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

Azufre Volcano (21 degrees 47 & PRIME;S, 68 degrees 15 & PRIME;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 & delta;18O values ranging from 7.9 to 9.8%o. The original magmas had & delta;18O values from 7.4 to 9.4%o (average -8.0%o) assuming that & UDelta;quartz-melt = 0.5%o.A three-stage magmatic evolution is proposed for Azufre. In the first stage, a mantle-derived magma (87Sr/86Sr = 0.70376; & delta;18O-5.7%o) 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 & delta;18O-6.5%o. 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 & delta;18O (6.4-11.8%o) at upper crustal levels (10-25 km depth). This upper crust cor-responds 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 & delta;18Omelt 8.5-9.4%o, which fractionates high-& delta;18O quartz (9.0-9.8%o). 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 & delta;18O value (& delta;18Omelt 7.4-8.5%o). During this stage, high-& delta;18O quartz of the previous stage remains as antecrysts, coexisting with the low-& delta;18O quartz (& delta;18Oquartz 7.9-9.0%o) fractionated at shallow depths.