Mineralogical-chemical alteration and origin of ignimbritic stones used in the old Cathedral of Nostra Signora di Castro (Sardinia, Italy)

The pyroclastic rocks belonging to the Late Eocene-Miocene volcanic activity occurred in Sardinia between 38 and 15 Ma ago were widely used as construction materials in several Romanesque churches of the easternmost Logudoro area, as well as in large part of the Sardinia territory. In this work, the ancient Cathedral of Nostra Signora di Castro (12th century) was taken as a representative case study. There are not historical or archaeological evidences of ancient quarries. Based on the geochemical, petrographic and volcanological data on several samples from an extensive field-area (approximately 150 km2), a geographical zoning of the volcanics has been recognized. In the Oschiri sector, there are three different sub-zones, which can be identified with different volcanic rocks: less fractionated rocks (Differentation Index ~70÷78); intermediately fractionated rocks (D.I. ~76÷79); more fractionated rocks (D.I. ~77÷82). To identify the origin of the ignimbrite rocks of the Church of Nostra Signora di Castro, two statistical methods have been used: stepwise linear discriminant and canonical analysis. Moreover, to define the geochemical transformation processes induced by the alteration, a comparative study of concentrations of major and trace elements measured by XRF and SEM-EDS analyses on the surface portion and the innermost of the stone has been made.