THE VOLCANIC AND MAGMATIC EVOLUTION OF VOLCAN OLLAGUE, A HIGH-K, LATE QUATERNARY STRATOVOLCANO IN THE ANDEAN CENTRAL VOLCANIC ZONE

Volcan Ollague is a high-K, calc-alkaline composite volcano constructed upon extremely thick crust in the Andean Central Volcanic Zone. Volcanic activity commenced with the construction of an andesitic to dacitic composite cone composed of numerous lava flows and pyroclastic deposits of the Vinta Loma series and an overlying coalescing dome and coulee sequence of the Chasca Orkho series. Following cone construction, the upper western flank of Ollague collapsed toward the west leaving a collapse-amphitheater about 3.5 km in diameter and a debris avalanche deposit on the lower western flank of the volcano. The deposit is similar to the debris avalanche deposit produced during the May 18, 1980 eruption of Mount St. Helens, U.S.A., and was probably formed in a similar manner. It presently covers an area of 100 km2 and extends 16 km from the summit. Subsequent to the collapse event. the upper western flank was reformed via eruption of several small andesitic lava flows from vents located near the western summit and growth of an andesitic dome within the collapse-amphitheater. Additional post-collapse activity included construction of a dacitic dome and coulee of the La Celosa series on the northwest flank. Field relations indicate that vents for the Vinta Loma and post-collapse series were located at or near the summit of the cone, The Vinta Loma series is characterized by an anhydrous, two-pyroxene assemblage. Vents for the La Celosa and Chasca Orkho series are located on the flanks and strike N55-degrees-W, radial to the volcano. The pattern of flank eruptions coincides with the distribution in the abundance of amphibole and biotite as the main mafic phenocryst phases in the rocks. A possible explanation for this coincidence is that an unexposed fracture or fault beneath the volcano served as a conduit for both magma ascent and groundwater circulation. In addition to the lava flows at Ollague, magmas are also present as blobs of vesiculated basaltic andesite and mafic andesite that occur as inclusions in nearly all of the lavas. All eruptive activity at Ollague predates the last glacial episode (approximately 11,000 a B.P.), because post-collapse lava flows are overlain by moraine and are incised by glacial valleys. Present activity is restricted to emission of a persistent, 100-m-high fumarolic steam plume from a vent located within the summit andesite dome. Sr and Nd isotope ratios for the basaltic andesite and mafic andesite inclusions and lavas suggest that they have assimilated large amounts of crust during crystal fractionation. In contrast, narrow ranges in Nd-143/Nd-144 and Sr-87/Sr-86 in the andesitic and dacitic lavas are enigmatic with respect to crustal contamination.