Plagioclase morphologies in assimilation experiments. Implications for disequilibrium melting in the generation of granodiorite rocks

Summary Assimilation experiments have been performed using a crustal contaminant and several basaltic compositions in order to study the morphologies of plagioclase crystals produced in the course of the assimilation reactions. A Crd-Bt anatectic gneiss was used as a crustal contaminant. Both a synthetic basaltic glass, with the composition of a high-Al ocean tholeiite, and a natural Hb gabbro were used as basaltic compositions. Experiments were carried out at temperatures in the range 900–1100 °C and at pressures of 4, 5 and 10 kbar. Plagioclase crystals with skeletal and honeycomb morphologies are produced in the assimilation experiments. These morphologies result from destabilisation of the crystal-melt interface caused by the disequilibrium production of a Ca-rich melt. The production of the Ca-rich metastable melt is the result of reaction between two compositionally dissimilar systems, pelite and basalt. This interpretation, based on the kinetics of plagioclase and melt production, may be applied to granodiorite rocks characterised by plagioclase with Ca-rich, skeletal cores.

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Zusammenfassung Plagioklas-Morphologie und Assimilations-Experimente: Die Bedeutung von Ungleichgewichts-Schmelzvorg?ngen bei der Entstehung von Granodioriten Assimilations-Experimente mit einer krustalen Komponente und verschiedenen basaltischen Zusammensetzungen wurden durchgeführt, um die Morphologien von Plagioklas-Kristallen, die im Laufe von Assimilations-Reaktionen entstehen, zu studieren. Ein CRD-PT anatektischer Gneis wurde als krustale Beimengung benützt. Ein synthetisches basaltisches Glas mit der Zusammensetzung eines Al-reichen ozeanischen Tholeiites und ein natürlicher Hb-Gabbro wurden als basaltische Zusammensetzungen verwendet. Die Experimente wurden bei Temperaturen zwischen 900 und 1.100° und bei Drucken von 4, 5 und 10 kb durchgeführt. Plagioklas-Kristalle mit skelettartigen und Bienenwaben-f?rmigen Morphologien entstehen bei den Assimilations-Experimenten. Diese Morphologien sind das Ergebnis der Destabilisation der Kristall-Schmelze-Kontaktfl?che, die durch die Entstehung einer im Ungleichgewicht befindlichen Ca-reichen Schmelze gef?rdert wird. Die Entstehung von Ca-reichen metastabilen Schmelzen geht auf die Reaktion zwischen zwei in der Zusammensetzung verschiedenen Systemen, Pelit und Basalt, zurück. Diese Interpretation, die auf der Kinetik von Plagioklas und Schmelz-Entstehung basiert, kann auf granodioritische Gesteine angewendet werden, die durch Plagioklasse mit Ca-reichen, skelettf?rmigen Kernen gekennzeichnet sind.

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Received December 7, 1999; revised version accepted April 14, 2000     

Neutral Flow Over A Series Of Rough Hills: A Laboratory Experiment

This paper presents laboratory experiments of aerodynamically fully rough, neutral flow over a series of sinusoidal hills. Two sets of hills, with maximum gradients (slopes) of 0.2 (10°) and 0.4 (20°), were considered.The flow remained attached in the former case while separation occurredin the latter. Characteristics of the mean flow and turbulence statistics are discussed and compared with profiles over a flat surface covered with the same roughness as the hills. Comparisons are made with linear theory predictions for the flow in the inner region and aloft. Accurate measurements of the surface pressure were also made, enabling the comparison between the measured pressure drag and predictions from theoretical and computational work with different turbulent closure schemes. Organised secondary flow in the spanwise direction, observed previously in both experimental and computational studies, was also observed here over the small hills.     

The stabilizing effect of Na2S on the collector coating of chrysocolla

The heterogeneous reaction between natural and sulphidized chrysocolla and potassium amyl xanthate in solution is investigated.It is found that, together with the consumption of large amounts of xanthate, particles composed of the reaction products are spontaneously released from the surface of chrysocolla giving rise to a colloidal dispersion. In principle the mechamism is related to the peptizing effect of the excess xanthate ions that are found when Cu²⁺ and X^? ions are made to react in a homogeneous phase.In sulphidized chrysocolla this phenomenon is markedly reduced, showing that Na₂S has a stabilizing effect on the collector coating.Flotation experiments performed with aqueous amyl dixanthogen emulsions show that stable adhesion occurs only on properly sulphidized chrysocolla. It is believed that the mechanism controlling adhesion in this case is similar to that which determines the stability of the collector layer.     

