Oxygen and radiogenic isotopes require different contamination paths in the Azufre volcano, Central Andes, northern Chile

Abstract

Azufre Volcano (21°47′S, 68°15′W), located in the Altiplano-Puna Volcanic Complex of the Andean Central Volcanic Zone (CVZ), is formed by two coalescent edifices (Northern and Southern) constructed between 120 and 1300 kaExtensive hydrothermal alteration occurred within a host sequence of andesites/dacites (120–700 ka in age) in the Northern edifice. The silica content of the erupted lavas of this volcano range from ∼60% m/m (andesite) to ∼67% m/m (dacite). These lavas have 87Sr/87Sr ratios ranging from 0.7066 to 0.7074, 143Nd/144Nd ratios ranging from 0.51229 to 0.51239, and quartz δ18O values ranging from 7.9 to 9.8‰. The original magmas had δ18O values from 7.4 to 9.4‰ (average ∼8.0‰) assuming that Δquartz-melt = 0.5‰.A three-stage magmatic evolution is proposed for Azufre. In the first stage, a mantle-derived magma (87Sr/86Sr = 0.70376; δ18O ∼5.7‰) ascended to plagioclase-dominated felsic crustal levels (25–50 km depth), assimilating up to 21% of crustal material. This generated a sub-arc parental melt with 87Sr/86Sr ∼0.705 and δ18O ∼6.5‰. The second stage involves contamination of the parental magma by assimilation of up to 25% of crust with higher 87Sr/86Sr ratios (0.714) and δ18O (6.4–11.8‰) at upper crustal levels (10–25 km depth). This upper crust corresponds to partially molten zones which coincides with the location of the Altiplano-Puna Magma Body. The generated melts have 87Sr/86Sr ratios up to ∼0.7075 and δ18Omelt 8.5–9.4‰, which fractionates high-δ18O quartz (9.0–9.8‰). The final stage occurred at shallow crustal levels (<10 km depth) where the magmas underwent fractional crystallization during assimilation of crustal material, which due to high-temperature hydrothermal alteration had low δ18O value (δ18Omelt 7.4–8.5‰). During this stage, high-δ18O quartz of the previous stage remains as antecrysts, coexisting with the low-δ18O quartz (δ18Oquartz 7.9–9.0‰) fractionated at shallow depths.