Sierra De Queupuniyeu

This volcanic province extending for 70 km north-south and 20 km east-west comprises flows and plugs of basanite and olivine basalt, with some pyroclastics, and plugs and necks of nephelinite, tephrite and phonolite. The basalts and basanites contain phenocrysts of olivine, augite and sometimes hornblende, in a matrix of olivine, pyroxene, plagioclase, magnetite, a little biotite and 5-20% nepheline plus analcime. Nephelinites are concentrated in the northern part of the province and form upstanding plugs; they comprise phenocrysts of olivine, augite, biotite and nepheline in a matrix predominantly of pyroxene and nepheline with accessory perovskite. There are rather fewer plugs of tephrite and phonolite, the former being characterized by about 10% analcime and some varieties containing aegirine-augite. The phonolites consist of alkali feldspar, 10-15% nepheline, 15% brown amphibole and 10% aegirine-augite.