Neolithic polished greenstone industry from Castello di Annone (Italy); minero-petrographic study and archaeometric implications

High-pressure (HP) meta-ophiolites - usually termed 'greenstones' by archaeologists - were used in the Neolithic to produce polished stone implements all over Western Europe. Their accurate petrographic characterization may help to infer the provenance of the raw materials, thus contributing to reconstruct the migratory routes of our ancestors. The lithic industry of Castello di Annone (northwestern Italy) was investigated by means of a multi-analytical approach including density measurements, X-ray powder diffraction, optical microscopy, scanning electron microscopy with energy-dispersive spectroscopy, and geothermometry. More than half of the studied tools (52 %) are made of fine-grained eclogites subdivided in three different groups - each with a peculiar metamorphic history. Another 26 % consists of Na-pyroxene-rich rocks, with mixed Na-pyroxenites (omphacite + jadeite-bearing rocks) more abundant than jadeitites. The remaining 22 % is made up of serpentinites and a variety of minor lithologies. In most greenstone implements, both pyroxenes and garnets show a complex compositional zoning, almost unknown in geologic samples due to the lack of detailed petrologic data on these rocks. Therefore, though it has been ascertained that these HP meta-ophiolites derive from the Piemonte Zone, more systematic field and laboratory data are necessary to refine the exact recognition of the raw material sources. A recent geologic survey in the Pellice Valley discovered small boudins of fine-grained eclogites and omphacitites, similar to those found in the prehistoric tools. The Castello di Annone eclogites, which appear to be poorly fashioned from fluvial pebbles, must be considered low quality materials. This confirms the marginal role of this site in the production and distribution network of greenstone implements in northern Italy during the Neolithic.