Date of Graduation

Spring 2020

Degree

Master of Science in Geography and Geology

Department

Geography, Geology, and Planning

Committee Chair

Gary Michelfelder

Abstract

Lascar volcano is one the most active volcano in the Central Volcanic Zone of the Andean Cordillera, with 36 Holocene eruptions including a VEI 4 eruption in April 1993. Activity has not been consistent throughout time, and therefore, the processes behind it are poorly understood. Lascar volcano has cyclic behavior and has had four stages of activity, each of which had a unique eruptive style and product composition. Stage I (<43 >ka) had primarily mafic andesite lavas erupted effusively from a stratocone. Stage II initiated with dome building events ESE of the original vent and culminated in the 26.45 ka Soncor deposit, which is the largest ignimbrite erupted from Lascar. After the Soncor ignimbrite, there was then a period of quiescence, ending with the Tumbres pyroclastic flow (9.2 ka) and Tumbres-Talabre lava (7.1 ka) deposits defining Stage III. The current Stage IV eruptions are Vulcanian to Plinian and produce lava domes, pyroclastic flows, and pumices. The aim of this project is to determine if the bimodal nature of the pumices is caused by rejuvenation of magma into Lascar’s reservoir via diffusion geochronometry of zircon, geochemistry of plagioclase, and petrography of samples. If so, that would explain the cyclic nature and dynamic compositions that Lascar exhibits. The paucity of zircon prevents geochronological study at this time, and results for age dating were inconclusive. However, plagioclase data from other methods suggests that Lascar is not sourced from the Altiplano Puna Magma Body; a model of multiple smaller mama chambers at a variety of depths matches more closely. Additionally, data supports a bimodal chamber in which magma mixing is a core mechanism in differentiation in the system. Geothermobarometry of pyroxenes and plagioclase suggest an average magma chamber depth of approximately 6km.

Keywords

volcanology, geology, mineralogy, geochemistry, plagioclase

Subject Categories

Geochemistry | Geology | Volcanology

Copyright

© Madelaine M. Stearn

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