Abstract

The ophiolitic suite of Monian rocks in Anglesey has been regarded as segments of oceanic crust or upper mantle and sediments that were tectonically emplaced by cold shearing or thrusting during plate subduction. These processes are inconsistent with the field evidence. Deformation related to emplacement is not found in either the igneous materials or the country rocks. Igneous textures and the relatively weak meshtexture serpentine occur at the margins of the serpentinite bodies. An aureole of epidote hornfels is commonly developed in the adjacent country rocks. Both the regional tectonic style and the local structures are inconsistent with major low-angle thrusting. If the ophiolitic rocks are related to oceanic plate consumption, the ultramafic and mafic materials must have ascended as magmas, perhaps along deep vertical fractures formed at the inflexion point of the subducting plate.

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