F. Díaz García

Early Ordovician orogenic event in Galicia (NW Spain): evidence from U–Pb ages in the uppermost unit of the Ordenes Complex

New U–Pb data (zircon, monazite and rutile) obtained from rocks of the uppermost allochthon in the Variscan belt of NW Spain indicate that the hangingwall to the suture includes an allochthonous unit with a pre-Variscan tectonothermal evolution. This evolution is characterised by an Early Ordovician (498–500 Ma) bimodal magmatism followed almost immediately (493–498 Ma) by a Barrovian style metamorphism up to the granulite facies. The metamorphism subsequent to the igneous intrusions requires convergence and crustal thickening in order to generate the Barrovian facies pattern shown by the lithologies of the uppermost allochthon. The almost coeval magmatism and metamorphism, and the chemistry of the metabasites suggest an accretionary complex, probably related to a volcanic arc, as the most probable setting for the origin of the uppermost unit. The implications of the existence of Early Ordovician convergent plate boundaries are discussed in the context of Gondwana–Laurentia–Avalon interactions.

P-T evolution of eclogites from the Agualada Unit (Ordenes Complex, northwest Iberian Massif, Spain): Implications for crustal subduction

Abstract Eclogite lenses in the Agualada Unit (western Ordenes Complex, Spain) contain the peak mineral assemblage garnet (prograde rim: Alm = 48 mol%, Prp = 30 mol%), omphacite (Jd max = 36 mol%), quartz, rutile and rare zoisite, which equilibrated at T = 700 °C and P > 12–14 kbar. Garnet shows discontinuous growth zoning, with a pyrope-poor intermediate zone (Alm = 51 mol%, Prp = 10 mol%) between a core zone where pyrope is slightly higher (Alm = 46 mol%, Prp = 16 mol%) and areas just inward from the rims where the maximum pyrope contents (Alm = 48 mol%, Prp = 30 mol%) are recorded. In atoll interiors, garnet contains inclusions of a first generation of omphacite (Jd max = 40 mol%). This omphacite is replaced in the matrix by a second generation (Jd max = 36 mol%) with higher Fe/Fe + Mg ratio. The compositions of garnet and omphacite suggest a complex syneclogitic tectonothermal evolution for the Agualada Unit, characterized by: (1) eclogite-facies metamorphism ( T = 585 °C, P > 12–13 kbar), followed by (2) cooling during a slight decompression ( T = 500 °C, P > 11–12 kbar), and (3) a final increase in P and T to form the garnet rim-matrix omphacite mineral assemblage. The Agualada Unit is part of a subduction complex which affected the Gondwana margin at the beginning of the Variscan cycle. The P-T evolution of the Agualada eclogites is closely related to the structural evolution of the accretionary complex and the whole orogenic wedge. The cooling event recorded by the Agualada eclogites may have resulted from the accretion of a new colder crustal slice under the unit, whereas the final progradation reflects the emplacement of the Agualada Unit directly under the mantle wedge. This evolution fits well with previously presented the retical models, both for the tectonothermal evolution of accretionary complexes and for the dynamic evolution of orogenic wedges. P-T paths such as the one for the Agualada Unit eclogites, probably reflect a prolonged structural evolution. Although evidently rarely preserved, such paths are probably the rule rather than the exception during plate convergence.

Tectonic setting of the Monte Castelo gabbro (Ordenes Complex, northwestern Iberian Massif): Evidence for an arc-related terrane in the hanging wall to the Variscan suture

The Ordenes Complex is the largest of the allochthonous assemblages containing the Variscan suture in the northwestern Iberian Massif. Its uppermost tectonostrati-graphic unit overlies the ophiolitic units, and consists of a thick metasedimentary sequence, the Ordenes Series, intruded by orthogneisses and gabbros. In the lower part of the Ordenes Series, the large Monte Castelo gabbro (∼150 km 2 ) is surrounded by high-grade migmatitic paragneisses. Several shear zones cutting across the gabbro massif depict intermediate-pressure granulite facies, indicating a metamorphic evolution common with the surrounding paragneisses. Recent U-Pb geochronological data prove that the main tectonothermal evolution of the Ordenes Series took place in Late Cambrian-Early Ordovician time. These data suggest that the intrusion of the Monte Castelo gabbro (499 ± 2 Ma; U-Pb in zircons) was immediately followed by a Barrov-ian metamorphic episode that reached the granulite facies (493-498 Ma; U-Pb in monazites). A later Variscan overprint is indicated by U-Pb rutile ages of 380-390 Ma. Considering the geochronological evidence for almost coeval magmatism and metamorphism during the Early Ordovician, together with the geochemical characteristics of the Monte Castelo gabbro, an accretionary complex related to a Late Cambrian-Early Ordovician island arc appears as the more probable setting for the uppermost allochthonous terrane in the northwest of the Iberian Massif. This implies the presence of a convergent plate boundary in the oceanic realm between Laurentia and Gondwana, or close to it, during the early Paleozoic. The island arc later became involved in the Variscan convergence and accretion.

Allochthonous Units in the Variscan Belt of NW Iberia: Terranes and Accretionary History

The allochthonous complexes of NW Iberia can be correlated with the paleogeographic realms involved in the Variscan collision. Assuming that the existing ophiolites represent the suture formed by the closure of a Paleozoic ocean, the units in the footwall to the suture correspond to the outer edge of the Gondwana continental margin, formed after the opening of the ocean. The units in the hangingwall to the suture are interpreted as pieces of a continental fragment detached from Gondwana, which registered an orogenic event in the Lower Ordovician, shortly after its separation. The correlation establishes a link between circum-Atlantic terranes and the Rheic ocean, and makes it possible to draw a relatively simple scenario of the successive recorded tectonothermal events. Following the amalgamation of Avalon to Laurentia, the remaining outboard terranes underwent a progressive accretion to the Laurentian continental margin that ended with the collision between this continent and Gondwana. The accretionary history, as deduced from the tectonothermal events recognised mainly in the Galician allochthonous complexes (NW Spain), includes partial subduction of the continental margin of Gondwana, during the Middle Devonian.

