The 2 Ga peraluminous magmatism of the Jacobina — Contendas Mirante Belt (Bahia) Brazil): Major and trace-element geochemistry and metallogenic potential

The Jacobina — Contendas Mirante belt represents a Transamazonian (2 Ga), N-S, 500-km long, elongated orogenic domain in the central part of the São Francisco craton, Bahia state. Numerous syntectonic to post-tectonic peraluminous leucogranites were emplaced along the major structures of the belt. Their mineralogical and geochemical and some of their metallogenetic characteristics are very similar to their Hercynian and Himalayan equivalents. However, their average peraluminous index varies from one granitic pluton to another and biotite is, on average, slightly more magnesian in the Transamazonian leucogranites. Higher oxygen fugacity is indicated by the general occurrence of magnetite, the stability of allanite and sometimes epidote in most of the plutons and by biotite chemistry. The peraluminous magmatism of the Jacobina-Contendas Mirante belt results from crustal partial melting during a continental collision event at 2 Ga.Trace-element geochemistry implies variable source composition and/or melting conditions for the different granitic plutons and some different facies within the same plutonic unit. The scarcity of ilmenite, the general occurrence of magnetite, and the relatively low peraluminous index of some of these granites suggest that graphite-beating sediments are not a significantly source material. From their mineralogical and geochemical characteristics, acid meta-igneous rocks such as the Sete Voltas TTG suite of presumed Archaean age, seem to represent a suitable source for these granites.Sn, W, Li, F and Be enrichment of most Transamazonian leucogranites is much weaker than in the mineralized Variscan equivalents. The Caetano-Aliança and Riacho das Pedras granites represent the most specialized granitic bodies. Beryl (emerald), molybdenite and scheelite mineralizations are related to some of these granites which intrude ultrabasic formations: the Campo Formoso and the Carnaiba granites. In many of these granites, uranium content is comparable to values measured in mineralized Hercynian leucogranites. The occurrence of hexavalent uranium minerals, mineralization and episyenitic alteration are favourable criteria for finding Variscan-type uranium ore deposits.     

Response of the Antarctic ice sheet to future greenhouse warming

Possible future changes in land ice volume are mentioned frequently as an important aspect of the greenhouse problem. This paper deals with the response of the Antarctic ice sheet and presents a tentative projection of changes in global sea level for the next few hundred years, due to changes in its surface mass balance. We imposed a temperature scenario, in which surface air temperature rises to 4.2° C in the year 2100 AD and is kept constant afterwards. As GCM studies seem to indicate a higher temperature increase in polar latitudes, the response to a more extreme scenario (warming doubled) has also been investigated. The mass balance model, driven by these temperature perturbations, consists of two parts: the accumulation rate is derived from present observed values and is consequently perturbed in proportion to the saturated vapour pressure at the temperature above the inversion layer. The ablation model is based on the degree-day method. It accounts for the daily temperature cycle, uses a different degree-day factor for snow and ice melting and treats refreezing of melt water in a simple way. According to this mass balance model, the amount of accumulation over the entire ice sheet is presently 24.06 × 10¹¹ m³ of ice, and no runoff takes place. A 1°C uniform warming is then calculated to increase the overall mass balance by an amount of 1.43 × 10¹¹ m³ of ice, corresponding to a lowering of global sea level with 0.36 mm/yr. A temperature increase of 5.3°C is needed for the increase in ablation to become more important than the increase in accumulation and the temperature would have to rise by as much as 11.4°C to produce a zero surface mass balance. Imposing the Bellagio-scenario and accumulating changes in mass balance forward in time (static response) would then lower global sea level by 9 cm by 2100 AD. In a subsequent run with a high-resolution 3-D thermomechanic model of the ice sheet, it turns out that the dynamic response of the ice sheet (as compared to the direct effect of the changes in surface mass balance) becomes significant after 100 years or so. Ice-discharge across the grounding-line increases, and eventually leads to grounding-line retreat. This is particularly evident in the extreme case scenario and is important along the Antarctic Peninsula and the overdeepened outlet glaciers along the East Antarctic coast. Grounding-line retreat in the Ross and Ronne-Filchner ice shelves, on the other hand, is small or absent.     

Regional estimates of heat and evaporation fluxes over non-homogeneous terrain. Examples from the HAPEX-MOBILHY programme

Questions related to the spatial integration of turbulent surface fluxes of heat and moisture up to horizontal scales of 100km are discussed. Results taken from the HAPEX-MOBILHY programme are presented, concerning either comparisons between surface-based and aircraft measurements, or estimates derived from numerical simulations of the large-scale hydrological balance, or finally area-averaged values obtained from atmospheric numerical mesoscale models. Conclusions are drawn from these results on how to estimate spatially-averaged surface fluxes efficiently.     

