Geochemistry of Late Cretaceous - Oligocenic shales from the varicolori formation, southern Apennines, Italy: implications for mineralogical, grain size control and provenance

Shales of the Varicolori Formation of Late Cretaceous-Oligocenic age from the southern Apennine Mountains, Italy, were analyzed for mineralogy, major element, and selected trace element concentrations, including the REE. In addition, size fractions of four samples of these shales were analyzed for Fe, Na, and the same trace elements as the whole shales. The shales consist mainly of illite and smectite with lesser chlorite, kaolinite, quartz, and feldspar. Calcite is quite variable (0 to 28 weight %). Accessory minerals are garnet, zircon, apatite, xenotime, and an unidentified Ti oxide mineral. Factor analysis of the whole rocks suggests that most trace elements are controlled by accessory minerals and are not due to variation in the major minerals. For example, mass balance calculations indicate that 50 % of La, 83 % of Sm, 100 % of Tb and 100 % of Yb in Varicolori shales can be due to the contribution of monazite, xenotime and apatite. The role played by zircon in controlling HREE is negligible. The total contribution of phosphatic phases and zircon to Eu contents is minor (16 %) whereas the feldspars contribution is limited to about 30 %. A significant amount of Eu can be thus controlled by 2:1 clay phases, likely as an adsorbed cation. These controls are also consistent with the mass balance performed on the size fractions of four selected samples. The lowest concentration densities for most trace elements are in the finer fractions dominated by clay minerals and in the > 64 mu m fraction dominated by quartz, illite and chlorite. In contrast, the highest concentration densities for most trade elements are in the silt-sized fractions dominated by chlorite, quartz or Calcite. These minerals generally contain low concentrations of these trace elements, thus supporting the idea that accessory minerals in the silt fractions control most trace element concentrations. The Eu/Eu* and the Th/Sc ratios are the most similar in the different size fractions, whereas, the Cr/Th and Co/Th ratios are the most variable in different size fractions, suggesting the latter ratios may be less reliable source indicators in these rocks than the former. The mineralogy and elemental ratios in these shales suggest a provenance of low- to medium grade metamorphic rocks of intermediate crustal composition, remnants of which presently form what is now the Calabrian-Peloritan Arc.