Ash-fall tuffs from Pliocene-Pleistocene deposits of Humahuaca and Casa Grande basins, Northwestern Argentina-Key to understanding intermountain basins evolution.

COIRA, B.L.; GALLI, C.I.; MAHLBURG KAY, S.; STOCKLI, D.; FLORES, P.; EVELING, E.

ANDEAN GEOLOGY

SERVICIO NACIONAL GEOLOGIA MINERVA

Lugar: Santiago de Chile; Año: 2020 vol. 1

0718-7092

Ash-fall tuffs in the Pliocene -Pleistocene Uquía, Mal Paso and Alonso Formations, studied in the Quebrada de Humahuaca and Casa Grande intermountain basins (23° and 24°S), can be shown to be linked to the Neogene ignimbrite fields and their corresponding calderas that were part of the Southern Central Andes ignimbrite flare-up and extended westward to near the Bolivian-Chilean and Argentine border. Two clusters of ages were recognized: 4.3 to 2.6 Ma corresponding to the Uquía and Mal Paso formations tuffs, and 2.2 to 2.1 Ma to the Alonso Formation tuffs, in agreement with pulses of ignimbrite eruptions in the Altiplano Puna Volcanic Complex (APVC), and Puna back arc Complex. We present new petrographic, geochemical and geochronological data of those tuffs. They are mainly vitreous to phenocryst-poor of rhyodacite-dacite composition with minor andesites to trachyandesites, characterized by 58-69% SiO2 contents, A/CNK 1-1.4, FeO/MgO (0.8-2.8), which plot in the calc-alkaline range. The major difference between the two groups lies in a slight enrichment in LREE and higher arc affiliation in the case of the Alonso Formation tuffs, although Sm/Yb ratios, indicative of sequestration of HREE in residual hornblende, are similar in both. The new geochemical and geochronological were of crucial importance in pinpointing potential effusive centers among the (APVC) for the ubiquitous ash-fall tuffs, and excellent tools as chronostratigraphic tracers to reconstruct the Pliocene-Pleistocene stratigraphic and tectonic evolution of the Humahuaca and Casa Grande intermountain basins, in the framework of the contemporary volcanic events.