Abstract

RAYO-ROCHA, Lorena  and  ZULUAGA, Carlos A.. MAGMATIC PROCESSES IN THE NEVADO DEL RUIZ VOLCANO: A QUANTITATIVE TEXTURAL ANALYSIS. bol.geol. [online]. 2011, vol.33, n.2, pp.59-72. ISSN 0120-0283.

A detailed petrographic analysis of lava flows of the Nevado del Ruiz Volcano (VNR), including determination of mineral assemblages and microstructures, geochemical analysis and a crystal size distribution (CSD) quantitative textural analysis in plagioclases, helps to constrain interpretations of magmatic processes occurring in this volcano during the last 1,8 Ma. The VNR is one of the northernmost active stratovolcanoes in the Colombian Central Cordillera and displays a typical behavior of a subduction zone volcano with calc-alkaline affinity. This volcano had an eruptive period dominated by effusive behavior during the Pleistocene and its products show a monotonous bulk composition, mainly andesitic to dacitic. Samples analyzed in this work are two-pyroxene andesites ± oxyhornblende ± biotite, with porphyritic and glomeroporphyritic microstructures. Plagioclase displays overgrow and resorption microstructures and crystal oscillatory zoning patterns. There are two plagioclase populations: one with crystals showing finely sieved interiors and clean rims, the other is a population of clean plagioclase crystals lacking any sieved microstructures. The CSD curves show patterns of curvilinear and concave distributions, similar for all samples, with the exception of the lava domes, that show a linear pattern. Pyroxene has poikilitic and resorption microstructures (coarse cellular microstructures) and amphibole shows reaction rims. These characteristics allow to infer abrupt changes in melt composition during the crystallization process probably related with magma mixing processes.

Keywords : Crystal Size Distribution; Colombian Central Cordillera; Nevado del Ruiz Volcano; Igneous Petrology.

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