Geochemistry of rare high-Nb basalt lavas: Are they derived from a mantle wedge metasomatised by slab melts?

Compositionally, high-Nb basalts are similar to HIMU (high U/Pb) ocean island basalts, continental alkaline basalts and alkaline lavas formed above slab windows. Tertiary alkaline basaltic lavas from eastern Jamaica, West Indies, known as the Halberstadt Volcanic Formation have compositions similar to high-Nb basalts (Nb > 20 ppm). The Halberstadt high-Nb basalts are divided into two compositional sub-groups where Group 1 lavas have more enriched incompatible element concentrations relative to Group 2. Both groups are derived from isotopically different spinel peridotite mantle source regions, which both require garnet and amphibole as metasomatic residual phases. The Halberstadt geochemistry demonstrates that the lavas cannot be derived by partial melting of lower crustal ultramafic complexes, metasomatised mantle lithosphere, subducting slabs, continental crust, mantle plume source regions or an upper mantle source region composed of enriched and depleted components. Instead, their composition, particularly the negative Ce anomalies, the high Th/Nb ratios and the similar isotopic ratios to nearby adakite lavas, suggests that the Halberstadt magmas are derived from a compositionally variable spinel peridotite source region(s) metasomatised by slab melts that precipitated garnet, amphibole, apatite and zircon. It is suggested that high-Nb basalts may be classified as a distinct rock type with Nb > 20 ppm, intraplate alkaline basalt compositions, but that are generated in subduction zones by magmatic processes distinct from those that generate other intraplate lavas.     

Organic geochemical investigation of far-field tsunami deposits of the Kahana Valley,O‘ahu,Hawai‘i

Far-field tsunami deposits observed in the Kahana Valley, O‘ahu, Hawai‘i (USA), were investigated for their organic-geochemical content. During short high-energy events, (tsunamis and storms) organic and chemical components are transported with sediment from marine to terrestrial areas. This study investigates the use of anthropogenic based organic geochemical compounds (such as polycyclic aromatic hydrocarbons, pesticides and organochlorides) as a means to identify tsunami deposits. Samples were processed by solid–liquid extraction and analyzed using gas chromatography–mass spectrometry. A total of 21 anthropogenic marker compounds were identified, of which 11 compounds were selected for detailed analysis. Although the tsunami deposits pre-date industrial activity in Hawai‘i by several hundred years, distinct changes were found in the concentrations of anthropogenic marker compounds between sandy tsunami deposits and the surrounding mud/peat layers, which may help in identifying tsunami deposits within cores. As expected, low overall concentrations of anthropogenic markers and pollutants were observed due to the lack of industrial input-sources and little anthropogenic environmental impact at the study site. This geochemical characterization of tsunami deposits shows that anthropogenic markers have significant potential as another high-resolution, multi-proxy method for identifying tsunamis in the sedimentary record.     

Quantitative modeling of compaction in the Holyoke flood basalt flow,Hartford Basin,Connecticut

Compaction in the Holyoke flood-basalt flow is modeled using a quantitative phase equilibrium routine based on the MELTS algorithm that is coupled with compaction-driven heat and mass transport equations. For a crystal-liquid mush that is cooling from both the top and the bottom, the compaction is qualitatively different in the upper and lower parts of the flow. The solid fraction in the lower part increases as a result of concurrent compaction and solidification. In contrast, cooling from the upper boundary results in an increase in viscosity and a decrease in permeability toward the top. This causes the velocity of the solid matrix to increase downward and the upper part to undergo dilation of the solid matrix. The result is an "S"-shaped profile for compatible elements, and an inverse profile for incompatible elements. The region of modeled maximum dilation matches the location of segregation sheets in the Holyoke flow. Discrepancy between observed deformation, as calculated from textural deformation of plagioclase chain networks versus compositional profiles, suggest some crystal accumulation and/or compaction prior to the formation of the plagioclase chains.     

