Evolution cristallochimique des amphiboles dans la série granitique de Fort-Trinquet (Mauritanie)

The precambrian postorogenic pluton of the Fort-Trinquet area (Northern Mauretania) is composed by a series of granitic rocks in which amphiboles are the characteristic mafic minerals. Twenty six amphiboles have been separated and chemically analysed; the optical properties and the unit-cell data are also given.The crystallographic and chemical differences between these minerals reflect the variations in bulk composition of the host-rocks. In the plutonic suite, two igneous trends have been recognized: 1) a granitic trend (quartz monzonite-adamellite-granite) and 2) a syenitic trend (syenite-quartz bearing syenite-alkali granite). The amphiboles of the first trend belong to the tremolite-hastingsite series; they become richer in hastingsite mole with increasing proportion of quartz and decreasing Mg/(Mg+Fe) ratio in the rocks. The granitoids of the second trend are characterized by the occurrence of two amphiboles: a primary prismatic green-coloured actinolitic hornblende generally surrounded by a dark blue rim of riebeckite composition; the riebeckite may also form some acicular crystals associated to needles of stilpnomelane. These coexisting amphiboles would result from autometasomatic reactions which affected the rocks of the syenitic trend and which gave rise to the late alkali pegmatites where the constituent is a low arfvedsonitic riebeckite. The major substitutions involved in that amphibole transformation are Na^x R³⁺Ca^x R²⁺ and Na^xSiCa^xAl^IV.Comparison with experimental data allows to estimate the physical conditions during the emplacement and the tardimagmatic evolution of this granitoid series.     

Considerations generales sur la distribution de l'uranium dans les roches granitoides

Using definite examples the authors show that it is possible to find, in a given granite, relations between uranium and the other elements, but these relations cannot be generalized. They cannot depend on the petrographical and mineralogical properties rather than on the chemical composition. One finds with the essential minerals some apparent relations but they are valid only for a given rock. The nature of a granite depends, mainly, on its formation and consequently on the original material from which it derived. The present data on radioactivity of granites cannot be generalized.     

Circumgranular bassanite in a gypsum crust from eastern Algeria – a potential palaeosurface indicator

Detrital sand grains are surrounded by thin bassanite coatings in the upper part of a coarse-crystalline gypsum crust from the Algerian Sahara. The bassanite developed by topotactic replacement of the surrounding gypsum in the absence of a liquid phase. Heating experiments using a gypsum crystal with sand inclusions produced similar patterns and textures. In one experiment, bassanite developed around quartz and carbonate grains but not along the sides of the heated gypsum crystal. This is the result of differences in heat capacity between gypsum, quartz and calcite. Bassanite formation in the crust from Algeria was not controlled by differences in thermal properties. Instead, the bassanite apparently formed under conditions of thermal equilibrium. The occurrence of bassanite as circumgranular coatings in the crust is interpreted as being related to the availability of submicroscopic space along the contact between the gypsum cement and the enclosed sand grains. The presence of coatings of this type, or derived relict features, is a potential criterion for the recognition of palaeosurfaces.     

Influence of Mediterranean sea-level changes on the Dacic Basin (Eastern Paratethys) during the late Neogene: the Mediterranean Lago Mare facies deciphered

A recently published scenario viewing the Messinian salinity crisis as two evaporitic steps rather than one has led to a search for new indices of the crisis in the Eastern Paratethys. Fluvial processes characterized the southwestern Dacic Basin (Southern Romania, i.e. the Carpathian foredeep) whereas brackish sediments were continuously deposited in its northern part. This is consistent with previously evidenced responses of the Black Sea to the Messinian salinity crisis. High sea‐level exchanges between the Mediterranean Sea and Eastern Paratethys are considered to have occurred just before and just after desiccation of the Mediterranean. This accounts for two successive Mediterranean nannoplankton‐dinocyst influxes into the Eastern Paratethys that, respectively, belong to zones NN 11 and NN 12. Meanwhile, two separate events that gave rise to Lago Mare facies (with Paratethyan Congeria, ostracods and/or dinoflagellate cysts) arose in the Mediterranean Basin in response to these high sea‐level exchanges and located 5.52 and 5.33 Ma (isotopic stages TG 11 and TG 5, respectively), i.e. just before and just after the almost complete desiccation of the Mediterranean). These Lago Mare facies formed independently of lakes with ostracods of the Cyprideis group that developed in the central basins during the final stages of desiccation. The gateway faciliting these water exchanges is not completely identified. A proto‐Bosphorus strait seems unlikely. A plausible alternative route extends from the northern part of the Thessaloniki region up to the Dacic Basin and through Macedonia and the Sofia Basin. The expression ‘Lago Mare’ is chronostratigraphically ambiguous and should be discontinued for this purpose, although it might remain useful as a palaeoenvironmental term.     

Shear Wave Anisotropy of the Upper Mantle Beneath the Region from Tingri of Tibet to Golmud of Qinghai

Measurements of shear wave splitting at 43 three-component seismic stationsshow very big difference in anisotropy on both sides of the Indus-Yarlung Zangbo suture(ITS), but little difference on both sides of the older Bangong-Nujiang suture (BNS) and theJinsha River suture (JS) to its north. Obvious discrepancy exists between the anisotropy direc-tion and the superficial tectonic trends, which is not explicable directly by the coherent uppermantle deformation usually supposed to occur in consistency with the trend of a collisional belt.On the other hand, strong spatial relationships are observed from the anisotropy results, such asthe orthogonal directions of anisotropy on both sides of ITS and the good correlation betweenthe region of larger magnitude of anisotropy and the zone of inefficient Sn propagation ofQiangtang as well as the systematic rotation of the directions of anisotropy, which should testifysome much more complicated aspects of the continental convergence mechanism. To the best ofour data, we tend to suppose that the Qinghai-Tibet plateau might result from a mechanismcomplicated by the coexistence of Argand's underthrusting and Dewey's diffuse deformation.     

