Abstract
Resolving time differences between successive magmatic pulses in composite granitoid plutons is often a difficult task. High-precision CA-ID-TIMS zircon ages obtained from such a pluton, the Variscan Karkonosze Granite (NE part of the Bohemian Massif), provide evidence that the crystallization of the two main granite facies, porphyritic and equigranular, happened between 312.5 ± 0.3 and 312.2 ± 0.3 Ma, thus unresolvable at the 0.08–0.1 % precision level of a single 206Pb/238U age. This finding is at odds with most other previous dating attempts and asks for a re-evaluation of the previous scattered geochronological data. The main reasons for the scatter of the earlier dates obtained by various techniques can include analytical causes, the presence of older inheritance and disturbance of the U–Pb isotopic system, due to zircon metamictization (enhanced by high-U content in zircon) or late- and post-magmatic alteration.
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Acknowledgments
This research was carried out under the Project N307 008 32/0314 of the Polish Ministry of Science and Education. Additional support came from internal grants 1017/S/ING and 2022/W/ING of the University of Wrocław. Kalina Dymna separated the zircons. The financial support of the isotope laboratories at University of Geneva by the Swiss Science Foundation is kindly acknowledged. We are grateful to Fernando Corfu and Monika Kusiak for their constructive reviews.
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Appendix: ID-TIMS analysis
Appendix: ID-TIMS analysis
Sample preparation Zircon grains were separated by standard methods (crushing and milling; concentration via Wilfley Table or panning; magnetic separation; heavy liquids). To minimize the effects of secondary lead loss, the zircons were treated by ‘‘chemical abrasion’’ prior to analysis, involving high-temperature annealing followed by a HF partial dissolution step (CA; Mattinson 2005). Annealing was performed by loading 20–30 zircon grains of each sample in quartz crucibles and placing them into a furnace at 900 °C for c. 48 h. Subsequently, zircons from each sample were transferred into 3 mL screw-top Savillex PFA vials together with c. 120 μL of concentrated HF and 20 μL of 7 N HNO3 for the leaching step. Savillex vials were placed into a 125 mL Teflon Parr™ vessel with 2 mL of concentrated HF and placed in an oven at 180 °C for 12–15 h. After the partial dissolution step, the leachate was completely pipetted out and the remaining zircons were rinsed in ultrapure water and then fluxed in 6 N HCl for several hours on a hotplate at a temperature of c. 80 °C. After removal of the acid, the fractions were again rinsed several times in ultra-pure water and acetone in an ultrasonic bath. Single zircon grains were selected, weighed and loaded for dissolution into pre-cleaned miniaturized Teflon vessels. After adding a mixed 205Pb–233U–235U spike (EARTHTIME, spike calibration described on www. earth-time.org), zircons were dissolved in 63 μL of concentrated HF with a trace of 7 N HNO3 at 206 °C for 6 days, then evaporated and the residue was redissolved overnight in 36 μL of 3 N HCl at 206 °C. Pb and U were separated by anion exchange chromatography (Krogh 1973) in 40 μL micro-columns, using minimal amounts of ultrapure HCl and H2O and finally dried down with 3 μL of 0.06 N H3PO4.
Mass spectrometry and procedural blank The isotopic analyses were performed at the University of Geneva on a TRITON mass spectrometer equipped with a MasCom discrete dynode electron multiplier. The linearity of the multiplier was calibrated using solutions of U, Sr and Pb isotope standards U500, SRM 987, SRM 982 and SRM 983. Measured Pb isotope ratios are externally corrected for a Pb mass fractionation of 0.13 ± 0.04 %/amu, which was determined by repeated measurements of the SRM 981 standard. Within-run correction for U mass fractionation is achieved by analyzing double-spiked samples and using the 233U/235U spike ratio of the double-spike solution. Both Pb and U were loaded with 1 microL of silica gel—phosphoric acid mixture (Gerstenberger and Haase 1997) on outgassed single Re-filaments. Pb isotope compositions were measured on the electron multiplier, whereas U (emitted as UO2 +) isotopic measurements were made in static Faraday mode (using amplifiers equipped with 1012 Ohm resistors) or, in case of insufficient U beam size, on the electron multiplier. Common Pb concentrations up to 1.8 pg in the zircon analyses were attributed to the procedural blank and corrected with the following isotopic compositions: 206Pb/204Pb = 18.30 ± 0.71; 207Pb/204Pb = 15.47 ± 1.03; 208Pb/204Pb = 37.60 ± 0.98 (all 1 s %); excess common Pb was corrected using the model of Stacey and Kramers (1975). Uranium blanks are <0.1 pg and do not influence the degree of discordance at the age range of the studied samples.
Data reduction, reporting ages and errors The initial statistical calculations were done using the TRIPOLI program (www.earth-time.org), followed by the data reduction and age calculation using the YourLab program (applying the algorithm of Ludwig 1980). Generation of Concordia plots and calculation of weighed averages was done with the Isoplot/Ex v.3 program of Ludwig (2005). All uncertainties reported are at the 2 s level, following the x/y/z systematic of Schoene et al. (2006). All data are reported in Table 2 with internal errors only, marked as [x], including counting statistics, uncertainties in correcting for mass discrimination and the uncertainty in the common Pb (blank) composition. The [y] uncertainty includes the external or systematic errors, comprising the uncertainty on Pb/U ratio of the mixed Pb–U tracer used in isotope dilution calculations (0.05 %, Condon and the Members of the Earthtime Working Group 2005), and [z] uncertainties consider the decay constants of 235U and 238U. The accuracy of the data was assessed by repeated analysis of the international R33 standard zircon (Black et al. 2004), which was pretreated by annealing, partial dissolution and measured at an average 206Pb/238U age of 419.08 ± 0.19 Ma (N = 27; MSWD = 0.70). In addition, a synthetic National Environment Research Council Isotope Geosciences Laboratory (NIGL) solution with a nominal age of 100 Ma was measured repeatedly and yielded an external reproducibility better than 0.1 % in 206Pb/238U ratio.
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Kryza, R., Schaltegger, U., Oberc-Dziedzic, T. et al. Geochronology of a composite granitoid pluton: a high-precision ID-TIMS U–Pb zircon study of the Variscan Karkonosze Granite (SW Poland). Int J Earth Sci (Geol Rundsch) 103, 683–696 (2014). https://doi.org/10.1007/s00531-013-0995-0
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DOI: https://doi.org/10.1007/s00531-013-0995-0