LASCAR VOLCANO, NORTHERN CHILE - EVIDENCE FOR STEADY-STATE DISEQUILIBRIUM

Lascar Volcano, located in the Central Volcanic Zone of Northern Chile , is a medium- to high-K calc-alkaline stratovolcano. Three stages are recognized in the construction of the edifice, with activity ranging from andesitic fissure eruptions to a large-volume dacitic pumice flow , the Soncor Flow. The eruption products are predominantly two-pyroxen e andesites and dacites, with some hornblende dacites occurring in the Soncor Flow. Disequilibrium textures resulting from mixing between ma fic and acid end-members are recognized in all the eruption products, and geochemical modelling has been carried out to quantify the effects of magma mixing and fractional crystallization. A model is developed which explains the observed textures by the existence of a continuousl y fractionating magma chamber which receives periodic influxes of basa ltic andesite. Primary anhydrite has been found in the Soncor pyroclas tic flow. Anhydrite formation and sulphur dioxide degassing are explai ned in terms of oxidation and quenching of sulphide-rich mafic magma d uring mixing with the resident more evolved magmas.