Factors affecting Uranium distribution in early Permian-late Variscan Sardinian basins: chemical and γ-ray data constraints.

The late, post-collisional evolution, of the European Variscides resulted in both extensional and transtensional dynamic responsible for volcanic and plutonic activity, development of intracratonic basins and overall high thermal flow. Between late Carboniferous and early Permian, equatorial conditions promoted the diffusion of rainforests that, inside the subsiding basins, gave rise to accumulations of organic matter and coal seems. The transition from the dark, reducing, organic matter-rich facies to red beds is not sharp, and traditionally has been related to drier conditions and to a new different palaeogeographic frame. The red bed-arid climate paradigm has been questioned in the Sardinia Permian basins (Sinisi et al., 2014 and references therein), whereas in other Variscan sites the dark facies-red beds dualism was referred to the subsidence history of the basin. Several geochemical and mineralogical proxies (e.g. REEs contents, V/Cr and Zn/Cu elemental ratios, and occurrence of both Fe-oxyhydroxides and phyllosilicates) have been used in order to establish environmental conditions in terms of sediments and terrane provenance, palaeoredox and draught/ wet balance. With this in mind we investigated a robust multielemental chemical dataset of sediments (n=56) having different grain size and different colour assuming the dark facies as broad proxy of wet vegetated environment, red beds as indicator of arid conditions and U/Th as palaeoredox indicator. The sampled successions consist of proximal to distal alluvial fan and lacustrine deposits. The U content in the sampled sediments ranges between 0.9 and 22.3 ppm with a median value of 3.95, higher than the average value associated to the Upper Continental Crust models (UCC: 2,80 ppm, Taylor & McLennan 1985; GLOSS: 1,68 ppm, Plank & Langmuir 1998). Univariate statistic allowed to individuate outliers (11-22.3 ppm) indicative of a reducing episode. This finding is supported by several γ-ray data (HPGe detector) that confirm the occurrence of an uraniumrich level. Principal Component Analysis (PCA), performed on the devoid database instead, suggests that U distribution is controlled by a mechanism mostly depending on differences in source areas. Finally, no meaningful differences have been observed in palaeoredox conditions between the red and grey siltite, as hematite - the sole Fe- oxidised phase detected - is stable in a wide range of Eh-pH conditions.