Chaîne varisque
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Fig. 16


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Pre-Variscan geodynamic configuration. A. During the Cambrian, the Gondwana margin is marked by the emplacement of calc-alkaline magmas attributed to an enriched mantle source above a subducting slab. The size of the Iapetus is about 2000–3000 km wide. B. During the Ordovician, the Avalonia and Armorica continental ribbons are separated from the Gondwana margin. The Rheic Ocean corresponds to the future Rheic suture exposed in southern Great Britain. The Medio-European Ocean corresponds to the Leptynite-Amphibolite Complex forming boudins and enclaves in high-grade nappes of the Moldanubian allochthonous terrane (see Fig. 3). The tholeiitic to calc-alkaline signature of the LAC is interpreted as reflecting an emplacement of the magmatic protoliths in a back-arc or immature oceanic setting. Alkaline magmas intrusive in Ediacaran sedimentary sequences correspond to orthogneisses preserved in the LGU and PAU and are attributed to opening of a series of rifts leading to hyperextension of the Gondwana margin. These features are consistent with retreat of the southward plunging Iapetus slab that will eventually lead to tectonic accretion of Avalonia to the Laurussia craton.

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