Date of Graduation

Summer 2023

Degree

Master of Natural and Applied Science in Geography, Geology, and Planning

Department

Geography, Geology, and Planning

Committee Chair

Gary Michelfelder

Abstract

Magma mixing is a common factor in the creation of intermediate composition magmas and a potential instigator of a volcanic eruption. Magmatic enclaves, physical evidence of magma mixing within a volcanic system, are a phenomenon whose mechanisms remain unclear and debated. Common hypotheses explaining the occurrence of magmatic enclaves within a host lava range from the repeated injection of a new magma into a shallow reservoir to the disruption of equilibrium within a stratified magma chamber. Within the Andean Central Volcanic Zone (CVZ), the occurrence of magmatic enclaves yields similar geochemical compositions to their respective host rocks, bringing more difficulty to the task of identifying magmatic end members before hybridization at these volcanic centers. In this study, whole rock and plagioclase feldspar phenocryst geochemical analysis is applied Volcán Ollagüe’s Chasca Orkho lava series’ magmatic enclaves and host lava to determine the magmatic evolution. This geochemical study utilizes major and trace element concentrations with electron probe microanalyzer (EPMA) and laser ablation ICP-MS on plagioclase feldspar phenocrysts and ICP-MS on whole rock samplesfrom both magmatic enclave and host lava. Trace element data from plagioclase feldspar phenocrysts depict triangular mixing trends, evidence of three magmatic end members within the Chasca Orkho volcanic system: a primitive magma (38%-63% An), magma from a hybridizedchamber (26% - 40% An), and a magma from an altered hybridized chamber (33%-55% An). Due to the petrologic textures and trace element mixing trends of the plagioclase feldspar phenocrysts, the magmatic evolution of the Chasca Orkho series is proposed to be evidence of a stratified magma chamber. The geochemical endmember originating from the hybridized reservoir is the first to be emplaced within the Chasca Orkho shallow reservoir, followed by the basaltic andesitic endmember from an isolated chamber, creating the stratified structure of the chamber. These two endmembers are the basis of the main mixing trend of the Chasca Orkho series. The third endmember is introduced to the hybridization reservoir and alters its composition, creating the muted third endmember of the Chasca Orkho series’ magmatic evolution.

Keywords

entral andes, volcán ollagüe, volcanology, petrology, geochemistry, plagioclase feldspar

Subject Categories

Geochemistry | Geology | Volcanology

Copyright

© Nathaniel W. Lenhard

Download

Open Access

Share

COinS