High-pressure meta-ophiolite boulders and cobbles from northern Italy as possible raw-material sources for “greenstone” prehistoric tools: petrography and archaeological assessment

Recent research and field surveys, performed on the Monviso massif as well as in the Po and Curone valleys, revealed the presence of high-pressure (HP) meta-ophiolites – namely “greenstones” – in the form of blocks extracted from primary outcrops or erratic cobbles/boulders in the alluvium, respectively. These rare lithotypes are important, as they may have been used by prehistoric people as raw materials for the manufacture of polished stone tools, in particular axes/adzes, blades and chisels, found all over western Europe and along a wide corridor running from southern Italy to the British Isles. The bulk chemistry and microstructural features of “greenstone” HP-meta-ophiolites collected during the geological surveys were characterized by polarizing microscopy and SEM-EDX on thin sections, as well as bulk prompt-gamma activation analysis. By comparing the most significant aspects recurring in specimens from a particular site of collection, a chart was provided indicating the distinctive features of each provenance. These discriminant features were also compared with those of Neolithic polished stone tools from archaeological sites, thus providing interesting outcomes about the supposed sources of their raw materials. In particular, the results suggest a possible local origin for the raw materials used in the Neolithic workshops for the production of “greenstone” artefacts of Rivanazzano (in the Staffora valley) and Brignano Frascata (Curone valley), collected as cobbles/boulders from the secondary deposits of HP-meta-ophiolites found in the same Curone valley and/or other adjacent sites. Our results contravene the results of previous explorations, performed in the same Curone and Staffora valleys, which failed in finding HP-metamorphic lithotypes in the alluvial detritus of the local streams, eroding the Oligocene conglomerates. These lithotypes are probably relics from the dismantling of HP-meta-ophiolites from the Voltri Massif (or an analogous palaeo-unit, now eroded) during the Oligocene, then redeposited as alluvial sediments in Pleistocene and more recent times.