Two-step magma flooding of the upper crust during rifting: The Early Paleozoic of the Ossa Morena Zone (SW Iberia)

Abstract

The Ossa Morena Zone of SW Iberia represents a continental arc accreted to the Iberian Autochthon during the Late Proterozoic–Early Cambrian Cadomian orogeny. A subsequent Cambrian–Ordovician rifting event is recorded in this zone, which was accompanied by intrusion/eruption of large volumes of igneous rocks. Exposed crustal segments show both volcanic and shallow plutonic rocks that according to their relationship with coeval sedimentary successions can be assigned to one of two periods of magma emplacement: i) an Early Igneous Event, exclusively comprised of acid peraluminous rocks associated with migmatite formation during development of core-complex structures in mid-upper crust environments; and ii) a Main Igneous Event, which produced predominantly basaltic and acid (rhyolite) rocks and minor amounts of intermediate (trachyte) rocks. Tholeiites and alkaline rocks predominate in this suite but minor calcalkaline peraluminous compositions are also present. Besides, a volumetrically unimportant but petrologically significant group of Mg-rich rocks also occurs within the Main Igneous Event. These latter rocks are interpreted to reflect high partial melting rates of a protolith similar to the primitive mantle. All the outlined characteristics provide evidence for large heterogeneity within the rift-related association that may be due to several causes, such as the involvement of various magma sources (asthenospheric, lithospheric, crustal) and/or involvement of various petrogenetic processes in their generation and evolution. Radiometric (U–Pb zircon) dating yielded c. 530 ± 5 Ma ages for the Early Igneous Event and a longer duration, 517–502 ± 2 Ma, for the Main Igneous Event. The large volume of magma emplaced into upper crustal environments, along with the presence of abundant dikes, suggest that magma ascent benefited from coeval extensional tectonism. It is suggested that they represent the igneous expression of rifting in connection with a severe thermal modification of the lithosphere, probably caused by an underlying thermal anomaly. A model is presented that considers a ridge/trench collision process as the triggering motor of the described evolution, which culminated in the opening of the Rheic Ocean in Early Ordovician time.