Geochemistry and mineralogy as indicators of parental affinity for Cenozoic Bentonites: a case study from S. Croce di Magliano (southern Apennines, Italy)

The major and trace element contents and mineralogical composition of Cenozoic bentonites from the southern Apennines (Italy) have been determined, for the whole-rocks and the <2 ~tm size-fractions, in order to constrain parental affinities. The main mineralogical and chemical differences have been recognized in eleven samples allowing them to be grouped into two distinct subsets. The differences are based on smectite abundance, occurrence or lack of detrital clay phases, different contents of Ti, Fe, Mn, K, P, Rb, Sc, V, Cr and Ni and differences in the Eu/Eu* and Ti/AI elemental ratios. These ratios indicate an affinity for felsic volcanics for the subset showing high smectite contents. The low smectite subset shows, instead, an affinity for Cretaceous-Oligocene southern Apennine shales. A similar result is obtained using the La-Th-Sc and Th-Sc-Zr/10 diagrams. We suggest that during the deposition of the southern Apennine shales, episodic volcanic events took place. These were associated with the suture stage of the Tethyan ocean that promoted accumulation of felsic ash in the related basin and the diagenetic alteration of these materials produced bentonitic layers interbedded with shales.