EVOLUTION OF THE LONG-LIVED AUCANQUILCHA VOLCANIC CLUSTER, NORTHERN CHILE, AS INDICATED BY VOLATILE AND MAJOR ELEMENT COMPOSITION

The Aucanquilcha Volcanic Cluster (AVC) is located at ~21ºS in the Central Volcanic Zone of the Andes. The AVC is a Miocene to recent cluster of ~20 volcanoes that have erupted andesite and dacite lavas over the last 11 M.y. and define four major, 1-3 million year pulses of volcanism. We here investigate the magmatic development of the AVC through the major and volatile element composition of amphibole.

Major element analyses of a temporally representative suite of amphiboles at the AVC fall into two compositional populations, one low and one high in Al. Amphiboles have a range of textures from euhedral to completely reacted. The range in amphibole compositions and textures is widest in the early volcanic groups with Al^IV ranging from 1.25 to 1.95. Amphibole composition becomes more restricted in the Polán Group (4-2 Ma), with Al^IV 1.35 to 1.85 for the group with individual grains spanning the entire range of Al^IV. In the most recent group, (~1 Ma-present), amphibole is strongly bimodal and textural diversity returns. Fluorine concentrations increase over the lifespan of the AVC from 1250 to 4200 ppm independent of Al. Chlorine in amphibole ranges from 200 to 800 ppm. High Al^IV generally corresponds to lower Cl, while low Al^IV corresponds to higher Cl concentrations. Sulfur ranges 11 to 168 ppm, with a high and a low S population that correspond with Al^IV in amphiboles. High Al^IV generally corresponds to high S, while low Al^IV corresponds to low S concentrations. The water concentration in amphiboles (measured using SIMS) varies from the 0.6-1.5wt% H₂O in the early groups to 1.5-2.5 wt % H₂O in the youngest. δD values in amphiboles display a wide range from -168 ‰ to -41 ‰ δD (+/-8 ‰). δD for the AVC ranges from typical arc magmatic values to lower values than observed in the region to date.

We interpret the overall amphibole compositional and textural trends to reflect early heterogeneity in both composition and thermal structure of the magmatic system beneath the AVC, followed by a stage of homogenization to dacite-dominant. The homogenization coincides with greatest magma productivity and textural homogeneity. In the last stage, an additional shallower, cooler magma reservoir is established and bimodal texturally diverse amphibole populations are produced by syneruptive mixing from this tiered magma reservoir.