SUPPLEMENTARY FILE. Whole-Rock Geochemistry

Transcription

1 GSA Data Repository Item Guzmán, S., Strecker, M.R., Martí, J., Petrinovic, I.A., Schildgen, T.F., Grosse, P., Montero- López, C., Neri, M., Carniel, R., Hongn, F.D., Muruaga, C., and Sudo, M., 2016, Construction and degradation of a broad volcanic massif: The Vicuña Pampa Volcanic Complex, southern Central Andes, NW Argentina: GSA Bulletin, doi: /b SUPPLEMENTARY FILE Whole-Rock Geochemistry At the Washington State University laboratory (Seattle, USA), samples were analyzed with a ThermoARL Advant XP+ sequential X-ray fluorescence (XRF) spectrometer. Samples were crushed, split and pulverized using an agate mill at the Universidad Nacional de Cordoba, Argentina. This rock powder was analyzed at the Washington State University lab with a dilithium tetraborate flux (2:1 flux:rock) to produce fused beads, fusing at 1000 C in a muffle oven, and cooling; the bead was then reground, refused and polished on diamond laps. Natural rocks of known composition and pure quartz reagent (blank) were used as reference standards (USGS standard samples using the values recommended by Govindaraju, 1994). Details on the procedure are given in Johnson et al. (1999). At ACME Labs (Vancouver, Canada), samples were crushed, split and pulverized to a grain size lower than 200 mesh and fused in a platinum gold crucible with a commercial lithium tetraborate flux. The molten material was cast in a platinum mold obtaining fused discs that were analyzed by XRF. Natural rocks of known composition and pure quartz reagent (blank) were used as reference standards. The analytical accuracy was controlled using the geological standard material STD SO-18 and STD SY-4(D) which represent similar materials; based on replicate analyses the estimated uncertainties for major element measurements are <0.04 wt. % for all elements. Detection limits are between 0.1 and 0.01%. Age Determinations Age determinations by the 40 Ar/ 39 Ar method were carried out in fresh biotite, amphibole and plagioclase samples at the geochronology laboratory in the University of Potsdam. Mineral separates were obtained at the Universidad Nacional de Salta laboratory in Argentina following standard techniques (i.e., crushing, milling, sieving, and mineral separation by a Frantz Magnetic Separator, followed by separation with paper shaking, cleaning in an ultrasonic bath, drying at C over 24 h, and handpicking under a binocular microscope). Neutron activation of the samples was performed at the RODEO facility of the reactor in NRG Petten, Petten, the Netherlands for sample VP-10 and VP-17, and also at the CLICIT facility of the Oregon State TRIGA Reactor (OSTR) of the Radiation Center of Oregon State University, USA, for the other samples. The samples were irradiated over 10 h with a fast neutron flux of n/cm 2 /s at the former reactor, while the samples were irradiated over 4 h with a fast neutron flux of n/cm 2 /s at the latter, with the aim of obtaining enough 39 Ar from the 39 K (n, p) 39 Ar nuclear reaction. As an age standard sample, sanidine from the Fish Canyon Tuff was irradiated together with our samples in order to obtain J values, which reflect the degree of neutron activation. The sanidine had been dated at the Geological Survey of Japan, providing an age of 27.5 Ma (Uto et al., 1997; Ishizuka, 1998). After a few weeks, the samples had been cooled down at the OSTR Page 1 of 7