Physiological responses of transplants of the freshwater angiosperm Vallisneria americana along a salinity gradient in the Caloosahatchee Estuary (Southwestern Florida)

Fluctuations in freshwater input may affect the physiology and survival of submerged aquatic vegetation (SAV) occurring in oligoaline to mesohaline estuarine regions. Controls on the distribution of the freshwater angiosperm Vallisneria americana, were investigated by transplanting ramets. Pots (3.8-1) containing ramets were distributed among four sites (upstream site [least saline], donor site, near downstream site, and far downstream site [most saline]) in the Caloosahatchee Estuary (Southwest Florida) during wet (May–August) and dry (October–February) seasons. During 2–4 mo of each season, physiological indicators were monitored, including photosynthesis, glutamine synthetase activity, and protein content in shoots, and carbohydrates and total nitrogen and carbon in shoot and subterranean tissues. Where the physical environment (light or salinity) was suboptimal, all physiological indices, except photosynthetic rate, showed similar stress responses, which ranged from a slow decline to a rapid drop in physiological function. Levels of soluble carbohydrates decreased in response to unfavorable conditions more rapidly than did insoluble carbohydrates. Shoot protein of V. americana declined prior to transplant death, suggesting that measuring protein content may provide a rapid assessment of physiological health. V. americana transplants at the low-salinity upstream site died during both wet and dry season experiments, likely in response to light limitation and/or partial burial by sediments. At the far downstream site, death occurred within 2–4 wk, and was attributable to elevated salinities (>ca. 15‰). Comparison of physiological responses with salinity and light regimes at the donor and near downstream sites suggest that light may ameliorate salinity stress. This study demonstrates that V. americana, nominally classed as a freshwater macrophyte, is capable of a remarkable degree of halotolerance.     

Tertiary tectono-metamorphic evolution of the European margin during Alpine collison: example of the Leventina Nappe (Central Alps,Switzerland)

The Leventina Nappe represents one of the lowermost exposed units in the Alpine nappe stack and corresponds to a slice of the European margin that was entrained into the Alpine continental accretionary prism during the Tertiary tectonic event. This study yields details regarding the tectonic and metamorphic history of the Leventina Nappe, through detailed analysis of structures and shear zone patterns, and the examination of the Si-content of white mica along a north-south profile. The Leventina Nappe underwent three phases of ductile deformation. Foliation S1 is mostly sub-parallel to the regionally dominant structural fabric (the S2 foliation). S2 foliation is penetratively developed in the structurally higher portions of the Leventina Nappe toward the Simano Nappe, while it is only weakly developed in the core of the Leventina Nappe. A 50 to 200 m wide mylonite zone, with a D2 top-to-NW sense of shear marks the boundary to the Simano Nappe. Throughout the Leventina Nappe only small-scale D2 shear bands (mm to cm wide) are observed, showing a top-to-NW sense of shear. Deformation phase D3 locally generated a vertical axial plane foliation (S3) associated with the large-scale D3 Leventina antiform.Microtextural evidence and phengite geobarometry were used to constrain the temperature and pressure conditions of equilibration of the Leventina Gneisses. Highest Si (pfu) values are preserved in the core of phengitic micas and reflect pressure and temperature conditions of around 8 kbar at 550 °C and 10 kbar at 650 °C in the northern and southern parts of the Leventina Nappe, respectively. Lower Si (pfu) values from the rims of white micas correspond to a metamorphic pressure of ca. 5 kbar during the exhumation of the unit. These metamorphic conditions are related to the underthrusting of the thinned European margin into the continental accretionary prism during late Eocene time. These new data allow us to propose a kinematic model for the Leventina Nappe during the Tertiary Alpine tectonics.     

Magmatic and metallogenetic episodes in the northern tin belt,cordillera real,Bolivia

New K-Ar determinations for granitoid rocks and ores from the northern Bolivian segment (ca. 15°-18° S.) of the Central Andean Cordillera Oriental (Eastern Cordillera) confirm that this region displays a remarkable repetition of magmatic activity and associated lithophile metal mineralization. Intrusion of monzogranites, in part peraluminous, and granodiorites took place in the Mid—to—Late Triassic (apparent age range: 225-202 Ma) and Late Oligocene-Early Miocene (28.4-ca. 19.2 Ma); the two plutonic domains are juxtaposed, with little or no areal overlap, along an essentially linear belt defining the inner boundary of the Central Andean magmatic arc. There is no conclusive evidence of granitoid intrusion during the intervening period, although at least two centres of Upper Cretaceous basic-intermediate hypabyssal-volcanic rocks are known. Whereas the Triassic episode simultaneously affected ca. 200 km of the belt, the Oligocene-Miocene domain experienced a longitudinal (southeastward) migration of activity which, in its later stages (22-19 Ma), coincided with the initiation of widespread volcanism and intrusion on the altiplano, to the west of the granitoid belt.Both intrusive domains may be regarded as the innermost expressions of relatively brief episodes of radical broadening of the Central Andean magmatic arc, possibly generated through abrupt, but minor, decrease in the inclination of eastward subduction at the western plate margin, and attended by a degree of crustal anatexis unattained in the outer parts of the arc. Our radiometric data permit the delimitation of two metallogenetic (W-Sn) sub-provinces in the northern part of the Bolivian tin belt. The tungsten-tin vein deposits associated with the Triassic and Tertiary plutons are very similar, although tungsten appears to be enriched relative to tin in the older domain. There is no radio-metric or petrographic evidence of significant reactivation of older hydrothermal systems in the Tertiary. The late-Early Miocene age (16.3 Ma) determined for the important Oruro Sn-Ag epithermal centre (17° 56' S.) represents an anomaly in the broad southward migration of volcanic and hydrothermal activity defined byGrant et al. (1979) in the central and southern, sub-volcanic, segments of the Bolivian tin-polymetallic belt. Minor Pb-Zn-Ag vein mineralization was probably associated with the Upper Cretaceous basic-intermediate magmatism.

