young basaltic magmatism.     

Deep crustal cumulates reflect patterns of continental rift volcanism beneath Tanzania

Magmatism on Earth is most abundantly expressed by surface volcanic activity, but all volcanism has roots deep in the crust, lithosphere, and mantle. Intraplate magmatism, in particular, has remained enigmatic as the plate tectonic paradigm cannot easily explain phenomena such as large flood basalt provinces and lithospheric rupture within continental interiors. Here, I explore the role of deep crustal magmatic processes and their connection to continental rift volcanism as recorded in deep crustal xenoliths from northern Tanzania. The xenoliths are interpreted as magmatic cumulates related to Cenozoic rift volcanism, based on their undeformed, cumulate textures and whole-rock compositions distinct from melt-reacted peridotites. The cumulates define linear trends in terms of whole-rock major elements and mineralogically, can be represented as mixtures of olivine?+?clinopyroxene. AlphaMELTS modeling of geologically plausible parental melts shows that the end-member cumulates, clinopyroxenite and Fe-rich dunite, require fractionation from two distinct melts: a strongly diopside-normative melt and a fractionated picritic melt, respectively. The former can be linked to the earliest, strongly silica-undersaturated rift lavas sourced from melting of metasomatized lithosphere, whereas the latter is linked to the increasing contribution from the upwelling asthenospheric plume beneath East Africa. Thus, deep crustal cumulate systematics reflect temporal and compositional trends in rift volcanism, and show that mixing, required by the geochemistry of many rift lava suites, is also mirrored in the lavas’ cumulates.     

Geodynamic conditions of volcanism and magma formation in the Kurile-Kamchatka island-arc system

The conditions of magma formation were reconstructed on the basis of characteristic features of the evolution of the Kurile-Kamchatka island-arc system, structural and chemical zoning patterns of volcanic complexes, and available published data on peridotite and basalt melting and stability of hydrous minerals. It was shown that the volcanic arc of the Sredinnyi Range of Kamchatka occurs now at the final stage of subduction, whereas subduction beneath the volcanic arc of eastern Kamchatka began at the end of the Miocene, after its jump into the present-day position. The volcanism of Southern Kamchatka and the Kuriles has occurred under steady-state subduction conditions since the Miocene and is represented by typical island-arc magmas. The latter are generated in a mantle wedge, where the melting of water-saturated peridotite occurs in a high-temperature zone under the influence of fluid. The formation of the frontal and rear volcanic zones was related to the existence of two levels of water release from various hydrous minerals. During the initial and final stages of subduction, as well as in the zone of Kamchatka—Aleutian junction, partial melting is possible in the upper part of the subducted slab in contact with a hotter mantle material compared with the mantle in a steady-state regime. This is responsible for the coexistence of predominant typical island-arc rocks, rocks with intraplate geochemical signatures, and highly magnesian rocks, including adakites.     

Late Miocene calc-alkalic volcanism in northwestern Mexico: an expression of rift or subduction-related magmatism?

Magmatism in NW Mexico records a Late Miocene transformation from convergence to extension in the Gulf of California rift system. Miocene calc-alkalic rocks in the Baja California peninsula are related to the final subduction of the Farallon plate system, but the heterogeneous nature of volcanism younger than 12.5 Ma has led to conflicting tectonic interpretations. Neogene volcanic rocks in the Sierra Santa Ursula, Sonora, were emplaced in three magma pulses, according to mapping, K–Ar geochronology, and geochemistry. From 23.5 to 15 and 14 to 11.4 Ma, calc-alkalic rocks show an arc-like signature. The 12–11 Ma calc-alkalic dacites, however, are characterized by higher K, Rb, ⁸⁷Sr/⁸⁶Sr, and light REE abundances than are the older rocks. The timing, petrography, and geochemistry of the 12–11 Ma rocks are interpreted to reflect postsubduction magmatism. A change in magma chemistry from predominantly calc-alkalic to tholeiitic rocks at 10.3 Ma corresponds to orthogonal extension during early Gulf of California evolution. Sr, Nd, and Pb radiogenic isotope signatures show minor changes over time. The volcanic record for 20–12.5 Ma at Sierra Santa Ursula and adjacent areas is consistent with the reconstructed history of the Guadalupe microplate. The interval of magmatism produced from 12 to 11 Ma appears to reflect changes in plate geometry during the transition from subduction to rifting.     

