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Intense progressive shear in Ile de Groix blueschists and compatibility with subduction or obduction

Abstract

THE Ile de Groix (South Brittany, France) is made up of a series of metapelitic rocks intercalated with metabasic horizons, most of which contain glaucophane1,2. Palaeozoic metamorphism2–4 in the basic rocks produced various assemblages, such as glaucophane + lawsonite5,6 or omphacite + garnet, characteristic of high pressure and low2 to medium1 temperatures. All these rocks were later slightly or completely recrystallised under greenschist facies conditions1,5,6. The presence of a contrasting metamorphic facies (high temperature/low to medium pressure), on the mainland north of Groix, has led several workers to propose the existence of a paired metamorphic belt2 of pre-Hercynian age4,7,8, formed as a result of subduction2,7–9. We present evidence here concerning the style of deformation contemporaneous with the high-pressure metamorphism.

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