'Race' as a scientific and organizational construct: a critique

'Race' for many years has been a major construct of science and society. While its importance as such has not historically been particularly pronounced on a global scale, the emergence of its most forceful architects, the Anglophone countries, to pre-eminence since World War II has significantly extended its geographical range and added to its significance as an idea within commercialized culture as well as within social organization.In the present paper, 'race' is critically examined from the following angles: 1) its role in the behavioral and medical sciences; b) its historical origins and manifestations within the Anglophone countries, particularly the United States; and c) its emergence as a 'liberal' concept and operating principle since World War II. Questions of why and how 'race' arose and its continued use in science, society and culture drive both the trajectory and depth of this research. 'Race' is found to be a modern construct which arose as a consequence of colonialism and slavery, and was substantially constructed in its present form and substance by England and its off-spring societies, particularly the United States. 'Race' was not used as a term expressing a social idea until modern times, and had no basis in the primordial civilizations which greatly influenced modern Western societies (e.g., ancient Greece and Rome). Efforts undertaken by liberals – particularly in the United States – to 'humanize' the concept of 'race' since the 1960's have been largely unsuccessful. 'race' is viewed as inherently hierarchical, a fact which is evident from its historical and present role in science and society.     

Dynamic-stiffness matrix of soil by the boundary-element method: Conceptual aspects

Starting from a weighted-residual formulation, the various boundary-element methods, i.e. the weighted-residual technique, the indirect boundary-element method and the direct boundary-element method, are systematically developed for the calculation of the dynamic-stiffness matrix of an embedded foundation. In all three methods, loads whose analytical response in the unbounded domain can be determined are introduced acting on the continuous soil towards the region to be excavated. In the weighted-residual technique and in the indirect boundary-element method, a weighting function is used; in the latter case, it is selected as the Green's function for the surface traction. In the direct boundary-element method, the surface traction along the structure-soil interface is interpolated. The same type of boundary matrices which have a clear physical interpretation are identified in the three formulations, each of which is illustrated with a simple static example. The indirect boundary-element method leads to the most accurate results. The guaranteed symmetry and the fact that the displacement arising from the applied loads can easily be calculated and compared to the prescribed displacement makes the indirect boundary-element method especially attractive for calculating the dynamic-stiffness matrix of the soil. Instead of calculating the dynamic-stiffness matrix of the embedded foundation with the boundary-element method, it can be determined as the difference of those of the regular free field and of the excavated part. The calculation of the former does not require the Green's function for the surface traction. The dynamic stiffness of the excavated part can be calculated by the finite-element method.     

Dynamic-stiffness matrix of soil by the boundary-element method: Embedded foundation

Green's influence functions are derived for a linearly distributed load acting on part of a layered elastic halfplane on a line which is inclined to the horizontal. Using these Green's functions as fundamental solutions in the boundary-element method, the dynamic-stiffness matrices of the unbounded soil with excavation, of the excavated part and of the free field are calculated. The indirect boundary-element method using distributed loads and no offset leads to more accurate results than the weighted-residual technique and the direct boundary-element method. At the natural frequencies of the undamped excavated part built-in along the structure-soil interface, the spring coefficients associated with the dynamic-stiffness matrices of the excavated part and of the free field will become infinite. If the dynamic-stiffness matrix of the soil with excavation is calculated as the difference of that of the free field and that of the excavated part, the difference of two large numbers will arise in the vicinity of these frequencies. A consistent discretization must then be used. In particular, the dynamic-stiffness matrix of the embedded part cannot be determined by the finite-element method in this case. A parametric study is performed for the dynamic-stiffness matrix of the free field for a rectangular foundation embedded in a halfplane and in a layer built-in at its base; the aspect ratio and the damping of the soil are varied.     

The relative deformation gradient and its application to cylindrical folds with deformed lineations

The relative deformation gradient is defined and used to compare deformations between any two points of a body. This concept is useful in the geometrical analysis of deformation in natural folds, where only deformed planes and straight lines can be observed. The case of cylindrical folds with deformed lineations is studied.     

The characteristics and dynamics of tar pollution on the beaches of Trinidad and Tobago

Trinidad and Tobago beaches were periodically sampled for tar between July 1980 and September 1981. This project was carried out by the Institute of Marine Affairs in cooperation with the United Nations. The Caribbean coasts of both islands were almost pristine, whereas pollution on the Atlantic coast beaches was comparable to that of other coasts along major tanker routes. The source of high tar concentrations on the Atlantic coasts may be attributed to the residues from tanker bilge cleanings, which are carried along the South Equatorial Current, eventually to strand on the islands' beaches. In the dry season (January to April) north-westerly currents and north-easterly winds prevail and more tar strands on Trinidad than on Tobago. In the wet season (June to November), however, when currents are more northerly and south-easterly wind prevail, the reverse situation holds and more tar strands on Tobago.