Obsidian-bearing lava flows and pre-Columbian artifacts from the Ecuadorian Andes: First new multidisciplinary data

Abstract

All known outcrops of obsidian flows in the Cordillera Real (Ecuador) have been mapped and sampled to reconstruct their eruptive history using geological observations, age determinations, and trace element data. Our results bear also on the recognition of the sources of obsidian artifacts, used in pre-Columbian tools, and on the reconstruction of ancient trading patterns in Ecuador. Three obsidian flow groups were identified on the basis of flow structures and state of preservation. The groups are further defined by differences in radiometric age, fission-track measurements made at two laboratories (Pisa, Italy, and Campinas, Brazil), and different trace element patterns, determined by neutron activation analysis (Pavia, Italy). The oldest obsidian flows form the upper part of the Basal Volcanic Complex (BVC), the basement of Quaternary Ecuadorian stratovolcanoes. Their ages fix the upper limit of the BVC at 1.5 Ma, in the central Cordillera Real. The two more recent episodes of obsidian rhyolitic volcanism are date at ∼0.85 Ma and slightly less than 0.2 Ma, corresponding in age to the present volcanic arc.