Genesis and evolution of a syn-orogenic basin in transpression: Insights from petrography, geochemistry and Sm-Nd systematics in the Variscan Pedroches basin (Mississippian, SW Iberia)

The Pedroches basin is an outstanding syn-orogenic Variscan (Mississippian) depocenter, located in the vicinity of, and overstepping onto the Neoproterozoic (Cadomian) suture between the Ossa Morena and Central Iberian zones of the SW Iberian Massif. Its formation appears to be related to transtensional processes at a major releasing bend in the reactivated suture during sinistral escape of the Ossa Morena Zone from the zone of frontal Variscan collision between northern Gondwana and Laurussia. Subsequent basin inversion resulted in compartmentalization and internal deformation, and was probably related to transpression at a restraining bend along the reactivated suture (Badajoz-Cordoba shear zone). Basin fill consists of both igneous and metasedimentary rocks generally deposited in a shallow marine, storm-dominated platform environment. By using new petrographic, geochemical and Sm-Nd isotopic data, we attempt to characterize: 1) the sources and geological evolution of the Pedroches basin fill, and 2) the processes involved in basin development and sediment supply and dispersal. Sampling was carried out in both sedimentary and igneous rocks belonging to three major structural units within the central pan of the basin: Guadiato unit, Guadalbarbo unit and South Pedroches Valley unit. The combined Nd isotopic and geochemical data from the Pedroches Mississippian metasedimentary rocks indicate an upper continental crust provenance and a high degree of sedimentary recycling of these materials throughout the basin; i.e., they are derived from old, recycled upper crust without any significant juvenile component despite the presence of interbedded basaltic rocks. The igneous rocks in turn correspond to submarine basalts and trachyandesitic basalts plus some hybrid rocks including crust-derived components. Concerning the mafic rocks, major element compositions reveal a tholefitic and transitional tholeiitic character for the Guadalbarbo unit rocks (MORB type) and an alkaline affinity for those of the Guadiato unit Trace element analyses, along with Sm-Nd isotopic ratios, suggest two kinds of provenance sources for the Pedroches basin syn-sedimentary igneous rocks: 1) an important crust-contaminated and/or old, enriched mantle source for the Guadiato unit and the Pedroches Batholith, and 2) a time-integrated depleted mantle source for the Guadalbarbo rocks, in which no significant interaction with continental crust components is discernible. These latter rocks suggest that a strong attenuation, or even almost complete tectonic and/or thermal erosion of the continental lithosphere was reached, at least locally, in this region. (C) 2008 Elsevier B.V. All rights reserved.