Eruptive history of Incahuasi, Falso Azufre and El Cóndor Quaternary composite volcanoes, southern Central Andes

Abstract

Volcanoes can stay dormant for much longer than 10 ka and hence many Quaternary volcanoes lacking Holocene activity have the potential to become active. Reconstructing the eruptive histories of these volcanoes is an important first step towards evaluating their long-term eruptive probabilities. The southern Central Volcanic Zone (CVZ) of the Andes at ~ 27° S latitude has a notable concentration of Quaternary volcanoes, several considered potentially active, but most of which are poorly known. We reconstruct the eruptive histories of three of these volcanoes, Incahuasi, Falso Azufre and El Cóndor, on the basis of field and satellite image mapping and unspiked K-Ar geochronology, supported by petrography and whole-rock geochemistry. Incahuasi (volume of 62 ± 6 km3) comprises a main conical edifice, capped by a summit crater, that was constructed between ~ 1.6 and 0.7 Ma by mostly andesitic lavas at a growth rate of ~ 0.07 km3/ka. At 0.8–0.7 Ma, activity shifted to the eastern flank, with the emplacement of a trachydacitic lava dome and a trachyandesitic lava field. At 0.35 Ma, a mafic center consisting of overlapping scoria cones and basaltic andesite lava flows was emplaced on the NE flank. Falso Azufre (98 ± 12 km3) is an arcuate-shaped massif with several vents aligned NW-SE and ENE-WSW. It contains the remnant of a Pliocene andesitic edifice. The bulk of the massif was constructed between ~ 0.9 and 0.5 Ma by andesitic and trachyandesitic lava flows. More recent activity (< 0.4 Ma) consists of andesitic to dacitic flows, coulées and domes restricted to the summit regions and the eastern flank. The average Quaternary growth rate is ~ 0.1 km3/ka. El Cóndor (109 ± 8 km3) is a NW-SE elongated massif with two distinct stages of activity. The Pliocene stage consists of andesitic to dacitic rocks. Recent activity (< 0.15 Ma) is among the youngest recorded in the region and consists mainly of trachyandesitic to trachydacitic lava flows. The estimated eruption rate of ~ 0.4 km3/ka is one of the highest of the CVZ. Incahuasi has the lowest long-term eruptive potential of the three volcanoes. Falso Azufre has an intermediate long-term eruptive potential and a minor felsic phase may be ongoing. El Cóndor has the highest long-term eruptive potential and its main constructive phase may still be ongoing. Most CVZ volcanoes, including Incahuasi and Falso Azufre, have longer lifespans and lower average growth rates compared to volcanoes from other arcs, suggesting a significant difference between the CVZ and other arcs.