Date of Graduation

Summer 2022

Degree

Master of Science in Geospatial Sciences

Department

Geography, Geology, and Planning

Committee Chair

Gary Michelfelder

Abstract

Aucanquilcha Volcanic Cluster (AVC) is an 11 m.y. volcanic system in the central Andes that is evolving over four distinct stages of activity. Stages include the Aloncha (11-8 Ma), Gordo (6-4 Ma), Polán (4-2 Ma), and Aucanquilcha (Ma) stages. The AVC evolved from a series of magmatic underpinnings during the Aloncha Stage to a larger zone of melting, assimilation, storage, and homogenization (MASH) by the Polán Stage. The transition from smaller underpinnings to MASH zones began during the Gordo Stage. At ~5-2 Ma the AVC reached thermal maturity before beginning its volcanic death during the Aucanquilcha Stage. This study focuses on elucidating the magma plumbing system architecture during the Gordo Stage to better understand the AVC’s evolving magmatic processes from 6-4 Ma. Magma evolution was determined by analyzing major and trace element chemistry of whole rock composition, plagioclase phenocrysts, and pyroxene phenocrysts. Populations of plagioclase include plagioclase phenocrysts with variation between Sr and Ba (Type 1), re-equilibrated Sr but not Ba (Type 2), and no variation in Sr or Ba (Type 3). The three types are further split into textural categories with patchy resorption, sieved rims, abundant patchy resorption in the core, or abundant sieving in the cores. All plagioclases have oscillatory zoning throughout and some normal zoning profiles in the outer mantle and rim. Molar % An ranges from ~30-90 and does not have a correlation with Sr/Ba values. Although some variation still exists, An composition across core-to-rim transects becomes more homogenous over time after multiple eruptions, except for one likely isolated eruption. In general pyroxene phenocrysts have MG# values ranging from ~0.45-0.75. Pyroxene populations include clinopyroxenes (Type 1) and orthopyroxenes (Type 2). Pyroxene population types are further classified based on if they have normal zoning, reverse zoning, oscillatory zoning and if they are phenocrysts, antecrysts, or xenocrysts. In addition to determining overlapping plagioclase phenocryst and pyroxene phenocryst populations, the magma plumbing system architecture was determined by analyzing rare earth element (REE) trends. REE trends for plagioclases and pyroxenes represent an open system for AVC magmas containing trends indicative of crystals residing in equilibrium, going through fractional crystallization, and being included from magma mingling. REE core patterns have multiple groupings of different trends that indicate AVC magmas have an interactive system of different source reservoirs in the crust. REE rim patterns have less variation indicating the magmas homogenized, but variation between the cores, mantles, and rims suggest a final magma mixing event triggered eruptions at the AVC. In summary, the AVC is long-lived volcanic system that began to evolve from a series of magmatic underpinnings from ~6-4 Ma to a series of connected, developing MASH reservoirs in the crust at ~15-30 km deep.

Keywords

Aucanquilcha Volcanic Cluster, Gordo, Polán, magma storage, magma plumbing system architecture, plagioclase, pyroxene, central Andes, Chile

Subject Categories

Geochemistry | Geology | Volcanology

Copyright

© Kasey Lynn Buckley

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