2 and were ready for Ar isotopic analysis at the University of Potsdam. The Ar analytical system in Potsdam integrates a New Wave Gantry Dual Wave laser ablation system with a 50W CO 2 laser (wavelength: 10.6 m) for heating samples and extracting gas. In addition, the system contains an ultra-high vacuum purification line with Zr Al alloy SAES getters and a cold trap maintained at the freezing temperature of ethanol, as well as a Micromass 5400 sector-type noble gas mass spectrometer, which has a high sensitivity and an ultra-low background. Samples were analyzed by stepwise heating. The biotite, amphibole and plagioclase grains were heated by a CO 2 laser beam for periods of between 1 and 1.5 min per step. The Ar isotopic analysis by stepwise heating was repeated until total fusion of the sample occurs. The age was then calculated following Uto et al. (1997). Several corrections were then applied to the results, including system blank, mass discrimination, post-irradiation decay of 37 Ar and 39 Ar, and neutron-induced interferences from Ca and K. We thus obtained plateau, isochron, and total-gas ages. The criteria generally used to determine the plateau steps are the ones summarized by McDougall and Harrison (1999). Used criterion to determine the plateau steps are those of Fleck et al. (1977). These are, (1) the ages of two consecutive steps should agree to within ± 2 sigma, excluding the J value error, (2) the total plateau length should include more than 50% of the total 39 Ar released, and (3) each degassing step making up the plateau should comprise at least 3% of the total 39 Ar released. Two samples were successfully dated. Root Complex sample VP-10 was dated in amphibole separates by stepwise heating; we obtained a plateau age of ± 0.05 Ma (Table DR1, Fig. DR1). Sample VP-17 from the Upper Lava Flows Succession was dated in plagioclase (Table DR2, Fig. DR2) and in biotite (Table DR3, Fig. DR3) giving substantial age differences of the order of 800 k.y., with plateau ages of ± 0.06 Ma and ± 0.04 Ma, respectively. We performed four additional age determinations (not shown) for other units of the Vicuña Pampa Volcanic Complex in plagioclase separates, but were not able to obtain reliable results because of relatively noiser Ar signal by aging the electron-multiplier for the detector of MM5400 mass spectrometer. Additionally, the ubiquitous presence of sieve textures in plagioclases may also have contributed to the inefficacy of these absolute age determinations. Thus, for the Upper Lava Flows Succession we consider that the biotite age of ± 0.04 Ma is a better estimation of the age of this unit. REFERENCES CITED Fleck, R.J., Sutter, J.F., and Elliot, D.H., 1977, Interpretation of discordant 40 Ar/ 39 Ar age-spectra of Mesozoic tholeiites from Antarctica: Geochimica et Cosmochimica Acta, v. 41, p , doi: / (77) Govindaraju, K., 1994, Geostandards Newsletter, Special Issue, v. 18, p. l l 58. Ishizuka, O., 1998, Vertical and horizontal variation of the fast neutron flux in a single irradiation capsule and their significance in the laser-heating 40 Ar/ 39 Ar analysis: Case study for the hydraulic rabbit facility of the JMTR reactor, Japan: Geochemical Journal, v. 32, p , doi: /geochemj Johnson, D.M., Hooper, P.R., and Conrey, R.M., 1999, XRF analysis of rocks and minerals for major and trace elements on a single low dilution Li-tetraborate fused bead: Advances in X- Ray Analysis, v. 41, p Page 2 of 7

3 McDougall, I., and Harrison, T.M., 1999, Geochronology and Thermochrology by the 40 Ar/ 39 Ar method, 2 nd edition: New York, Oxford University Press, 269 p. Uto, K., Ishizuka, A., Matsumoto, A., Kamioka, H., and Togashi, S., 1997, Laser-heating 40 Ar/ 39 Ar dating system of the Geological Survey of Japan: System outline and preliminary results: Bulletin of the Geological Survey Japan, v. 48, p Page 3 of 7

4 Table 1 Ar/Ar analytical result of VP-10 Unit: Root Complex Dated mineral: amphibole Laboratory ID C11025 Reactor: NRG Petten Steps #Laser output J= Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar (x10 3) K/Ca 40 Ar* 39 Ar K 40 Ar*/ 39 Ar K Age (±1s) Ma ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.19 Plateau age (5 steps from 3.8 to 3.2%) ± 0.05 Total gas age ± 0.13 Normal isochron age (of plateau steps) 12.5 ± 0.3 Inverse isochron age (of plateau steps) 12.5 ± 0.3 #100% corresponds to 50W output of CO2 laser. All the errors indicate 1 sigma error. *radiogenic 40 Ar Age (Ma) ± 0.05 Ma 5 Total gas age: ± 0.13 Ma VP-10 amphibole (C11025) Ar released (Cumulative %) Fig. 1 Page 4 of 7

