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Sr and O Isotope Geochemistry of Volcán , Andean Central Volcanic Zone, : 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 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 ) from andesitic to dacitic composition 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 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 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 mixing models for plagioclase cores 0.712

10O 18 2 versus 87Sr/86Sr ratios for Uturuncu volcanic 0.711 from select Utruruncu lavas and domes. D= 1.5 and D= 3.0. Uturuncu magmas must therefore contain high δ O crustal material. This hypothesis is further supported by textures and isotopic variation (87Sr/86Sr= 0.7098-0.7165) within single plagioclase suggesting 8 Trachy- rocks compared to and 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 (b) features are interpreted to re ect assimilation of heterogeneous upper 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 in uence 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 re ect 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 - 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 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 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 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 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 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. In situ Sr isotope ratios from plagioclase phenocrysts record the pathway of magma Figure 10. GSM 23 UTDM108 Rims of Quaternary volcanic rocks. Modied from Michelfelder et al., UTDM15P3 Figure 9. 40 0.712 0.710 30 n=9 35 n=10 UTDM108 Cores 25 migration through the crust. Core Sr isotopic ratios from old lavas and domes erupted from (2014). 30 0.709 25 0.710 20 20 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.0005 0.001 0.0015 0.002 0.0025 Percent 15 Uturuncu exhibit more variation than cores observed in the youngest lavas and domes Figure 2. Simplied geologic map of Cerro Uturuncu based on Percent Cores 15 Rims Age (Ma) 1/Sr 10 eld mapping and satellite imagery interpretation. 10 5 5 0.7140 0.716 0 suggesting hybridization of the crust over time. 0 Modied from Sparks et al. (2008). 40 45 50 55 60 65 70 75 80 85 90 95 100 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 3. AFC and mixing models suggest that even though whole rock trends are linear, mixing 0.7138 DM90 Cores 0.715 0Microns 50 25 60 UTDM108 Cores Sr Sr UTDM 112 played a minor role in the generation and accumulation of plagioclase phenocryst. Only UTDM 58 0.7136 0.714 86 Figure 5. Figure 7. 50 86 UTDM63 Cores 20 n=8 n=6 Sr/ 40 Sr/ 0.7134 GSM18 Cores 0.713 15 samples DM90 and DM112 show textural and compositional evidence of extensive mixing. 87 87 30 UTDM112 Cores UTDM112 Rims 10 UTDM15P9 Percent Percent 0.7132 0.712 UTDM112 Cores Figure 6. 20 GSM14 Cores 5 10 UTDM28 Cores UTDM112 BR 0.711 0 0 0.7130 40 45 50 55 60 65 70 75 80 85 90 95 100 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.0005 0.0015 0.0025 0.0035 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 Age (Ma) 1/Sr 87Sr/86Sr 35 UTDM 93B Figure 13 and Figure 15. In situ Sr isotope ratios of plagioclase cores and rims from Uturuncu lavas and domes. Core 30 n=9 25 compositions show a more diverse range in ratios in older samples and are more restricted ratios in younger samples. 9. Acknowledgements: 20 0 Microns 100 15 Figure 14. Histograms of Sr isotopic ratios showing the distribution of plagioclase rim and core isotopic variation for Percent The authors wish to thank Frank Ramos for use of and assistance with the thermal ionization mass spectrometer at New Mexico 10 representative lava ows and domes. Samples were selected at random to illustrate the variation between lava 5 State University; the sta at the Geoanalyitical lab at Washington State University and all members of the PLUTONS Research UTDM15P5 0 ows and domes and to express the complexity of the Cerro Uturuncu subvolcanic 40 45 50 55 60 65 70 75 80 85 90 95 100 group for insightful discussions, especially Shan de Silva, Matt Pritchard and Duncan Muir. Jamie Kern for assistance mapping and plumbing system. collecting samples in the eld, and the residents of Quetena Chico, Bolivia and Lipiko Tours for logistical support and hospitality. 40 35 UTDM 112 Support for this work came from the U.S. National Science Foundation EAR-0901148 to TCF and grants to GSM from the n=16 4 90 0.7136 90 0.714 Figure 3. Modal percent phenocrysts versus SiO2 for representative Uturuncu lava and domes. 30 Figure 15. 25 85 Department of Earth Sciences at Montana State University. 80 0.7135 Figure 4. A. Photomicrgraphs of Uturuncu lavas. Pl, plagioclase, Opx, orthopyroxene, Sp, spinel, Bt, . Note 20 0.7132 Sr 80 Sr Percent 86 15 1 86 sieve texture of Pl core in top right image. D. Cross-polarised photomicrographs of plagioclase-hosted MI from 70 1 75 0.713 Sr/

