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

Abstract

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. At a relatively mature stage, the magma chambers may be compositionally, thermally, and density stratified. Temperatures estimated from Fe-Ti oxide and pyroxene thermometry for the chambers beneath Ollagüe range from 1000 to 790°C with increasing SiO2 from 59 to 67 wt.% in the upper reaches, and from 1150 to 1020°C with increasing SiO2 from 53 to 59 wt.% in the lower reaches. The occurrence of basaltic andesite magmatic inclusions within the intermediate lavas and the repeated eruption of monotonous composition andesitic magmas indicate that the shallow chambers are periodically replenished with parental basaltic andesite magmas. Ubiquitous, reversely zoned plagioclase and pyroxene phenocrysts in the lavas at Ollagüe suggest that convective cooling of the basaltic andesite releases buoyant derivative liquid that mixes with the overlying intermediate-composition body of the chambers. Further crystallization and differentiation of the intermediate magmas may take place in solidification zones at the boundaries of the magma chambers. If so, the return of residual liquid from the crystallizing margins and mixing with the interior are highly efficient such that magma differentiation can be modeled as a simple, homogeneous, fractional crystallization process.