Physical and Chemical Models of Zoned Silicic Magmas: The Loma Seca Tuff and Calabozos Caldera, Southern Andes

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Abstract
The late Pleistocene Calabozos ash-flow and caldera complex lies in central Chile in a section of the Andean cordillera that is transitional between dominantly andesitic-to-rhyolitic volcanism to the north and mafic andesitic and high-alumina basaltic volcanism to the south. The Calabozos rocks range in composition from basaltic andesite to rhyodacite and define a high-K calcalkalic suite. They contain 2–25% phenocrysts of plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides, and apatite, ± minor biotite or amphibole. More than 1000km3 of rhyodacitic to dacitic magma erupted at the Calabozos caldera complex as three major compositionally zoned ash-flow sheets, Unit L (0·8 Ma), Unit V (0· 30Ma), and Unit S (0·15 Ma) of the Loma Seca Tuff. Phenocryst modes, trace-element contents, inferred magmatic volatile contents, and oxygen fugacities vary systematically with major-element composition in the tuffs. In the cases of Units V and S, it is possible to reconstruct compositional, thermal, and volatile gradients that existed in density-stratified magma chambers shortly prior to their eruption. The magma graded from crystal-poor, water-rich, and biotite-bearing rhyodacite in the upper reaches of the chamber to more crystal-rich, water-poor, and amphibole-bearing dacite at deeper levels. Fe-Ti oxide equilibration temperatures are ∼800 to 900°C for rhyodacite and ∼900 to 950°C for dacite. Magma that erupted as Unit S was slightly hotter and more oxidized than magma that gave rise to Unit V. More mafic magmas were associated with the voluminous rhyodacitic to dacitic magma reservoir, as indicated by the presence of andesite and basaltic andesite lava flows and by scoria inclusions in Unit V. The compositional suite from basaltic andesite to rhyodacite can be simulated satisfactorily by crystal-fractionation calculations based on major-element phenocryst and rock compositions, and is consistent with modes of the Calabozos rocks. Rhyodacites of Units V and S, however, are enriched in elements such as Rb, Ba, and Zr relative to trace-element contents predicted by crystal-fractionation models. The enrichment can be achieved by assimilation of wall rock or a partial melt of the wall rock. The latter requires that the ratio of assimilation rates to crystallization rates be between ∼0·1 and 0·3. Rates of assimilation versus crystallization were greater for Unit S than for Unit V, which is consistent with the lower Fe-Ti oxide temperatures and less oxidized state of the latter. The Loma Seca Tuff is similar in bulk composition to sanidine-bearing ash-flow sheets erupted on ‘mature’ continental crust, but it is mineralogically akin to ash-flow tuffs erupted on ‘immature’ crust. The difference is attributed, in part, to the effect of the density of the crust on the rate of magma ascent at shallow levels. The ascent of large bodies of silicic magma is slower in silicic (less dense) crust than in mafic crust, causing the magmas to be erupted at a later point in the crystallization history.

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