Les minéralisations Cu(NiBiUAuAg) d'Ifri (district du Haut Seksaoua,Maroc) : apport de l'étude texturale au débat syngenèse versus épigenèse

The Cu ore of Ifri is a chalcopyrite stockwork hosted by Cambrian formations and was until now interpreted as a syngenetic massive sulphide deposit. Textural studies highlight two generations of pyrite early (Py I) and late (Py II) with respect to the regional deformation. The chalcopyrite stockwork overprinted Py II, outlining the epigenetic nature of the Cu mineralization. Regarding the origin of Cu-depositing fluids, the presence in the stockwork paragenesis of an U, W, Sn association and preliminary Pb/Pb dating of a brannerite belonging to this association suggest a contribution of the Tichka granite. To cite this article: L. Barbanson et al., C. R. Geoscience 335 (2003).     

Le métamorphisme Tardi-Crétacé à Éocène des zones internes de la chaı̂ne Indo-Birmane (Myanmar occidental) : implications géodynamiquesLate Cretaceous to Eocene metamorphism of the internal zone of the Indo-Burma range (western Myanmar): geodynamic implications

Metamorphic study on Triassic schists in the internal zone of the Indo-Burma range, essentially based on chlorite–mica equilibrium in metapelites, allows a P–T path to be quantified. During the prograde metamorphism, the geothermic gradient evolves from that of a ‘normal’ crust (30 °C km^?1) to that of a thickened crust (18 °C km^?1). The peak conditions are around 8 kbar and 450 °C. This thickening (25–30 km) is probably made in a wedge set up between the Late Cretaceous and the Eocene, in front of the obduction. The obtained cold retrograde path requires a mechanism allowing thermal re-equilibration, implying slow exhumation. It occurred along a shear zone that put into contact the micaschists of the core with the Triassic schists of the roof. To cite this article: A. Socquet et al., C. R. Geoscience 334 (2002) 573–580.     

Some characteristics of atmospheric gravity waves observed by radio-interferometry

Observations of atmospheric acoustic-gravity waves (AGWs) are considered through their effect on the horizontal gradient G of the slant total electron content (slant TEC), which can be directly obtained from two-dimensional radio-interferometric observations of cosmic radio-sources with the Nançay radioheligraph (2.2^°E, 47.3^°N). Azimuths of propagation can be deduced (modulo 180^°). The total database amounts to about 800 h of observations at various elevations, local time and seasons. The main results are:a) AGWs are partially directive, confirming our previous results.b) The propagation azimuths considered globally are widely scattered with a preference towards the south.c) They show a bimodal time distribution with preferential directions towards the SE during daytime and towards the SW during night-time (rather than a clockwise rotation as reported by previous authors).d) The periods are scattered but are larger during night-time than during daytime by about 60%.e) The effects observed with the solar radio-sources are significantly stronger than with other radio-sources (particularly at higher elevations), showing the role of the geometry in line of sight-integrated observations.     

Factors controlling stratal pattern and facies distribution of fluvio‐lacustrine sedimentation in the Sivas mini‐basins,Oligocene (Turkey)

The Sivas Basin, located in the Central Anatolian Plateau of Turkey, is a foreland basin that records a complex interaction between sedimentation, salt tectonics and regional shortening during the Oligo‐Miocene leading to the formation of numerous mini‐basins. The Oligocene sedimentary infill of the mini‐basins consists of a thick continental succession, the Karayün Formation, comprising a vertical succession of three main sub‐environments: (i) playa‐lake, (ii) fluvial braided, and (iii) saline lacustrine. These sub‐environments are seen as forming a large Distributive Fluvial System (DFS) modified through time as a function of sediment supply and accommodation related to regional changes in climate and tectonic regime. Within neighbouring mini‐basins and despite a similar vertical stratigraphic succession, subtle variations in facies assemblages and thickness are observed in stratigraphic units of equivalent age, thus demonstrating the local control exerted by halokinesis. Stratigraphic and stratal patterns reveal in great detail the complex interaction between salt tectonics and sedimentation including different types of halokinetic structures such as hooks, wedges and halokinetic folds. The regional variations of accommodation/sediment supply led to coeval changes in the architectural patterns recorded in the mini‐basins. The type of accommodation regime produces several changes in the sedimentary record: (i) a regime dominated by regional accommodation limits the impact of halokinesis, which is recorded as very small variations in stratigraphic thickness and facies distribution within and between mini‐basins; (ii) a regime dominated by localized salt‐induced accommodation linked to the subsidence of each individual mini‐basin enhances the facies heterogeneity within the DFS, causing sharp changes in stratigraphic thickness and facies assemblages within and between mini‐basins.     

