100 Ma cycles of oceanic lithosphere generation in periGondwana: Ophiolites from Morocco and Iberia

Trabajo presentado en el X Congreso Geológico de España, 5-7 julio 2021, Vitoria-Gasteiz[EN] The Variscan Orogen in Iberia and the Anti-Atlas mountains in Morocco contain a set of ophiolites formed between Neoproterozoic and Devonian times during the complex evolution of the NW African margin of Gondwana. During this time interval the margin evolved from an active margin (c. 750-500 Ma, the Avalonian-Cadomian arc), to the final collision with Laurussia to form Pangea. In this context, different ophiolites have been dated at c. 700, 600, 500 and 400 Ma. The composition of all these peri-Gondwanan ophiolites is of supra-subduction zone type, showing no evidence for preserved MORB-type oceanic lithosphere. Consequently, these ophiolites were generated in the peri-Gondwanan realm during the opening of fore-arc or back-arc basins. The peri-Gondwanan realm has been a domain where generation of oceanic or transitional lithosphere seems to have occurred at intervals of c. 100 Ma. These regularly spaced time intervals may indicate cyclic events of mantle upwelling in the peri-Gondwanan mid-ocean ridges, with associated higher subduction rates at the peri-Gondwanan trenches and concomitant higher rates of partial melting in the associated mantle wedges.[ES] El Orógeno Varisco en Iberia y el Anti Atlas de Marruecos contiene un conjunto de ofiolitas generadas entre el Neoproterozoico y el Devónico. Durante este intervalo temporal, el margen de Gondwana evolucionó desde un margen activo (c. 750-500 Ma, el arco Avaloniense-Cadomiense), hasta la colisión final con Laurussia para formar Pangea. En este contexto, diferentes ofiolitas han sido datadas en c. 700, 600, 500 y 400 Ma. La composición de todas estas ofiolitas peri-Gondwánicas es de tipo zona de supra-subducción, sin preservación de litosferas oceánicas de tipo MORB. En consecuencia, estas ofiolitas fueron generadas durante la apertura de cuencas de tipo fore-arc o back-arc. El ámbito peri-Gondwánico ha sido pues un dominio donde la generación de litosfera oceánica o transicional se produjo en intervalos de c. 100 Ma. Estos intervalos temporales parecen indicar episodios cíclicos de ascenso astenosférico (mantle upwelling) en las dorsales oceánicas peri-Gondwánicas, con un incremento asociado de la velocidad de subducción en las trincheras peri-Gondwánicas y concomitantes mayores tasas de fusión parcial en las cuñas de manto litosférico asociadas.Peer reviewe

100 myr cycles of oceanic lithosphere generation in peri-Gondwana: Neoproterozoic–Devonian ophiolites from the NW African–Iberian margin of Gondwana and the Variscan Orogen

