Reorganization of Northern Peri-Gondwanan Terranes at Cambrian–Ordovician Times: Insights from the Detrital Zircon Record of the Ossa-Morena Zone (SW Iberian Massif)

Abstract

The Ossa-Morena Zone constitutes a fringe Gondwana-related terrane all along the Paleozoic. This continental block has been classically interpreted as being attached to a portion of the northern Gondwanan margin located close to the West African Craton. We report here the results of U-Pb dating on detrital zircon grains from 15 metasedimentary rocks collected in two well-exposed and well-dated Cambrian sections (Córdoba and Zafra) of the Ossa-Morena Zone. The studied samples show a dominant late Tonian-Ediacaran population peaked at c. 600 Ma. Secondary populations are Rhyacian-early Orosirian and late Orosirian-Statherian in age, with maxima at c. 2.1 and 1.9 Ga. Minor detrital zircon populations are Mesoarchean-early Siderian in age, with peaks between c. 3.05 and 2.45 Ga. Most of the studied samples lack a Stenian-early Tonian population, except for two of them with a minor peak at c. 1 Ga. Our results corroborate previous studies that locate the Ossa-Morena Zone close to the West African Craton and/or the Tuareg Shield (i.e., in a western position with respect to other Variscan zones of the Iberian Massif) at the onset of the rifting stage that opened the Rheic Ocean. Nevertheless, the absence of a significant Stenian-early Tonian population in the Cambrian Ossa-Morena rocks contrasts with the reported results on middle Ordovician-Lower Devonian rocks of this zone, which systematically contain an important population with a peak at c. 1 Ga. We relate this change to the latest Ediacaran-early Ordovician paleogeographic/paleotectonic evolution of the Ossa-Morena Zone, which might have recorded a significant eastward displacement, together with a reorganization of the drainage systems. Thus, the vanishing stages of the Cadomian orogeny could have translated eastward the Ossa-Morena Zone terrane at latest Ediacaran-earliest Cambrian time due to change in plate kinematics from subduction to right-lateral shearing. This translation would have shifted the Ossa-Morena Zone from an Ediacaran location close to the West African Craton to a Cambrian position close to the Tuareg Shield. Finally, the rift-to-drift transition occurred at late Cambrian-early Ordovician time along the northern Gondwanan margin would have reorganized the drainage systems, facilitating sediment supply from an eastern source with abundant Stenian-early Tonian detrital zircon grains, probably the Saharan Metacraton.