Geochemistry of shales from the southern Apennines (Italy): source-area weathering, sorting, provenance, and tectonic setting.

The southern Apennines orogenic wedge is a fold-and-thrust belt mainly derived from the deformation of the African-Apulian passive margin. Several wide belts have been recognised in the chain area including shallow-water, basinal and shelf-margin facies successions (middle Triassic-Miocene) and deep-water ophiolite-bearing successions (Jurassic-Miocene). In these successions the occurrence of fine-grained siliciclastic sediments is the rule and a thorough geochemical analysis, including lanthanides, actinides and transition metals is especially helpful as other signatures of source-area weathering, sorting, provenance and tectonic environment are not preserved. In the current paper the geochemistry of 164 shales is inspected to decipher the influence of the above cited factors on their composition. Relative to the UCC, the shales are depleted in Si, Ca, Na, K, and LILE, enriched in transition elements and REEs, whereas HFSE contents are close to those of the UCC. Chondrite-normalised REE diagrams are PAAS- like. The geochemical proxies point toward an upper continental source(s) affected by moderate, non-steady state, weathering. The weathering trends depart from micaceous-rich protoliths evolving toward illite and illite-smectite compositions. Volcaniclastic contribution and hydrothermal supply are not observed at all. In some cases an additional biogenic siliceous supply is observed. The sorting effect appears negligible. The shales mainly record the tectonic setting of the source(s) and not those of the basins where deposition occurred. The source area, a former continental arc, can be envisaged in the Paleotethys suture zone, consistent with recent palinspastic restorations.