Different alterations paths of alluvium derived from the Variscan basement in different time: Cretaceous and Messinian alterites of Sardinia (Italy)

Mineralogical and chemical analyses on two clayey deposits, derived from weathering of alluvial material, were performed in order to compare their provenance and the weathering path experienced in two different time intervals considered hot and wet and hot and dry, respectively: the Upper Cretaceous and the Messinian. The Cretaceous alterites (CP) are mostly composed by kaolinite, subordinate illite+/‐ montmorillonite, quartz, Ti‐oxides, minor Fe‐oxyhydroxide and boehmite, and vertically pass into bauxite. The Messinian alterites (MP) are composed by illite and kaolinite, quartz, Tioxides, with minor content of Fe‐oxyhydroxide and gibbsite. The Subset CP is strongly enriched in Ti and Al (2‐5 x GLOSS values from Plank and Langmuir, 1998) and, to a minor extent, in Fe (1‐4 x GLOSS) whereas relevant depletion (<1 x GLOSS) is observed for Si and especially Mn, Na and P. The subset MP is moderately enriched (1‐2 x GLOSS) in Ti, Al, Fe, and K. Chemical index of alteration (CIA ‐ Nesbitt and Young, 1982) of CP alterites yield a mean value of 82.3 suggesting strong weathering whereas that of MP alterites is 74.2 pointing to a less strong alteration. Chondrite‐normalized REEs patterns show homogeneous and typical UCC pattern for MP samples with average Lach/Ybch = 12,91 and average Eu/Eu*= 0.70. Chondrite‐normalized REEs patterns of CP samples show fractionation effects likely due to intense weathering, in agreement with CIA values and ACN‐ K ternary plot, including scattered Lach/Ybch values (from 5 to 25). It is interesting to observe that the conservative provenance Eu/Eu* proxy has the same mean value (0,70) of the MS subset. As MS alterites patently derives from alluvium fed by the Variscan basement (Mongelli et al., 2012), it can be assumed that this was also the provenance of CP alterites. However MP and CP alterites shared similar alteration in the first stages of the weathering path, MP, despite some high Al enrichment values, never ended in bauxite: although unexpectedly wet, the Messinian climate was not warm enough to generate ferrallitic alteration.