Mixed-magma pyroclastic surge deposits associated with debris avalanche deposits at Colima volcanoes,Mexico

The transition between the terminal cones and the ancestral edifices of Nevado de Colima and Fuego de Colima volcanoes is marked by the deposits of gigantic volcanic debris avalanches of the Mount St. Helens (MSH) or Bezymianny type. Unusual mafic juvenile fragments and cauliflower bombs as well as juvenile fragments of mixed and more evolved composition are abundant in dune-bedded pyroclastic-surge deposits directly associated with these catastrophic events at both volcanoes. At Nevado, these mafic juvenile fragments represent the most primitive magma ever erupted by the volcano (SiO₂52.50%). The lavas directly preceding and following the debris-avalanche event are silicic andesites (SiO₂59%). At Fuego these juvenile fregments have 56% SiO₂. The lavas from the upper parts of the caldera wall are dacites (65% SiO₂), whereas the terminal cone is composed of andesites (57% to 62% SiO₂). At Nevado, petrologic evidence for interaction of mafic magma with andesitic or dacitic magma in a high-level magma chamber, just before the eruption of pyroclastic surge deposits, consists of: (1) banded juvenile bombs of intermediate composition; (2) the range of composition of these bombs from SiO₂52% to 58%; (3) the presence of highly magnesian olivine with reaction rims; (4) inverse zoning in clinopyroxene with strong Mg enrichment towards the rim; (5) resorption of plagioclase; and (6) significant compositional heterogeneity in the vitric phase. Volcanic debris-avalanche events at Nevado and Fuego de Colima may thus correspond with major breaks in the petrological evolution of the volcanoes and the start of a new magmatic cycle. Injection of mafic magma into the presently perched viscous surface dome of the active Fuego cone, as occurred in 1818 and 1913, could enhance the likelihood of southward collapse of the flank of an already unstable edifice, and it must be considered in future hazard assessment of this active volcano. Risk to life and property for the entire Colima region associated with such catastrophic phenomena would be immeasurably greater in comparison with hazards related to the last explosive outburst in 1913, which resulted in emplacement of pyroclastic flows over uninhabited areas of the upper flanks of the volcano.     

Nutrient Enrichments of Waters of "Golfo de San Jose" (Argentina, 42 oS), Growth and Species Selection of Phytoplankton

Abstract. Coastal Patagonian sea waters had been differentially nutrient-enriched to determine (i) the magnitude of the algal biomass supported and (ii) which species were able to best compete for the available nutrients. Nitrogen was found to be the nutrient limiting growth of the natural phytoplankton population. The addition of NO₃ at 25 μg at 1^-1 led to a 10 to 30 μg 1^-1 increase in chlorophyll a in all but those treatments containing trace metal additions. The best yield coefficients were obtained when a chelator (EDTA) was added with NO₃. A pool of nine diatom species were common in the N0₃ supplemented waters and the addition of vitamins along with NO₃ allowed two of the nine species to dominate.     

Solidus and liquidus temperatures and mineralogies for anhydrous garnet-lherzolite to 15 GPa

A review of experimental data for systems, pertaining to anhydrous fertile garnet-lherzolite shows strong convergence in the liquidus and solidus temperatures for the range 6.5–15 GPa. These can converge either to a common temperature or to temperatures which differ by only ～ 100°C. The major-element composition of magmas generated by even minor degrees of partial melting may be similar to the primordial bulk silicate Earth composition in an upper-mantle stratigraphic column extending over 160 km in depth.The convergence of the solidus and liquidus temperatures is a consequence of the highly variable $\text{d}\text{T}\text{d}\text{P}$ of the fusion curves for minerals which crystallize in peridotite systems. In particular, $\text{d}\text{T}\text{d}\text{P}$ for the forsterite fusion curve is much less than that for diopside and garnet. Whether or not the solidus and liquidus intersect, the liquidus mineralogy for undepleted garnet-lherzolite compositions changes from olivine at low pressures to pyroxene, garnet, or a complex pyroxene-garnet solid solution at pressures in excess of 10–15 GPa. Geochemical data for the earliest Archean komatiites are consistent with an upper-mantle phase diagram having garnet as a liquidus phase for garnet-lherzolite compositions at high pressures. All estimates of the anhydrous solidus and liquidus for the range 10–15 GPa are consistent with silicate liquid compressibility data, which indicate that olivine may be neutrally buoyant in ultramafic magmas at these pressures.