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Paleo- and Neoproterozoic magmatic and tectonometamorphic evolution of the Isla Cristalina de Rivera (Nico Pérez Terrane, Uruguay)

Abstract

The Isla Cristalina de Rivera crystalline complex in northeastern Uruguay underwent a multistage magmatic and metamorphic evolution. Based on SHRIMP U–Pb zircon, Th–U–Pb monazite (CHIME-EPMA method) and K–Ar age data from key units several events can be recognized: (1) multistage magmatism at 2,171–2,114 Ma, recorded on zircon of the granulitic orthogneisses and their 2,093–2,077 Ma overgrowths; (2) a distinct amphibolite facies metamorphism at ~1,980 Ma, recorded by monazite; (3) greenschist facies reworking and shearing at ca. 606 Ma (monazite and K–Ar on muscovite) along the Rivera Shear Zone, and finally (4) intrusion of the post-tectonic Sobresaliente and Las Flores granites at around 585 Ma. Lithological similarities, geographic proximity and coeval magmatic and metamorphic events indicate a similar tectonometamorphic evolution for the Isla Cristalina de Rivera, the Valentines Block in Uruguay and the Santa María Chico Granulitic Complex in southern Brazil, since at least 2.1 Ga.

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Acknowledgments

The authors thank M. A. S. Basei for the fruitful and open discussions. P. Oyhantçabal gratefully acknowledges grants provided by the German Science Foundation (DFG, GZ: SI 438/40-1) and the Programa 720-Contrapartida de Convenios of the Universidad de la República (240011-000274-11) for a research stay at the Georg-August University of Göttingen. Thanks to Georg Schroer, Devin den Boer, Sandra Cazaux, Hernán Vidal and Nicolas Viana of Uruguay Mineral Exploration Inc. for sharing their knowledge of the geology of the ICR. The electron microprobe monazite dating required long-term analytical sessions, which were facilitated by M. Göbbels at the Geozentrum Nordbayern in Erlangen and accompanied by N. Langhof and A. Richter. We thank K. Saalmann and J. Konopasek for their thoughtful review that helped to improve the manuscript.

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Appendix

Appendix

Geochronological procedures

Analytical procedures

SHRIMP-II U–Pb dating procedure

Zircon grains were hand selected and mounted in epoxy resin together with chips of the TEMORA (Middledale Gabbroic Diorite, New South Wales, Australia) (Black et al. 2003) and reference zircons (91500 Geostandard zircon) (Wiedenbeck et al. 1995). The grains were sectioned approximately in half and polished. Reflected and transmitted light photomicrographs and cathodoluminescence (CL) SEM images were prepared for all zircons. The CL images were used to decipher the internal structures of the sectioned grains and to target specific areas within these zircons. The U–Pb analyses of the zircons were made using SHRIMP-II ion microprobe at the Research School for Earth Sciences of the Australian National University (RSES-ANU) in Canberra. Each analysis consisted of five scans through the mass range. The spot diameter was about 25 microns, and the primary beam intensity was about 2 nA. The data have been reduced in a manner similar to that described by Williams (1998, and references therein), using the SQUID Excel Macro of Ludwig (2000). The Pb/U ratios have been normalized relative to a value of 0.0668 for the 206Pb/238U ratio of the TEMORA reference zircons, equivalent to an age of 416.75 Ma (Black et al. 2003). Uncertainties given for individual analyses (ratios and ages) are at the one σ level; however, the uncertainties in calculated Concordia ages are reported at the two σ level. The Ahrens-Wetherill Concordia plot (Wetherill 1956) has been prepared using ISOPLOT/EX (Ludwig 2001).

Pb/Pb stepwise leaching experiments (PbSL)

Slightly modified procedures by Frei and Kamber (1995) were applied. Mineral separates were obtained by standard techniques using jaw crusher and sieve analysis. The 150- to 250-μm sieve fractions were then purified by hand-picking followed by repeated rinsing in deionized water; 200 mg of these materials was transferred to 7-ml Savillex1 screw-cap beakers for step-leaching. Successive 120°C acid leach steps (seven in total) involving various mixtures of HBr, HNO3, and HF were performed on each separate, to extract Pb selectively from the phases. The leaching scheme is given in Table A-5. Purified Pb extracts were mounted on Re filaments, and Pb isotopic ratios were determined by thermal ionization mass spectrometry (TIMS) at the University of Copenhagen. A similar PbSL procedure was previously applied to garnets in metapelites (Dahl and Frei 1998). Fractionation for Pb was controlled by repeated analysis of the NBS 981 standard and amounted to 0.103 ± 0.007% per amu (2r; n = 5) relative to the values proposed by Todt et al. (1996). Procedural blanks for Pb remained below 87 pg, an amount that insignificantly affects the isotopic data for the samples.

