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Tracing two orogenic cycles in one eclogite sample by Lu–Hf garnet chronometry

Nature Geoscience volume 4, pages 178–183 (2011)Cite this article

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Abstract

Subduction of rocks into the mantle results in high-pressure metamorphism and the formation of eclogites from basaltic precursor rocks. At the Earth’s surface, eclogites often occur as isolated fragments embedded in crustal rocks that lack evidence for high-pressure conditions1,2,3. The high-pressure rocks are therefore often viewed as dismembered fragments that have been assembled and intercalated with rocks devoid of any high-pressure history at shallow crustal levels4,5,6,7,8, forming a tectonic mélange. Such mélange models were supported by age discrepancies among high-pressure rocks from the Adula nappe (Central Alps)9,10,11,12, which was thought to represent a classic example of such a situation4,5. Here we present Lu–Hf age data from two populations of the high-pressure mineral garnet, found within a single eclogite sample taken from Trescolmen, in the Central Adula nappe. We report a minimum Variscan age of 332.7 Myr and a maximum Alpine age of 38 Myr for the two populations. We suggest that the Trescolmen eclogite was subducted to mantle depth and subsequently exhumed, becoming part of a continental crust during the Variscan orogeny. Later, during the Alpine orogeny, the Adula nappe must have been subducted to—and exhumed from—mantle depth a second time, as one coherent unit. We conclude that the Adula nappe is not a mélange, and therefore, the crustal rocks that envelope the eclogites have also been subjected to high-pressure conditions through deep subduction during the Alpine event13,14.

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Figure 1: Two contrasting models for the exhumation mechanism of geological units comprising (ultra) high-pressure rocks.
Figure 2: Location of the Adula nappe and the Alp Trescolmen.
Figure 3: Illustration of the different garnet generations in Trescolmen sample TRC1.
Figure 4: Isochron plots illustrating the different ages obtained for two garnet generations present in eclogite sample TRC1 from Trescolmen.

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Acknowledgements

We thank S. Weber for his help with preparing mineral separates, S. Oppel for processing thin sections, A. Luguet for technical assistance with the MC-ICP-MS instrument, J.E. Hoffmann for critical discussion and S. Kramer for proofreading. We are grateful for the thorough and constructive reviews by J. Kramers, H.K. Brueckner and C. Beaumont.

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Authors and Affiliations

  1. Steinmann-Institut, Universität Bonn, Bonn 53115, Germany

    Daniel Herwartz, Thorsten J. Nagel & Nikolaus Froitzheim

  2. Institut für Geologie und Mineralogie, Universität zu Köln, Köln 50674, Germany

    Carsten Münker

  3. Institut für Mineralogie, Universität Münster, Münster 48149, Germany

    Erik E. Scherer

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  1. Daniel Herwartz
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  4. Erik E. Scherer
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Contributions

D.H. acquired the Lu–Hf isotope data including MC-ICP-MS measurements and data processing. T.J.N. collected, processed and refined electron microprobe analyses. C.M., N.F. and E.E.S. substantially contributed to the study design, data evaluation and interpretation. E.E.S. developed the selective digestion method and C.M. developed the column chemistry and the analytical protocol. D.H. and T.J.N. wrote the manuscript and contributed equally to the study.

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Correspondence to Daniel Herwartz.

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Herwartz, D., Nagel, T., Münker, C. et al. Tracing two orogenic cycles in one eclogite sample by Lu–Hf garnet chronometry. Nature Geosci 4, 178–183 (2011). https://doi.org/10.1038/ngeo1060

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