EARLY PALEOZOIC RIFTING AND BIMODAL VOLCANISM IN THE OSSA-MORENA ZONEOF SOUTH-WEST SPAIN

Lower Paleozoic volcanic members have been investigated by geological, petrographical and geochemical means in a traverse across the Ossa-Mo rena Zone (OMZ) in south-west Spain. The volcanism lasted from the Ear ly Cambrian to the Early Ordovician, with a peak in the Middle Cambria n. The volcanism is bimodal, starting up with acidic and ending with b asic compositions. From north to south, peralkaline rhyolites change t o rhyolites, and strongly enriched alkali basalts change via transitio nal basalts to mid-ocean ridge basalt (MORB-type basalts). The geologi cal and magmatic evolution suggests an extensive Early Paleozoic rifti ng with its center along the southern boundary of the OMZ. Temporal, s patial and crustal aspects of the rifting event are presented in a geo dynamic model.