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Eclogites of the Snowbird tectonic zone: petrological and U-Pb geochronological evidence for Paleoproterozoic high-pressure metamorphism in the western Canadian Shield

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Abstract

Eclogite occurs within the southern domain of the East Athabasca mylonite triangle in northern Saskatchewan. Situated at the boundary between the Archean Rae and Hearne Provinces of the western Canadian Shield, the East Athabasca mylonite triangle is a fundamental exposure of the ~3,000-km-long Snowbird tectonic zone. The eclogite occurs in association with a variety of lower crustal high-pressure granulites that record a complex metamorphic history from 2.6 to 1.9 Ga. Temperatures of the eclogite facies metamorphism are constrained by garnet-clinopyroxene exchange thermometry at 920–1,000 °C. Minimum pressure conditions are recorded by the jadeite+quartz=albite geobarometer at 1.8–2.0 GPa. A near-isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene-plagioclase and pargasite-plagioclase. U-Pb IDTIMS zircon geochronology of the eclogite yields a weighted mean 207Pb/206Pb date of 1,904.0±0.3 Ma, which we interpret as the time of peak eclogite facies metamorphism. SHRIMP in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207Pb/206Pb date of 1,905±19 Ma. Inclusion suites of high-pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism. Zircon from one eclogite sample has older cores that are 2.54 Ga, which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith. U-Pb rutile data indicate slow cooling at ~1°C/Ma below ~500 °C from 1.88 to 1.85 Ga. The formation and exhumation of the eclogites at ca.1.9 Ga has important implications for the tectonic significance of the Snowbird tectonic zone during the Paleoproterozoic. The eclogites described here are consistent with transport of continental crust to mantle depths during the Paleoproterozoic, followed by rapid buoyancy-driven exhumation to normal lower crustal depths.

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Acknowledgements

This research was supported by National Science Foundation grant EAR-0001131 to SAB and MLW and a research grant from the Mineralogical Society of America to JAB. We thank J. Hanchar for providing the CL image shown in Fig. 11 and T. Grove, P.J. O’Brien, R. Berman, G. Ross for helpful discussions. We also appreciate N. Chatterjee’s assistance with the electron microprobe. Thorough and constructive reviews by F. Corfu and an anonymous reviewer greatly improved the manuscript.

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  1. Julia A. Baldwin

    Present address: Department of Geology, Laboratory for Crustal Petrology, University of Maryland, College Park, MD, 20742, USA

Authors and Affiliations

  1. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA

    Julia A. Baldwin & Samuel A. Bowring

  2. Department of Geosciences, University of Massachusetts, Amherst, MA, 01003, USA

    Michael L. Williams

  3. Research School of Earth Sciences, The Australian National University, ACT 0200, Canberra, Australia

    Ian S. Williams

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Baldwin, J.A., Bowring, S.A., Williams, M.L. et al. Eclogites of the Snowbird tectonic zone: petrological and U-Pb geochronological evidence for Paleoproterozoic high-pressure metamorphism in the western Canadian Shield. Contrib Mineral Petrol 147, 528–548 (2004). https://doi.org/10.1007/s00410-004-0572-4

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