Eclogites, jades and other HP-metaophiolites employed for prehistoric polished stone implements in Italy and Europe

Abstract

Prehistoric polished stone tools are now fairly well-known from a petrographic point of view in Northern Italy. Many more than one thousand implements sampled from the most important sites and collections (axes, adzes, chisels, some ornaments and various tools fragments), have been analysed. These artefacts were commonly employed from the Neolithic period onwards for working wood and cutting the forest trees. Surface optical observations, density, thin sections, XRD, microprobe analyses and bulk chemistry have been employed, alone or in combination, for petrographic study oriented to archaeometric interpretation. Alpine eclogites, jades (Na-pyroxenites) and other minor HP (High Pressure) metaophiolites dominate the polished stone lithology of Northern Italy, being at least 70% and often surpassing 90% of stone materials in single sites. Petrographic, geochemical, minerochemical, textural data of the studied rocks are described and discussed in some detail. A number of new definitions are introduced. The provenance of the raw material is identified as being NW Italy, essentially Piedmont and Liguria. These regions represent one of the few geological zones in the world where alpine eclogites and jades occur, both as primary outcrops in the High Alps, alluvial and morainic deposits along the valleys, and Oligocene conglomerates in the Northwestern Apennines. The dominance of eclogites and jades among the prehistoric polished stone tools represents a lithic selection of cultural significance. This selection seems to be justified by litho-technological (best mix of hardness, toughness and density) and aesthetic (fine green colours, translucency) reasons, which caused the exclusion of other, elsewhere common, lithologies. The remarkable presence of jade and alpine eclogite lithologies among the Western and rarely Central European axe blades (mostly status symbol or ceremonial axes) gives evidence of a relevant long distance exportation of HP metaophiolite materials from NW Italy to France, Germany, Benelux, Great Britain, etc., up to1000-1500 km far from the source areas. The still unsystematic petrographic knowledge of the HP-metaophiolitic stones of the axe blades in Europe, as well as in the Italian Peninsula, allows only a preliminary comparison with the better known Northern Italian implements.

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