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New constraints on the origin and age of Variscan eclogitic rocks (Ligurian Alps, Italy)

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Abstract

Gabbro and eclogite boudins are preserved within the amphibolites of the composite para- and ortho-gneiss Variscan basement of the Savona Crystalline Massif (Ligurian Briançonnais, Italy). Whole rock trace element patterns, low initial εNd (+5.4 to +8.8) data and trace element analyses on relict igneous clinopyroxene revealed that the mafic rocks were derived from depleted mantle melts, which most likely underwent crustal contamination during emplacement. Gabbros have a cumulus origin controlled by clinopyroxene and plagioclase segregation, whereas the eclogites represent evolved melts. U-Pb and trace element micro-analyses on zircons separated from one amphibolitised gabbro and one eclogite help to constrain coeval ages at ~468 Ma for their igneous protoliths. The occurrence of a few inherited zircons confirms the involvement of a crustal component in the petrogenesis of the mafic rocks. In the eclogite, concordant zircon ages younger than the protolith age testify to metamorphic re-crystallisation (or new growth) from about 420 to 305 Ma. Zircon textures and trace element compositions indicate that eclogite facies metamorphism occurred 392–376 Ma ago. The younger zircon portions yielding a mean Concordia age of 333 ± 7 Ma are related to equilibration or new growth during the post-eclogite, amphibolite-facies equilibration.

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Acknowledgments

We are grateful to Bruno Messiga that in different times introduced us to the geology of the Savona Crystalline Massif. Angela Ivaldi and Stefano Maffeo are also acknowledged for precious help in the field. Special thanks are due to Claudio Ghezzo for stimulating discussions. Suggestions and comments by Felix Oberli and an anonymous referee led to a considerable improvement of the original manuscript. This work was financially supported by Ministero dell’Università e della Ricerca Scientifica (Progetti di Ricerca di Interesse Nazionale, Ghezzo and Tribuzio fundings).

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Authors and Affiliations

  1. Dipartimento di Scienze della Terra, Università di Siena, via Laterina 8, Siena, Italy

    Folco Giacomini

  2. Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, P.za San Donato 1, Bologna, Italy

    Roberto Braga

  3. CNR-IGG Unità di Pavia, via Ferrata 1, Pavia, Italy

    Massimo Tiepolo

  4. Dipartimento di Scienze della Terra, Università di Pavia, via Ferrata 1, Pavia, Italy

    Riccardo Tribuzio

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  1. Folco Giacomini
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  2. Roberto Braga
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  3. Massimo Tiepolo
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  4. Riccardo Tribuzio
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Correspondence to Folco Giacomini.

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Communicated by J. Hoefs.

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Giacomini, F., Braga, R., Tiepolo, M. et al. New constraints on the origin and age of Variscan eclogitic rocks (Ligurian Alps, Italy). Contrib Mineral Petrol 153, 29–53 (2007). https://doi.org/10.1007/s00410-006-0131-2

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  • DOI: https://doi.org/10.1007/s00410-006-0131-2

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