U-Pb detrital zircon analysis of the lower allochthon of NW iberia: Age constraints, provenance and links with the variscan mobile belt and Gondwanan cratons

摘要

Detrital U-Pb laser ablation inductively coupled plasma mass spectrometry zircon ages from six siliciclastic samples from the lower allochthon of NW Iberia are analysed to constrain their maximum sedimentation age and provenance, and to evaluate the connections to the adjacent tectonostratigraphic domains. Deposited in the external sections of the Gondwana platform, their maximum depositional age is latest Neoproterozoic (c. 560 Ma). Comparison of the age populations of the lower allochthon with those of the rest of the allochthonous and autochthonous units of NW Iberia suggests that the terranes located in the footwall of the Variscan suture should not be considered as exotic elements, but as contiguous pieces of the same continental margin transported onto the adjacent Gondwana mainland in Variscan times. The data are in agreement with the regional trend defined by the drop in Early Neoproterozoic and Mesoproterozoic zircon content upward in the tectonic pile, which had been previously proposed as a marker of proximity to the eastern part of the West African Craton. Based on the age spectra, the palaeoposition for the time of sedimentation is placed in northern Africa, between the West African and Saharan cratons. Particular attention is paid to the occurrence of an Early Neoproterozoic input, probably derived from the Pan-African Hoggar suture.