The geochemical evolution of the Cerro Uturuncu magma chamber, sw Bolivia and its relation to the Andean Central Volcanic Zone
Michelfelder, Gary Scott
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Cerro Uturuncu is a composite volcano located in the back-arc of the Andean Central Volcanic Zone (22.27°S, 67.18°W). The volcano has exclusively erupted crystal-rich andesite and dacite lava flows over the ~800,000 year life span of eruptive activity. This study provides new bulk-rock major- and trace- element, 18 O/16 O isotope ratios, Sr, Nd, and Pb radiogenic isotopic ratios of lava flows, domes, magmatic inclusions and xenoliths. The study also adds major and trace element and Sr isotopic ratios of plagioclase phenocrysts in order to examine the evolution of the magmatic plumbing system beneath Cerro Uturuncu and place the volcanic center in the regional context of the CVZ. Plagioclase crystals from silicic (andesitic to dacitic) lavas and domes at Volcan Uturuncu exhibit large variations in An contents, textures, and core to rim 87 Sr/86 Sr ratios. Many of the isotopic variations cannot have existed at magmatic temperatures for more than a few thousand years. The crystals likely derived from different locations in the crustal magmatic system and mixed just prior to eruption. Uturuncu magmas initially assimilated crustal rocks with high 87 Sr/86 Sr ratios. The magmas were subsequently modified by frequent recharge of more mafic magmas with lower 87 Sr/86 Sr ratios. A typical Uturuncu silicic magma therefore only attains its final composition just prior to or during eruption. On an arc-wide scale silicic lavas erupted from three well-characterized composite volcanoes between 21°S and 22°S (Aucanquilcha, Ollague, and Uturuncu) display systematically higher K 2O, LILE, REE and HFSE contents and 87 Sr/86 Sr ratios with increasing distance from the arc-front. In contrast, the lavas have systematically lower Al 2O 3, Na 2O, Sr, and Ba contents; LILE/HFSE ratios; 143 Nd/144 Nd ratios; and more negative Eu anomalies. Silicic magmas along the arc-front apparently reflect melting of relatively young, mafic composition amphibolitic source rocks with the continental crust becoming increasingly older with a more felsic bulk composition toward the east. This results from progressively smaller degrees of mantle partial melting, primary melt generation, and crustal hybridization with distance from the arc-front.