Abstract

Two bimodal meta-igneous complexes from the southern Massif Central provide important tectonic constraints on the initial stage of the Palaeozoic orogen. The metabasites may be subdivided into two groups according to the distribution of the Large Ion Lithophile (LIL) and High Field Strength (HFS) elements: N-type MORBs with low LIL/HFS ratios and subduction-type basalts with high LIL/HFS ratios. These associations suggest a complex tectonic setting such as an extensional zone or a back-arc basin above a subduction zone. The great volume of felsic rocks with low incompatible trace element contents indicates partial melting of an underlying continental crust. The association of terrigeneous sediments with N-type MORBs, subduction-type basalts and high-silica peraluminous rhyolites argue against an intra-oceanic setting, and suggest that the area was one of ensialic crustal tension during middle Ordovician. The two complexes are identified as remnants of ensialic back-arc basins rather than relicts of major oceanic sutures. Such marginal basins could have been located in a mainly continental domain, such as the Armorican microplate.

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