Palaeogeography and crustal evolution of the Ossa–Morena Zone, southwest Iberia, and the North Gondwana margin during the Cambro-Ordovician: a review of isotopic evidence

Abstract

Cambro-Ordovician palaeogeography and fragmentation of the North Gondwana margin is still not very well understood. Here we address this question using isotopic data to consider the crustal evolution and palaeogeographic position of the, North Gondwana, Iberian Massif Ossa–Morena Zone (OMZ). The OMZ preserves a complex tectonomagmatic history: late Neoproterozoic Cadomian orogen- esis (ca. 650–550 Ma); Cambro-Ordovician rifting (ca. 540–450 Ma); and Variscan orogenesis (ca. 390–305 Ma). We place this evolution in the context of recent North Gondwana Cambro-Ordovician palaeogeographic reconstructions that suggest more easterly positions, adjacent to the Sahara Metacraton, for other Iberian Massif zones. To do this we compiled an extensive new database of published late Proterozoic–Palaeozoic Nd model ages and detrital and magmatic zircon age data for (i) the Iberian Massif and (ii) North Gondwana Anti-Atlas West African Craton, Tuareg Shield, and Sahara Metacraton. The Nd model ages of OMZ Cambro-Ordovician crustal-derived magmatism and Ediacaran- Ordovician sedimentary rocks range from ca. 1.9 to 1.6 Ga, with a mode ca. 1.7 Ga. They show the greatest affinity with the Tuareg Shield, with limited contribution of more juvenile material from the Anti-Atlas West African Craton. This association is supported by detrital zircons that have Archaean, Palaeoproterozic, and Neoproterozoic radiometric ages similar to the aforementioned Iberian Massif zones. However, an OMZ Mesoproterozoic gap, with no ca. 1.0 Ga cluster, is different from other zones but, once more, similar to the westerly Tuareg Shield distribution. This places the OMZ in a more easterly position than previously thought but still further west than other Iberian zones. It has been proposed that in the Cambro-Ordovician the North Gondwana margin rifted as the Rheic Ocean opened diachronously from west to east. Thus, the more extensive rift-related magmatism in the westerly OMZ than in other, more easterly, Iberian Massif zones fits our new proposed palaeogeographic reconstruction.