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Book Chapter

Styles of zoning in central Andean ignimbrites; Insights into magma chamber processes

By
S. L. de Silva
S. L. de Silva
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Published:
January 01, 1991

Calc-alkaline high-K ignimbrites from the Altiplano-Puna Volcanic Complex (APVC) of the Central Volcanic Zone (CVZ) of the Andes exhibit a variety of compositional zonations. The Caspana ignimbrite member (∼8 km3) is zoned from andesite (59 percent SiO2) to rhyolite (~75 percent SiO2), with a large compositional gap between these two compositions. The 1.3 Ma Lower Purico ignimbrite member (∼80 to 100 km3) is zoned from andesite (∼59 percent SiO2), through a large volume of dacite (∼65 percent SiO2), to a rhyodacite (∼72 percent SiO2); small compositional gaps of ∼4 to 5 percent SiO2 are suggested. The Puripicar ignimbrite (>500 km3) is a crystal-rich dacite that has a small proportion (<0.1 percent by volume) of a compositionally distinct, second pumice population. The largest volume ignimbrites (∼1,000 km3) are crystal-rich dacites that show no evidence of compositional heterogeneity and appear to represent huge volumes of homogeneous magma. These “monotonous ignimbrites” represent the dominant eruptive style of ignimbrites in this region.

The characteristics of the juvenile material from the zoned and heterogeneous ignimbrites suggest that crystallization of the observed phenocrysts occurred in prezoned magma chambers consisting of two or more layers. The spectrum of compositions and layering can be produced by fractional crystallization accompanied by assimilation in closed magma chambers. The preferred mechanism producing the layering through fractional crystallization processes is convective fractionation. In the case of the large-volume Puripicar magma chamber, open-system recharge is preferred. The characteristics of the ignimbrites from the APVC indicate that in general the smaller eruptions (<100 km3) show the largest range in compositions and are commonly characterized by a large compositional gap, while the largest volumes erupted (>1,000 km3) are homogeneous. This relationship appears to be typical for ignimbrites in general and suggests an inverse volume-zonation relation (e.g., Smith, 1979).

The role of the aspect ratio (width/height) or w/h of a magma chamber is highlighted as a critical control on the style of zonation that may develop in a closed magma chamber. Small-volume magma chambers are approximately equant (w/h <2) and sidewall cooling and crystallization will be important leading to zonation in the system through convective fractionation. However, larger-volume chambers will tend to be more slab-like (w/h >2) where roof and floor cooling and crystallization will be dominant. In such cases convective fractionation is not efficient in generating zonation of the magma body and homogeneous magmas develop. Changes in the magma chamber aspect ratio and shape with volume is suggested to be an important factor in controlling the inverse volume/zonation relationship apparent for ignimbrites. Furthermore, for a given composition of parent magma, it appears possible to generate a spectrum of zonation styles simply due to variation in aspect ratio, shape, and geometry.

The evidence of large volume “monotonous ignimbrites” is taken to suggest that not all pyroclastic eruptions tap chambers that are compositionally zoned, that homogeneous magma chambers exist, and their contents are erupted.

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Contents

GSA Special Papers

Andean Magmatism and Its Tectonic Setting

Russell S. Harmon
Russell S. Harmon
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Carlos W. Rapela
Carlos W. Rapela
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Geological Society of America
Volume
265
ISBN print:
9780813722658
Publication date:
January 01, 1991

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