Lower Pliensbachian caldera volcanism in high-obliquity rift systems in the western North Patagonian Massif,Argentina

In the Cerro Carro Quebrado and Cerro Catri Cura area, located at the border between the Neuquén Basin and the North Patagonian Massif, the Garamilla Formation is composed of four volcanic stages: 1) andesitic lava-flows related to the beginning of the volcanic system; 2) basal massive lithic breccias that represent the caldera collapse; 3) voluminous, coarse-crystal rich massive lava-like ignimbrites related to multiple, steady eruptions that represent the principal infill of the system; and, finally 4) domes, dykes, lava flows, and lava domes of rhyolitic composition indicative of a post-collapse stage.The analysis of the regional and local structures, as well as, the architectures of the volcanic facies, indicates the existence of a highly oblique rift, with its principal extensional strain in an NNE–SSW direction (∼N10°).The analyzed rocks are mainly high-potassium dacites and rhyolites with trace and RE elements contents of an intraplate signature. The age of these rocks (189 ± 0.76 Ma) agree well with other volcanic sequences of the western North Patagonian Massif, as well as, the Neuquén Basin, indicating that Pliensbachian magmatism was widespread in both regions. The age is also coincident with phase 1 of volcanism of the eastern North Patagonia Massif (188–178 Ma) represented by ignimbrites, domes, and pyroclastic rocks of the Marifil Complex, related to intraplate magmatism.     

Late cenozoic rift development and intra-plate volcanism in Northern New Zealand inferred from geochemical discrimination diagrams

Major- and trace-element discrimination diagrams are used in an attempt to discover the tectonic settings in which the late Cenozoic basalts and basalt-andesites of northern New Zealand evolved. The rocks of the Northland area are shown to have rift affinities, whereas those of Auckland and South Auckland appear to have evolved in an intraplate environment. It is proposed that the common link between the two groups of volcanic rocks is the Hauraki Rift. The Northland basalts are directly associated with the rift, whereas the Auckland and South Auckland rocks either developed on the flanks of the rift or their origin is linked to the evolution of membrane stresses during the opening of this rift. The basalts and basalt-andesites of the Waikato district also show an affinity with the rift, but the geochemistry is complicated by the influence of the developing Taupo Volcanic Zone which adds a shoshonitic character to these rocks.     

Early Cretaceous trachybasalt-trachyte-trachyrhyolitic volcanism in the Nyalga basin (Central Mongolia): sources and evolution of continental rift magmas

As shown by geological, mineralogical, and isotope geochemical data, trachybasaltic-trachytic-trachyrhyolitic (TTT) rocks from the Nyalga basin in Central Mongolia result from several eruptions of fractionated magmas within a short time span at about 120 Ma. Their parental basaltic melts formed by partial melting of mantle peridotite which was metasomatized and hydrated during previous subduction events. Basaltic trachyandesites have high TiO₂ and K₂O, relatively high P₂O₅, and low MgO contents, medium ⁸⁷Sr/⁸⁶Sr(₀) ratios (0.70526-0.70567), and almost zero or slightly negative ε_Nd(T) values. The isotope geochemical signatures of TTT rocks are typical of Late Mesozoic basaltic rocks from rift zones of Mongolia and Transbaikalia. The sources of basaltic magma at volcanic centers of Northern and Central Asia apparently moved from a shallower and more hydrous region to deeper and less hydrated lithospheric mantle (from spinel to garnet-bearing peridotite) between the Late Paleozoic and the latest Mesozoic. The geochemistry and mineralogy of TTT rocks fit the best models implying fractional crystallization of basaltic trachyandesitic, trachytic, and trachyrhyodacitic magmas. Mass balance calculations indicate that trachytic and trachydacitic magmas formed after crystallization of labradorite-andesine, Ti-augite, Sr-apatite, Ti-magnetite, and ilmenite from basaltic trachyandesitic melts. The melts evolved from trachytic to trachyrhyodacitic and trachyrhyolitic compositions as a result of prevalent crystallization of K-Na feldspar, with zircon, chevkinite-Ce, and LREE-enriched apatite involved in fractionation. Trachytic, trachyrhyodacitic, and trachyrhyolitic residual melts were produced by the evolution of compositionally different parental melts (basaltic trachyandesitic, trachytic, and trachyrhyodacitic, respectively), which moved to shallower continental crust and accumulated in isolated chambers. Judging by their isotopic signatures, the melts assimilated some crustal material, according to the assimilation and fractional crystallization (AFC) model.     

