SHRIMP U–Pb geochronological constraints on the timing of the intra-Alcudian (Cadomian) angular unconformity in the Central Iberian Zone (Iberian Massif, Spain)

Abstract

New SHRIMP U–Pb ages of detrital zircons from Neoproterozoic low-grade metasandstones of the Schist–Graywacke Complex (Central Iberian Zone, Iberian Massif) sampled just below and above the intra-Alcudian unconformity at two selected locations contribute to reconstruct the geodynamic evolution of Iberia during the Cadomian orogeny in the north Gondwana margin. The youngest zircons (i.e., maximum depositional age) in the Lower Alcudian are c. 580–576 Ma, while those in the Upper Alcudian are c. 555–552 Ma. The obtained remarkable time gap of about 21 Ma supports the existence of a tectonic event in between. This event resulted in moderate folding (without related foliation/metamorphism) that verticalized the Lower Alcudian previous to the deposition of the Upper Alcudian. Additional evidence of late Cadomian tectonothermal events elsewhere in Iberia also fit in the interval c. 560–550 Ma. Combined with other geological data, the most probable maximum depositional ages are c. 580–560 Ma for the Lower Alcudian (previous to the late Cadomian folding event) and c. 550–540 Ma for the Upper Alcudian (previous to the deposition of the overlying Pusian Group and Lower Cambrian sandstones and limestones). A comparison of the new zircon age spectra with possible source areas verifies recent studies that point to the Cadomian foreland in the north Gondwana continent affected by the Pan-African orogeny: the West African Craton and/or the Saharan Metacraton. Furthermore, ongoing Cadomian arc-related magmatism in Iberia (c. 605–545 Ma) could have contributed as a local zircon source. The end of the Cadomian activity is marked by a transient stage (ephemeral Lower Cambrian platform) which preceded widespread Cambro–Ordovician rifting of north Gondwana.