Nouvelles données pétrographiques et structurales sur le magmatisme du Sud du bassin de Morondava (Sud-Ouest de Madagascar)

New petrological and geochemical data are presented on basic igneous rocks from the south of the Morondova basin located in the southwestern part of Madagascar. The structural setting is well defined and, with the aid of a schematic map, the spatial relationships between the magmatic centres and major faults can be deduced from seismic data. On the other hand, new petrographic and geochemical data enable the different types of basaltic magmatism manifested in this area to be distinguished. It appears that tholeiitic magmatism evolved towards transitional magmatism from the south to the north of the Morondava basin. This geochemical evolution of basaltic magmatism can be linked to the initiation of rifting between Madagascar and Africa along the Mozambique channel and the length of the Davie ridge associated with the opening of the Indian ocean. These observations, together with structural data and recently obtained geochronological data, contribute to a better understanding of the mechanism and evolution of Madagascan rifting.     

U-series disequilibrium in arc magmas induced by water-magma interaction

Volcanic rocks from subduction zones are widely believed to originate by partial melting of mantle lherzolite modified by the addition of a fluid or melt extracted from the down-going slab. U-series disequilibrium in such magmas is commonly attributed to this particular melting process. A detailed study of U-series isotopes in the 650 y. B.P. eruptive sequence of Mt. Pelée (Martinique) shows that plinian products are in radioactive equilibrium, whereas dome-forming products of the same eruption are characterized by ²³⁸U-²³⁰Th disequilibrium. The same features apply to other plinian and dome-forming products of this volcano and systematically correspond to different eruptive styles. We attribute these characteristics to variable superficial interaction of magmas with the hydrothermal system during the final stages of eruption rather than to deep magma genesis processes. This conclusion might be generally applicable to arc magmas.     

Reappraisal of great historical earthquakes in the northern Chile and southern Peru seismic gaps

A critical reappraisal of great historical interplate earthquakes in the occidental margin of South America, including southern Peru and northern Chile, is carried out.A spacetime distribution of the earthquakes associated to the seismotectonics regions defined by the rupture zones of the greatest events (1868, Mw = 8.8 and 1877, Mw = 8.8) is obtained. Both regions are seismic gaps that are in the maturity state of their respective earthquake cycles. The region associated to the 1868 earthquake presents a notable seismic quiescence in the present century.     

On the product of legender functions as encountered in geodynamics

Résumé On rencontre souvent des produits de fonctions de Legendre en géodésie mathématique, en mécanique céleste et quantique, et en géophysique. Nous donnons les expressions générales des coefficients de la décomposition d'un produit quelconque sur la base des fonctions de Legendre ellesmêmes. Quelques applications immédiates sont envisagées.     

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.