Zircons from a >45-cm-thick K-bentonite (altered ash-fall tuff) bed within the upper Barrios Formation (Ordovician Armorican Quartzite facies), in the Cantabrian zone of the Iberian Variscan belt were dated by isotope dilution–thermal ionization mass spectrometry. U-Pb analyses of six highly abraded single grains yielded concordant and overlapping error ellipses with a pooled concordia age of 477.47 ± 0.93 Ma. This age provides the depositional age of the Armorican Quartzite facies in the studied sector and establishes an absolute minimum age for the rifting that led to the opening of the Rheic Ocean in this section of northern Gondwana. This age is within error of the currently accepted interpolated age for the Tremadocian–Lower Ordovician Stage 2 (Floian) limit at 478.6 ± 1.7 Ma ( Gradstein et al., 2004 ).

The Granjeno Schist of northeastern México is the oldest component of the Sierra Madre terrane and comprises polydeformed, pelitic metasedimentary and metavolcaniclastic rocks that enclose lenses of serpentinite-metagabbro. This low-grade Paleozoic assemblage is exposed in the core of a NNW-trending frontal anticline of the Laramide fold-thrust belt where it is tectonically juxtaposed against ca. 1 Ga granulites of the Novillo Gneiss. Silurian strata that unconformably overlie the Novillo Gneiss are unmetamorphosed and contain fauna of Gondwanan affinity. LA-ICPMS (laser ablation inductively coupled plasma mass spectroscopy)U-Pb ages for detrital zircons from a Granjeno phyllite yield age populations that cluster in the ranges ca. 1375–880 Ma, ca. 650–525 Ma, and ca. 460–435 Ma and slightly discordant grains with individual ages of ca. 1435 Ma, ca. 1640 Ma, ca. 2105 Ma, and ca. 2730 Ma. The youngest detrital zircon indicates a maximum depositional age for the Granjeno Schist of ca. 435 Ma (Lower Silurian). Detrital zircons of Neoproterozoic–Cambrian age suggest a provenance in the Maya terrane beneath the Yucatan Peninsula or the Brasiliano orogens of South America, and a source for the detrital zircons of Ordovician–Silurian age is present in the Acatlán Complex of southern México. Provenance of the Mesoproterozoic detrital zircons is likely to have been the adjacent Novillo Gneiss, which has yielded ages of ca. 990–980 Ma, ca. 1035–1010 Ma, and ca. 1235–1115 Ma. These detrital ages closely match those recorded from the Cosoltepec Formation of the Acatlán Complex and support correlation of the two units, which are both interpreted to be vestiges of the southern margin of the Rheic Ocean.

Recent advances in geochemical studies of igneous rocks, isotopic age data for magmatism and metamorphism, quantitative pressure-temperature (P-T) estimates of metamorphic evolution, and structural geology in the northwestern Iberian Massif are integrated into a synthesis of the tectonic evolution that places the autochthonous and allochthonous terranes in the framework of Paleozoic plate tectonics. Because northwestern Iberia is free from strike-slip faults of continental scale, it is retrodeformable and preserves valuable information about the orthogonal component of convergence of Gondwana with Laurentia and/or Baltica, and the opening and closure of the Rheic Ocean. The evolution deduced for northwest Iberia is extended to the rest of the Variscan belt in an attempt to develop a three-dimensional interpretation that assigns great importance to the transcurrent components of convergence. Dominant Carboniferous dextral transpression following large Devonian and Early Carboniferous thrusting and recumbent folding is invoked to explain the complexity of the belt without requiring a large number of peri-Gondwanan terranes, and its ophiolites and high-pressure allochthonous units are related to a single oceanic closure. Palinspastic reconstruction of the Variscan massifs and zones cannot be achieved without restoration of terrane transport along the colliding plate margins. A schematic reconstruction is proposed that involves postcollisional strike-slip displacement of ∼3000 km between Laurussia and Gondwana during the Carboniferous.

Allochthonous ophiolitic units in the northwestern Iberian Massif are remnants of peri-Gondwanan Paleozoic oceans sandwiched among other exotic terranes of continental and volcanic-arc derivation. All these terranes define an intricate suture zone that marks the convergence and collision between Laurussia and Gondwana. The suture is defined by three different ophiolitic ensembles: upper ophiolitic units, lower ophiolitic units, and the Somozas mélange. The lower ophiolitic units were derived from an alternation of basalts and sediments intruded by gabbros and scarce granitoids, and they formed during the opening of a marginal basin, the Galician ocean, during Late Cambrian to Early Ordovician time. This ocean was created as a back arc by the severance of a volcanic arc that had developed at the northern margin of Gondwana and formed part of the Rheic oceanic realm. The upper ophiolitic units formed during the Early Devonian from intraoceanic subduction in the early Paleozoic lithosphere of the Rheic Ocean. These suprasubduction ophiolites were formed just before the ocean closed, preceding the collision between Gondwana and Laurussia. The Somozas mélange appears in an anomalous position at the base of the Cabo Ortegal Complex. The ophiolites involved in this tectonic mélange represent an imbricate of highly dismembered oceanic lithosphere, slivers of subducted outer edge of the Gondwanan continental margin, and Paleozoic metasediments of the northern Gondwanan platform. The ophiolites might either record the development of a different peri-Gondwanan oceanic domain, or they might be equivalent to any of the other ophiolitic ensembles, and their anomalous structural position is simply a consequence of complex thrusting.