Petrographic, mineralogic, geochemical and geo-morphological provenience study on granite stelae from the Beira Alta (Beira Interior, Portugal)

Of twelve stelae from the Late Bronze Age (c. 1200-800 BC) that are exhibited in five museums in the Beira Interior (Portugal), ten are carved in granitoid rocks of mainly leucocratic composition. Between the Serra da Estrela and the river Tejo, 70 % of the area is covered by granitoid intrusions of pre-, syn- and post-orogenic Variscan age. Plutons intruded in metamorphic and meta-sedimentary rocks older than Late-Carboniferous. Only one stele each was extracted from the largest intrusions of the area, from the Guarda biotite-monzogranite, and one from the pre-orogenic Fundão hornblende bearing biotite-granodiorite to monzogranite. A precise provenience study is thus not possible.

Eight stelae where isotropic fine to medium grained granitoids with predominantly muscovite and rarely containing biotite. Microstructures, mineralogy, textural characteristics and geochemical analyses points to highly differentiated micro-plutonic rocks or dykes. Having realized that all twelve large granitoid intrusions in the Beira are less differentiated and porphyry, 16 aplite dykes and apophyses could be detected in a field study. Some aplite dykes in the northern part of the study area show the same composition variations as the Malhada Sorda pluton and its apophyses. But some dykes cannot be related with a late or post-orogenic intrusion. Seven syenogranite aplite rocks and apophyses could be related with a stelae rock type. Remarkably, all potential extraction sites are close to the place of stelae discovery, at the border of old communication routes, or near Late Bronze Age sites.

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