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 basalti c andesite to dacitic lava flows erupted at Volcan Ollague in the Ande an Central Volcanic Zone. Variation of delta(18)O values (7.1-8.1 part s per thousand relative to SMOW) for the entire lava suite is small an d 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 coma gmatic 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 andesi tic lavas erupted from the summit relative to older flows erupted low on the flanks of the volcano. The oxygen isotope compositions of Ollag ue lavas are explained by a two-stage intracrustal contamination model . In the first stage, parental basalt and basaltic andesite magmas ass imilate large amounts of high-O-18/O-16 lower continental crust. In th e second stage, the andesitic and dacitic magmas melt and assimilate l ow-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 contami nant for the young andesitic flows erupted from the summit probably ha d delta(18)O approximate to -4 parts per thousand and Sr-isotope ratio s (0.707) identical to lavas erupted during the initial cone-building phase of Ollague. In contrast, older lavas erupted from vents low on t he flanks require a crustal source with delta(18)O between 2 parts per thousand and -1 parts per thousand, and Sr-isotope compositions (simi lar to 0.711) similar to the Miocene ignimbrites upon which the volcan o is constructed. The differences in the isotopic compositions of the crustal contaminants can be explained by increasing degrees of hydroth ermal alteration and hybridization of the shallow crust with time or t oward the center of the volcano, or both. Although the delta(18)O valu es for the Ollague lavas are not low, this model for upper crustal con tamination is similar to models advanced for the generation of low-del ta(18)O magmas at other continental volcanic centers. We explain the h igh delta(18)O values at Ollague as resulting from contamination of pa rental basaltic andesite magmas during the earlier episode of differen tiation in high-delta(18)O lower crust. This process may have general application in explaining why eruption of low-delta(18)O silicic magma s is a relatively rare phenomenon.