Petrographic classification of unusual high-pressure metamorphic rocks

A great number of prehistoric stone implements from Western Europe is made of high-pressure (HP) metamorphic lithotypes, such as Na-pyroxenites and fine grained eclogites. These rocks represent minor “accessory” lithologies that occur as either small primary outcrops or secondary clastic deposits derived from erosion of the formers. Due to their scarcity, these lithologies have been partly neglected by the Subcommission on the Systematics of Metamorphic Rocks (SCMR) of the International Union of Geological Sciences (IUGS). Their archaeometric relevance, however, is important. A new petrographic classification of these HP metamorphic lithotypes is presented here, which is consistent with the recommendations of the SCMR. Two simple diagrams are suggested, which are useful to classify “jades”, mainly consisting of Na-pyroxenes, and rocks mainly consisting of Na-pyroxenes and garnets. Other unusual HP rocks, sometimes used to make archaeological implements, are also considered and classified according to a serious petrographic approach.