Geochemistry and Petrography of Western Tethys Cretaceous sedimentary covers (Corsica and Northern Apennines): from source areas to configuration of margins
Luca Pandolfi e Laura Bracciali
Dipartimento di Scienze della Terra, Università di Pisa, Italy
Provenance studies most commonly apply the classical approach based on petrographic modal analysis of arenites. In this paper a modal analysis of arenites is combined with both a petrographic study on conglomerate clasts and a geochemical investigation of major and trace elements of pelites. The Ligure-Piemontese oceanic basin, a branch of Western Tethys, and its continental margins were consumed during the Eocene collisional events that led to the formation of the Alpine-Apennine belt. Remnants of Cretaceous sedimentary successions supplied by the continental margins are today preserved as tectonic units in the Alpine-Apennine belt: Balagne Nappe in Alpine Corsica and Internal and External Ligurian units in the Northern Apennines. The petrography of pebbles from rudites and lithic fragments from arenites shows that Corsica and Internal Ligurian units contain debris from granitoids, low grade metamorphic rocks and carbonate platform rocks, while the External Ligurian units contain debris from low to high grade metamorphic rocks, a mantle-rock source, carbonate platform and pelagic siliceous/carbonate rock sources. Geochemical data on pelites indicate a more mafic/ultramafic character for External Ligurian units (enrichment in Cr, Co, Ni and Th/Sc/Cr/V/Ni relationships showing a systematic shift towards an ultramafic contribution). Petrographic and chemical data indicate that the source for sediments of Corsica and Internal Ligurian units were made up of the upper part of a continental basement and its carbonate sedimentary cover (the Corsica/Europe continental margin). On the other hand, the External Ligurian units were supplied by a source area where a complete lithospheric section was exposed, from the upper mantle up to the deep sea sedimentary cover (the Adria continental margin). These findings are useful in order to unravel the processes related to the opening mechanisms of the Ligure-Piemontese oceanic basin: among the different rifting models in existence, our data support an asymmetric mechanism dominated by a west-dipping detachment fault, with the Adria margin acting as the lower plate.