大陆花岗岩的地球动力学意义

大陆花岗岩的地球动力学意义是一个有争议的话题。笔者认为,花岗岩可分为大洋和大陆两个系列,产于洋盆内及其边缘的花岗岩属于大洋系列,产于大陆内(不包括造山带)的花岗岩属于大陆系列。大洋系列花岗岩最重要的地球动力学意义是判断花岗岩形成的构造环境;大陆系列花岗岩最重要的地球动力学意义是判断地壳状况,包括花岗岩形成时的地壳厚度和温度状况。花岗岩按照Sr-Yb含量可分为埃达克型、喜马拉雅型、浙闽型、广西型和南岭型5类。产于大陆内的不同类型的花岗岩与其形成的深度有关:埃达克型花岗岩富Sr贫Yb,与榴辉岩相处于平衡,产于加厚的地壳;喜马拉雅型花岗岩贫Sr和Yb,与麻粒岩相处于平衡,产于较厚的地壳;浙闽型(贫Sr富Yb)和广西型(富Sr和Yb)花岗岩与角闪岩相处于平衡,产于正常或较薄的地壳;南岭型花岗岩也与角闪岩相处于平衡,地壳厚度最薄。喜马拉雅型花岗岩属于低温系列,浙闽型花岗岩为中或高温系列,广西型和南岭型花岗岩属于高温系列。埃达克型花岗岩则可以出现在各个温度系列。应用花岗岩分类可以恢复古代地壳厚度和下地壳底部温度状况,还可以追踪某些地区随时间变化地壳厚度和温度变化的情况和趋势。     

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 , , , , .

     

Mesozoic-Cainozoic volcanism of Australia

Mesozoic—Cainozoic volcanism was concentrated on the youngest eastern Australian craton. Basaltic activity (with some felsic fractionation) has predominated over Mesozoic interludes of calcalkaline volcanism (rhyolites, dacites, trachytes andesites) and more isolated shoshonitic activity (now represented by appinitic, syenitic, granitic and lamprophyric complexes).Epeirogenic movements and associated sea-floor spreading and orogenic episodes at the continental margins, initiated and controlled much of the volcanism. Basin edges, faults, lineaments and their intersections were important in locating sites of volcanism; some fundamental structural lines have focussed volcanism over 300–600 km.The eastern Mesozoic basaltic volcanism shows a late Jurassic N-S trend from undersaturated to saturated compositions, with increasing intensity of melting towards a major Tasmanian-Antarctic thermo-tectonic event. A late Jurassic-late Cretaceous E-W trend may extend from possible ‘kimberlites’ through shoshonitic to calcalkaline activity with increasing proximity to orogenic movements along the New Zealand ‘Geosyncline’.Cainozoic basaltic volcanism reflects the NNE drift of Australia under Atlantic-Indian-Southern Ocean sea-floor spreading, with a debatable role for subduction along the Tasman Sea margin. The ultimate mechanisms of volcanism are not clearly understood. Drift of cratonic structural weaknesses over thermal anomalies in the mantle, with generation of magmas from a geochemically zoned Lower Velocity Zone under influence of uplifts, lithospheric thickness and periodic release of thermal energy, seems to partly explain observed patterns of E. Australian volcanism.     

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.