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Zusammenfassung Neue K-Ar Bestimmungen für die granitoiden Gesteine und Erze aus dem nord-bolivischen Segment (ca. 15°-18° S) der mittleren andeanischen Cordillera Oriental (östlichen Cordillera) bestätigen, daß dieses Gebiet eine beachtenswerte Wiederholung der magmatischen Aktivität und der damit verbundenen Erzmineralisierung aufweist. Granodiorite und teilweise Monzogranite mit Aluminium-Überschuß sind in der Mittel- und Spät-Trias (225-202 Ma) und im Spät-Oligozän bis Früh-Miozän (28.4-ca. 19.2 Ma) eingedrungen; die beiden plutonischen Bereiche liegen mit nur geringer oder fehlender Überlappung benachbart längs einem im wesentlichen linearen Gürtel, der den inneren Rand des mittelandinischen magmatischen Bogen abgrenzt. Es gibt keine entscheidende Anzeichen für granitische Intrusionen in der dazwischen liegenden Zeit, obwohl wenigstens zwei Zentren der oberkretazischen basischen bis intermediären, hypabyssalen bis vulkanischen Gesteinen bekannt sind. Während die triassische Episode gleichzeitig ungefähr 200 km des Gebiets beeinflußte, erfuhr der oligozän-miozäne Bereich jedoch eine longitudinale (südöstliche) Wanderung der Aktivität, die, in den jüngeren Stufen, mit der Einleitung des ausgedehnten Vulkanismus und der Intrusion auf dem Altiplano, westlich des granitoiden Gebietes zusammenfiel.Die beiden intrusiven Gebiete könnte man als die innersten Vorgänge verhältnismägig kurzer Episoden einer drastischen Verbreitung des mittleren andinischen magmatischen Bogens betrachten. Diese könnten vielleicht durch die plötzliche Verringerung des Winkels der östlichen Subduktion an der westlichen Plattengrenze entstanden sein, mit einem Grad der Anatexis, der in den äußeren Teilen des Bogens nicht erreicht wurde. Unsere radiometrischen Daten ermöglichen die Abgrenzung zweier metallogenetischen (W-Sn) Unterprovinzen im nördlichen Teil des bolivischen Zinn-Gebiets. Obwohl die mit den triassischen und tertiären Plutonen verbundenen Wolfram-Zinn-Erzgänge einander sehr ähneln, scheint das Wolfram im älteren Bereich dem Zinn gegenüber etwas angereichert zu sein. Es liegen keine radiometrischen oder petrographischen Zeugnisse für eine erhebliche Reaktivierung von älteren hydrothermalen Systemen im Tertiär vor. Ein spätes früh-miozänes Alter (16.3 Ma) wurde für das wichtige Sn-Ag epithermale Zentrum von Oruro bestimmt. Dieses weicht von der allgemeinen nach S gerichteten Wanderung der vulkanischen und hydrothermalen Aktivität in den zentralen und südlichen subvulkanischen Segmenten der bolivischen Zinn-polymetallischen Gürtel ab, die vonGrant et al. (1979) beschrieben wurde. Geringere Pb-Zn-Ag-Erzgangbildung stand wahrscheinlich mit dem oberkretazischen basisch-intermediären Magmatismus in Verbindung.