K–Ar dating of muscovite samples

Mica separation was performed by the standard techniques such as crushing, sieving, Frantz magnetic separation and selection by hand. The pure micas were ground in alcohol and sieved to remove altered rims, which might have suffered argon loss. The argon isotopic composition was measured in a Pyrex glass extraction and purification line coupled to a VG 1200 C noble gas mass spectrometer operating in static mode. The amount of radiogenic 40Ar was determined by the isotopic dilution method using a highly enriched 38Ar spike from Schumacher, Bern (Schumacher 1975). The spike is calibrated against the biotite standard HD-B1 (Fuhrmann et al. 1987). The age calculations are based on the constants recommended by the IUGS quoted in Steiger and Jaeger (1977). Potassium was determined in duplicate by flame photometry using an Eppendorf Elex 63/61. The samples were dissolved in a mixture of Hf and HNO3 according to the technique of Heinrichs and Herrmann (1990). CsCl and LiCl were added as an ionization buffer and internal standard, respectively. The analytical error for the K–Ar age calculations has a 95% confidence level of 2σ. The procedural details for argon and potassium analyses at the laboratory in Göttingen are given in Wemmer (1991).

Th–U–Pb dating of monazite by electron microprobe (EMP-monazite)

In situ analysis of Th, U and Pb for calculation of monazite model ages, as well as for Ca, Si, LREE and Y for corrections and evaluation of the mineral chemistry were carried out with a JEOL JXA 8200 at the University of Erlangen-Nürnberg (Schulz et al. 2007). The Mα1 lines of Th and Pb and the Mβ1 lines for U of the same PETH crystal were selected, and these elements relevant for age calculations were analyzed in the sequence Th–U–Pb. For the analysis of the Pb Mα1 line, the background positions were evaluated by linescans on Madmon monazite with known compositions, vanadinite, REE-orthophosphates and the Paleoproterozoic monazite with PbO contents at ~3 wt% reported in this study. A negative background position far from the Pb-Mα1 peak, −9.650 mm, was chosen. A choice of the positive background position also far from the Pb-Mα1 peak (+7.500 mm) minimizes the problem of uncertainty of possible background curvature at the Pb-Mα1 position. Combined with a long counting time in the background (2 × 120 s), this provided the best results in sight of the arguments on background positions raised by Spear and Pyle (2002), Williams et al. (2006) and Spear et al. (2009). Repeated control of background positions on Madmon revealed stable counting rates through long terms, and a linear interpolation of the background curve was applied. The measurements were performed during several sessions with the same conditions and using the same spectrometer and crystal for each element. Resulting absolute errors (2σ) at 20 kV acceleration voltage, 100 nA beam current, 5 μm beam diameter and counting times of 320 s (Pb), 50 s (U) and 40 s (Th) are typically 0.008–0.012 wt% for Pb, 0.020–0.025 wt% for U and 0.02–0.03 wt% for Th. The lines Lα1 for La, Y, Ce; Lβ1 for Pr, Sm, Nd, Gd and Kα1 for P, Si and Ca were chosen. Orthophosphates of the Smithsonian Institution were used as standards for REE analysis (Jarosewich and Boatner 1991; Donovan et al. 2003). Calibration of PbO was carried out on a vanadinite standard. The U was calibrated on an appropriate glass standard with 5 wt% UO2. The age of the Madmon monazite (Schulz et al. 2007), dated by SHRIMP at 496 ± 9 Ma and numerous Pb–Pb-TIMS monazite evaporation data (K. Bombach, Freiberg, unpublished analytical method) at 497 ± 2 Ma, was also determined at 503 Ma by the EMP-monazite dating routines established at facilities in Salzburg and BRGM Orléans (Finger and Helmy 1998; Cocherie et al. 1998). The Madmon contains ThO2 at around 10 wt%, as determined by LA-ICPMS and by the microprobe at University of Salzburg. Madmon was used for calibration and offline re-calibration of ThO2 as well as for the control of data. Interference of YLγ on the PbMα line was corrected by linear extrapolation after the measure of several standards with different Y-contents, as proposed by Montel et al. (1996). An interference of ThMγ on UMβ was also corrected by using a Th-glass standard. Interference of a Gd-line on UMβ needs correction when Gd2O3 in monazite is >5 wt%. These parameters matched the analytical problems and limits of the method discussed in Williams et al. (2006) and Spear et al. (2009) in the best way. Representative data are given in Table A-4.

The monazite chemical model ages were gained following two different approaches. First, for each single analysis, an age was calculated using the equations given by Montel et al. (1996). The error resulting from counting statistics was typically on the order of ±10 to ±30 Ma (1σ). Using these apparent age data, weighted average ages for monazite populations in the samples were then calculated using Isoplot 3.0 (Ludwig 2001). Secondly, the ages were determined using the ThO2*–PbO isochron method (CHIME) of Suzuki et al. (1994), where ThO2* is the sum of measured ThO2 and ThO2 equivalent to the measured UO2. In all samples analyzed, the model ages obtained by the two different methods agree exceptionally well.

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Oyhantçabal, P., Wagner-Eimer, M., Wemmer, K. et al. Paleo- and Neoproterozoic magmatic and tectonometamorphic evolution of the Isla Cristalina de Rivera (Nico Pérez Terrane, Uruguay). Int J Earth Sci (Geol Rundsch) 101, 1745–1762 (2012). https://doi.org/10.1007/s00531-012-0757-4

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Keywords

  • Paleoproterozoic
  • Neoproterozoic
  • Granulites
  • Shear zones
  • Granites
  • Geochronology
  • U–Pb SHRIMP
  • K–Ar
  • Zircon
  • Monazite
  • Th–U–Pb monazite EMP method