Abstract

ARNOSIO, Marcelo. Textural and geochemical evidence for magma mixing in the Chimpa Volcano, Puna Salteña. Rev. Asoc. Geol. Argent. [online]. 2010, vol.66, n.1-2, pp.253-270. ISSN 0004-4822.

The eruptive history of the Chimpa volcano (24º00´S-66º08´O) comprises an explosive cycle that erupted andesitic ignimbrites and blocks and ash flow deposits, and an effusive cycle, represented by the discharge of andesitic and basaltic andesite lava flows. The first material emitted during the explosive cycle corresponds to ignimbrites, which do not show a clear evidence of basic magma interaction. On the contrary, the blocks and ash flow deposits show field and petrographic evidence suggesting a process of interaction with mafic magmas (mingling). The latter is represented by the presence of mafic inclusions and development of disequilibrium textures in plagioclase and biotite. However, the andesitic magma composition was not modified by the mingling process. During the second cycle, a new magma is generated due to magma mixing between andesitic and more mafic magmas. This hybrid magma has a disequilibrium mineral association, including olivine and quartz with well-developed clinopyroxene reaction rims; hornblende and biotite with reaction rims of plagioclase + orthopyroxene + magnetite; and orthopyroxene with an overgrowth of clinopyroxene. All these phases coexist with other crystals of the same mineralogy, but in apparent equilibrium. In geochemical diagrams, analyses of the hybrid lavas occur in an intermediate position between andesitic and basaltic andesite end members, defining a linear tendency. The proportion of end members involved in the mixture is 56% for the andesitic magma and 44% for the basic magma. The petrographic and chemical data suggest an increasing interaction between raising basic magmas and a resident andesitic magma with time.

Keywords : Chimpa volcano; Puna; Magma mixing; Hybridization; Disequilibrium textures.

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