Eruptive history and petrology of Mount Drum volcano,Wrangell Mountains,Alaska

Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80×200 km) of southcentral Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to rhyolite. The last constructional activity occurred in the vicinity of Snider Peak, on the south flank of the volcano, where extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top and a part of the south flank of the volcano, produced more than 7 km³ of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and 72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks, but have higher MgO contents at a given SiO₂, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small, but significant, range in ⁸⁷Sr/⁸⁶Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages. In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas. Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks. Received March 13, 1993/Accepted September 10, 1993     

Debris-fan constrictions and flood hydraulics in river canyons: Some implications from two-dimensional flow modelling

Rapids in river canyons are frequently found at sites where debris fans constrict flow along the channel. Whereas some fans may have persisted in the same location with unchanging geometry for centuries to millennia, others have changed in response to flow conditions imposed by successive floods. Such a change in boundary conditions may alter local flow hydraulics. This paper utilizes two-dimensional flow modelling to compare flood hydraulics along two alternative versions of an idealized reach of a river canyon: one with uniform width, gradient and cross-section, and a second perturbed by a prominent debris fan along the valley wall. The flow pattern along the reach with the fan is far more complex than the pattern along the uniform reach. Maximum velocity along the debris-fan reach is up to 50 per cent higher than along the uniform reach, maximum bed shear stress is up to three or four times higher, and an area of supercritical flow is predicted extending from the nose of the fan into the zone of flow expansion immediately downstream. Comparison of model output along longitudinal profiles of the two reaches indicates that the backwater effect of the fan extends several valley widths upstream. Predicted flows based on the same stage are as much as 190 to 230 per cent greater along the uniform reach than along the debris-fan reach. Reconstruction of palaeoflood discharge based on remnant flood marks in the vicinity of the fan would be sensitive to assumptions about boundary conditions that existed in the past; this effect relaxes over a longitudinal distance of several hundred metres. Furthermore there are significant cross-stream gradients that change slope and direction several times in the vicinity of the fan, calling into question the utility of one-dimensional step-backwater hydraulic models for predicting high-water marks in areas of complex valley morphology.     

Strategies of lake sanitation

Lake sanitation measures must be planned in such a way that under consideration of the ecological requirements the trophic state which is needed in order for the lake to fulfill its designated purpose can be achieved with a sufficient degree of certainty. This is obtainable solely by limiting the influx of phosphorus compounds. The influencing factors as well as a knowledge of the local conditions which must be taken into consideration so that planning can be undertaken are described and discussed. The limits and effectiveness of technical and administrative single and joint control measures are presented with regard to the source of the phosphorus compounds. Presented at the International Conference on Lake Restoration at Zürich, 3–4 November 1986     

Kritische Betrachtungen zum geologischen Zusammenhang zwischen den chilenischen Anden und der Westantarktis

Zusammenfassung Die Entwicklung der Westantarktischen Geosynklinale läßt sich eher mit der der Anden-Geosynklinale vergleichen als mit der der Magallanes-Geosynklinale. Südast und Nordast des Südantillen-Bogens gehören verschiedenen Bereichen an. Analogien zum Westindischen Inselbogen werden aufgezeigt.

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Summary The West Antarctic geosyncline is more similar to the Andean geosyncline than to the Magallanes geosyncline. Northern and southern trace of the Scotia Arc form parts of distinct units. Analogies to the West Indian Island Arc are pointed out.

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Resumen El geosinclinal Antártico se asemeja más al geosinclinal Andino que al geosinclinal Magallánico. El brazo meridional y el brazo septentrional del arco de las Antillas Australes, pertenecen a diferentes unidades geológicas. Se señalan algunas analogías con el arco de las Antillas de América Central.

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Résumé Le géosynclinal de l'Antartide Occidentale est comparable avec le géosynclinal des Andes plutôt qu'avec celui de Magallanes. La branche Nord et la branche Sud de l'Arc de Scotia font partie de différentes unités. Des analogies avec l'Archipel des Indes Occidentales sont soulignées.

