Petrology of Calc-Alkaline Lavas at Volc´n Ollagüe and the Origin of Compositional Diversity at Central Andean Stratovolcanoes

Volcán Ollagüe (21°17'S) is a large stratovolcano that lies slightly east of the main axis of Quaternary Volcanoes in the Andean Central Volcanic Zone (CVZ). Euptive products range from basaltic andesite to dacite and define a high-K, calc-alkaline suite. This compositional range is similar to the collective compositional range of the other stratovolcanoes in the CVZ, and it provides a record of both early and late-stage differentiation processes operating at the stratovolcanoes. The volumetrically dominant andesitic and dacitic lavas are divided into four eruptive series on the basis of vent locations and petrography. In ascending stratigraphic order they are: the Vinta Loma, Chasca Orkho, post-collapse, and La Celosa series. Whole-rock compositions of the lavas are remarkably similar regardless of eruptive series. Variations in phenocryst assemblages and magmatic fo2 however, suggest differences in subliquidus volatile contents for magma chambers developed beneath the summit of the volcano versus those developed beneath the flanks. Basaltic andesite magmas are principally preserved as quenched inclusions within the andesitic and dacitie lava flows. Large ranges in isotopic ratios over a narrow compositional range indicate that the basaltic andesites were derived by crystal fractionation coupled with large amounts of crustal assimilation. Increasing Ce/Yb ratios with decreasing Yb contents further suggest that this initial stage of differentiation occurred at deep crustal levels where garnet was stable. Additional supporting evidence for differentiation in the deep crust includes isotopic and trace element compositions that indicate assimilation by the basaltic andesite magmas of a crust different from upper-crustal rocks exposed at present in the region. Whole-rock major and trace element trends of the dacitic lavas can be simulated largely by fractional crystallization of parental andesitic magma. The fractionating assemblages for the different eruptive series are consistent with the observed modes of the parent magmas. Small increases in Sr isotope ratios with increasing Rb contents indicate that the fractionating magmas also assimilated small amounts of wall rocks similar in composition to the upper-crustal basement to the volcano. Consideration of the chemical trends, mineral compositions, and eruptive history of Ollagüe rocks permits construction of a model for the evolution of shallow crustal magma chambers beneath the stratovolcanoes in the CVZ. A