Effects of seismic wave propagation upon buried pipelines

The behaviour of long straight buried pipelines subjected to seismic wave propagation is investigated. Well-known relationships for determining upper bounds for the axial strain and curvature in the pipeline as well as relationships for relative displacement and rotation at the pipeline joints are discussed. The assumption that the seismic excitation can be modelled as a travelling wave having a shape which remains unchanged as it traverses the pipeline is examined in detail. It is shown that this assumption is unconservative when the effective propagation velocity of the seismic waves with respect to the pipeline is such that the actual time lag (separation distance between points divided by effective propagation velocity) is less than a ‘cross-over’ time lag. Cross-over time lags for 22 pairs of ground displacements recorded during the 1971 San Fernando Earthquake are presented in this paper. Finally, methods for estimating the propagation speed of the seismic waves along or with respect to the pipeline are discussed.     

Interbasin underflow between closed Altiplano basins in Chile

Interbasin ground water movement of 200 to 240 L/sec occurs as underflow beneath a mountainous surface water divide separating the topographically higher Salar de Michincha from the topographically lower Salar de Coposa internally drained basins in the Altiplano of northern Chile. Salt-encrusted flats (salars) and saline lakes occur on the lowest parts of the basin floors and comprise the principal evaporative discharge areas for the basins. Because a surface water divide separates the basins, surface water drainage boundaries do not coincide with ground water drainage boundaries. In the region, interbasin ground water movement is usually not recognized, but occurs for selected basins, and at places is an important component of ground water budgets. With increasing development of water for mining industry and potential exportation of ground water from the Altiplano for use at coastal cities, demonstration and quantification of interbasin movement is important for assessment of sustainable ground water development in a region of extreme aridity. Recognition and quantification of interbasin ground water underflow will assist in management of ground water resources in the arid Chilean Altiplano environment.     

Application of a statistical water-table model reveals connections between dunes and vegetation at Doñana

We present a simple statistical model of the water-table fluctuations based on a set of multiple regressions for a dune slack of the Doñana National Park, and its utility to understand some ecological processes. The variation in the coefficients of the regressions for different locations allows discrimination between different zones in the dune slack, and explains the spatial pattern of heath vegetation observed in the stabilized sands. Model simulations were compared with an existing sparse data set of water-table levels, and suggests possible mechanisms responsible for the catastrophic pine tree mortality, which happened during the drought of 1979–83.     

Burying processes carried out by a mobile transversal dunefield, Paracuru County, State of Ceará, Brazil

The migration of dunes and the consequent burying of the city of Paracuru by a transversal dunefield with perpendicular ridges oriented to the dominant windstream have been investigated. Overlapping aerial photographs taken during different flights have been used as a reference. Two sediment transportation directions were identified: the first is fed by the prevailing easterly winds; the second corresponds to the secondary southeasterly winds. With the maintenance of current wind conditions and pluvial precipitation, dunes will bury part of Paracuru town in approximately 120 years. Within this context, the geological evolution model that was applied proved to be satisfactory for the comprehension of sediment transportation mechanisms along the transversal dunes. Considering the prognosis in relation to Paracuru and its urban growth towards the dunefield, it is necessary to continue to study aeolian dynamics, aiming to develop support for the elaboration of future projects directed towards environmental management of the area.     

Turbulence Over Urban-type Roughness: Deductions from Wind-tunnel Measurements

Turbulence data from experiments conducted over a staggered cube array, modelling a neutrally stable atmospheric boundary layer in an urban environment, are presented. The results support the contention that organised eddy structures in the near-wall region differ significantly from those in regular smooth-wall flows or in rough-wall boundary layers with much smaller h/δ ratios (where δ and h are the boundary-layer thickness and the height of the roughness elements, respectively). Attention is concentrated on spatial correlations, spectra (and thus the dominant length and time scales), maps of anisotropy invariants and quadrant analyses of the stress tensor. Results are obtained within both the roughness sublayer (i.e. the region above the roughness but within which the flow is spatially inhomogeneous) and the canopy region (i.e. below the height of the roughness elements) and discussion includes consideration of the turbulence kinetic energy balance at various heights.     

The Central Extremadura batholith: Geotectonic implications (European Hercynian Belt)—an outline

Study of the Central Extremadura batholith and shear zone, described for the first time herein, provides new insights into the tectonomagmatic evolution of the Ibero-Armorican arc.The main plutonic event may be closely related to intracontinental shear-zone development. The Central Extremadura shear zone may provide a model for the interpretation of collision structures related to the origin of the Ibero-Armorican arc.