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Sr and O Isotope Geochemistry of Volcán Uturuncu, Andean Central Volcanic Zone, Bolivia

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Sr and O Isotope Geochemistry of Volcán Uturuncu, Andean Central Volcanic Zone, Bolivia Sr and O Isotope Geochemistry of Volcán Uturuncu, Andean Central Volcanic Zone, Bolivia: Resolving Crustal and Mantle Contributions to Continental Arc Magmatism 1 2 2 Missouri State University Gary S. Michelfelder ; Alicia D. Wilder and Todd C. Feeley Volcanology, Petrology and Geochemistry Department of Geography, 1. Department of Geography, Geology and Planning, Missouri State University, Springfield, MO 65897 Geology and Planning 2. Department of Earth Sciences, Montana State University, Bozeman, MT 59717 Research Group 1. Abstract: 3. Bulk Rock Geochemistry: 5. Oxygen Isotope Geochemistry: 7. AFC and Mixing Models: Figure 16. (A) Nd Isotope and Sr isotope constraints on bulk mixing and This study reports oxygen isotope values determined by laser uorination of mineral separates and in situ Sr Figure 8. A. Total alkali versus silica (TAS) 13 4 0.718 Figure 17. Uturuncu Figure 11. Figure 12. isotope ratios (mainly plagioclase) from andesitic to dacitic composition lava ows erupted from Volcán Uturuncu 16 diagram for Uturuncu volcanic rocks. 0.719 assimilation fractional crystallization (AFC) models for Uturuncu lava ows 18 87 86 0.716 in the Andean Central Volcanic Zone (CVZ). Variation in δ O values (6.6-11.8‰ relative to SMOW) for the lava suite 11 3 0.717 and domes. (B) Sr/ Sr ratios versus Rb/Sr ratios for Uturuncu lava ows, 14 B. Major-element oxide concentrations Sr Sr versus SiO for Uturuncu lavas and domes. domes and inclusions with AFC models of primary melts discussed in text and 86 is large and the data as a whole exhibit no simple correlation with any parameter of compositional evolution. 2 86 0.715 0.714 QTZ ‰ QTZ 9 2 - 18 Sr/ 12 MgO C. Trace-element concentrations versus SiO for Sr/ in Michelfelder et al., 2014. Plagioclase separates from nearly all rocks have δ O values (6.6-11.8‰) higher than expected for production of O 2 87 87 0.713 143 144 18 the magmas by partial melting of little evolved basaltic lavas erupted in the back arc regions of the CVZ. Most Trachyte Uturuncu lavas and domes. D. Nd/ Nd ratios δ 7 1 Figure 17. AFC and magma mixing models for plagioclase phenocryst cores 0.712 10O 18 2 versus 87Sr/86Sr ratios for Uturuncu volcanic 0.711 from select Utruruncu lavas and domes. Dacite D= 1.5 and andesite D= 3.0. Uturuncu magmas must therefore contain high δ O crustal material. This hypothesis is further supported by Rhyolite textures and isotopic variation (87Sr/86Sr= 0.7098-0.7165) within single plagioclase phenocrysts suggesting 8 Trachy- rocks compared to andesites and dacites from 5 0 0.709 Figure 18. Hypothetical models to produce core compositions observed. 0.71 O+Na andesite 87 86 7 8 9 10 11 7 8 9 10 11 12 7.5 8.0 8.5 9.0 9.5 10.0 10.5 0.0005 0.0015 0.0025 0.0035 2 the Central Volcanic Zone (CVZ). Sr/ Sr ratios 18 18 18 repeated mixing followed by crustal contamination events occurring in the shallow crustal reservoir. The dacite K δ O-PLG ‰ δ O ‰ δ O ‰ 1/Sr 6 versus Sr concentration for Uturuncu lava composition rocks show more variable and extend to higher δ18O ratios than andesite composition rocks. These 14 3.60 10.5 Figure 18. ows and domes compared to andesites UTU QTZ 18 4 10.0 (a) upper crust (b) features are interpreted to reect assimilation of heterogeneous upper continental crust by low δ O value Dacite UTU PLG C C1 Andesite and dacites from the CVZ. Analytical 12 Figure 16. C1 1 9.5 crystallisation andesitic magmas followed by mixing or mingling with similar composition hybrid magmas with high δ18O values. 2 errors are with the size of the data point. 