Effet de la température sur les distributions de Na,Rb et Cs entre la sanidine,la muscovite,la phlogopite et une solution hydrothermale sous une pression de 1 kbar

The distribution of trace amounts of Na, Rb and Cs, between muscovite, phlogopite, sanidine and hydrothermal solution have been studied by ion exchange in a temperature range from 400 to 800°C.These distributions have been expressed with a partition ratio P^aq?m_x = $\text{(}\text{X}\text{K}\text{)}{}^{\mathrm{aq}}\text{(}\text{X}\text{K}\text{)}{}^{\mathrm{m}}$ (where X is Na, Rb or Cs).In the case of Na and Cs in muscovite, even for the dilute solutions, the ratio P^aq?m_x is not the equilibrium constant k_x of exchange reactions. In other cases, P^aq?m_x does not depend on the trace alkali ion concentration in silicates (X) and is equal to k_x. Variations of P_x or k_x with T are greater for Na and Cs than for Rb. Generally, k_x decreases with increase in T. The function log $\text{P}{}_{\mathrm{x}}\text{= f(}\text{1}\text{T}\text{)}$ is not linear for Na or Cs, but in the case of Rb, $\text{f(}\text{1}\text{T}\text{)}$ is linear and the standard enthalpy and entropy of exchange reactions have been estimated by applying the Arrhenius relation.The distribution relations obtained between silicate and vapour phase permit the determination of distributions of Na, Rb and Cs between two minerals mI and mII, relative to K. These have been expressed with the partition ratio Q_x =$\text{(}\text{X}\text{K}\text{)}{}^{\mathrm{mI}}\text{(}\text{X}\text{K}\text{)}{}^{\mathrm{mII}}$. Variations of Q_x with T are not remarkable, and even for Rb between phlogopite and feldspar are negligible. Nevertheless, one may use the distributions of Rb and Cs between muscovite and feldspar for geothermometry. Experimental results have been applied to some rocks by effecting corrections from the major element composition of the natural minerals. Estimated temperatures are near to 400°C in the granites and pegmatite studied here.     

Geodynamic aspects of the basic intrusions of central and Western Nepal

A petrological study of the basic rocks of magmatic activity in the Pokhara and Beri Khola regions of Central and Western Nepal suggests that the volcanic activities were not related to collision or continental subduction.     

Detailed debris flow hazard assessment in Andorra: A multidisciplinary approach

In many mountainous areas, the rapid development of urbanisation and the limited space in the valley floors have created a need to construct buildings in zones potentially exposed to debris flow hazard. In these zones, a detailed and coherent hazard assessment is necessary to provide an adequate urban planning. This article presents a multidisciplinary procedure to evaluate the debris flow hazard at a local scale. Our four-step approach was successfully applied to five torrent catchments in the Principality of Andorra, located in the Pyrenees. The first step consisted of a comprehensive geomorphologic and geologic analysis providing an inventory map of the past debris flows, a magnitude–frequency relationship, and a geomorphologic–geologic map. These data were necessary to determine the potential initiation zones and volumes of future debris flows for each catchment. A susceptibility map and different scenarios were the principal outcome of the first step, as well as essential input data for the second step, the runout analysis. A one-dimensional numerical code was applied to analyse the scenarios previously defined. First, the critical channel sections in the fan area were evaluated, then the maximum runout of the debris flows on the fan was studied, and finally simplified intensity maps for each defined scenario were established. The third step of our hazard assessment was the hazard zonation and the compilation of all the results from the two previous steps in a final hazard map. The base of this hazard map was the hazard matrix, which combined the intensity of the debris flow with its probability of occurrence and determined a certain hazard degree. The fourth step referred to the hazard mitigation and included some recommendations for hazard reduction. In Andorra, this four-step approach is actually being applied to assess the debris flow hazard. The final hazard maps, at 1 : 2000 scale, provide an obligatory tool for local land use planning. Experience achieved during the study showed that the collaboration between geologists, geomorphologists, engineers, and decision makers is essential and that only a multidisciplinary approach allows for solving all the problems of such a complex process as debris flows. Finally, we propose that our approach may be applied to other mountainous areas, adapting the hazard matrix to new local conditions.     

