Paper No. 171-8
Presentation Time: 9:00 AM-1:00 PM
DYNAMICS AND TIMESCALES OF MAGMATIC PROCESSES AT UTURUNCU VOLCANO, BOLIVIA
In the Andean Central Volcanic Zone (CVZ), parental and mantle magmas migrate through 70-80 km of continental crust. As a result, these magmas are temporarily stored, mixed, and homogenized in mid-crustal magma bodies. Storage within the largest of these magma bodies, the Altiplano Puna Magma Body (APMB), is suggested to have fueled caldera forming eruptions in the Altiplano Puna Volcanic Complex (APVC). Cerro Uturuncu is a dormant stratovolcano constructed above the center of the APMB. Storage pressures and whole rock compositions are similar the four stages of activity: Puntas Negras (250-316 ka), Uturuncu (387-458 ka), Lomo Escapa (505-595 ka), and Cerro Agua de Pajarito (1016-1050 ka). Here we employ in situ trace element analysis and textural data on plagioclase phenocrysts to obtain a clear picture of magma assembly and storage and resolve the magma plumbing architecture over the volcanic history of Uturuncu. We use Sr, Ba, Fe and Mg contents to track the thermal and compositional changes experienced by the plagioclase over time. Uturuncu Stage plagioclase are mostly homogenous with high LREE/HREE ratios and pronounced positive Eu anomalies. Puntas Negras Stage plagioclase display two populations: one like Uturuncu Stage plagioclase and a second with low LREE/HREE ratios and minor positive Eu anomalies. Lomo Escapa and Cerro Agua de Pajarito plagioclase exhibit a large range in LREE/HREE with many demonstrating low Eu anomalies. Low Eu anomalies are inherited from a more radiogenic source and become less prevalent over time in the Uturuncu and Puntas Negras stages. All stages illustrate a similar range in Mg, Ba, Sr, Li, and Rb compared to increasing Fe. However, Cerro Agua de Pajarito and Lomo Escapa plagioclase trace elements are not homogenous. Uturuncu and Puntas Negras display 2-3 distinct homogenous groups of Mg, Ba, Sr, Li, and Rb. These results suggest a steady-state balance of intrusion and crystallization during magma storage at mid and shallow crustal levels which over time modifies the composition of magma stored to become more homogeneous. Overall, Uturuncu and Puntas Negras stage dacites are the product of increased homogenization of the plumbing system, while Cerro Agua de Pajarito Stage magmas are the result of a poorly developed plumbing system with significant influence of the local crust.