Quizapu volcano, Chile
Magmatic processes under Quizapu Volcano, Chile identified from geochemical and textural studies.
Contributions to Mineralogy and Petrology, 2015
Michael
D Higgins1, Stéphanie Voos2, Jacqueline Vander
Auwera2
1 Université du Québec à Chicoutimi, Canada. 2 Université de Liège,
Belgium
Abstract
Quizapu is part of a linear system of active volcanos in central Chile. The volcanic petrology and geology have been used to infer the plumbing system beneath the volcano. The 1846-7 eruption (~5 km3) started with small flows of dacite, then changed to a range of andesite-dacite compositions and finally terminated with large flows of dacite. Andesitic enclaves (<10%) occur in some of these flows. Activity restarted explosively in 1932 (~5 km3 DRE) with an initial andesite-dacite ash, followed by uniform dacite ash and then a terminal andesite ash. All samples, including the enclaves, have chemical compositions that lie on an almost perfect mixing line, with a few exceptions. The abundant plagioclase macrocrysts in the matrix were divided into five petrographic classes on the basis of colour in cold-cathode cathodoluminescence images and zonation in visible light. All populations of macrocrysts have CSDs characteristic of coarsening, although they differ in detail. Two classes can be ascribed to growth in andesite and dacite magmas, but the 3 other classes are associated with particular magma batches. A model is developed which started when, andesite magma ponded in the crust and differentiated to produce a dacite magma, most of which probably solidified to make a granodiorite batholith. Activation of a N-S fault enabled volcanism: Andesite magma traversed the dacite-filled chamber, heating and raising it up into storage areas hosted by the fault. Here, bubble growth on plagioclase promoted mixing. A short time before the 1846-7 eruption more andesite magma was injected into the shallow part of the system where it mingled with existing mixed magmas. The first magma to be erupted from Quizapu was a dacite, but soon other storage areas along the fault started to feed the system - first mixed magmas, then back to dacites. The eruption then terminated until 1932 when renewed injection of andesite into the system created a conduit that tapped a deeper, undegassed dacite chamber and resulted in a strong explosive eruption. The whole story is one of continual andesite magmatism, modulated by storage, degassing and mixing.