The church is located in the site of the ancient village of Otti (Coroneo, 1993) abandoned at the end of the sixteenth century and now situated in the country surrounding the center of Oschiri. The construction technique is typical of the Romanesque period, with the use of squared stone ashlars, and is ascribable to the second half of the XII century. The mono-nave room has apse facing northeast and wooden roof. The ignimbrites employed in the Nostra Signora di Otti church are very similar to those used in other medieval churches of the territory of Oschiri (i.e. San Demetrio, Santa Maria di Castro) of considerable historical interest due to their aesthetic characteristics and to their geographically circumscribed and simple architectural style. The ignimbrites, belonging to the Oligo-Miocenic volcanic cycle with calcalkaline affinity occurred in Sardinia between 32 and 11 Ma (Beccaluva et al., 1985; Columbu et al., 2014), have been widely used in the historical buildings of the medieval period similarly to many other localities from ancient times to present. Based on the macroscopic features (structures, presence of pumice, lithic, sialic and mafic minerals, welding degree, etc.) and microscopic characteristics observed in polarized microscopy (texture, paragenesis, etc.) a mapping of the main ignimbritic types found in the church was performed. According to De La Roche classification diagram (1980), chemical data obtained by XRF analysis indicate that these volcanics have mainly a composition of quartz-latite and rhyolites. The lithotypes of monument show a clear similarity to the volcanic rocks outcropping around the church. However, although widely used in the construction of other churches, of these rocks there is no historical or archaeological evidences of the ancient quarries from which the building materials were extracted. Based on the geochemical, petrographic and volcanological characters of numerous samples from an extensive field-area (approximately 150 kmq) a geographical zoning of the Oligo-Miocenic volcanics has been recognized. The area has been divided into two distinct main sectors: Chilivani/Tula and Oschiri, with variable composition of pyroclastic products, from trachyte, qz-trachyte, qz-latite, to rhyodacite and rhyolite. Internally to the Oschiri sector, from which the construction materials are mostly coming, according to TiO2 vs Differentation Index diagram (where D.I. = normative Q + Ab + Or + Ne + Kp + Lc; Thornton & Tuttle, 1960) there are three different sub-zones with pyroclastics: less fractionated (LFW, with D.I. ~70÷78); intermediate fractionated (IFC, D.I. ~76÷79); more fractionated (MFE, D.I. ~77÷82). In order to identify the origin of the ignimbrites of church from the areas identified on the geochemical and petrographic basis, two statistical methods have been used: stepwise linear discriminant and canonical analysis.
Geochemical characterization and approaches for provenance of the ignimbrite used in the mediaeval church of Nostra Signora di Otti (north.Sardinia, Italy)
COLUMBU, STEFANO
2016-01-01
Abstract
The church is located in the site of the ancient village of Otti (Coroneo, 1993) abandoned at the end of the sixteenth century and now situated in the country surrounding the center of Oschiri. The construction technique is typical of the Romanesque period, with the use of squared stone ashlars, and is ascribable to the second half of the XII century. The mono-nave room has apse facing northeast and wooden roof. The ignimbrites employed in the Nostra Signora di Otti church are very similar to those used in other medieval churches of the territory of Oschiri (i.e. San Demetrio, Santa Maria di Castro) of considerable historical interest due to their aesthetic characteristics and to their geographically circumscribed and simple architectural style. The ignimbrites, belonging to the Oligo-Miocenic volcanic cycle with calcalkaline affinity occurred in Sardinia between 32 and 11 Ma (Beccaluva et al., 1985; Columbu et al., 2014), have been widely used in the historical buildings of the medieval period similarly to many other localities from ancient times to present. Based on the macroscopic features (structures, presence of pumice, lithic, sialic and mafic minerals, welding degree, etc.) and microscopic characteristics observed in polarized microscopy (texture, paragenesis, etc.) a mapping of the main ignimbritic types found in the church was performed. According to De La Roche classification diagram (1980), chemical data obtained by XRF analysis indicate that these volcanics have mainly a composition of quartz-latite and rhyolites. The lithotypes of monument show a clear similarity to the volcanic rocks outcropping around the church. However, although widely used in the construction of other churches, of these rocks there is no historical or archaeological evidences of the ancient quarries from which the building materials were extracted. Based on the geochemical, petrographic and volcanological characters of numerous samples from an extensive field-area (approximately 150 kmq) a geographical zoning of the Oligo-Miocenic volcanics has been recognized. The area has been divided into two distinct main sectors: Chilivani/Tula and Oschiri, with variable composition of pyroclastic products, from trachyte, qz-trachyte, qz-latite, to rhyodacite and rhyolite. Internally to the Oschiri sector, from which the construction materials are mostly coming, according to TiO2 vs Differentation Index diagram (where D.I. = normative Q + Ab + Or + Ne + Kp + Lc; Thornton & Tuttle, 1960) there are three different sub-zones with pyroclastics: less fractionated (LFW, with D.I. ~70÷78); intermediate fractionated (IFC, D.I. ~76÷79); more fractionated (MFE, D.I. ~77÷82). In order to identify the origin of the ignimbrites of church from the areas identified on the geochemical and petrographic basis, two statistical methods have been used: stepwise linear discriminant and canonical analysis.
I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