Genesis of the Dordogne (France) high-purity silica placers

This article describes the formation of the principal French high-purity silica placer (source for electrometallurgy of silicon and ferrosilicon), the Boudeau deposit in Dordogne, and of one of its subordinate deposits, the Pays Brûlé deposit. Boudeau has always been considered to be a colluvial deposit, formed in a trap produced by extensional tectonism. The present work suggests a new genetic interpretation, based on a combined study of the deposits and their underlying rocks. It shows that the quartz gravel deposits, laid down by stream flow within braided channels, were reworked by karstic progressive sinking, and that the mechanism responsible for the increase in the thickness of the economic formations is cryptokarstic corrosion. This undercovers weathering process, which involves dissolution of karstifiable rocks (oolitic and bioclastic limestones) at their interface with a permeable, non-karstifiable cover composed of chemically pure quartz gravels in a sandy-argillaceous matrix. A typological study of the gravels provides a basis for tackling the problem of their origin and their purity. They seem to be derived mainly from the synmetamorphic quartz mobilisates of the stripped metamorphic units (the lower and upper gneiss units forming the western edge of the Massif Central). Lastly, a dynamic model is suggested for the genesis of the high-purity silica deposits of Dordogne.     

Organic petrology and geochemistry of the Carboniferous coal seams from the Central Asturian Coal Basin (NW Spain)

This paper presents for the first time a petrological and geochemical study of coals from the Central Asturian Coal Basin (North Spain) of Carboniferous (Pennsylvanian), mainly of Moscovian, age. A paleoenvironmental approach was used, taking into account both petrographic and organic geochemical studies. Vitrinite reflectance (R_r) ranges from 0.5% to 2.5%, which indicates a high volatile bituminous to semianthracite and anthracite coal rank. The coal samples selected for paleoenvironmental reconstruction are located inside the oil–gas-prone phase, corresponding to the interval between the onset of oil generation and first gas generation and efficient expulsion of oil. This phase is represented by coals that have retained their hydrocarbon potential and also preserved biomarker information. Paleodepositional reconstruction based on maceral and petrographic indices points to a swamp environment with vitrinite-rich coal facies and variable mineral matter content. The gelification index (GI) and groundwater influence index (GWI) indicate strong gelification and wet conditions. The biomarkers exhibit a high pristane/phytane ratio, suggesting an increase in this ratio from diagenetic processes, and a high diterpanes ratio. This, in turn, would seem to indicate a high swamp water table and a humid climate. The maximum point of coal accumulation occurred during the regressive part of the Late Moscovian sequence and in the most humid climate described for this period of time in the well-known coal basins of Europe and North America.     

Contrôle structural du partage de Rb entre fluide et micas potassiques dans les pegmatites du Brésil

A geochemical study of pegmatitic micas from Minas Gerais State in Brazil was performed with an electron microprobe, in order to examine the variations of Rb, K, Al and F contents. It is observed a linear decreasing relationship between the [Rb/K] ratio of the micas and their contents in Al^VI. The interpretation is based on the hypothesis that the partition coefficient C_Rb/K^mica/fl between fluid and mineral does not vary significantly as a function of temperature and pressure in the narrow conditions of crystallization of pegmatites. It is suggested that the relation: C_Rb/K^mica/fl=0.55×(5?[Al^VI]) is of crystallochemical order. Micas with low contents in Al take higher contents in Rb because the potassic sites where Rb enters are larger. This relation gives another way of calculating the values [Rb/K] of the fluids knowing Rb, K, Al and Si in the micas. This crystallochemistry also allows us to foresee a direct correlation between Rb and F in the pegmatitic micas. To cite this article: J. Quéméneur et al., C. R. Geoscience 336 (2004).     

Dinosaur remains from the type Maastrichtian: an update

Isolated cranial and post-cranial remains of hadrosaurid dinosaurs have been collected from various outcrops in the type area of the Maastrichtian stage during the last few years. In the present contribution, dentary and maxillary teeth are recorded from the area for the first time. Post-cranial elements comprise a newly collected, fragmentary, large right metatarsal III and a broken ?right humerus, recently recognised in the collections of Teylers Museum (Haarlem). Unfortunately, none of these remains can be identified to species level. The available material suggests, however, that more than one taxon of non-lambeosaurine hadrosaurid and a possible euhadrosaurian are represented. Most of the new finds are stratigraphically well documented, which means that they may be linked to the recently published sequence-stratigraphic interpretation of the type Maastrichtian. Dinosaur remains recorded previously from the Maastrichtian type area are tabulated.