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Microsyenitic dykes from the Northern Part of the Ilimaussaq Peninsula,Southern Greenland

Summary Among the igneous rocks of the Gardar period of the Precambrian of Southern Greenland numerous dykes occur. Microsyenitic types are an outstanding feature among these. Many of them can be traced over 15 km in length; widths over 10 m are not uncommon. There are porphyritic and non-porphyritic varieties. Alkali feldspars form the main-constituents and are accompanied by aegirine-augite, riebeckite, fayalitic olivine, ilmeno-magnetite, apatite. Riebeckite seems to appear in quartz-bearing types only. Quartz or nepheline appear in slight amounts in some of these rocks.There is no differentiation in the dykes. Undersaturated, saturated and slightly oversaturated types can be distinguished. The undersaturated types are closely connected with the augite syenites. of the main-masses which are supposed to be the original magma of the alkaline province. In many places the dykes are prior to the main intrusions. The saturated and slightly oversaturated dykes are thought to belong to the main quartz-syenites which are hybridization products of the augite syenite magma contaminated by later alkalic acidic magmas.

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Zusammenfassung Unter den magmatischen Gesteinen der Gardar Periode im Präkambrium Südgrönlands sind Ganggesteine sehr zahlreich vertreten, unter denen wiederum Mikrosyenite eine ausschlaggebende Rolle spielen. Manche lassen sich über 15 km hin verfolgen; Breiten von über 10 m sind keine Seltenheit. Man kann porphyrische und nicht-porphyrische Typen unterscheiden. Alkalifeldspate sind die Hauptgemengteile, die von Aegirin-Augit, Riebeckit, fayalitischem Olivin, Ilmeno-Magnetit, Apatit begleitet werden. Riebeckit scheint nur in Quarz führenden Typen aufzutreten. Quarz oder Nephelin sind in untergeordnetem Maße in einigen Gesteinen vorhanden.Es ist keinerlei Differentiation festzustellen. Es werde untersättigte, gestättigte und übersättigte Typen unterschieden. Die untersättigten sind eng mit den Augitsyeniten der Hauptmassive verbunden, von denen angenommen wird, daß sie aus dem ursprünglichen Magma dieser Alkaliprovinz erstarrien. In vielen Fällen sind die Gänge vor den Hauptintrusionen eingedrungen. Die gesättigten und leicht übersättigten Typen werden den Quarzsyeniten der Hauptmassive genetisch zugeordnet. Diese werden als Hybridisationsprodukte aus dem Originalmagma der Augitsyenite mit späteren kieselsäurereichen, alkalischen Magmen gedeutet.

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Monzonitic plutons,California, and a model for generation of alkali-rich,near silica-saturated magmas

Petrochemical data suggest that early Mesozoic alkali-rich monzonite plutons in California were generated by partial melting of a garnet-bearing, virtually feldspar-free source which was rich (compared to abyssal basalt or sub-oceanic mantle peridotites) in LIL elements. These monzonites are characterized by high Na₂O+K₂O (near 10 wt% total) and Sr (1000 to 2000 ppm), moderate SiO₂ (53 to 63 wt%), steep negatively sloping REE patterns (from >100 to <10 × chondritic abundance), and moderate initial ⁸⁷Sr/⁸⁶Sr ratios (0.705 to 0.707). Most are very slightly oversaturated in silica, but one pluton includes both oversaturated and undersaturated units.The simplest petrogenetic model which can account for the compositions of these rocks involves partial melting of quartz eclogite. Any differentiation of the resulting magma must take place under pressures at which plagioclase is unstable. The eclogite could be equivalent to a wide variety of continental or oceanic island basalts.Rocks which are chemically similar to these monzonites, though not extremely abundant, are widespread in both space and time and occur in a wide variety of tectonic settings. These rocks are anomalous in terms of crustal compositions and phase equilibria, but many could be explained as partial melting products of eclogite. Relatively minor thermal events or early stages of major events may tap alkalic intermediate magmas from eclogite pockets as the lowest temperature melting fractions of the upper mantle.