Variscan ophiolites and high-pressure metamorphism in southern Iberia

The Variscan ophiolites in southern Iberia occur both as a thin belt along the boundary between the Ossa-Morena (OMZ) and South Portuguese Zones (SPZ) (Beja-Acebuches ophiolite) and as dismembered. scattered allochthonous kli ppen on top of lower Palaeozoic sequences within the internal areas of the OMZ. The Beja-Acebuches ophiolite corresponds to a thin amphibolite-serpent inite belt displaying internal lithological organisation including, from bo ttom to top: metaperidotites (harzburgitic/dunitic) and cumulate pyroxenite s, flaser gabbros with trondhjemitic intrusions, amphibolites (locally deri ved from a sheered dike complex) and fine-grained greenschists (locally pre serving pillowed structures). Deformation structures result from three main deformation phases: D-1 (early Devonian) corresponds to high-temperature o phiolite obduction towards N-NE, D-2 (middle Devonian) is related to retrog ression during transpressive sinistral shearing to WNW, and finally, D-3 is a more brittle event, and involved sinistral south-westwards thrusting rea ctivating D-2 structures. The ophiolite is bounded to the north by a thrust that brought OMZ infra-crustal rocks over the ophiolitic sequence; towards the south the ophiolitic complex has been thrust over the SPZ units and is unconformably overlain by a late Devonian flysch sequence. The Beja-Acebuc hes amphibolites were originally tholeiitic gabbros/dolerites/basalts displ aying considerable geochemical variations that range from MORB-type to thos e transitional to are tholeiites, suggesting derivation from a back-are bas in oceanic crust. The internal ophiolitic klippen were emplaced contemporaneously with the ob duction of the Beja-Acebuches ophiolite. They comprise small, dismembered t ectonic slices that were imbricated within a high-pressure (eclogite/bluesc hist), early Palaeozoic passive continental margin sequence, and then thrus t onto the OMZ. The high-pressure metamorphism was polyphase; early (pre- t o syn-D1) eclogite recrystallization is interpreted as reflecting type-A su bduction and initial D1-thrusting; late blueschist facies overprinting corr esponds to tectonic imbrication related to the nappe emplacement. Ophiolite geochemistry dis plays wide variations in incompatible clement fractionati on, ranging from N-MORB type LREE-depleted to LREE-enriched T/P-MORE; contr asting with similar lithologies from the Beja-Acebuches ophiolite, the orog enic (island arc-like) characteristics were not detected in these internal ophiolitic occurrences. The contrasting characteristics of the Ossa-Morena ophiolite types are reminiscent of those already described from other ophio lite belts and suggest that they probably represent different oceanic basin s.