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Detrital zircons in blueschist-facies meta-conglomerates : Implications for the Early Permian palaeo-topography of the Western Alps

Paola Manzotti 1 Marc Poujol 2 Michel Ballevre 2
1 Terre, Temps, Traçage
Institute of Geological Sciences [Bern], GR - Géosciences Rennes
2 Terre, Temps, Traçage
GR - Géosciences Rennes
Abstract : In theWestern Alps, the Money Unit is a monocyclic unit, metamorphosed at blueschist facies during the Alpine cycle. It crops out as a tectonic window below the overthrusted polycyclic, eclogite-bearing unit of the Gran Paradiso and its present position is the result of the Alpine tectonics. The Money Unit is considered to be Permo-Carboniferous in age, but no palaeontological or radiometric data constrain this interpretation. The Money unit exposes a sequence of clastic sediments and volcanics (Money Complex) that are intruded by a granitoid body (Erfaulet metagranite). In the Money Complex a polygenic (graphite-rich) and a monogenic (graphite-poor) meta-sedimentary formation have been recognized. The two sequences are separated by fine-grained biotite-amphibole gneisses, and by albite-bearing gneisses and amphibolites. Detrital zircon U-Pb geochronology in both meta-sedimentary formations shows that (i) the main population is Cambrian and Ordovician in age, (ii) the youngest grains are Silurian and Early Devonian, and (iii) Carboniferous zircons are lacking. A study of the age distributions in the Alps suggests that potential source for the detrital material in the Money Complex is the Briançonnais basement. Late Carboniferous magmatism is widespread in the Helvetic Zone of the Alps. Permian magmatism is dominant in the Briançonnais, in the Austroalpine, and in the Southern Alps. The lack of Carboniferous zircons in the Money Complex suggests that the detrital material did not derive from the erosion of the Helvetic Zone. The Helvetic Zone was separated from the Money basin by the Zone Houillère basin, where the main drainage pattern was developed from south to north and where the depocenters migrated northwards from the Namurian to the Stephanian.We suggest that the Money Complex may had been located to the east of the main river drainage inside the Zone Houillère basin or alternatively may represent a small basin, located on the east of the Zone Houillère.
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Submitted on : Thursday, November 6, 2014 - 11:22:18 AM
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Paola Manzotti, Marc Poujol, Michel Ballevre. Detrital zircons in blueschist-facies meta-conglomerates : Implications for the Early Permian palaeo-topography of the Western Alps. 24 ème Réunion des sciences de la Terre 2014, Oct 2014, Pau, France. pp.195. ⟨insu-01080778⟩

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