The geochemical evolution of volcan Donna Ines in the central volcanic zone of Chile
Moulds, Tari Noelani
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Volcanic rocks from the Mid-Miocene Doha Ines complex vary from medium to high-K andesites and dacites. The most abundant lavas are the medium-K, hornblende andesites from the central vent, Doha Ines proper, which also erupted ignimbrites containing medium-K andesitic pumice. A sister cone, Gemelas, which is presumed to have been active early in the history of Doha Ines, produced medium-K, 2- pyroxene dacites. The lavas are higher in K, Rb, and Ba than the rocks from the central vent. High-K lavas and welded tuffs are found low on the flanks of the volcano and may have erupted from Loma Doha Ines. These rocks have strong crustal signatures including high K, Rb, and Ba concentrations. The variable incompatible- element ratios of the Loma Dona Ines flows, such as K/Rb and Ba/Rb, can only be explained by crustal assimilation. Magma mixing was fundamental in the development of plagioclase resorbtion textures. Two distinct types of magma have been identified which in turn control the degree of crustal assimilation. Magma mixing between chemically similar melts may produce hybrid melts which are near the plagioclase liquidus. These hybrid melts are presumed to have little capacity for crustal assimilation. This scenario is envisioned for the medium-K andesites erupted from the central vent. In contrast, magma mixing between chemically distinct melts may produce hybrid melts which are above the plagioclase liquidus. These hybrid melts are presumed to have assimilated crustal material and produced the evolved flows which erupted from Gemeles and Loma Dona Ines. Compositional variations in rocks from the Doha Ines group are similar to compositional variations observed in volcanic rocks in the Miocene arc-, of which Doha Ines is a part. Lavas erupted from central vents along the Mid-Late Miocene front increase in crustal affinity southward as a major tectonic discontinuity is approached. As at Dona Ines, the greater crustal affinity may be due to combined crustal assimilation and magma mixing.