The datasets generated during and/or analysed during the current study are availablein the DIGITAL.CSIC repository: http://doi.org/10.20350/digitalCSIC/12510.[EN] The Variscan Orogen in Iberia and the Anti-Atlas Mountains in Morocco contains a set of ophiolites formed between Neoproterozoic and Devonian times, during the complex evolution of the NW African–Iberian margin of Gondwana. During this time interval, the margin evolved from an active margin (c. 750–500 Ma: the Reguibat–Avalonian–Cadomian arc) to the final collision with Laurussia in Devonian times to form Pangaea. In this context, one of the oldest recognized ophiolites is the Bou Azzer Ophiolite from the Anti-Atlas Mountains, dated at c. 697 Ma and containing two types of mafic rocks, the youngest of which has a boninitic composition. To the north, in the SW Iberian Massif, the Calzadilla Ophiolite contains mafic rocks also of boninitic composition dated at c. 598 Ma. Farther north, in the NW Iberian Massif, the Vila de Cruces Ophiolite is formed by a thick sequence of mafic rocks with an arc tholeiitic composition and minor alternations of tonalitic orthogneisses dated at c. 497 Ma. In the same region, the Bazar Ophiolite has a similar age of c. 495 Ma. Also in NW Iberia, there is a group of ophiolites with varied lithologies and dominant mafic rocks with arc tholeiitic composition (Careón, Purrido and Moeche ophiolites) dated at c. 395 Ma. The composition of all these peri-Gondwanan ophiolites is of supra-subduction zone type, showing no evidence of preserved mid-ocean ridge basalt type oceanic lithosphere. Consequently, these ophiolites were generated in the peri-Gondwanan realm during the opening of forearc or back-arc basins. Forearc oceanic lithosphere was promptly obducted or accreted to the volcanic arc, but the oceanic or transitional lithosphere generated in back-arc settings was preserved until the assembly of Pangaea. Based on the ages of the described ophiolites, the peri-Gondwanan realm has been a domain where the generation of oceanic or transitional lithosphere seems to have occurred at intervals of c. 100 myr. These regularly spaced time intervals may indicate cyclic events of mantle upwelling in the peri-Gondwanan mid-ocean ridges, with associated higher subduction rates at the peri-Gondwanan trenches and concomitant higher rates of partial melting in the mantle wedges involved. The origin of the apparent cyclicity for mantle upwelling in the peri-Gondwanan ocean ridges is unclear, but it could have possibly been related to episodic deep mantle convection. Cycles of more active deep mantle convection can explain episodic mantle upwelling, the transition from low- to fast-spreading type mid-ocean ridges and, finally, the dynamic context for the episodic generation of new supra-subduction zone type oceanic peri-Gondwanan lithosphere.Financial support has been provided bySpanish projects CGL2016-76438-P and CGL2015-65824 (Ministerio de Economía, Industria y Competitividad) anda FPU grant to Núria Pujol-Solà (Ministerio de Educación)Peer reviewe

Allochthonous terranes involved in the Variscan suture from NW Iberia: a review of their origin and tectonothermal evolution [Abstract]

NW Iberia includes a rather complete section of the Variscan suture, where different terranes with continental or oceanic affinities appear with clear structural relationships. Three groups of terranes, namely upper, ophiolitic and basal units and a frontal tectonic mélange appear in Galicia, in Cabo Ortegal, Órdenes and Malpica-Tui complexes. They constitute a huge allochthonous pile thrust over the Iberian parautochthonous and autochthonous domains, which represent the section of the Gondwanan margin that escaped continental subduction during the Variscan cycle (Schistose Domain of Galicia-Trás-os-Montes and Central Iberian Zone)Departamento de Petrología y Geoquímica, Universidad Complutense de Madrid, EspañaInstituto de Geociencias, Universidad Complutense de Madrid, EspañaInstituto de Geociencias, Consejo Superior de Investigaciones Científicas, EspañaDepartamento de Geología, Universidad de Salamanca, EspañaInstitut für Geowissenschaften, Mineralogie, Johann Wolfgang Goëthe-Universität, AlemaniaUnidad de Tres Cantos, Instituto Geológico y Minero de España, EspañaCentro de Apoyo a la Investigación de Geocronología y Geoquímica Isotópica, Universidad Complutense de Madrid, Españ

