Multianalytical characterization of silica-rich megabreccias from the proposed natural area of Richat (Sahara desert, Mauritania)

RESEARCH JOURNAL OF CHEMISTRY AND ENVIRONMENT (Impact Factor: 0.64). 09/2011; 15(3):49-54.


Some terrestrial areas have favourable environmental features that allow the preservation and scientific study of processes which can be used as geomarkers. The crater-like Richat structure (Sahara desert, Mauritania) is a recently proposed protected natural area, which can be considered as one of these sites. Volcanic, hydrothermal and arid environments merge together, making easy their investigation in the field and their later characterization by the use of a set of geochemical and mineralogical techniques (XRD, transmitted and reflected light microscopy, SEM-EDX, laser-Raman spectroscopy, XRF, ICP-MS and stable isotope spectroscopy). The study presented here focuses on the spectacular silica-rich megrabreccias which crop out, as a large and thick lenticular body, at the centre of the Richat structure. Based on paragenetic relationships, six types of silica-rich megabreccias were identified. The link between the geochemical and mineralogical characteristics was established, from macro to micro scale, showing the existence of: a) variable distributions of the major, minor and trace elements; b) polymorphic phases; c) micrometric cataclastic features and d) different textural intergrowths and degrees of crystallinity of some minerals. The oxygen and carbon isotopic signatures of the carbonates, within the silica-rich megabreccias, point out a meteoric (low-temperature hydrothermal) origin for the source waters.

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    ABSTRACT: The Richat dome is a spectacular circular structure located in the Mauritanian part of the Sahara Desert. The current erosion level of this igneous complex presents a wide variety of contrasting extrusive and intrusive rocks from shallow to deep source regions providing insight into the magmatic process at the origin of the complex. The Richat is the superposition of a bimodal tholeiitic suite crosscut by carbonatitic and kimberlitic magmatic rocks. The bimodal series is characterized by two concentric gabbroic ring dikes and two extrusive rhyolitic centers representing the remnant of two maar systems. Silica undersaturated magmas occur as carbonatite dikes, a kimberlite plug, and kimberlite sills extruded along the old regional anisotropies filling NNE-SSW dextral strike-slip faults and en-echelon tension gashes. An intense low-temperature hydrothermal event affected the Richat area. It is responsible, notably, for the karst-collapse central mega-breccia, the alteration of the rhyolites, the potassic alteration of the gabbros and the stable isotope enrichment in the carbonatites. A piston-like collapse is proposed to explain the contrast existing between the central and outer part of the Richat. Structural inheritance played an important role in the history of the Richat complex. Pre-existing anisotropies acted as a pathway for the ascent of asthenospheric and sub-continental melts and allowed the coexistence of alkaline and tholeiitic magmas within the same igneous complex.
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