Antisana volcano: A representative andesitic volcano of the eastern cordillera of Ecuador: Petrography, chemistry, tephra and glacial stratigraphy

Antisana volcano is representative of many active andesitic strato-volcanoes of Pleistocene age in Ecuador's Eastern Cordillera. This study represents the first modern geological and volcanological investigation of Antisana since the late 1890's; it also summarizes the present geochemical understanding of its genesis. The volcano's development includes the formation and destruction of two older edifices (Antisana I and II) during some 400 + ka. Antisana II suffered a sector collapse about 15,000 years ago which was followed by the birth and growth of Antisana III. During its short life Antisana III has generated ≥50 eruptions of small to medium intensity, often associated with andesitic to dacitic lava flows and tephra, as well as with late Pleistocene and Holocene glacial advances. Throughout its long history Antisana's lavas have been characterized by a persistent mineral assemblage, consisting of 30-40 vol% phenocrysts of plagioclase, both clino- and orthopyroxene, and Fe-Ti oxides, with rare occurrences of olivine or amphibole, frequently in a microcrystalline to glassy matrix. This uniformity occurs despite the magma's progressive chemical evolution over ≥400 ka from early basic andesites (53-58 wt% SiO2) to intermediate and Si-rich andesites (58-62% SiO2), and recently to dacites (63-67% SiO2). Chemical diagrams suggest that crystal fractionation was the most likely magmatic process of evolution. The exception to this slowly evolving history was the short-lived emission at ∼210 ka of the Cuyuja lavas from Antisana II that generated a 73 km long andesitic lava flow. Contrasting with Antisana's general magmatic trend, Cuyuja lava (∼11 km3) is a high-Mg andesite with unusually high concentrations of incompatible elements. Antisana developed within the Chacana caldera complex, a large active siliceous center that began ∼3 Ma ago, however its lavas are chemically distinct from coeval lavas of Chacana.