The S. Antioco of Bisarcio Basilica (NE Sardinia, Italy): Water-rock interaction in ignimbrite monument decay

The Basilica is constructed of pyroclastic rock dating from the calcalkaline-affinity volcanic cycle occurring in Sardinia between 32 and 11 Ma ago. Very small amounts of epiclastic products have also been detected. Petrographic and geochemical data indicate that the composition of these volcanic rocks ranges from dacite to rhyolite. Variations in total open porosity in the external part of the hewn rock permitted the singling out of two extreme trends pointing to reprecipitation within the pores and surface disaggregation. Visible signs of weathering include exfoliation, enucleation and alveolation, as, well as chromatic surface alterations, with organic films and salts efflorescence formed as a result of water rise and evaporation.