     

Rythme et Modalités des Plissements Superposés dans les Chaînes Subalpines Méridionales des Alpes Occidentales Françaises

Résumé Les chaînes subalpines des Alpes françaises offrent un bon exemple de plis de couverture décollés au niveau du Keuper à évaporites. Leur partie méridionale, au S du Vercors et au N et NE des Chaînes Provençales, montre en outre l'entrecroisement de deux systèmes plissés presque orthogonaux: un système E-W, dit pyrénéoprovençal, dont la genèse s'étale du Crétacé supérieur à l'Eocène moyen, et un système N-S à NW-SE, dit alpin, qui date de l'Oligocène et du Néogène. Il y a donc eu deux étapes majeures de plissements qui se sont succédées dans le temps; mais on doit se demander si chacune de ces étapes était faite d'une succession de phases tectoniques courtes séparées par des intervalles relativement plus longs de calme tectonique, ou bien si elles correspondent à de longues périodes de déformations continues; ces deux postulats paraissent ici également vraisemblables. La première période, celle des plis pyrénéens, semble avoir fait naître des plis droits assez simples, parfois de style éjectif, dont les anticlinaux ont souvent, dès l'Eocène, été érodés jusqu'au Jurassique supérieur. A l'Oligocène et au Miocène, des plis N-S à NW-SE sont nés, dans les domaines qui n'étaient pas déjà plissés, tandis que les plis E-W déjà existants ont été accentués et rompus, dégénérant parfois en chevauchements caractérisés par des troncatures des flancs inverses des plis. A l'échelle des Alpes Occidentales comme à celle des Chaînes Subalpines Méridionales et des « arcs » élémentaires qui les composent, les structures plissées disposées en arc ne sont pas simples, mais sont l'aboutissement d'une évolution tectonique longue et complexe.

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The Subalpine Chains of the French Western Alps give a good example of cover folds, which are detached along evaporite-bearing upper-Triassic beds. Their southern half shows also the intersection of two successive folded systems: E-W pyrenean folds (upper Cretaceous-middle Eocene), followed by N-S to NW-SE alpine folds (Oligocene—Miocene). It is therefore possible to distinguish two successive stages of tectonic deformation, but the problem remains to decide whether these stages can be divided into a certain number of short tectonic phases, or are long and continuous periods of tectonic deformation. It is shown that these two answers to the problem are two postulates, which seem to be equally probable. The first pyrenean stage of deformation gave rise to relatively simple folds, often of the ejective style type. The second, alpine, stage, occurred after and during subaerial erosion. In the areas where earlier E-W folds did not occur, new alpine N-S to NW-SE folds rose, whereas in the areas where earlier E-W folds were present, refolding of these folds only occurred, often accompanied by breaks, i. e. wrench faults and overthrusts, the latter often showing subhorizontal truncation of nearly vertical beds. In the same manner as for the entire Western Alps arc, the fold arcs in the southern subalpine chains are not simple virgations, but the result of a long and complex tectonic evolution.

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Zusammenfassung Die subalpinen Ketten der französischen Westalpen sind ein gutes Beispiel von abgescherten Falten einer sedimentären Serie. Ihr südlicher Teil zeigt die Kreuzung von zwei Faltensystemen: Pyrenäische E-W-Falten (Obere Kreide -Mittleres Eozän) und alpine N-S-oder NW-SE-Falten (Oligozän-Miozän). Es bleibt das Problem, ob diese zwei Faltungsperioden in verschiedene tektonische Phasen gegliedert werden müssen oder nicht. Beide Hypothesen scheinen gleich wahrscheinlich zu sein. Während der ersten pyrenäischen Faltungsperiode entstehen nur einfache ejektive Falten. Die zweite, alpine Periode verlief während und nach kontinentaler Erosion; die ersten E-W-Falten wurden überkippt und zerbrachen, so daß Überschiebungen entstanden. Für die Westalpen sowie für die einzelnen Ketten der subalpinen Ketten sind Faltenbögen nicht einfache Virgationen, sondern verwickelte Strukturen, deren Geschichte lang und verschieden war.

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