Cadomian and Variscan sutures of Iberia: a comparison

p.11-12. -A meeting held as a tribute to Teodoro Palacios on his retirement as Professor of Palaeontology at the University of Extremadura, Badajoz, Spain, 26th & 27th January, 2022, University of Extremadura, Badajoz. Edited by Sören Jensen[EN] The Iberian Massif holds evidence of two pre-Mesozoic orogenies, namely the Cadomian and Variscan. The Cadomian Orogeny resulted from long-lived subduction under the periphery of Gondwana during the Neoproterozoic and early Paleozoic. The Variscan Orogen resulted from the progressive collision of Gondwana, Laurussia and their pericontinental terranes during the Devonian and Carboniferous, after the closure of the Rheic Ocean and other marginal basins located along their mainlands. Despite these two orogens differ from one another in the global context from which they emerged, in Iberia they share some characteristics that make them intriguingly alike. The Variscan Orogen contains two major sutures zones. One that separates mainland Gondwana from peri-Gondwanan terranes (intra-Gondwana suture), and another one separating Laurussia from the latter terranes (Rheic suture). The Variscan intra-Gondwana suture is Tectonically dismembered and separates a collection of terranes with continental crust affinity that were transported inland from the periphery of Gondwana during the closure of a (Devonian) marginal basin opened during ongoing convergence between Gondwana and Laurussia (e.g., Careón Ophiolite), ⁓15 million years after the onset of the Variscan Orogen. This process was the result of subduction polarity towards Laurussia, i.e. away from mainland Gondwana. The exhumation of the ophiolites and high-P rocks in this suture was largely controlled by syn-convergence extensional tectonics. The current structure of the Rheic suture, on the other hand, is the result of reworking after the opening of an ephemeral oceanic basin (Beja-Acebuches Ophiolite). Subduction polarity during both the closure of the primary suture zone and the closure of the ephemeral basin were beneath mainland Gondwana. However, the closure of the ephemeral basin developed flake tectonics and obduction of pieces of the ocean basin onto the upper plate. Suture zones in the Cadomian Orogen went unnoticed until few years ago. Despite being intensely reworked by Variscan deformation, the ongoing structural, tectonometamorphic, geochemical and geochronological studies provide first-order constrains on their primary (Cadomian) geometry as well as insight on the paleogeographic location of subduction zones that led to their formation. A collective, yet preliminary analysis of these sutures, pictures a major architecture of the Cadomian Orogen that contain, at least, two sutures zones. One Cadomian suture is identified in the Mérida Ophiolite, which separates an upper and lower plate, both with continental crust affinity and likely Gondwanan derivation (intra-Gondwana suture). This intra-Gondwana suture was formed after the closure by subduction away from mainland Gondwana of a marginal basin that opened during ongoing convergence between Gondwana and an oceanic plate, millions of years after the onset of the Cadomian Orogen. The exhumation of the ophiolite and mid-P rocks that make this suture was largely controlled by syn-convergence extensional tectonics. Another Cadomian suture is represented by the Calzadilla Ophiolite, whose protoliths formed in a fore-arc basin to the most external part of Gondwana. The location of this suture zone is explained by flake tectonics, which contributed to the obduction of the ophiolite onto the upper plate while ongoing subduction was beneath mainland Gondwana. Cadomian and Variscan sutures share fundamental characteristics regarding the paleolocation of the ocean basins they derive from and the overall resulting geometry and tectonic processes involved in their formation. The suture zones that represent the closure of basins located at the outermost section of peri-Gondwana, and closely facing subduction underneath Gondwana (Calzadilla and Beja-Acebuches ophiolites), were obducted inwards onto mainland Gondwana (upper plate). The intra-Gondwana suture zones (Careón and Mérida ophiolites) formed after subduction of a marginal basin beneath the periphery of Gondwana, and the exhumation of rock units of the subduction system was largely assisted by syn-convergence extensional tectonics following subduction-accretion. The Variscan and Cadomian orogens, despite being formed in different contexts (oceanic subduction vs. continental collision), share two major features. Both are (i) mostly built by Gondwanan lithosphere, and (ii) occupy the upper plate of a subduction zone that consumed a large ocean. These two orogens alternate phases of contraction and extension (mostly concentrated in the upper plate). In both cases, extension was intense enough as to create marginal ocean basins and to favor exhumation of deep-seated rocks (quite common in upper plates). Perhaps, these major features they share may explain the resemblance of the final global architecture of these two orogens, and provide additional arguments to consider Gondwana as resistant to subduction and recycling in the mantle and prone to crustal growth, being the upper plate to the orogenic systems it was involved in during at least 300 m.y.Peer reviewe

