La theorie des deux magmas fondamentaux dans le volcanisme intracontinental et l'evolution geochimique des laves du Mont-Dore (France)

Résumé Les auteurs utilisent toutes les données chimiques, volcanologiques, chronologiques et tectonophysiques fournies pour un certain nombre de massif continentaux (Mont-Dore et autres). La synthèse de toutes ces études montre qu'il existe deux magmas continentaux indépendants: le magma basaltique venant du manteau (sima) et le magma rhyolitique fourni par la croûte supérieure (sial). Le mélange de ces deux magmas et l'action secondaire de la' gravité et de la pneumatolyse expliquent logiquement la totalité des phénomènes géochimiques des volcans continentaux.

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The authors used all the chemical, volcanological, chronological and tectonophysical data supplied for a certain number of continental mountain massifs (Mont-Dore and others). The synthesis of these studies shows that there are two independent continental magmas: the basaltic magma coming from the mantle (sima) and the rhyolitic magma supplied by the upper crust (sial). The mixing of these two magmas, with the secondary action of gravity and pneumatolysis logically explains the totality of the geochemical phenomena in continental volcanoes.

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Zusammenfassung Die Autoren benutzten alle chemischen, vulkanologischen, chronologischen und tektonisch-physikalischen Daten, die von einer bestimmten Anzahl von kontinentalen Massiven (Mont-Dore und andere) entnommen wurden.Das Ergebnis dieser Studien zeigt, daß es zwei voneinander unabhängige kontinentale Magmen gibt.Das basaltische Magma, vom Sima stammend, und das rhyolitische Magma aus der oberen Sial-Kruste. Die Vermischung dieser beiden Magmen und die zusätzliche Wirkung der Gravitation und Pneumatolyse erklären auf natürliche Weise die Gesamtheit der geochemischen Phänomene des kontinentalen Vulkanismus.

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Age-dependent Large-scale Fabric of the Mantle Lithosphere as Derived from Surface-wave Velocity Anisotropy

—Systematic variations of the seismic radial anisotropy ξ to depths of 200–250 km in North America and Eurasia and their surroundings are related to the age of continental provinces, and typical depth dependences of ξ _R are determined. The relative radial anisotropy ξ _R in the mantle lithosphere of Phanerozoic orogenic belts is characterized by ν _SH > ν _SV , with its maximum depth of about 70 km, on the average, while beneath old shields and platforms, it exhibits a maximum deviation from ACY400 model (Montagner and Anderson, 1989) at depths of about 100 km with ν _SV ≥ν _SH signature. An interpretation of the observed seismic anisotropy by the preferred orientation of olivine crystals results in a model of the mantle lithosphere characterized by anisotropic structures plunging steeply beneath old shields and platforms, compared to less inclined anisotropies beneath Phanerozoic regions. This observation supports the idea derived from petrological and geochemical observations that a mode of continental lithosphere generation may have changed throughout earth's history.     

Weak Contrast PP Wave Displacement R/T Coefficients in Weakly Anisotropic Elastic Media

—Approximate PP plane wave displacement coefficients of reflection and transmission for weak contrast interfaces separating weakly but arbitrarily anisotropic elastic media are presented. The PP reflection coefficient for such an interface has been derived recently by Vavry?uk and P?en?ík (1997). The PP transmission coefficient presented in this paper was derived by the same approach. The coefficients are given as a sum of the coefficient for the weak contrast interface separating two nearby isotropic media and a term depending linearly on contrasts of the so-called weak anisotropy (WA) parameters (parameters specifying deviation of properties of the medium from isotropy), across the interface. While the reflection coefficient depends only on 8 of the complete set of the WA parameters describing P-wave phase velocity in weakly anisotropic media, the transmission coefficient depends on their complete set. The PP reflection coefficient depends on "shear-wave splitting parameter" γ. Tests of accuracy of the approximate formulae are presented on several models.     

Palaeogeographical implications of CretaceousBenthic Foraminifera recovered by the Deep Sea Drilling project in the Western South Atlantic ocean

Albian/Cenomanian benthic foraminiferal faunas recovered by the DSDP in the western South Atlantic Ocean (Leg 36) are described and analyzed from the palaeogeographic and palaeo-environmental points of view. In doing this the author compares Leg 36 assemblages in the western South Atlantic Ocean with coeval benthic foraminiferal faunas recovered in the eastern South Atlantic Ocean (Leg 40) and in the eastern Indian Ocean (Legs 26 and 27). The specific composition of these assemblages, except for Leg 27, is virtually the same. Consequently, they are considered to indicate the same depositional water depth at all relevant sites studied, whether located in the Angola Basin, the northern flank of the Walvis Ridge, the eastern margin of the Falkland Plateau or on the Naturaliste Plateau. All the assemblages indicate shallow environments around 100 m and not exceeding 300–400 m in the deepest parts, corresponding to the inner shelf and the inner part of the outer shelf. By contrast the foraminiferal associations of Leg 27 (especially Site 259) indicate a greater depth, of the order of 200–600 m (but not exceeding 1000 m) corresponding to upper slope of Sliter & Baker (1972) and Sliter (1972). These bathymetrical conclusions are in remarkable accord with those of Sliter (1976), based on planktic Foramini fera of Leg 36.Late Cretaceous (Campanian-Maastrichtian) material with benthic Foraminifera was limited to two positive samples; however, these faunas indicate much the same palaeo-environment as do the planktic ones analyzed by Sliter (1976).