On the entropy of glaucophane Na2Mg3Al2Si8O22(OH)2

The heat capacity of glaucophane from the Sesia-Lanza region of Italy having the approximate composition (Na_1.93Ca_0.05Fe_0.02) (Mg_2.60Fe_0.41) (Al_1.83Fe_0.15Cr_0.01) (Si_7.92Al_0.08)O₂₂(OH)₂ was measured by adiabatic calorimetry between 4.6 and 359.4 K. After correcting the C _p ⁰ data to values for ideal glaucophane, Na₂Mg₃Al₂Si₈O₂₂(OH)₂ the third-law entropy S ₂₉₈ ⁰ -S ₀ ⁰ was calculated to be 541.2±3.0 J·mol^-1·K^-1. Our value for S ₂₉₈ ⁰ -S ₀ ⁰ is 12.0 J·mol^-1·K^-1 (2.2%) smaller than the value of Likhoydov et al. (1982), 553.2±3.0, is within 6.2 J·mol^-1·K^-1 of the value estimated by Holland (1988), and agrees remarkably well with the value calculated by Gillet et al. (1989) from spectroscopic data, 539 J·mol^-1·K^-1.     

Tectonic setting of Western Pacific marginal basins

Global kinematics as well as magnetic anomalies of marginal basins, with continental geology and paleomagnetic data as additional constraints, are used to present a set of reconstructions of the Western Pacific marginal basins between 56 Ma and the Present at key periods (56, 43, 32, 20, 12 and 3 Ma). Our model accounts for the rapid motion of “exotic terranes” along the whole of the Western Pacific convergent zone.

Marginal basins appear to open in a great variety of tectonic settings, the two extreme examples being the Mariana trough where trench suction may be the predominant driving force and the South China Sea where intracontinental deformation appears to be the major mechanism. The study of marginal basins is a 3-D problem which must take into account the whole tectonic context (subduction related tectonics in cross-section and upper plate deformation in map view) and not only a 2-D problem (the classical trench-arc-back-arc problem).     

L'événement intraviséen dans la zone moldanubienne de la chaîne varisque d'Europe: les données des formations volcano-sédimentaires dévono-dinantiennes du Massif Central Français,des Vosges du Sud (France) et de la Forêt Noire (R.F.A.)

Resumé L'existence d'une phase tectonique intraviséenne dans la chaîne varisque d'Europe a été admise par divers auteurs. Le présent travail apporte des précisions sur la nature de cet événement dans le Nord du Massif Central, les Vosges et la Forêt Noire, c'est à dire dans la zone interne varisque. Des données nouvelles concernent les Vosges et la Forêt Noire. Dans le domaine étudié, cet événement géodynamique n'est pas phcatif et se caractérise par une période d'instabilité au cours du Viséen supérieur qui conduit à d'importants phénomènes de resédimentation, résultats de l'érosion de paléoreliefs. Cette période d'instabilité se prolonge au cours du Viséen supérieur où une emersion généralisée se produit dans la zone moldanubienne. Dans les zones externes septentrionales, la déformation synmétamorphique qui se déplace vers l'extérieur de la chaîne est contemporaine de cet événement. Dans le massif armoricain (zone externe), il se traduit par des mouvements décrochants. Cet événement s'explique par un épaississement crustal, conséquence directe de la collision varisque et se traduit par un soulèvement qui précède la phase de déformation paroxysmale d'âge tardi-viséen et carbonifère supérieur.

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The existence of an Intravisean tectonic phase in the variscan belt of Europe is admitted by some authors. The present work specifies the characteristics of such an event in the northern part of the French Massif Central, in the Vosges mountains and in the Black Forest, i.e. in the internal variscan zone. New data are given for the Vosges and the Black Forest. In the considered domain the geodynamic event does not have a folding effect. It leads to a period of instability during Late Visean, inducing important resedimentation processes as a result of erosion of the paleoreliefs. A generalized emersion of the Moldanubian Zone occurs at the same time. This event coincides with the migration toward the North of the syn-metamorphic deformation in the extern parts of the belt. In the Armorican Massif (external zone) strike slip faults are active during the same period. That event can be explained by the crustal thickening resulting from the variscan collision. The uplift preceds the paroxysmal Late Visean and Upper Carboniferous tectonic phase.

