O-18/O-16 ISOTOPE GEOCHEMISTRY OF SILICIC LAVA FLOWS ERUPTED FROM VOLCAN OLLAGUE, ANDEAN CENTRAL VOLCANIC ZONE

Twenty-one new oxygen isotope analyses have been obtained from basaltic andesite to dacitic lava flows erupted at Volcan Ollague in the Andean Central Volcanic Zone. Variation of delta(18)O values (7.1-8.1 parts per thousand relative to SMOW) for the entire lava suite is small and the data as a whole exhibit no simple correlation with any parameter of compositional evolution, except that values for basaltic andesite lavas are lower than those of andesitic and dacitic lavas. Within comagmatic suites of the andesitic and dacitic lavas, however, delta(18)O values are negatively correlated with major and trace element contents and Sr-isotope ratios. Furthermore, the rate of decrease in delta(18)O as a function of compositional evolution is greater for young andesitic lavas erupted from the summit relative to older flows erupted low on the flanks of the volcano. The oxygen isotope compositions of Ollague lavas are explained by a two-stage intracrustal contamination model. In the first stage, parental basalt and basaltic andesite magmas assimilate large amounts of high-O-18/O-16 lower continental crust. In the second stage, the andesitic and dacitic magmas melt and assimilate low-O-18/O-16 hydrothermally altered wall rocks during differentiation in shallow crustal magma chambers. Modeling of magma evolution trends for delta(18)O and Sr-87/Sr-86 suggests that the upper crustal contaminant for the young andesitic flows erupted from the summit probably had delta(18)O approximate to -4 parts per thousand and Sr-isotope ratios (0.707) identical to lavas erupted during the initial cone-building phase of Ollague. In contrast, older lavas erupted from vents low on the flanks require a crustal source with delta(18)O between 2 parts per thousand and -1 parts per thousand, and Sr-isotope compositions (similar to 0.711) similar to the Miocene ignimbrites upon which the volcano is constructed. The differences in the isotopic compositions of the crustal contaminants can be explained by increasing degrees of hydrothermal alteration and hybridization of the shallow crust with time or toward the center of the volcano, or both. Although the delta(18)O values for the Ollague lavas are not low, this model for upper crustal contamination is similar to models advanced for the generation of low-delta(18)O magmas at other continental volcanic centers. We explain the high delta(18)O values at Ollague as resulting from contamination of parental basaltic andesite magmas during the earlier episode of differentiation in high-delta(18)O lower crust. This process may have general application in explaining why eruption of low-delta(18)O silicic magmas is a relatively rare phenomenon.