Letter | Published:

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

Nature Geoscience volume 4, pages 178183 (2011) | Download Citation

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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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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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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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.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Daniel Herwartz.

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DOI

https://doi.org/10.1038/ngeo1060

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