Since prehistoric time, the architecture of the human settlements has been deeply influenced by the lithologies occurring on their territory and, above all, by their physico-mechanical features. Among the most important archeological sites in Sardinia are the hypogean necropolises, called Domus de Janas (Home of the Witches), in Sardinian language. These artificial caves, which date back to the Neolithic Age (4000-2400 BP), are rock-cut tombs constituted by a sequence of underground chambers often decorated by sculptures, paintings and engravings. Domus de Janas have been discovered in several outcrops within volcanoclastites related to the Tertiary parossistic manifestations of calc-alkaline magmatism (Oligo-Miocene, 32-14 Ma). The volcanic products of this cycle crop out along the Tertiary Rift of Sardinia that crosses the Western part of the island from South (Cagliari) to North (Sassari-Porto Torres). This acid volcanism is linked to the Cenozoic geodynamic evolution of the western Mediterranean. In this work, the minero-petrographic features of the volcanoclastic suite in the Logudoro region (Northern Sardinia) are investigated with the aim to characterize the stones where these prehistoric underground structures were carved. In the study area, the landscape is characterised by volcanic plateaux some hundreds of metres thick and north-westward dipping, overlying a Paleozoic basement complex composed of metamorphic and granite rocks. The volcanoclastic succession, Aquitanian to Burdigalian in age, is composed of a sequence of dacitic to rhyolitic lava flows, ignimbrites and pyroclastic tuffs in which two ignimbrite episodes with different degrees of welding and separated by an epiclastic layers have been identified. The upper volcanic unit is overlaid by a transgressive conglomerate which marks the end of this magmatic cycle and the base of the Miocene marine succession. From a petrographic point of view these ignimbrites are defined as rhyolites, rhyodacites and dacites composition ranging from reddish and brown in color. They generally show an eutaxitic glassy texture, sometimes with collapsed scoriae along with lithic clasts often occur. Some autigenic phases, such as clinoptilolite associated with opal-CT and smectite, derive from the transformation of precursor rhyolitic glass with a post-deposition zeolitizzation process. The secondary mineralization was favored by hydrothermal fluids preferentially circulating through fault system. As expected, the analysis of the tombs host rock has concurred to recognize that the graves were carved in the most workability lithotypes represented by unwelded pyroclastic flow, likely exploiting previous natural erosional voids. These results also provide a petrographic background to define the state of conservation of these Neolithic monuments and to investigate the weathering and restoration of their decorations.
Petrographic features of Tertiary volcanic products used for Neolithic human settlements in Logudoro region (Northern Sardinia, Italy)
Laura Sanna
2017
Abstract
Since prehistoric time, the architecture of the human settlements has been deeply influenced by the lithologies occurring on their territory and, above all, by their physico-mechanical features. Among the most important archeological sites in Sardinia are the hypogean necropolises, called Domus de Janas (Home of the Witches), in Sardinian language. These artificial caves, which date back to the Neolithic Age (4000-2400 BP), are rock-cut tombs constituted by a sequence of underground chambers often decorated by sculptures, paintings and engravings. Domus de Janas have been discovered in several outcrops within volcanoclastites related to the Tertiary parossistic manifestations of calc-alkaline magmatism (Oligo-Miocene, 32-14 Ma). The volcanic products of this cycle crop out along the Tertiary Rift of Sardinia that crosses the Western part of the island from South (Cagliari) to North (Sassari-Porto Torres). This acid volcanism is linked to the Cenozoic geodynamic evolution of the western Mediterranean. In this work, the minero-petrographic features of the volcanoclastic suite in the Logudoro region (Northern Sardinia) are investigated with the aim to characterize the stones where these prehistoric underground structures were carved. In the study area, the landscape is characterised by volcanic plateaux some hundreds of metres thick and north-westward dipping, overlying a Paleozoic basement complex composed of metamorphic and granite rocks. The volcanoclastic succession, Aquitanian to Burdigalian in age, is composed of a sequence of dacitic to rhyolitic lava flows, ignimbrites and pyroclastic tuffs in which two ignimbrite episodes with different degrees of welding and separated by an epiclastic layers have been identified. The upper volcanic unit is overlaid by a transgressive conglomerate which marks the end of this magmatic cycle and the base of the Miocene marine succession. From a petrographic point of view these ignimbrites are defined as rhyolites, rhyodacites and dacites composition ranging from reddish and brown in color. They generally show an eutaxitic glassy texture, sometimes with collapsed scoriae along with lithic clasts often occur. Some autigenic phases, such as clinoptilolite associated with opal-CT and smectite, derive from the transformation of precursor rhyolitic glass with a post-deposition zeolitizzation process. The secondary mineralization was favored by hydrothermal fluids preferentially circulating through fault system. As expected, the analysis of the tombs host rock has concurred to recognize that the graves were carved in the most workability lithotypes represented by unwelded pyroclastic flow, likely exploiting previous natural erosional voids. These results also provide a petrographic background to define the state of conservation of these Neolithic monuments and to investigate the weathering and restoration of their decorations.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
