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

作者：El Korh Afife^*; Schmidt Susanne Th; Ballevre Michel; Ulianov Alexey; Bruguier Olivier

来源：International Journal of Earth Sciences, 2012, 101(5): 1169-1190.

DOI：10.1007/s00531-011-0732-5

• Cambro-Ordovician rifting
• Ile de Groix
• Incompatible trace elements
• In situ U-Pb geochronology
• LA-ICP-MS
• Albite orthogneiss
• Titanite
• Variscan orogeny
• Zircon

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 +/- A 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 +/- A 33 Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP-LT peak metamorphism.

• 出版日期2012-7