Testing the Avalonian affinity of the South Portuguese Zone and the Neoproterozoic evolution of SW Iberia through detrital zircon populations

Abstract

New SHRIMP U-Pb geochronogical data on detrital zircons from the South Portuguese Zone (SPZ) in SW Iberia and the Sehoul Block in northwest Morocco are provided, aimed at deciphering the presumed Avalonian affinity of these two regions. Despite the limitation imposed by the absence of Ordovician-Silurian rocks in outcrop, indirect support for an Avalonian affinity comes from the presence of Mesoproterozoic (≈1 and 1.3–1.7Ga) and Silurian-Early Devonian (≈400Ma) zircons in sediments of the SPZ. Moreover, the Ribeira de Limas formation of the Pulo do Lobo belt (northern SPZ) shows the same zircon pattern (dominated by a 580–630Ma population) and tectonic deformation as the Ronquillo formation in the inner SPZ, thus challenging the classical interpretation of the Pulo do Lobo belt as exotic with respect to the SPZ. On the contrary, the Horta da Torre formation, cropping out just south of the Ossa-Morena Zone/SPZ boundary, is the only element that can be considered exotic from a sedimentary perspective, due to presence of ≈1 and 1.4–1.6Ga zircon populations. Finally, a close similarity in detrital zircon content has been found between the Santa Iría (upper Pulo do Lobo rocks) and PQ formations, both featuring a missing Variscan volcanic arc denoted by a population at 365–375Ma. Regarding the Sehoul Block, its Caledonian tectonic imprint strongly supports an Avalonian derivation, although the zircon spectrum of its Cambrian sediments is dominated by Cadomian/Pan-African ages (560–605Ma). On a broader scale, we have explored the Neoproterozoic evolution of SW Iberia through a synthesis of available published detrital zircon data. The difference in Proterozoic zircon inheritance between Ossa-Morena and Central Iberian samples (and their central European correlatives) is highlighted and explained in terms of right-lateral transcurrent tectonics along a northern Gondwana late Cadomian subduction zone.