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Monazite EMP-Th-U-Pb age pattern in Variscan metamorphic units in the Armorican Massif (Brittany, France)

[Monazit-Altersmuster (EMS-Th-U-Pb) in den variskischen metamorphen Einheiten des Armorikanischen Massivs (Bretagne, Frankreich)]

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The Armorican Massif in Brittany is composed of metamorphic domains, which underwent different pressure-temperature (P-T) paths during Variscan subduction and collision. Monazites in orthogneisses, paragneisses, micaschists and hornfelses were analysed by the electron microprobe (EMP-CHIME) Th-U-Pb dating method. Carboniferous metamorphism and monazites (315–305 Ma) are omnipresent in the autochthonous Lower Group (Morbihan) and in the allochthonous parts (Baie d'Audierne) of the South Armorican Domain (SAD), in the Brioverian micaschists in the Pontivy region of the Central Armorican Domain (CAD) and in the Léon Domain enclosing Ile d'Ouessant to the north. Devonian monazites (405–370 Ma) in the Brioverian micaschists and also in the high-pressure unit of Baie d'Audierne are related to metamorphism along the Eo-Variscan suture zone at the southern margin of the Armorica Microplate. In the Léon Domain to the north of Armorica, a crystallisation of Cambrian (~515 Ma) monazites with high Y contents at presumably low pressure-high temperature metamorphic conditions can be assigned to a late Cadomian event. The lacking Devonian monazite population could signal an Early Carboniferous age of the eclogite-forming event in the Léon Domain. The monazite age data underline the contrasting P-T-time paths along the northern and southern margins of the Palaeozoic Armorica Microplate.


Bei der variskischen Subduktion und Kontinent-Kollision durchliefen die metamorphen Domänen des Armorikanischen Massivs in der Bretagne verschiedenartige Druck-Temperatur-(P-T)-Pfade. Die Monazite in Ortho- und Paragneisen, Glimmerschiefern und Hornfelsen wurden mit der Elektronenstrahl-Mikrosonde altersdatiert (EMP-CHIME Th-U-Pb-Methode). Karbonische Metamorphose und Monazite (315–305 Ma) dominieren in der autochthonen Lower Group (Morbihan) und in den allochthonen Serien (Baie d'Audierne) der Südarmorikanischen Domäne (SAD), den Glimmerschiefern des Brioverien um Pontivy in der Zentralarmorikanischen Domäne (CAD) und in der Léon-Domäne mit der Insel Ouessant im Norden. Devonische Monazite (405–370 Ma) in den Brioverien-Glimmerschiefern und der Hochdruck-Serie der Baie d'Audierne lassen sich einer Metamorphose in der eo-variskischen Suturzone am Südrand der Armorika-Mikroplatte zuordnen. In der Léon-Domäne nördlich von Armorika kann man kambrische Monazite (∼515 Ma) mit hohen Y-Gehalten wahrscheinlich einer spätcadomischen Niedrigdruck-Hochtemperatur-Metamorphose zuordnen. Die fehlende devonische Monazit-Population könnte auf ein unterkarbonisches Alter des Eklogit-bildenden Ereignisses im Léon hinweisen. Die Monazit-Altersdaten belegen die unterschiedlichen P-T-Zeit-Pfade am Nord- und Südrand einer paläozoischen ArmorikaMikroplatte.
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Document Type: Research Article

Publication date: 01 June 2013

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  • Zeitschrift der Deutschen Gesellschaft für Geowissenschaften (ZDGG) is an international peer reviewed journal that accepts papers on research and applied topics in the Earth Sciences. It is published online and in print. One volume, consisting of four issues is published annually. The journal is continously published by the German Geological Society since its foundation in 1848. It was relaunched in 2005.

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