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Résumé De nouvelles determinations K-Ar sur des roches granitiques et sur du minerais provenant du segment bolivien septentrional (ca. 15°-18° S) de la Cordillère orientale des Andes centrales (Cordillère est) confirme que cette région fut affectée par une activité magmatique répétée auquelle s'associa une activité metalifère riche en métaux lithophiles. Des intrusions de monzogranites, en partie peralumineux et de granodiorites se mirent en place durant le Triassique moyen et supérieur (intervalle d'âge apparent: 225-202 Ma) et durant l'Oligocène moyen et le début du Miocène (24.4-ca. 19.2 Ma). Ces deux domaines plutoniques se juxtaposent avec un recouvrement spatial faible ou nul, le long d'une ceinture essentiellement linéaire définissant la bordure interne de l'arc magmatique des Andes centrales. Bien qu'aucune indication concluante nesitue la mise en place d'intrusions granitiques durant la période intermédiaire, au moins deux centres volcaniques hypabyssaux de composition basique à intermédiaire se sont formés au Crétacé supérieur. L'épisode magmatique du Triassique a affecté simultanément ca. 200 km de la ceinture. Le domaine Oligocène-Miocène, pour sa part, a subi une migration longitudinale (sudest) de l'activité magmatique dont les étapes tardives (22-19 Ma) coîcident avec le déput d'une longue période de volcanisme et d'intrusions sur l'altiplano, à l'ouest de la ceinture granitique.celle des aggregats de grande taille, par des mécanismes de dislocation. La transition est Les deux domaines intrusifs peuvent être causés par un fort élargissement répété et bref de l'arc magmatique des Andes centrales. neut-être dû à une diminution abrupte de l'inclinaison de la zone de subduction orientale à la bordure ouest de la plaque, accompagnée par un degré d'anatexie crustale qui n'atteignit pas la partie externe de l'arc. Les résultats radiométriques obtenus permettent de délimiter deux sous-provinces métallogéniques (W-Sn) dans la partie septentrionale de la ceinture bolivienne stannifère. Les gisements filoniens d'étain et de tungsténe associés aux plutons triassiques et tertiaires sont très semblables; cependant le tungstène semble être plus abondant rapport à l'étain dans le plus vieux domaine. Il n'y a aucune évidence radiométrique ou pétrographique d'une réactivation signicative du système hydrothermal tertiaire. L'age Miocène (16.3 Ma) obtenu pour l'important centre épithermal Sn-Ag d'Oruro (17° 56' S.) représente une anomalie dans la vaste migration sud-est de l'activité volcanique et hydrothermale définie parGrant et al. (1979) dans les segments «sub-volcaniques» central et sud-est de la ceinture polymétallique et stannifére bolivienne. Une faible minéralisation filonienne Pb-Zn-Ag était probalement associée avec le magmatisme intermédiaire à basique du Crétacé supérieur.

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Tectonic correlations of pre-Mesozoic crust from the northern termination of the Colombian Andes, Caribbean region

Reconnaissance zircon U/Pb SHRIMP, Ar–Ar, and Sm–Nd geochronology, petrological, and geochemical data were obtained from selected localities of two pre-Mesozoic metamorphic belts from the northern termination of the Colombian Andes in the Caribbean region. The older Proterozoic belt, with protoliths formed in a rift- or backarc-related environment, was metamorphosed at 6–8 kb and 760–810 °C during Late Mesoproterozoic times. This belt correlates with other high-grade metamorphic domains of the Andean realm that formed a Grenvillian-related collisional belt linked to the formation of Rodinia. The younger belt was formed over a continental arc at <530–450 Ma in a Gondwanide position and metamorphosed at 5–8 kb and 500–550 °C, probably during the Late Paleozoic–Triassic, as part of the terranes that docked with northwestern South America during the formation of Pangea. A Mesozoic Ar–Ar tectonothermal evolution can be related to regional magmatic events, whereas Late Cretaceous–Paleocene structural trends are related to the accretion of the allocthonous Caribbean subduction metamorphic belts. Lithotectonic correlations with other circum-Caribbean and southern North American pre-Jurassic domains show the existence of different terrane dispersal patterns that can be related to Pangea’s breakup and Caribbean tectonics.     

Spatial patterns of Holocene glacier advance and retreat in Central Asia

Glaciers in the southern Himalayas advanced in the early Holocene despite an increase in incoming summer solar insolation at the top of the atmosphere. These glacier advances are in contrast to the smaller alpine glaciers in the western and northern regions of Central Asia. Two different glacier mass-balance models are used to reconcile this Holocene glacier history with climate by quantifying the change in equilibrium-line altitudes (ELA) for simulated changes in Holocene climate. Both ELA models clearly show that the lowering of ELAs in the southern Himalayas is largely due to a decrease in summer temperatures, and that an increase in monsoonal precipitation accounts for less than 30% of the total ELA changes. The decrease in summer temperatures is a dynamic response to the changes in solar insolation, resulting in both a decrease in incoming shortwave radiation at the surface due to an increase in cloudiness and an increase in evaporative cooling. In the western and northern zones of Central Asia, both ELA models show a rise in ELAs in response to a general increase in summer temperatures. This increase in temperatures in the more northern regions is a direct radiative response to the increase in summer solar insolation.     

