Rift-related volcanism predating the birth of the Rheic Ocean (Ossa-Morena zone, SW Iberia)

Two very different periods of magma emplacement in the crust of the Ossa-Morena zone (early and main events) in SW Iberia have been previously interpreted to record a Cambrian/Early Ordovician rifting event that is thought to have culminated in the opening of the Rheic Ocean during the Early Ordovician New stratigraphic, petrographic, geochemical and Sm-Nd isotope data from Cambrian volcanic rocks included in six key low-grade sections in both Portugal and Spain considerably improve our understanding of these events. These data (1) confirm the existence of two rift-related magmatic events in the Cambrian of the Ossa-Morena zone, (2) demonstrate that the early rift-related event was associated with migmatite and core-complex formation in the mid-upper crust and is represented by felsic peraluminous rocks. the parent magmas of which were derived mainly from crustal sources, and (3) show the main rift-related event to be represented by a bimodal association of felsic and mafic rocks with minor amounts of intermediate rocks. Some of the mafic rocks show N-MORB affinity, whereas others have OIB or E-MORB affinities, suggesting different heterogeneous mantle sources (depleted and enriched, asthenospheric and lithospheric, plume-like and non-plume-like) The acid and intermediate rocks appear to represent hybrid mixtures of crust and mantle-derived magmas This new data supports the hypothesis that the onset of rifting was associated with a process of oblique ridge-trench collision We interpret the significant differences between the early and main events as reflecting the evolution from a wide rift stage with passive extension mainly accommodated by lower-crust flow in a high heat-flow setting, to a narrow rift stage with active extension characterized by extension rates that outpaced thermal diffusion rates (C) 2009 International Association for Gondwana Research. Published by Elsevier B V All rights reserved