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|Title: ||Close or not so close? Provenance studies of megalithic monuments from Alentejo (Portugal)|
|Authors: ||Boaventura, Rui|
|Issue Date: ||30-Aug-2012|
|Publisher: ||Mineralogical Magazine|
|Citation: ||BOAVENTURA, R. & P. MOITA (2012) - Close or not so close? Provenance studies of megalithic monuments from Alentejo (Portugal). EGU - Austria (Viena) Geophys. Res. Abs, v. 14, EGU2012-13128, 1 p.|
|Abstract: ||There has been a significant amount of studies about megalithic tombs conducted in the Alentejo region. However
the geological provenance of monoliths used in the construction of those tombs usually was not a priority among
researchers with rare exceptions (Dehn, Kalb and Vortisch, 1991; Boaventura, 2000). Recent studies of dolmens
(Oliveira, 1997 and 2006; Gonçalves, 2003) refer only to a brief characterization of rocks, such as "granite or
schist slabs", highlighting certain types if the geological stratum is identical or not to the stone blocks. On the
other hand, when the type of raw material appears to be similar with the bedrock, it is common and empirically
assumed its local provenance.
With the aim of testing and expand the knowledge about the provenance of the slabs used in the construction
of megalithic tombs, several lithic samples from dolmen slabs and outcrops in their surroundings were
collected for analysis and comparison. The samples were characterized by petrographic studies in thin section as
well with a geochemical analyses performed by XRF that gives major elements as well some trace elements.
The dolmens tested for this project are located roughly between the northeast to west of the town of Monforte
(Upper region of Alentejo, Portugal) and are named, from south to north, as Serrinha, Rabuje group (1 to
5), Geodésico de Besteiros 3 and Velho. The field work and petrographic studies revealed that the slabs are constituted
mainly by several types of granitoids (gnaissic, red, white, tonalitic), amphibolites and mottled schist shale.
The comparison of chemical analyses between slabs and selected outcrops revealed that the provenances
are in most of the cases from the nearby geological stratum. In fact, major elements (e.g. MgO, SiO2, CaO)
as well trace elements (e.g. Sr, Y, Zr, Nb) compositions are similar on slab samples and in rocks from the
outcrops. If in terms of major elements a similarity was already expectable, or easier to obtain, the trace elements
(namely immobile elements such as Y or Nb) compositions corroborated that slabs and geological bedrock were
alike. The capstone slab that covers the dolmen of Rabuje 1 group does not belong to the nearby geological
stratum. Nevertheless, a probable matching source-outcrop was located sampled and characterized in terms of
geochemistry and petrograpphy and compared with the megalithic capstone.
This work allowed a better characterization of the rocks used in megalithic tombs as well as corroborat a
pragmatic attitude of Neolithic populations in the search of the appropriate slabs for construction as proposed
previously (Boaventura, 2000). When available, the megalithic stones were likely collected from the nearby
stratum and therefore the distances traveled were small (in situ or less than 1-2 km). Nevertheless, when the
type of stone needed was not available in the vicinity (e.g fracturing provided only smaller stones) it would be
necessary to travel longer distances, up to 8 km (Boaventura, 2000), as in the case of the dolmen of Rabuje 1.
Boaventura, R. (2000) - A geologia das Antas de Rabuje (Monforte, Alentejo), Revista Portuguesa de Arquelogia.
Dehn, W.; Kalb, P.; Vortich, W. (1991) - Geologisch-Petrographische Untersuchungen an Megalithgräbern
Portugals. Madrider Mitteilungen, 32, p. 1-28.
Oliveira, J. (1997) - Monumentos megalíticos da bacia hidrográfica do Rio Sever. Ibn Maruan. Castelo de
Vide. Special Edition.
Oliveira, J. (2006) – Património arqueológico da Coudelaria de Alter e as primeiras comunidades agropastoris.
|Appears in Collections:||GEO - Comunicações - Em Congressos Científicos Internacionais|
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