Metamorphic mapping and geo-morphologic considerations to locate the extraction sites of meta-psammite Late Bronze Age stelae from the Beira Interior (Portugal)

A petrographic and geochemical provenience analysis of meta-psammite Iberian Stelae from the Final Bronze Age will be presented. A provenience analysis of archaeological objects is a Sisyphean task or impossible when dealing with uniform rock composition and structure along strike as they occur in sedimentary formations. If there are no observable singularities along vertical and horizontal inter-digitations of facies, the specification of a possible extraction site is like searching a needle in a haystack. Geo-morphologic consideration helps to enhance the probability to find the outcrop if strike of formation is perpendicular to valleys or mountain ranges. Furthermore, post metamorphic folding and tectonic dismembering reduces the possible areas, and increases the probability to find appropriate outcrops.

The Beira Interior was metamorphosed during the Variscan Orogeny showing a metamorphic field gradient from amphibolite to sub-greenschist facies. The orogenic tectono-metamorphic pattern is disturbed by pluton intrusions and by aplite dykes. Metamorphic aureole zonation crosscuts formation strike and the orogenic metamorphic facies zonation. Therefore, areas with the same sedimentary lithotype (educt) and transformed to meta-sedimentary and metamorphic rocks (product) of a specific contact-metamorphic grade often postdates orogenic metamorphism and deformation. All these observations allow to drastically reduce the potential raw-material areas and to find outcrops showing the same educt-product genetic history as found in the rock-types of stelae. The very first application of metamorphic mapping techniques in combination with geo-morphology will be presented to clarify the provenience of archaeological objects.

*(DFG-project AR 1305/2-1), https://www.experimentalarchaeology.uni-freiburg.de/