The amphibolites from the Ossa–Morena/Central Iberian Variscan suture (Southwestern Iberian Massif): geochemistry and tectonic interpretation

Abstract

Basic rocks included in the Ossa–Morena/Central Iberian Variscan suture of SW Iberia have been studied in order to decipher the pre-orogenic evolution along this major boundary. These rocks appear as garnet-free amphibolites and garnet-bearing amphibolites. Previous geochronological data on these basic rocks suggest the existence of two different protolith ages: Upper Precambrian and Lower Palaeozoic (Ordovician). Upper Precambrian amphibolites form small-size (up to several metres thick), lense-, dike- or elongated-shaped bodies, while Lower Palaeozoic ones form elongated bodies with thicknesses up to several hectometres. The chemical compositions of the amphibolites enable us to separate cumulate rocks from those representing more basaltic magmas. As for the originally basaltic rocks, their geochemical characteristics indicate a shallow melting with different proportions of at least three end-members, namely (i) a MORB-like asthenospheric source, (ii) a more enriched (plume-derived or subcontinental lithosphere) mantle source, and (iii) a continental crust component. Moreover, Upper Precambrian and Lower Palaeozoic amphibolites have different geochemical affinities. In the former group, we envisage an advanced intra-continental rifting situation or an intra-continental back-arc setting related to the Cadomian orogeny, while for the latter, we propose an environment of oceanic crust formation predating Variscan subduction and collision. The recognition of these remains of oceanic-affinity basic rocks along the OMZ/CIZ boundary allows us to propose that some sort of ocean may well have existed between the two zones during Early Palaeozoic times, and that this ocean was presumably closed at a later stage, probably during the Devonian, by oceanic subduction under the CIZ crust.