Search

International audience; In Europe, exploration geochemistry is mainly restricted to classical soils geochemical analyses based on total digestion of samples, despites the good results obtained in other areas with approaches related to selective and partial leaches, in particular to reveal anomalies linked to deeply buried deposits. Similarly, examples of biogeochemistry usages in Europe remain anecdotic, but it is now very efficient to help in focusing exploration efforts with the actual level of element detection reached in a large spectrum of plants. Within the framework of the UpDeep project (EIT Raw Materials), which aims at developing the geochemical expertise on deep buried exploration into Europe, we performed a comparison of some of these new geochemical methods on the Vendée antimony district (France). One of the challenging questions of exploration geochemistry in Western Europe is the long-term anthropogenic contamination related to past mining activities but also agriculture and forestry, which can strongly affect primary geochemical anomalies. Within the scope of testing different well-documented approaches, we collected around 160 samples of soils on which we performed separately the following treatment: deionized water leach, sodium pyrophosphate leach, and Aqua Regia digestion on Ah horizon; and hot hydroxylamine leach, Ionic LeachTM (ALS) and Aqua Regia digestion on B horizon. Each soil samples have been analyzed by pXRF. We also collected 184 samples of oak and 94 samples of bramble twigs in order to compare their geochemical signal. In this Western European context, identification of a homogenously distributed vegetation media represents a complex issue.

International audience; The French Massif Central (FMC) forms the hinterland of the west-European Variscan belt and was subject to numerous tectonic and metamorphic events from Devonian to Permian times. The accreted tectonic units belonging to Gondwana and Laurussia were variably involved in subduction, nappe stacking and syn-to post-orogenic extension. Besides, plutons and migmatites showing syn-collision crust melting are very common in the Variscan belt. Such orogenic evolution implies that the resulting finite structure likely reflects the interference between fabrics formed at different stages of the orogenic cycle with different degrees of overprinting, depending on e.g. the spatial strain gradients, lithologies, structural level or partial melting grade. We present a model of finite strainfield of the western FMC based of the analysis/interpolation of ductile and brittle fabrics, at regional to local scale, using geostatistics. Ductile fabrics within autochthonous units and syn-collisional plutons are poorly disturbed and reflect the transition from syn-thickening to extensional lateral flow of the partially molten mid-lower crust. In contrast, allochtonous units are affected by numerous perturbations including polyphased ductile shearing, folding and faulting inducing local bloc tilting and rotations. Taking into account the overprint of late tectonic events, we show that early fabrics related to High Pressure metamorphism or nappes' stacking have suffered intense reorientations. The finite trend of the related lineations L1 must be treated with extreme caution and may not reflect the original direction of stretching and shearing that occurred during the initial compressional stages. Taking into account i) the spatial distribution and amount of reorientation of these D1 fabrics and ii) the deconvolution of these disturbances, we propose that:-The high dispersion of lineation trend can be simply explained by passive rotations, during late isoclinal folding, rather than two superposed tectonic phases, as previously inferred.-The allochtonous units formed one single tectonic unit.-This D1 event looks related to the exhumation of the subducted continental crust due to partial melting-The formation of Variscan orogenic prism in western FMC only results of continuous south-directed thrusting rather than a switch from NE-SW to NW-SE thrusting.