Hydrothermal influences in the Late Triassic Lagonegro Basin (southern Italy): evidence of contemporaneous mid-ocean-ridge spreading in the western Tethys

The south-western branch of the Tethys Ocean was interested, during the Late Triassic, by the opening and spreading of an oceanic basin, the Ionian Ocean. Its north-western termination was characterized by carbonatic/siliceous deposition inside the Lagonegro Basin, which was imposed on transitional crust. Three stratigraphic sections of the Lagonegro Basin sedimentary record, representing a proximal-to-distal transect (e.g., Casacci et al., 2016) from the continental margin, have been investigated to evaluate the interaction of this basin with the adjacent Ionian Ocean. This ocean was actively spreading since the Middle Triassic (Passeri et al., 2014) but no investigation has been done on the influence of the ridge activity on its peripheral portion situated northward, represented by the pelagic sediments of the Lagonegro Basin. Three major inputs into the sediment were recorded: detrital clay, biogenic silica and carbonates, and most of the samples here studied consist of mixtures of these components. In order to discriminate the chemical signature of the main fractions, bulk and partition geochemical investigations have been carried out in relation of the three main components of the sediments. The rare earth elements (REEs) reflect different pattern for each sediment fraction with the SiO2-bio group showing middle REEs enrichment, suggesting Fe oxyhydroxy influence; the clays group shows a flat pattern typical of shale from siliciclastic input; the carbonate group indicates heavy REEs enrichment and Ce negative anomaly, pointing out seawater influence. Enrichments in hydrothermally derived elements, from the submarine weathering of middle oceanic ridge basalts (MORB), were found in both the clay and carbonate fractions indicating a moderate mixing of the two end-member solutions, hydrothermal fluids and seawater, during the deposition of the Lagonegro Basin sediments. The tectonics of the basin and the different setting for each section left the shallower and more proximal section to the continental margin, without any record of the hydrothermal influence. The results can be considered as a first geochemical evidence of the hydrothermal activity related to the Ionian Ocean ridge, in the Lagonegro Basin deposits.