Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): geochemistry and LA-ICP-MS U-Pb geochronology of its Ordovician protolith

Alifé El Korh 1 Susanne Schmidt Michel Ballevre 2 Alexey Ulianov 3 Olivier Bruguier 4
1 Department of Mineralogy
2 Terre, Temps, Traçage
GR - Géosciences Rennes
3 Institute of Mineralogy and Geochemistry
4 Géosciences Montpellier
Abstract : For the first time, an albite orthogneiss has been recognised and dated within the HP-LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U-Pb age of 480.8 ± 4.8 Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6-647.4 Ma. Zircon and titanite U-Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U-Pb age of 366 ± 33 Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP-LT peak metamorphism.
keyword : Cambro-Ordovician rifting Ile de Groix Incompatible trace elements In situ U-Pb geochronology LA-ICP-MS Albite orthogneiss Titanite Variscan orogeny Zircon
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Submitted on : Thursday, July 5, 2012 - 9:37:50 AM
Last modification on : Friday, April 5, 2019 - 8:22:38 PM

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Alifé El Korh, Susanne Schmidt, Michel Ballevre, Alexey Ulianov, Olivier Bruguier. Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): geochemistry and LA-ICP-MS U-Pb geochronology of its Ordovician protolith. International Journal of Earth Sciences, Springer Verlag, 2012, 101 (5), pp.1169-1190. ⟨10.1007/s00531-011-0732-5⟩. ⟨insu-00714570⟩

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