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Mojanda volcanic complex (Ecuador): development of two adjacent contemporaneous volcanoes with contrasting eruptive styles and magmatic suites

Abstract:

In Ecuador, Volcan Mojanda, previously thought to be a single edifice, consists of two contemporaneous volcanoes, Mojanda and Fuya Fuya. Despite their proximity and contemporaneity, these volcanic centres continuously showed contrasting eruptive dynamics and geochemistry. Andesitic lava flows form the main part of basal Mojanda (Moj I). Following caldera collapse, a small andesitic stratocone (Moj II) was built, consisting mainly of basic andesite lava flows, scoria flow deposits and a thick summit series of vitric brecias. This cone was partly destroyed by phreatoplinian eruptions that led to the formation of a small, summit caldera. Fuya Fuya grew on the western flank of basal Mojanda and was contemporaneous with Mojanda II. Its activity began with andesitic and dacitic viscous lava flows and domes (FF I) and continued with a period of intense pyroclastic activity (FF II), during which two voluminous Plinian airfalls of rhyolitic Pumice (R1 and R2) were erupted. Later, the activity of Fuya Fuya became effusive with the building of an intermediate andesitic edifice, the San Bartolo cone (FF III). Subsequently, a Mount St. Helens-like collapse event occurred in Fuya Fuya which was responsible for the loss of a large part of FF III and the western part of Mojanda. This avalanche (unit FF IV) was accompanied by voluminous pyroclastic flows and was followed by the construction of a dacitic dome complex (FF V) within the avalanche caldera. Both the Fuya Fuya and Mojanda magmatic suites show adakitic characteristics. This adakitic character is more marked for the Fuya Fuya volcanics than for the Mojanda rocks, suggesting different sources. Nevertheless, several arguments suggest that both volcanoes are related in their development and magmatic evolution. For example, the older rhyolitic Plinian deposits (R1) of Fuya Fuya contain juvenile andesitic clasts that have petrogenetic affinity with the Mojanda suite, a characteristic also observed in the ash flow deposits emitted after the avalanche event in Fuya Fuya.