Fabric Development in a Middle Devonian Intraoceanic Subduction Regime: The Careon Ophiolite (Northwest Spain)

A Middle Devonian suprasubduction zone ophiolite, the Careón Unit (northwest Spain), displays amphibolite‐facies ductile deformation fabrics related to the onset of the Rheic Ocean closure. Two different fabrics, an early high‐T foliation and a subsequent lower‐T foliation, each of which characterized by distinct deformation mechanisms, have been identified in two distinct crustal‐scale shear zones of the same ophiolitic thrust sheet. Combined quantitative texture analysis by electron backscattered diffraction and time‐of‐flight neutron diffraction, were carried out on the shear zones and correlated with micro‐ and macrostructural data. The results indicate that the regional lineation and shear zone kinematics (east‐west, top‐to‐the‐east) represent fabrics developed essentially during the intraoceanic subduction of the Rheic Ocean, and their orientation may be considered a reference vector for convergence models in this part of the Variscan belt.

Contrasting high-pressure metabasites from the Santiago unit (Ordenes Complex, northwestern Iberian Massif, Spain)

The Santiago unit forms part of the Basal unit of the Ordenes Complex. It is part of the Gondwana continental paleomargin, partially subducted under Laurentia at the beginning of the collisional episode that gave rise to the European Variscan chain. It is composed mainly of pelitic-semipelitic schists, orthogneisses, and intercalations of metabasites. The metabasites intercalated in orthogneisses preserve evidence of their transformation into low-temperature (T) eclogites during an initial syn-D 1 high-pressure (P) episode (P min = 14.8 ± 0.3 kbar; T = 502 ± 10 °C). The metabasites intercalated in the schists conserve an internal schistosity included in albite porphyroblasts, corresponding to garnet amphibolites developed during the same high-P episode (P = 15.8 ± 0.3 kbar; T = 466 ± 14 °C). Petrographic evidence, the chemical zoning of the syn-D 1 garnets, and the thermometry indicate that the basic rocks included in the pelitic schists underwent metamorphic recrystallization prior to the peak pressure, whereas those included in the orthogneisses went directly from slightly hydrated igneous conditions to eclogitization. The subsequent extensional relaxing of the accretional prism produced the superposition of a hot mantle material layer over the Santiago unit. The development of an inverted metamorphic gradient is reflected in the metabasites included in the schists by the appearance of three metamorphic syn-D 2 zones: (1) tschermakite + oligoclase + chlorite; (2) tschermakite + andesine + biotite; and (3) ferrotschermakite + labradorite + garnet. The peak conditions reached by the top of the unit during D 2 are P = 9.3 ± 0.8 kbar and T = 528 ± 34 °C.

Geometry and regional significance of Neoproterozoic (Cadomian) structures of the Narcea Antiform, NW Spain

The Narcea Antiform is located at the hinterland-to-foreland transition of the Variscan belt in the NW of the Iberian massif. It consists of an antiformal stack of fault-propagation folds that developed in the hanging wall of allochthonous thrust sheets during the first Variscan deformation event in the Palaeozoic rocks. The Neoproterozoic Narcea Slates Formation, cropping out in the core of the Narcea Antiform, records a Cadomian deformation event that formed large asymmetric NW-vergent folds, with related axial planar cleavage, developed in the lower greenschist facies. These folds are truncated by the Lower Cambrian angular unconformity. Lower Cambrian conglomerates contain pebbles with predepositional microstructures that are widespread in the underlying Neoproterozoic rocks. The Narcea Antiform provides valuable information concerning major tectonic Cadomian events. The large-scale, near-recumbent Cadomian folds developed between 560 and 540 Ma. Top-to-the-NW kinematics can be deduced for the Cadomian plate convergence (transverse to the Variscan convergence) in present coordinates. It was followed shortly afterwards by large displacements along the northern Gondwana margin that resulted in its accretion alongside the essentially undeformed Neoproterozoic rocks of the Central Iberian Zone before Early Cambrian times.The Narcea Antiform is located at the hinterland-to-foreland transition of the Variscan belt in the NW of the Iberian massif. It consists of an antiformal stack of fault-propagation folds that developed in the hanging wall of allochthonous thrust sheets during the first Variscan deformation event in the Palaeozoic rocks. The Neoproterozoic Narcea Slates Formation, cropping out in the core of the Narcea Antiform, records a Cadomian deformation event that formed large asymmetric NW-vergent folds, with related axial planar cleavage, developed in the lower greenschist facies. These folds are truncated by the Lower Cambrian angular unconformity. Lower Cambrian conglomerates contain pebbles with predepositional microstructures that are widespread in the underlying Neoproterozoic rocks. The Narcea Antiform provides valuable information concerning major tectonic Cadomian events. The large-scale, near-recumbent Cadomian folds developed between 560 and 540 Ma. Top-to-the-NW kinematics can be deduced for the Cadomian plate convergence (transverse to the Variscan convergence) in present coordinates. It was followed shortly afterwards by large displacements along the northern Gondwana margin that resulted in its accretion alongside the essentially undeformed Neoproterozoic rocks of the Central Iberian Zone before Early Cambrian times.