Graduate Thesis Or Dissertation
 

Volcanology and petrology of Volcán Miño, Andean Central Volcanic Zone

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  • Volcán Miño (21°11'S) is located on the westernmost periphery of a longlived complex of stratovolcarioes and domes called the Aucanquilcha Complex. The Aucanquilcha Complex ranges in age from 11 Ma to 1-lolocene and lies along the main N-S trending axis of Quaternary volcanoes in the Andean Central Volcanic Zone (CVZ). Volcán Aucanquitcha lies at the center of the complex and forms a ridge extending 10 km in an east-west direction; defined by a distinct cluster of andesite and dacite stratocones, dacite domes and a prominent collapse structure and two debris avalanche deposits. In contrast to the main edifice, Volcán Miño (5611 m) is a steep-sided, symmetric andesitic stratovolcano. Volcán Miño lavas range in age from 3.0 to 3.7 Ma and eruptive products are dominantly two-pyroxene ± hornblende andesites. Basaltic andesites and dacites are rare. Volcán Miño lavas conform to regional med- to high-potassium caic-alkaline trends and are characterized by subduction-related light rare earth and large ion lithophile-element enrichments and high field strength element depletions. Miño lavas are distinctive in that they display a restricted range in whole-rock composition, 60±2 weight percent SiO₂. Despite this wholerock compositional homogeneity, lavas are texturally and mineralogically diverse as evidenced by variations of proportions and textures of clinopyroxene, orthopyroxene, and amphibole in assemblages with similar weight percent SiO₂. Volcán Miño lavas exhibit textural evidence for both thermal and chemical disequilibrium including mixed phenocryst populations, xenocrysts, and amphibole breakdown textures. Petrographic observation suggests that these lavas have undergone complicated magmatic histories involving combined mixing, assimilation, fractionation, and hybridization. Major and trace element modeling and petrographic interpretation of disequilibrium phenocryst assemblages elucidate the importance of magma mixing in buffering whole-rock composition in an open magmatic system. Amphibole disequilibrium textures preserve important information about the preeruptive conditions of Volcán Miño's magma chamber and are used here to constrain the range in pressure, temperature, fₒ₂, and water concentration responsible for the generation of Miño andesites.
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