10 Sr/ 87 3 2 0.7128 70 87 5 Mol An % various Uturuncu lavas. Scale bar is 100 microns. From Muir et al., 2014. 2 60 An Molar % 0.7125 0 UTDM106P9 65 UTDM112P10 0 Microns 500 40 45 50 55 60 65 70 75 80 85 90 95 100 Figure 5. Representative views of Cerro Uturuncu geology. (A) View southeast towards the edice of Uturuncu; 3 50 0.7124 60 0.712 %An 0 -500 -300 -100 100 300 500 10. References Cited: 200 400 600 800 -800 -600 -400 -200

(B) Typical ow folding in lava ows; (C) Hydrothermally altered remains of the edice of Uturuncu produced by -1000 Distance (microns) Distance (microns) Mamani, M., Tassara, A., and Woerner, G., 2008, Composition and structural control of crustal domains in the central : Geochemistry Geophysics Geosystems, v. 9. Figure 9. Nomarski dierential interference contrast (NDIC) images and interpretative line drawings of representative plagioclase Mamani, M., Worner, G., and Sempere, T., 2010, Geochemical variations in igneous rocks of the Central Andean orocline (13o S to 18o S): Tracing crustal thickening and magma ; (D) Typical prismatically jointed block from exterior wall of a collapsed dome; (E) Andesite inclusions in an 95 0.7106 94 0.7124 crystals from rocks of Cerro Uturuncu. The images demonstrate the variety of equilibrium and disequilibrium textures associated 92 generation through time and space: Geological Society of America Bulletin, v. 122, no. 1-2, p. 162-182. Uturuncu lava ow; (F) Typical ow front of an Uturuncu lava ow overlying APVC ; (G) Oxidized 90 0.7104 0.7122 4 Michelfelder, G., Feeley, T., Wilder, A., and Klemetti, E., 2013, Modication of the Continental Crust by Subduction Zone Magmatism and Vice-Versa: Across-Strike Geochemical with this complex magmatic system. The line drawings illustrate dissolution surfaces or zones. See text for description for each 3 90

4 Sr autobrecciation found at the terminus of lava ows; (H) Pressure ridge found commonly found in lava ows over 2 1 85 0.7102 Sr 2 0.712 Variations of Silicic Lavas from Individual Eruptive Centers in the Andean Central Volcanic Zone: Geosciences, v. 3, no. 4, p. 633-667. 88 86 crystal. 3 86 Sr/ 80 0.7100 Sr/ 86 0.7118 Michelfelder, G. S., Feeley, T. C., and Wilder, A. D., 2014, The Volcanic Evolution of Cerro Uturuncu: A High-K, Composite in the Back-Arc of the Central Andes of SW Bolivia: 20 m thick. 87 87 84 Molar An Molar %

Molar An Molar % International Journal of Geosciences, v. 5, no. 11, p. 1263. Figure 10. Histograms of anorthite mole percent showing the distribution of plagioclase rim and core anorthite compositions for 1 0.71161 Figure. 6. Temperature and fO calculated from coexisting oxides in natural dacitic lavas and domes. 75 0.7098 82 UTDM15P5 Muir, D. D., Blundy, J. D., Hutchinson, M. C., and Rust, A. C., 2014, Petrological imaging of an active pluton beneath Cerro Uturuncu, Bolivia: Contributions to Mineralogy and Petrology, 2 UTDM58P6 representative lava ows and domes. Samples were selected at random to illustrate the variation between lava ows and domes 70 0.7096 80 0.7114 v. 167, no. 3, p. 1-25. Figure 7. Modal distributions of andestite and dacite plagioclase, orthopyroxene and biotite compositions from 0 200 400 600 800 1000 0 100 200 300 400 500 600 700 800 900 1000 5 Sparks, R. S. J., Folkes, C. B., Humphreys, M. C. S., Barfod, D. N., Clavero, J., Sunagua, M. C., McNutt, S. R., and Pritchard, M. E., 2008, Uturuncu volcano, Bolivia: Volcanic unrest due to mid- and to express the complexity of the Cerro Uturuncu subvolcanic plumbing system. Distance (microns) Distance (microns) Uturuncu lavas and domes. From Muir et al. 2014. crustal magma intrusion: American Journal of Science, v. 308, no. 6, p. 727-769.