New ideas on the Proterozoic-Early Palaeozoic evolution of NW Iberia: insights from U-Pb detrital zircon ages

U-Pb ages were obtained on single detrital zircon grains separated from six samples of Neoproterozoic and Lower Palaeozoic sedimentary and volcanosedi mentary rocks from NW Iberia using the laser ablation microprobe-inductivel y coupled plasma mass spectrometry (LAM-ICP-MS) method. Precambrian greywac kes yielded abundant zircons with Neoproterozoic (800-640 Ma) and Mesoprote rozoic (0.9-1.2 Ga) ages, and a smaller proportion of Palaeoproterozoic (1. 8-2 Ga) and Archaean zircons. Palaeozoic samples (Lower Cambrian and Ordovi cian) yielded abundant zircons with younger Neoproterozoic (ca. 550 and 620 Ma) and Mesoproterozoic (0.9-1.2 Ga) ages. Palaeoproterozoic (1.8-2 Ga) an d Archaean zircons were also found. This data set, used in conjuction with previous paleogeographic and isotopic studies sheds new light on the Precam brian-early Palaeozoic evolution of NW Iberia and is consistent with the fo llowing sequence of events: (1) Early Cadomian-Avalonian subduction and are construction (ca. 800-640 Ma). This magmatic episode created the main are edifice (Avalonia); (2) full development of a back are basin upon which the Neoproterozoic sediments were deposited (ca. 640-600 Ma). The combined U-P b ages of detrital zircons and Nd isotopic features of these sedimenary roc ks suggest that they were mostly shed from the main magmatic are. On the ba sis of the presence of Grenvillian age detrital zircons with short waterbor ne transport before incorporation in the sediment, we propose that the basi n was possibly located in a peri-Amazonian realm close to West Avalonian te rranes. These basins were developed upon a cratonic basement that possibly involved both Grenvillian (ca. 0.9-1.2 Ga) and Transamazonian (ca. 1.9-2.1) igneous rocks. The reported zircon ages suggest a long-lived subduction, s tarting at ca. 800 Ma and terminated by ca. 580-570 Ma with no geological r ecord of a final collision event; (3) the continuation of extension gave ri se to the undocking of Avalonia from the back-are. Detrital zircon ages in Lowermost Cambrian strata suggest that the main are edifice had drifted awa y by ca. 550-540 Ma and was no longer shedding detritus into the back-are b asin. (4) During the Lower Ordovician, further extension of an already thin ned crust gave rise to the Lower Ordovician 'Ollo de Sapo' magmatic event ( ca. 480 Ma). Coeval volcanism in neighbouring areas displaying within-plate geochemical signatures is consistent with an extensional setting for the g eneration of the Lower Ordovician igneous and sedimentary rocks. Detrital z ircon ages and Nd isotopic features of the Ordovician greywackes reflect bo th an increase in the contribution from older crustal components and the ad dition of newly accreted crust. A progressively thinning crust is a likely scenario that would explain the simultaneous exhumation of lower crustal (G renvillian + Transamazonian/Icartian) material and the generation of coeval magmatism. This latter scenario is consistent with models proposed for oth er circum-North Atlantic Avalonian-Cadomian terranes where repeated episode s of melting occurred in response to subduction and subsequent rifting even ts. (C) 2000 Elsevier Science B.V. All rights reserved.