COMPOSITIONAL AND TEXTURAL CHARACTERIZATION OF CERRO GORDO, CENTRAL VOLCANIC ZONE, CHILE

Cerro Gordo, at 21°15'S, 68°34'W, is a highly glaciated, Miocene stratovolcano from the Central Volcanic Zone in the Andes of northern Chile. It lies in the southwestern part of a cluster of 12 volcanoes, ranging from about 12 to 2 Ma, which surround and underlie Aucanquilcha Volcano, a major Quaternary edifice. Cerro Gordo has an age around 5.3 Ma, based on 40Ar/39Ar dating of plagioclase, and it has a volume of approximately 29 km3.

A prominent W-SW trending collapse scar exposes crudely conformable stacks of intercalated lavas and scoria layers, cut by compositionally similar dikes. Cerro Gordo's fine-grained and generally crystal-poor lavas commonly display flow layering of dark blue-gray aphanitic groundmass with variable sized phenocrysts of plagioclase (dominant mineral phase), clinopyroxene and olivine, and Fe-Ti oxides. In general, olivine is more abundant in the older flows while clinopyroxene is prevalent in the younger flows; amphibole is also present to a lesser degree. Glomerocrysts of plagioclase, cpx, and oxides are widespread, making approx. 1% of most samples. Compositionally similar, but texturally discordant, magma inclusions are also present in a few samples.

Chemical analyses of Cerro Gordo's flows and dikes reveal a range of composition from andesite and trachy-andesite to dacite with silica values from 58 to 64wt% and K2O distribution from 1.5 to 2.7wt% which includes both the medium and high-K series. Systematically, with increasing weight percent silica, olivine content decreases. Plagioclase occurs in approx. 4 textural varieties among phenocrysts: euhedral spongy; twinned euhedral and spongy; spongy resorbed; and plain euhedral to anhedral (uncommon). Also, compositional zonation and flow alignment of plagioclase is widespread. The pyroxenes, often twinned, and olivine are generally, but not always, subhedral and resorbed. Amphibole occurs mostly as relict hornblende with oxide replacement and resorbtion rims.