Significance of the biotite-rích enclaves in the Mora-Gálvez pluton (Toledo)

Biotite-rich enclaves are conspicuos in the peraluminous granites. They had been interpreted as either melting restites or trapped xenoliths at different crustal levels. This work studies the biotite chemical composition in two mica-rich enclaves from Mora-Galvez pluton (Toledo). Based on these data, these mica-rich enclaves are interpreted as metamorphic xenoliths trapped at mid-crustal levels and variably reequilibrated with the host granite magma. Thus, a restitc origin is ruled ou

Caracterización geoquímica de los granitoides de Mora Gálvez (Toledo)

In the plutonic complex of Mora-Gálvez (south Toledo) we have distinguished three types of granitoids: coarse grained granites, porphiritc granites and cordierite nodules leucogranites. Al/ of them are biotite-bearing and they usually have minor cordierite. Geochemically are peralouminous and their origin could be from melting of crustal rock

La mélange tectónica basal del Complejo de Cabo Ortegal (NW del Macizo Ibérico): una unidad clave en la sutura de Pangea

[EN] Recent field work and mapping in the lower units of the Cabo Ortegal Complex provided new data about the tectonic mélange that appears in the lowest structural position: the Somozas Mélange. This mélange 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 mélange: 1) an ophiolitic mélange 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.[ES] Nuevos datos de campo y cartográficos en las unidades inferiores del Complejo de Cabo Ortegal, han proporcionado información relevante sobre la mélange tectónica situada en la posición estructural inferior: la Mélange de Somozas. Esta unidad de mélange tiene una potencia media de unos 1800 m y su aparición está restringida a la parte oriental del complejo; representa por tanto una gran unidad de mezcla localizada en el frente de avance de los complejos alóctonos del NW de Iberia. Tres conjuntos litológicos diferentes se distinguen en la mélange: 1) un conjunto formado por rocas ígneas mezcladas con serpentinitas, formando una mélange ofiolítica típica; 2) un conjunto metasedimentario con filitas y filonitas, con escasos conglomerados, mármoles y cuarcitas; 3) rocas metamórficas de alta-T con tipos variados de anfibolitas y ortogneises.Financial support for this research has been provided by Spanish project CGL2007-65338-CO2-01/BTE (Ministerio de Ciencia e Innovación).Peer reviewe

Mineral chemistry of the gabbroic Messejana – Plasencia dyke in the north-eastern sector

In the gabbroic Messejana–Plasencia dyke we have distinguished three types of pyroxenes: enstatite, pigeonite and augite. Enstatite and augite have been the first minerals that have crystallized at high T (1100ºC – 1150ºC). Pigeonites (@1000ºC) are later and finally appear symplectic two-pyroxene intergrowths around the orthopyroxene. Plagioclase have a continuous zonation with cores of An75 towards rims of An18. Occasionally, there is a micrographic intergrowth of orthoclase–quartz or albite-quartz interstitially to the plagioclase and pyroxene framework. Three types of amphibole have been distinguished: ferroedenite, magnesiohornblende and ferroactinolite. The estimated temperature range of amphibole crystallization is low suggesting that most amphiboles are subsolidus. Finally, biotite are FeO and TiO2 rich, its crystallization being late when residual liquids were enriched in those element

Late Cambrian magmatic arc activity in peri-Gondwana: geochemical evidence from the Basal Allochthonous Units of NW Iberia

