Provenance and alteration of pyroclastic rocks from the Romanesque Churches of Logudoro (north Sardinia, Italy) using a petrographic and geochemical statistical approach

Abstract

The pyroclastic rocks, belonging to the calcalkaline volcanic activity occurred in Sardinia between 32 and 11 Ma ago, were widely used as construction material in several Romanesque Churches of the easternmost Logudoro, as well as in the Sardinia territory. In this work was taken as a case study the Church of Nostra Signora di Otti (12th cent.). There are no historical or archaeological evidences of the ancient quarries. Based on the geochemical, petrographic and volcanological characters of several samples from an extensive field area (approximately 150 kmq), a geographical zoning of the Oligo-Miocenic volcanics has been recognised. Internally to the Oschiri sector, according to TiO2 vs Differentiation Index diagram, there are three different sub-zones with pyroclastics: less fractionated (D.I. ~70 ÷ 78); intermediate fractionated (D.I. ~76 ÷ 79); more fractionated (D.I. ~77 ÷ 82). To identify the origin of the ignimbrites of the Church, two statistical methods have been used: stepwise linear discriminant and canonical analysis. Moreover, to define the geochemical transformation processes induced by the alteration, by XRD analysis and XRF, SEM–EDS data, a comparative study of concentrations of major and trace elements measured between the surface portion and the innermost of the stone has been made.