Volcanology and geochronology of very long Neogene-Quaternary basaltic flows from southeast Payenia, central-west Argentina

Abstract

The basaltic lava flows located in the southeast of Payenia Volcanic Province, Argentina, show lengths between 40 and 181 km. These flows have transversal sections characterized by external vesicular crusts that surround a massive core hosting vesiculated structures. These characteristics defined them as P-type pahoehoe flows. Based on their surface topography, 2 types of flows were determined: sheet flows and hummocky flows. The first were emplaced through inflation and lateral coalescence of lobes in the front of the flow, giving rise to flat roofs and large lateral width. The second ones exhibit a microrelief given by local inflation structures (e.g., tumuli). It is proposed that the lava flows were initially emplaced as sheet flows on low angle slopes (<1.5º), under a regime of high effusion rates which led to a generalized inflation process. The gradual reduction of the lava supply restricted the circulation of lava to an internal network of tunnels that led to localized inflation processes. It is further stated that the remarkable longitudinal development of these flows is the result of the lava delivery through tunnels together with very low viscosity values (9 to 38 Pa s) and low cooling rates. New K-Ar (unspiked) ages were obtained for some of the flows with values of 2.85±0.18 Ma (Pampa de Luanco flow), 0.862±0.056 Ma (basal flow of the Valle del Colorado Basaltic Plateau) and 0.200±0.027 Ma (El Puesto flow) while in conjunction with other previous radiometric data, a time lapse between ~5 Ma and ~0.2 Ma was established for the emission of these flows.