The North African section of the Gondwana margin was the site of voluminous, arc-related magmatism during the Late Neoproterozoic (Avalonian–Cadomian orogen). The lower (and older) metasedimentary sequence that constitutes the Basal Units of the Allochthonous Complexes of NW Iberia was deposited in that setting. In these units, sedimentation was followed by the intrusion of tonalites and granodiorites in the late Cambrian (ca. 493–489Ma). In the Late Paleozoic, the collision of Gondwana and Laurussia (Variscan orogeny) deformed and metamorphosed the whole ensemble.New whole rock geochemical analysis performed in seven samples of metatonalites and fourteen samples of metagranodiorites are characterized by: i) slight enrichment in incompatible elements (Rb, Ba, Th, U), ii) negative anomalies in Nb, Ta, P, and Ti, and iii) negative anomalies in Eu. These chemical features are in agreement with a subduction-related setting for the genesis of both types of magma, which is also supported by chemical discrimination using tectonic setting diagrams. Positive anomalies of Pb suggest a crustal component. The new geochemical data reveal that the convergent orogen that ruled the paleogeography of the Gondwana periphery during the Neoproterozoic (Cadomian orogen) remained active beyond the Neoproterozoic-Cambrian transition, up to at least late Cambrian times

Geochemistry of mafic rocks in a Cambrian peri-Gondwanan magmatic arc (Órdenes Complex, NW Iberian Massif)

Trabajo presentado en el IX Congreso Geológico de España, celebrado en Huelva (España), del 12 al 14 de septiembre de 2016[ES] Las Unidades Superiores de los Complejos alóctonos del NO del Macizo Ibérico, han sido interpretadas como parte de un arco magmático peri-Gondwánico, activo desde el Cámbrico medio hasta el Ordovícico inferior. Los principales cuerpos plutónicos, con una edad de ca. 500 Ma., son las gabronoritas de Monte Castelo, y los ortogneises de Corredoiras, que incluyen pequeños cuerpos de metagabronoritas. Estas rocas máficas presentan en efecto características geoquímicas compatibles con su generación en un arco magmático, pero ocupando distinta posición dentro del mismo. Las metagabronoritas de Monte Castelo tienen afinidad toleítica, anomalía negativa de Nb, altas relaciones 143Nd/144Nd (0.5143119–0.513019), bajas de 87Sr/86Srinicial (0.702562–0.703174) y valores positivos de εNdi (+7.8 a +5.4). Sin embargo, las rocas máficas de Corredoiras son de afinidad calco-alcalina, con anomalía negativa de Nb, pero con valores bajos en la relación 143Nd/144Nd (0.512575–0.512436), altos en 87Sr/86Srinicial (0.705082–0.706684) y valores de εNdi entre -0.65 y +1.83. Estas características geoquímicas permiten interpretar estas rocas máficas como generadas en distintas posiciones del arco magmático, localizándose las de Monte Castelo más próximas a la trinchera y las de Corredoiras en una posición más distal.[EN] The Upper Units of the allocthonous complexes of the NW Iberian Massif, have been interpreted as a section of a peri-gondwanan magmatic arc, active from Middle Cambrian to Early Ordovician times. The main plutonic bodies (500 Ma.) intruding the arc metasedimentary rocks, are Monte Castelo gabbronorites, and Corredoiras orthogneisses, which included small metagabbronorites bodies. These mafic rocks have geochemical features consistent with generation in different locations of a same magmatic arc. Monte Castelo metagabbronorites have tholeiitic affinity, with negative Nb anomaly; its 143Nd/144Nd ratios are high (0.5143119–0.513019), whereas initial 87Sr/86Sr ratios are low (0.702562–0.703174), with positive εNdi values (+7.8 to +5.4). However Corredoiras mafic rocks have calcalkaline affinity, also with negative Nb anomaly, low 143Nd/144Nd (0.512575– 0.512436) and high initial 87Sr/86Sr (0.705082–0.706684), εNdi values ranging between −0.65 and +1.83. In the εNd versus age diagram, Monte Castelo samples show compositions equivalent to the contemporaneous depleted mantle, while Corredoiras samples suggest isotopic mixing of the original mafic mantle-derived magmas with a continental crust. These geochemical features can be linked to the setting of a volcanic arc, in which Monte Castelo metagabbros were located closer to the trench, while Corredoiras metagabbros would be located in a more distant position.Este trabajo ha sido financiado por el proyecto CGL 2012-34618 del Ministerio de Economía y CompetitividadPeer reviewe