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Génesis de cristobalita en andesitas alteradas (Los Escullos, Almería)

2015; Issue: 58 Linguagem: Espanhol

2173-6545

María de los Ángeles Bustillo Revuelta, Alfredo Aparicio,

Geological and Geophysical Studies Worldwide

espanolEl presente trabajo aborda el estudio de la composicion mineralogica y petrologica de un afloramiento de andesitas alteradas en la region volcanica de Cabo de Gata (Los Escullos), mediante Difraccion de rayos X, Microscopia Optica (MO) y Electronica (MEB) con Energia Dispersiva. El objetivo es determinar los procesos de alteracion que fueron capaces de formar gran proporcion de cristobalita, independientemente de los procesos de silicificacion que generaron venas y diques siliceos en este afloramiento. La microscopia optica revela que la alteracion afecto solo a la matriz de la andesita, quedando los fenocristales (anfibol, biotita, plagioclasa y opacos) preservados. La matriz esta constituida mayoritariamente por cristobalita y pequenas cantidades de zeolitas y esmectitas, que se formaron posteriormente a la cristobalita. Las maclas de cristales de cristobalita con habitos cuadrangulares y tetragonales, identificados en MEB, sugieren que pudo haber una precipitacion en fase de vapor. Se considera que la cristobalita se constituye por un proceso de desvitrificacion, planteando la hipotesis de que tambien pudo existir cristalizacion de cristobalita en fase de vapor, durante la percolacion de gases en el comienzo del enfriamiento del vidrio volcanico. EnglishThis paper studies the mineralogical and petrological composition of an outcrop of altered andesite in the volcanic region of Cabo de Gata (Los Escullos), using X-ray diffraction, optical microscopy and scanning electron microscopy (SEM) with energy dispersive analyses. The aim is to determine the alteration processes that were able to develop a large amount of cristobalite in the andesites. This cristobalite grew independent of the silicification that generated siliceous veins and dikes in this outcrop. Optical microscopy reveals that the alteration affected only the andesite groundmass, leaving the phenocrysts (amphibole, biotite, plagioclase and opaques) preserved. The paste is mainly composed of cristobalite and small amounts of zeolites and smectites, which were formed in a later stage. The twinnings of cubic and tetragonal cristobalite crystals, identified in SEM, suggest that they might be formed as a vapour-phase. It is considered the cristobalite was formed by devitrification and perhaps by vapour-phase mineralization and that may be necessary a cristobalite precursor formation, in order to provide the nuclei. This process would occur during the percolation of gas at the start of cooling of the glass in the andesitic flow.