3.40 9.0 R1 c, r 18 whole rock C2 O ‰ Conversely, the δ O values of the andesites suggest contamination of the magmas by continental crust modied 10 O ‰ C2 0 CVZ andesite and dacite data from Mamani C 18 2 compositionally compositionally crust stratied 18 Ba/Zr 8.5 δ by intrusion of mantle derived basaltic magmas. These results demonstrate on a relatively small scale the strong δ lower crust 60 65 70 75 3.20 R1 ratio Isotope SiO et al. (2008; 2010). L1 contamination contaminantion 2 8 8.0 inuence that intrusion of mantle-derived mac magmas can have on modifying the composition of pre-existing 2.8 0.717 15.67 L1 Crystals grow from magma 7.5 mantle after contamination 2.7 0.716 continental crust in regions of melt production. Given this result, similar, but larger-scale, regional trends in Isotope ratio R2 R2 2.6 6 3.00 7.0 magma compositions may reect an analogous but more extensive process wherein the continental crust 0.715 60 62 64 66 68 70 7 8 9 10 11 12 0.3 0.5 0.7 0.9 1.1 1.3 1.5 2.5 15.66 18 0.714 Pb SiO2 δ O ‰ Age (Ma) Sr O 2.4 86 becomes progressively hybridized beneath frontal arc localities as a result of protracted intrusion of subduction- 204 (c) (d) 2 2 0.713 14 C1 K C1 Sr/ 2.3 Pb/ r r related basaltic magmas. 87 0.712 13 2 207 2.2 15.65 Figure 11. Oxygen isotope values for plagioclase and 2 contaminantion & crystallisation 0.711 12 2 2.1 & contamination crystallization quartz phenocrysts from Utruruncu lavas and domes R1 C2 1 R1 C2 2.0 0.710 10 C 1 1 1.9 0.709 15.64 1 O ‰ 10 contamination compared to major and trace element concentrations. C 18 L1 60 62 64 66 68 70 60 62 64 66 68 70 18.78 18.80 18.82 18.84 18.86 18.88 18.90 Rb/Nb L1 & crystallization δ 2. Background: 206 204 Figure 12. A. In situ Sr isotope ratios from plagioclase contaminantion SiO2 SiO2 Pb/ Pb 7 Two-stage contamination Crystallisation after lower 8 during crystallization crustal contamination 70o W 4.0 0.512 39.00 phenocrysts versus O isotope values. B. Stratigraphic PERU Figure 2. R2 R2 15o S Figure 1. 3.5 0.512 corelation of O isotope values for Uturuncu lavas and La Paz BOLIVIA 6 4 0.512 0.708 0.710 0.712 0.714 0.716 0.718 7 8 9 10 11 12 3.0 38.95 domes. (e) L (f) 87 86 18 2 L2 Nd Pb Sr/ Sr δ O ‰ 0.512 144 204 C 2.5 2 C2 MgO Pb/ Nd/ 0.512 r,C o 3 C3 contaminantion contaminantion & crystallisation 208 20 S 143 R1 2.0 38.90 R1 contaminantion & crystallisation 0.512 C1 CHILE N 1.5 0.512 C1 L 6. In Situ Sr Isotope Geochemistry: 1 L1 Peru-Chile Trench 1.0 0.512 38.85 Magma - Magma Magma - Magma 60 62 64 66 68 70 0.708 0.710 0.712 0.714 0.716 0.718 18.78 18.80 18.82 18.84 18.86 18.88 18.90 20o S Mixing Mingling 87 86 206 204 GSM53 50 PACIFIC Uturuncu SiO2 Sr/ Sr Pb/ Pb 0.714 UTDM90 Rims 0.719 Figure 13. 45 Figure 14. R2 R2 OCEAN DM106 UTDM90 Cores 40 Uturuncu Andesites 0.72 70 35 0.513 CVZ Andesites UTDM90 Aucanquilcha 0.717 0.713 UTDM90 BR 30 CVZ Dacites 60 25 Uturuncu Dacites UTDM108 Cores Sr Sr Percent 20 Antofagasta 25o S 0.716 Ollagüe 86 50 86 0.715 UTDM63 0.712 15 Rims 8. Conclusions: Nd 0.5126 Uturuncu 10 GSM 23 Sr/ Sr 40 Sr/ GSM18 144 5 87 87 0.713 n=9 /86 0.712 0 Sr/Y UTDM112 Sr Nd/ 30 0.711 87 143 GSM14 Crust > 60 km 0.711 0.709 0.711 0.713 0.715 25o S 0.5122 20 0.7094 0.7098 0.7102 0.7106 0.7114 0.7118 0.7122 0.7126 0.7134 0.7138 0.7142 0.7146 0.7154 0.7158 0.708 UTDM28 Quaternary 0.710 30 18 volcanic series 10 0.709 1. The range in δ O values of Uturuncu lavas and domes are above accepted mantle values International 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0 0.001 0.002 0.003 25 Border ARGENTINA 0.5118 0.704 0 Age (Ma) 1/Sr 20 Active 0.702 0.707 0.712 0.717 0 500 1000 1500 0 10 20 30 40 50 suggesting that the magmas underwent signicant dierentiation prior to eruption. The Stratovolcanoes 87 86 0.720 15 70o W Sr/ Sr Sr Y 0.714 18 Figure 3. Figure 4. Percent 10 variation of the δ O values between erupted units and between the quartz and plagioclase 0.718 0.713 5 UTDM 58 n=6 Figure 1. Map showing location of Andean Central Volcanic 0 phenocryst suggest Sr Sr 0.716 0.712 86 Zone (CVZ). Shaded area shows the region where crustal 86 0.709 0.711 0.713 0.715 0.7094 0.7098 0.7102 0.7106 0.7114 0.7118 0.7122 0.7126 0.7134 0.7138 0.7142 0.7146 0.7154 0.7158 that the magma interacted with a variety of dierent contaminants during magma migration. 4. Mineral Chemistry and Textures: Sr/ Sr/ 0.714 0.711 87 thickness exceeds 60 km; stippled region illustrates distribution 87 40 45 35 UTDM 93B 2.
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