Palaeomagnetic investigations of the tertiary and quaternary igneous rocks: VIII a palaeomagnetic and petrologic study of volcanics of the valley of Mexico

The basin of Mexico lies in the center of the Trans-Mexican Volcanic Belt, an east-west structure which dissects the North American Continent along the 19th parallel. Paleomagnetic measurements suggest that two of the seven eruptive phases —Guadalupe Group and Chichinautzin Group — that formed the basin, probably were of short duration; the Chichinautzin Group, which finally closed the basin to the south, must have been formed principally in the last 700,000 years.The volcanic activity which began in Oligocene time and has continued until the present, has been proved to be essentially andesitic in composition.

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Zusammenfassung Das Becken von Mexiko befindet sich im Zentrum des Transmexikanischen Vulkangürtels, einer Ost-West-Struktur, die den nordamerikanischen Kontinent entlang des 19. Breitengrades durchschneidet.Paläomagnetische Messungen legen die Vermutung nahe, daß 2 (Guadalupe- und Chichinautzin-Gruppe) der 7 vulkanischen Phasen, die das Becken formten, von wahrscheinlich kurzer Dauer waren. Die Chichinautzin-Gruppe, die das Becken im Süden abschnürte, muß sich grundsätzlich in den letzten 700 000 Jahren gebildet haben.Die vulkanische Tätigkeit, die im Oligozän begann und sich kontinuierlich bis in dic heutige Zeit fortsetzte, förderte nachweislich im wesentlichen andesitische Produkte.

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Résumé Le bassin de Mexico se trouve au centre de la ceinture volcanique de Mexico, une structure est-ouest qui recoupe le continent nord-Americain près du 19^e parallèle. Les mesures paléomagnétiques conduisent à supposer que deux des sept phases d'activités volcaniques, celles de la Guadaloupe et du Chichinautzin, étaient courtes. Les roches du Groupe de Chichinautzin, fermant le bassin au sud, ont été formées pendant les derniers 700,000 ans.L'activité volcanique, qui a débuté à l'Oligocène et se continue encore actuellement a une charactère essentiellement andésitique.

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— , - 19- . , , Guadalupe Chichinautzin, , , . Chichinautzin, , 700000 . , , .

     

Very high strains recorded in mylonites along the Alpine Fault, New Zealand: implications for the deep structure of plate boundary faults

Oblique displacement on the Alpine Fault, which forms the principal structure along the Australian–Pacific plate boundary in South Island, New Zealand, has resulted in exhumation of a kilometre-wide mylonite zone in the hanging wall adjacent to the current brittle fault trace. The mylonites formed under amphibolite facies conditions at depths of ca. 25 km and have been uplifted during the past 5 Ma. A suite of 65–70 Ma pegmatite veins in the hanging wall Alpine schists has been progressively deformed within the mylonite zone and sheared out over a strike length of ca. 100 km. Measurements of the thickness distribution of the pegmatite veins within the non-mylonitised schists and at three localities within the progressively strained mylonites have been used to estimate strain values within the mylonites. The thicknesses approximate a log-normal distribution, with a mean value that is progressively reduced through the protomylonites, mylonites and ultramylonites. By assuming that the thickness distribution currently observed in the schists was the same for the pegmatites within the mylonites before strain, a model of deformation incorporating simple shear and simultaneous pure shear is used to strain the undeformed veins until a fit is obtained with the strained distributions. Shear strains calculated range from 12 to 22 for the protomylonites, 120 to 200 for the mylonites and 180 to 300 for the ultramylonites, corresponding to pure shear values of 1–3 in each case. These values are compatible with the strains predicted if most of the surface displacement on the fault over the past 5 Ma were accommodated within a 1–2-km-wide mylonite zone through the middle and lower crusts. The results suggest that processes such as erosional focussing of deformation and thermal weakening may cause intense strain localisation within the lower crust, with plate boundary deformation restricted to narrow zones rather than becoming increasingly distributed over a widening shear zone with depth.