New geological and geochronological constraints on the evolution of the Cotacachi - Cuicocha volcanic complex (Ecuador)

Abstract

Extensive fieldwork at the Cotacachi-Cuicocha Volcanic Complex (CCVC, North of Ecuador) resulted in a new collection of geological data including cartography, chronology, petrography, geochemistry, and morphology. This volcanic complex is formed by a central volcano (Cotacachi: 4939 m asl, current bulk volume of 56 ± 4 km3), several peripheral domes, and a 3 km wide volcanic caldera (Cuicocha: 4.2 ± 0.1 km3). CCVC comprises three stratigraphic members: The first, Cotacachi Basal, represents the initial phase of construction, which started at 173 ± 4 ka with a basal andesitic lava flow succession (∼500 m-thick) including isolated basaltic-andesitic lavas (Verde Tola unit; NE: 113 ± 6 ka, SE: 133 ± 9 ka), the construction of some peripheral amphibole-bearing andesitic domes, such as Muyurcu and Loma Negra (138 ± 4 ka and <108 ka, respectively), and a debris-avalanche deposit to the north-west (0.5–1.8 km3, older than 108 ka). The second member, Upper Cotacachi, consists of an andesitic lava flow succession (∼300 m-thick), younger than 108 ± 6 ka. A gap of activity occurred afterwards from 100 to 70 ka, during which a second debris-avalanche (0.2–1.1 km3, 108 to 65 ka) occurred to the NE, followed by the effusion of the dacitic Piribuela dome (65 ± 2 ka). Afterwards, several superimposed andesitic lava flows were emplaced at the summit, possibly around 15–10 ka since they lack glacial erosion. The third member includes the extrusion of the andesitic Cuicocha pre-caldera domes, which marks the beginning of a new eruptive stage of activity of CCVC during the Holocene, resulting in a violent eruption (3525 ± 35 to 2980 ± 30 a BP; VEI = 5) that partially destroyed the young dome and formed a funnel-shaped caldera (Cuicocha Caldera-Lake), ending with the emplacement of the Wolf and Yerovi post-caldera domes.