First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France)
We report new LA-ICP-MS U-Pb zircon ages of varied crystalline rocks occurring in the Serre Chevalier crystalline terrain, an allochtonous basement unit located at the top of the Briançonnais nappes stack ∼5 km west of Briançon city. Mapped as an undifferentiated metasedimentary basement on the geol...
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EDP Sciences
2023-01-01
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Series: | BSGF - Earth Sciences Bulletin |
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Online Access: | https://www.bsgf.fr/articles/bsgf/full_html/2023/01/bsgf230009/bsgf230009.html |
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author | Thiéblemont Denis Jacob Jean-Baptiste Lach Philippe Guerrot Catherine Leguérinel Mathieu |
author_facet | Thiéblemont Denis Jacob Jean-Baptiste Lach Philippe Guerrot Catherine Leguérinel Mathieu |
author_sort | Thiéblemont Denis |
collection | DOAJ |
description | We report new LA-ICP-MS U-Pb zircon ages of varied crystalline rocks occurring in the Serre Chevalier crystalline terrain, an allochtonous basement unit located at the top of the Briançonnais nappes stack ∼5 km west of Briançon city. Mapped as an undifferentiated metasedimentary basement on the geological map at 1/50,000 scale, this unit actually displays varied lithologies including alternating micaschist, paragneiss, quartzite, coarse-grained conglomerate and felsic (leptynite) to mafic (amphibolite) gneiss of magmatic origin. All rocks were metamorphosed in response to a dominant pre-Alpine event under amphibolite facies conditions. Partial recrystallization under low-grade amphibolite to greenschist facies conditions was associated with alpine events. Zircon ages were obtained on four types of rocks: (i) a micaceous quartzite from the core of the metasedimentary unit displays a dominant age population around 610 Ma, with a minimal age of 580 Ma, and subordinated age populations at c. 940 Ma, 1825 Ma and 2100-2560 Ma ; (ii) a granite boulder from a coarse-grained conglomerate yields a well-defined age of 582 ± 5 Ma and subordinated inherited ages between 1800 and 2200 Ma. Zircon rims of probable metamorphic origin provide a concordant age of 492 ± 4 Ma ; (iii) a gneissic band in the vicinity of the conglomerate is dated at 597 ± 4 Ma ; (iv) a coarse-grained garnet amphibolite yields an age of 517 ± 3 Ma interpreted as the emplacement age of the protolith, either as intrusive unit cutting accross the sedimentary succession or lava intercalated within the sediments.
The mainly Ediacarian record in the metasediments (quartzite and granite boulder in conglomerate) lead us to conclude that this material resulted from the erosion of a nearby Ediacarian (Cadomian ?) basement. The age of 517 Ma obtained on the amphibolite provides a minimum age for the sedimentary succession which deposition is therefore bracketed between 582 Ma and 517 Ma. This interval is comparable to that of the metasedimentary units of the Brioverian group of the Armorican Massif. Based on their low Th/U ratio, the 492 Ma-old zircon rims in the granite boulder are attributed to an Upper Cambrian metamorphic event. |
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spelling | doaj.art-c8fa5158dd944ab4a6f578eb1c6d9ec62024-01-17T15:04:03ZengEDP SciencesBSGF - Earth Sciences Bulletin1777-58172023-01-011941610.1051/bsgf/2023011bsgf230009First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France)Thiéblemont Denis0Jacob Jean-Baptiste1Lach Philippe2Guerrot Catherine3Leguérinel Mathieu4BRGM, B.P. 6009The Njord Centre, Department of Geoscience, University of OsloBRGM, B.P. 6009BRGM, B.P. 6009BRGM, B.P. 6009We report new LA-ICP-MS U-Pb zircon ages of varied crystalline rocks occurring in the Serre Chevalier crystalline terrain, an allochtonous basement unit located at the top of the Briançonnais nappes stack ∼5 km west of Briançon city. Mapped as an undifferentiated metasedimentary basement on the geological map at 1/50,000 scale, this unit actually displays varied lithologies including alternating micaschist, paragneiss, quartzite, coarse-grained conglomerate and felsic (leptynite) to mafic (amphibolite) gneiss of magmatic origin. All rocks were metamorphosed in response to a dominant pre-Alpine event under amphibolite facies conditions. Partial recrystallization under low-grade amphibolite to greenschist facies conditions was associated with alpine events. Zircon ages were obtained on four types of rocks: (i) a micaceous quartzite from the core of the metasedimentary unit displays a dominant age population around 610 Ma, with a minimal age of 580 Ma, and subordinated age populations at c. 940 Ma, 1825 Ma and 2100-2560 Ma ; (ii) a granite boulder from a coarse-grained conglomerate yields a well-defined age of 582 ± 5 Ma and subordinated inherited ages between 1800 and 2200 Ma. Zircon rims of probable metamorphic origin provide a concordant age of 492 ± 4 Ma ; (iii) a gneissic band in the vicinity of the conglomerate is dated at 597 ± 4 Ma ; (iv) a coarse-grained garnet amphibolite yields an age of 517 ± 3 Ma interpreted as the emplacement age of the protolith, either as intrusive unit cutting accross the sedimentary succession or lava intercalated within the sediments. The mainly Ediacarian record in the metasediments (quartzite and granite boulder in conglomerate) lead us to conclude that this material resulted from the erosion of a nearby Ediacarian (Cadomian ?) basement. The age of 517 Ma obtained on the amphibolite provides a minimum age for the sedimentary succession which deposition is therefore bracketed between 582 Ma and 517 Ma. This interval is comparable to that of the metasedimentary units of the Brioverian group of the Armorican Massif. Based on their low Th/U ratio, the 492 Ma-old zircon rims in the granite boulder are attributed to an Upper Cambrian metamorphic event.https://www.bsgf.fr/articles/bsgf/full_html/2023/01/bsgf230009/bsgf230009.htmlwestern alpsbriançonnaisprecambrianediacarianalpin orogencadomian orogen |
spellingShingle | Thiéblemont Denis Jacob Jean-Baptiste Lach Philippe Guerrot Catherine Leguérinel Mathieu First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France) BSGF - Earth Sciences Bulletin western alps briançonnais precambrian ediacarian alpin orogen cadomian orogen |
title | First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France) |
title_full | First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France) |
title_fullStr | First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France) |
title_full_unstemmed | First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France) |
title_short | First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France) |
title_sort | first report of an ediacarian basement in the western alps the serre chevalier crystalline unit brianconnais domain france |
topic | western alps briançonnais precambrian ediacarian alpin orogen cadomian orogen |
url | https://www.bsgf.fr/articles/bsgf/full_html/2023/01/bsgf230009/bsgf230009.html |
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