5 Table 2 Ar/Ar analytical result of sample VP-17 Unit: Upper Lava Flows Succession Dated mineral: plagioclase Laboratory ID: C11026 Reactor: NRG Petten Steps #Laser output 40 Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar (x10 3) K/Ca 40 Ar* 39 Ar K 40 Ar*/ 39 Ar K Age (±1s) Ma J= ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.16 Plateau age (8 steps from 2 to 3.6%) 11.4±0.06 Total gas age 11.41±0.06 Normal isochron age (of plateau steps) 11.26±0.12 Inverse isochron age (of plateau steps) 11.26±0.12 #100% corresponds to 50W output of CO 2 laser. All the errors indicate 1 sigma error. *radiogenic 40 Ar ± 0.06 Ma Age (Ma) 10 5 Total gas age: ± 0.06 Ma VP-17 plagioclase (C11026) Ar released (Cumulative %) Fig. 2 Page 5 of 7

6 Table 3 Ar/Ar analytical result of sample VP-17 Unit: Upper Lava Flows Succession Dated mineral: biotite Laboratory ID C11024 Reactor: NRG Petten Steps #Laser output 40 Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar (x10 3) K/Ca 40 Ar* 39 Ar K 40 Ar*/ 39 Ar K Age (±1s) Ma J= ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.14 Plateau age (6 steps from 2.2 to 3.6%) 12.19±0.04 Total gas age 11.16±0.04 Normal isochron age (of plateau steps) 13.0±0.3 Inverse isochron age (of plateau steps) 13.0±0.3 #100% corresponds to 50W output of CO 2 laser. All the errors indicate 1 sigma error. *radiogenic 40 Ar 15 Age (Ma) ± 0.04 Ma 5 Total gas age: ± 0.04 Ma VP-17 biotite (C11024) Ar released (Cumulative %) Fig. 3 Page 6 of 7

7 Structural data We measured faults and striations on basement rocks at the eastern inner wall of the depression. In the following table we present the data. Location Fault plane Striation Fault number Latitude S Longitude W Dip direction/dip ( ) Trend/plunge ( ) movement ' 10.4'' 66 58' 20.4'' 264/54 282/49 right, normal ' 10.4'' 66 58' 20.4'' 258/55 268/49 right, normal ' 10.4'' 66 58' 20.4'' 266/54 282/52 right, normal ' 10.4'' 66 58' 20.4'' 252/62 280/60 right, normal ' 10.4'' 66 58' 20.4'' 240/64 258/61 right, normal ' 10.4'' 66 58' 20.4'' 238/57 274/48 right, normal ' 10.4'' 66 58' 20.4'' 244/70 282/62 right, normal ' 10.4'' 66 58' 20.4'' 248/56 278/54 right, normal ' 10.4'' 66 58' 20.4'' 254/48 268/42 right, normal ' 10.4'' 66 58' 20.4'' 246/45 272/44 right, normal ' 10.4'' 66 58' 20.4'' 262/60 272/45 right, normal ' 10.4'' 66 58' 20.4'' 266/52 276/54 right, normal ' 10.4'' 66 58' 20.4'' 268/65 290/65 right, normal ' 10.4'' 66 58' 20.4'' 246/70 294/60 right, normal ' 10.4'' 66 58' 20.4'' 264/50 286/58 right, normal ' 21.1'' 66 58' 23.0'' 256/88 338/86 left, reverse ' 21.1'' 66 58' 23.0'' 254/89 328/82 left, reverse ' 21.1'' 66 58' 23.0'' 264/82 318/66 left, reverse ' 21.1'' 66 58' 23.0'' 250/82 322/78 left, reverse ' 21.1'' 66 58' 23.0'' 72/86 24/79 normal ' 21.1'' 66 58' 23.0'' 248/82 312/75 reverse ' 21.1'' 66 58' 23.0'' 250/76 308/72 reverse ' 21.1'' 66 58' 23.0'' 252/82 322/74 reverse ' 21.1'' 66 58' 23.0'' 248/86 320/75 reverse ' 21.1'' 66 58' 23.0'' 266/66 292/68 reverse Page 7 of 7