Pilar Andonaegui

The Vila de Cruces Ophiolite: A Remnant of the Early Rheic Ocean in the Variscan Suture of Galicia (Northwest Iberian Massif)

The Vila de Cruces ophiolite is one of the ophiolitic units involved in the Variscan suture of the northwest Iberian Massif. This ophiolite consists of a tectonically repeated succession of greenschist facies volcanic rocks, common alternations of metasediments of pelitic or siliceous character, and scarce orthogneisses, metagabbros, and serpentinites. The protolith age of a granitic orthogneiss that intruded the mafic rocks is dated at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

The basal tectonic mélange of the Cabo Ortegal Complex (NW Iberian Massif): a key unit in the suture of Pangea

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Space and time in the tectonic evolution of the northwestern Iberian Massif: Implications for the Variscan belt

\end{document} Ma (U‐Pb in zircons). This age can be considered a reference for the generation of the ophiolite. According to their contents of some of the most immobile trace elements, the greenschist and the metagabbros are derived from basaltic magmas with compositions similar to those of island‐arc tholeiites. The influence of a subduction zone in the generation of the original basaltic magmas can be deduced from the marked negative Nb anomaly observed in all the metabasic rocks of this ophiolite. The granitic orthogneisses can also be genetically related to the basic rocks because they are similar to granitic rocks generated in volcanic arcs. The Vila de Cruces ophiolite is interpreted as a suprasubduction zone ophiolite generated in Late Cambrian times, during the early stages of the opening of the Rheic Ocean. The ophiolite was probably generated in a back‐arc basin developed during the first stages of the pulling apart and later drift of one or more peri‐Gondwanan terranes, one of them represented by the upper allochthon of the northwest Iberian Massif.

A rootless suture and the loss of the roots of a mountain chain: The Variscan belt of NW Iberia

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.

Paleozoic ophiolites in the Variscan suture of Galicia (northwest Spain): Distribution, characteristics, and meaning

Recent field work and mapping in the lower units of the Cabo Ortegal Complex provided new data about the tectonic melange that appears in the lowest structural position: the Somozas Melange. This melange unit with average thickness of 1800 m is restricted to the eastern part of the complex, and is located at the advancing front of the allochthonous complexes of NW Iberia. Three rock units are involved in the melange: 1) an ophiolitic melange consisting of igneous rocks mixed with serpentinites; 2) a metasedimentary unit with phyllites and phyllonites, with scarce conglomerates, marbles and quartzites; 3) high-T metamorphic rocks with varied types of amphibolites and orthogneisses. Two granitic rocks within the ophiolitic melange were dated using U-Pb zircon geochronology at 527 ± 2 Ma and 499 ± 1 Ma. Two different series of igneous rocks can be distinguished in this melange. The first series consists of gabbros, diorites, granitoids and basalts-basaltic andesites with calc-alkaline affinities. The second series contains common basaltic rocks, diabasic dikes and gabbros with chemical compositions typical of island-arc tholeiites. Both igneous series shared a common geographic setting, but the island-arc tholeiites are younger than the calc-alkaline igneous rocks. The two igneous series were probably generated in a mature volcanic arc located along the periphery of Gondwana. In the metasedimentary unit, a conglomerate from a large tectonic block included in serpentinites yielded age populations of detrital zircons suggesting that the sediments were deposited along the periphery of the West-African Craton. This conglomerate contains a large number of zircons (n = 24) with ages ranging 630-464 Ma, probably representing the chronology of the Pan-African event, including the magmatic activity in the volcanic arc where the igneous lithologies involved in the melange were generated. The maximum age of sedimentation for this conglomerate is estimated as latest Cambrian – earliest Ordovician, and constraints the end of the magmatic activity in the volcanic-arc. Within the unit of high-T rocks, an orthogneiss yields a U-Pb protolith age of 485 ± 6 Ma, which is similar to other ages of igneous rocks in the basal allochthonous terrane in NW Iberia. The three rock assemblages forming part of the Somozas Melange may be linked to the evolution of a mature peri-Gondwanan volcanic arc. This volcanic arc was affected by pronounced extension which caused the opening of intra-arc basins, culminating the rifting and subsequent drift of the external parts of the arc during the opening of the Rheic Ocean. This opening started during a time interval constrained by the peak activity in a mature volcanic arc (c. 527-499 Ma) and the generation of intraarc basins around the Cambrian-Ordovician boundary. Tectonic melanges including high-P rocks have been classically related to subduction zone environments. Regional relationships in NW Iberia and the nature of the rock units involved in the Somozas Melange, suggest that two different subduction zones generated during oblique convergence and collision between Gondwana and Laurussia were active during the final stages of the assembly of Pangea. The first related to the underthrusting beneath Laurussia of the most external Gondwana margin (c. 370 Ma). The second subduction zone was a new one which accreted later remnants of a peri-Gondwanan arc and sediments of the continental margin below a layer of exhumed high pressure rocks. This oblique collision finished the closure of the Rheic Ocean and contributes to define the new oceanic domain located to the East of Pangea, the Palaeotethys.

Re-interpreting the Devonian ophiolites involved in the Variscan suture: U–Pb and Lu–Hf zircon data of the Moeche Ophiolite (Cabo Ortegal Complex, NW Iberia)

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.