Provenance variability along the Early Ordovician north Gondwana margin: Paleogeographic and tectonic implications of U-Pb detrital zircon ages from the Armorican Quartzite of the Iberian Variscan belt


Detrital zircon laser ablation–inductively coupled plasma–mass spectrometry U-Pb age data from the Lower Ordovician Armorican Quartzite (deformed passive margin strata of Gondwanan affinity) of the Iberian Massif are presented herein. The S-shaped coupled Iberian oroclines defined within these zones palinspastically restore to a 2300 km linear Variscan orogen with a paleomagnetically constrained Late Carboniferous north-south trend. Detrital zircons are used to assess paleogeography and interpreted geometry of the Iberian portion of the Gondwana passive margin. A common signature is identified by (1) Neoproterozoic (ca. 500–850 Ma), (2) Stenian–Tonian (ca. 0.9–1.1 Ga), and lesser (3) Paleoproterozoic and (4) Archean populations (ca. 1.8–2.15 and 2.5–2.7 Ga, respectively). Minor site-to-site variation in relative proportion of widely ranging age groups suggests near-uniform distribution of a highly varied detrital input. Provenance analysis reveals strong correlations with Cambro-Ordovician clastic rocks from northeast African realms. Similarity with underlying sequences suggests a common paleogeography from the Ediacaran through early Paleozoic and persistence of a provenance distinction within the autochthonous Iberian Massif. Consistent northward paleoflow within widespread northeast African lower Paleozoic sedimentary cover suggests long-distance sedimentary transport across a North African peneplain from outlying basement terranes. We propose that the 2300-km-long Cantabrian–Central Iberian portion of the early Paleozoic Gondwana margin stretched east-west along the northern limits of the then low-lying Saharan Metacraton and Arabian-Nubian Shield. Accepting paleomagnetic constraints, a 90° counterclockwise rotation is required to reorient the Iberian portion to a pre-oroclinal (Late Carboniferous) north-south trend. The mechanisms for accommodating such a rotation are unclear.

International Journal of Earth Sciences 103, 5, 1335-1357, DOI:10.1007/s00531-013-0923-3