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Zusammenfassung Verschiedene Autoren vertreten die Meinung, daß das mitteleuropäische Variszikum innerhalb des Visé von einer tektonischen Phase betroffen wurde. Die gegenwärtige Arbeit soll eine genauere Vorstellung von den Ereignissen geben, welche sich um diese Zeit im nördlichen Zentralmassiv, den Vogesen und dem Schwarzwald abspielten. Innerhalb des angeführten Bereiches läßt sich nirgends eine Faltung beobachten. Es handelt sich vielmehr um ein geodynarmsches Ereignis, welches sich im obereren Visé als eine Periode der Instabilität mit sehr beachtlichen Erosionen einerseits und ebenso bedeutenden Resedimentationen andererseits äußert. Diese Zeit der Instabilität mündet noch im Laufe des obersten Visé in einer allgemeinen Heraushebung des Moldanubikums. Das Intra-Visé Ereignis ist zeitgleich mit der nordwärtigen Wanderung der synmetamorphen Deformation in den externen Zonen. Im armorikanischen Massiv fällt es anscheinend mit bedeutenden Blattverschiebungen zusammen. Offensichtlich steht hinter den Vorgängen als Ursache eine relativ plötzliche Krustenverdickung im Zusammenhang mit der variszischen Kollision. Die Heraushebung geht der paroxysmalen Deformation im späten Visé bzw. frühen Oberkarbon voraus.

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Structure pattern analysis of the Noctis Labyrinthus-Valles Marineris regions of Mars

The Coprates and Phoenicis Lacus quadrangles of Mars contain the Valles Marineris, Noctis Labyrinthus, and Claritas Fossae areas, each of which shows distinctive structural patterns. Analyses of the structural trends seen within these quadrangles show four principal trend directions. The chronological relationships among these trends and their relation to the stratigraphy has been determined. It appears that the two oldest trends (essentially WSW/ENE and NNE/ SSW), on the basis of transection relations, are best defined in what have been mapped on stratigraphic criteria as older Martian terrains (troughed and furrowed and cratered terrains). Younger trends (WNW/ESE and N/S), also on the basis of transection relations, appear to be related to opening and widening of the canyon. These are present only in the younger stratigraphic units. A comparison between the structural pattern of the Valles Marineris region and that of the Eastern African Rift system at the same scale reveals regional similarities. These suggest that a common major process, lateral extensions in the crust, was involved in the formation of both features.     

Detection of underground cavities with monofrequency electromagnetic tomography between boreholes in the frequency range 100 MHz to 1 GHz1

In order to detect underground cavities, we have designed a geophysical method based on the interaction of a monochromatic electromagnetic wave in the frequency band 100 MHz to 1 GHz with the ground situated between two boreholes. Three different approaches are involved in the design of this EM tomographic method.

• 1 The application of an ‘exact’ theory is used to calculate artificial data, based on an integral equation method. These data are inverted using a tomographic algorithm whose basic assumptions are rather coarse. The results show that, however, cavities can very well be detected and their position recovered.
• 2 Data are obtained with a physical reduced-scale model in a water tank, in which all parameters are known in advance. The inversion process confirms the validity of the method.
• 3 Underground cavities are actually detected.

The above approaches are described and discussed and results are shown. The equipment involved and its operation is quite simple: the surface devices are commercially available and only the transmitter and receiver antennae must be specially built. The method is shown to be quite efficient and its cost should be reasonably low.     

VIRGO: Experiment for helioseismology and solar irradiance monitoring

The scientific objective of the VIRGO experiment (Variability of solar IRradiance and Gravity Oscillations) is to determine the characteristics of pressure and internal gravity oscillations by observing irradiance and radiance variations, to measure the solar total and spectral irradiance and to quantify their variability over periods of days to the duration of the mission. With these data helioseismological methods can be used to probe the solar interior. Certain characteristics of convection and its interaction with magnetic fields, related to, for example, activity, will be studied from the results of the irradiance monitoring and from the comparison of amplitudes and phases of the oscillations as manifest in brightness from VIRGO, in velocity from GOLF, and in both velocity and continuum intensity from SOI/MDI. The VIRGO experiment contains two different active-cavity radiometers for monitoring the solar constant, two three-channel sunphotometers (SPM) for the measurement of the spectral irradiance at 402, 500 and 862 nm, and a low-resolution imager (LOI) with 12 pixels, for the measurement of the radiance distribution over the solar disk at 500 um. In this paper the scientific objectives of VIRGO are presented, the instruments and the data acquisition and control system are described in detail, and their measured performance is given.died 13 October 1994     

Optical properties of silicates in the far ultraviolet

Near-normal incidence reflectance measurements in the interval 1026–1640 Å were performed on four silicates already studied in the visible and infrared by Pollack et al. (1973). We use a Kramers-Kronig analysis of these data to calculate the complex index of refraction m = n ? ik. New transmission measurements improve the determination of k in the interval 2500–4500 Å, except for andesite, which is more opaque than found by Pollack et al.