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

Abstract

U-Pb ages were obtained on single detrital zircon grains separated from six samples of Neoproterozoic and Lower Palaeozoic sedimentary and volcanosedimentary rocks from NW Iberia using the laser ablation microprobe-inductively coupled plasma mass spectrometry (LAM-ICP-MS) method. Precambrian greywackes yielded abundant zircons with Neoproterozoic (800-640 Ma) and Mesoproterozoic (0.9-1.2 Ga) ages, and a smaller proportion of Palaeoproterozoic (1.8-2 Ga) and Archaean zircons. Palaeozoic samples (Lower Cambrian and Ordovician) yielded abundant zircons with younger Neoproterozoic (ca. 550 and 620 Ma) and Mesoproterozoic (0.9-1.2 Ga) ages. Palaeoproterozoic (1.8-2 Ga) and Archaean zircons were also found. This data set, used in conjuction with previous paleogeographic and isotopic studies sheds new light on the Precambrian-early Palaeozoic evolution of NW Iberia and is consistent with the following sequence of events: (1) Early Cadomian-Avalonian subduction and arc construction (ca. 800-640 Ma). This magmatic episode created the main arc edifice (Avalonia); (2) full development of a back arc basin upon which the Neoproterozoic sediments were deposited (ca. 640-600 Ma). The combined U-Pb ages of detrital zircons and Nd isotopic features of these sedimenary rocks suggest that they were mostly shed from the main magmatic arc. On the basis of the presence of Grenvillian age detrital zircons with short waterborne transport before incorporation in the sediment, we propose that the basin was possibly located in a peri-Amazonian realm close to West Avalonian terranes. 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, starting at ca. 800 Ma and terminated by ca. 580-570 Ma with no geological record of a final collision event; (3) the continuation of extension gave rise to the undocking of Avalonia from the back-arc. Detrital zircon ages in Lowermost Cambrian strata suggest that the main arc edifice had drifted away by ca. 550-540 Ma and was no longer shedding detritus into the back-arc basin. (4) During the Lower Ordovician, further extension of an already thinned 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 generation of the Lower Ordovician igneous and sedimentary rocks. Detrital zircon ages and Nd

Gondwana-derived microcontinents — the constituents of the Variscan and Alpine collisional orogens

“…Hegner and Kröner (2000) report mainly 1.7 -1.4-Ga Nd model ages for the areas of the northeastern Bohemian Massif, and U -Pb ages of detrital zircons or zircon xenocrysts clustering from 600 to 850 Ma, 1.0 to 1.3 Ga, and 1.7 to 2.3 Ga. Following the model of Fernández-Suárez et al (2000), they place most of the source areas in an active margin setting along the Amazonian craton, only few areas showing Grenvillian sources. In the Cambrian of the Lysogory Massif, U -Pb single-grain dating of detrital zircons reflects sources of 600, 1.8-2.0 and >2.5 Ma attributed to Gondwana (ValverdeVaquero et al, 2000).…”

“…Chen et al, 2000;Crowley et al, 2000;Fernández-Suárez et al, 2000;Haydoutov, 1989;Schaltegger and Gebauer, 1999;Schaltegger et al, 1997;von Quadt, 1992;Zulauf et al, 1999). Initial rift zones became apparent along the Gondwana margin.…”

Paleozoic evolution of pre-Variscan terranes: From Gondwana to the Variscan collision

“…A Neoproterozoic active margin setting with formation of volcanic arcs is observed along the entire length of the future microcontinents at the Gondwanan border (e.g., Fernández Suárez et al, 2000;Schaltegger et al, 1997;Zulauf et al, 1999). Granites of Neoproterozoic age (ca.…”

Stratigraphic and structural framework of the Neoproterozoic Paracuellos Group, Iberian Chains, NE Spain

“…During the last two decades, the understanding of the Neoproterozoic geological framework and evolution of the Iberian Peninsula has greatly advanced [Quesada et al, 1990;San José et al, 1990;Quesada, 1991;Valladares, 1995;Fernández-Suárez et al, 2000;Valladares et al, 2000]. However, these efforts have not yet solved the stratigraphic and structural problems of some isolated Precambrian outcrops, such as those of the Iberian Chains (NE Spain).…”