Some aspects of a horizontal schistosity

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AbstractA Pre‐Cambrian horizontal schistosity was formed in regionally metamorphosed rocks around Lannilis, Finistére, during repeated fold movements. Only horizontal movement occurred since the superimposed load due to deep burial was great. Biotite crystallisation along horizontal planes resulted in a schistosity except in the principal thrust zones where decussate biotite textures and large garnets developed. During the early movements, ‘ac’ girdles of quartz axes were formed but these girdles were rotated during the later movements along the schistosity. The degree of rotation and the form of the associated structures cannot be interpreted easily either by ‘Sow’ on planes perpendicular to a compression axis or by ‘slip’ along the schistosity.