PLAGIOCLASE TRACE ELEMENT ZONING REVEALS CHANGES IN MAGMA STORAGE AT CERRO UTURUNCU

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. In the CVZ storage in largest of these magma bodies, the Altiplano Puna Magma Body (APMB) is suggested to have fueled caldera forming eruptions in southern Bolivia. Cerro Uturuncu is a dormant stratovolcano constructed above the center of the APMB. To obtain a clear picture of magma assembly and storage, we employ a sub-crystal-scale trace element analysis combined with textural and Sr isotopic data, on plagioclase phenocrysts. Storage pressures, whole rock compositions and phase assemblages are similar across the volcanic history represented by 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). 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. Additionally, Puntas Negras plagioclase demonstrate decreasing Fe and Mg and increasing Sr and Ba with increasing Li. Uturuncu Stage plagioclase phenocrysts have no correlation between Li, Fe, Mg, Sr, or Ba. Plagioclase ⁸⁷Sr/⁸⁶Sr ratios are more variable than observed whole rock ratios. Variation in some plagioclase crystals is comparable to that of the entire population. High ⁸⁷Sr/⁸⁶Sr ratios are related to relic phenocryst cores, while crystals with lower ratios are in equilibrium with the whole rock. This suggests plagioclase cores with high ⁸⁷Sr/⁸⁶Sr ratios and low Eu anomalies are inherited from a more radiogenic source becoming less prevalent over time. 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.