Abstract
THE Ile de Groix series consists of metavolcanic and metasedimentary rocks, with or without glaucophane. These rocks belong to a South Brittany polymetamorphic complex which is named the ‘domaine de l'Anticlinal de Cornouaille’ by Cogne1. In this complex, anatectic granites occur in the midst of a migmatitic and gneissic terrain1,2. Some meta-sedimentary and metavolcanic rocks which occur in the terrain are cut by Hercynian granites. The Ile de Groix series1,3,4 is characterised by high pressure mineral assemblages with glaucophane, jadeitic pyroxene and lawsonite5—these assemblages are unknown on the adjacent mainland. The series is considered to have formed during two metamorphic periods: the first produced high pressure–low temperature mineral assemblages and the second produced a recrystallisation of mineral assemblages in greenschist facies6. In this note, we shall present new isotopic age data on various rocks and minerals from Ile de Groix, hoping to delineate the time sequence of original magmatism, sedimentation and subsequent metamorphism.
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