Contrasting fractionation patterns for sequential magmas from two calc-alkaline volcanoes in Central America

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Two sequences of alternating units of lavas and pyroclastic rocks from two volcanoes in Central America were sampled and analysed. The rocks from Izalco, the smaller, more mafic cone, show a continuous increase in SiO2, K2O, Zr and La, and a decrease in CaO, MgO, Co and Cr with time. These trends are shown to be consistent with progressive crystal fractionation. Atitlán, a larger and more silicic volcano, displays a discontinuous trend in which geochemical reversals are thought to reflect the development of a series of small bodies of zoned magma. The lavas of each zoned section show decreases in SiO2, K2O, Hf and Ba, and increases in CaO, MgO, Co and V from the bottom of the section to its top. The different trends at Izalco and Atitlán are believed to reflect the restrictions cone height places on the effects of crystal fractionation. As cone height increases, the intervals between eruptions progressively lengthen, allowing more time for crystal fractionation to proceed. The two volcanoes may represent an early and a later stage in the general evolutionary development of large composite cones. Despite the remarkable regularity of the alternating flows and pyroclastic eruptions, no systematic chemical variations were recognized between the two types of lava.

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© 1979 Published by Elsevier B.V. Publisher's version of record: https://doi.org/10.1016/0377-0273(79)90003-9

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Journal of Volcanology and Geothermal Research