Graduate Thesis Or Dissertation
 

Dynamics of a long-lived magmatic system as indicated by variations in amphibole composition and textures in dacites erupted over 11 M.y. at the Aucanquilcha Volcanic Cluster, Central Andes, Chile

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  • The Aucanquilcha Volcanic Cluster (AVC) is the erupted part of a magmatic system with a complex and long-lived history. The AVC lies at 21°S in the high Andes and is built on thick continental crust. The thick crust in the area combined with the prolonged magmatic activity make it an excellent natural laboratory for examining long-term evolution of a continental arc volcanic system. The eruptive products deposited over the last 11 million years of volcanic activity preserve snapshots of the developing, dominantly dacitic, magmatic system and gives indications of the processes occurring at depth. In this study, the textural and compositional diversity of amphiboles from selected dacites inform the development of the intensive magmatic parameters including pressure, temperature and volatile content, during the protracted magmatism observed at the AVC. There are 4 dominant amphibole compositions erupted at the AVC, higher aluminum magnesiohastingsite, pargasite, and tschermakite and the relatively low aluminum magnesiohornblende, using the nomenclature of Leake et al., 1997. Amphiboles in early erupted dacites (11-8 Ma) occur in two compositionally distinct aluminum populations and have diverse textures. During voluminous dacite volcanism between 6 and 2 Ma, amphiboles are most strongly compositionally zoned, and while still displaying textural diversity, some equilibrium textures common in the early and late stages are rare. In the youngest stage (1-0.24 Ma), amphiboles in many dacites have two compositional populations distinguished by alunimum; equilibrium crystals with thin or no reaction rim are most common in the amphiboles from this stage. Changes in Cl, F and S and stable isotopes of the system were used as indicators of the evolution of the magmatic system. Fluorine increases in amphiboles over the 11 million year magmatic history independent of amphibole composition, implying a system-wide increase in F. Sulfur and chlorine in amphiboles correlate well with aluminum in amphibole: low aluminum amphiboles have low S (up to 40 ppm), whereas higher aluminum amphiboles had sulfur contents from 70-160 ppm. Amphiboles with lower aluminum have lower Cl contents than amphiboles with higher aluminum. Coupled amphibole geothermometry and geobarometry are utilized in this study to investigate pressure and temperature of the magmas at the AVC. Amphiboles from the early group are consistent with eruption of dacite from discrete magma batches: some residing at shallow levels of ~1-2 kbar and ~700-800 °C and some deeper at ~4-6.5 kbar and ~750-850 °C. It is interpreted that with time, the dacitic magma reservoir becomes integrated at relatively shallow levels (1.8-3.5 kbar and ~800-900°C). In waning, increasingly silicic stage of volcanism (~1-0.24 Ma), dacite magma is erupted from a shallow and cooler system of ~0.5-1.8kbar and ~700-800 °C.
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