In the Galicia Region of the NW Iberian Massif several allochthonous complexes(Cabo Ortegal,Órdenes and Malpica-Tui)contain a rootless Variscan suture that can be traced along the belt,from Iberia to the Bohemian...In the Galicia Region of the NW Iberian Massif several allochthonous complexes(Cabo Ortegal,Órdenes and Malpica-Tui)contain a rootless Variscan suture that can be traced along the belt,from Iberia to the Bohemian Massif in Central Europe.Within these allochthonous complexes are several ophiolite zones bounded by two different continental terranes.There exist in NW Iberia two different ophiolite groups with different chemical compositions,isotopic signatures and structural positions.The Bazar and Vila de Cruces ophiolites,characterized by c.500 Ma protolith ages,represent the Lower Group,whereas the Careón,Purrido and Moeche ophiolites containing 395 Ma maficultramafic sequences represent the Upper Group.This younger group constitutes the most widespread ophiolites in the Variscan Belt.A thick serpentinite mélange(Somozas Mélange)occurring at the base of the Cabo Ortegal Complex also belongs to the ophiolite zones of the Variscan suture.In this paper we describe the Galician ophiolites of the Variscan suture and discuss their tectonic setting of formation.We interpret the generation of the Galician ophiolites within the geodynamic and paleogeographic evolution of the Rheic Ocean and the Pangea supercontinent.展开更多
The c.1000-m-thick pre-orogenic Devonian mainly metapelitic sequence of North Menorca Island shows a fairly complete stratigraphic succession.The rocks of this sequence indicate gradually increasing deeper marine cond...The c.1000-m-thick pre-orogenic Devonian mainly metapelitic sequence of North Menorca Island shows a fairly complete stratigraphic succession.The rocks of this sequence indicate gradually increasing deeper marine conditions of sedimentation towards its uppermost levels.Furthermore,the obtained sedimentary characteristics resemble those related to a deep and narrow basin-associated deposit.Thin sills of Ti-augite-bearing alkaline gabbros occur within the Devonian sequence.The intensity of Variscan deformation increases downwards through the structure.According to the characteristics of the Devonian sequence and its location within the Variscan Orogen,a correlation with similar units located in the southern sectors of the Central Iberian Zone(Iberian Massif)is suggested.The Devonian metapelitic rocks have geochemical characteristics suggesting that they represent moderately recycled mature siliciclastic sediments,generated from erosion of distant source areas belonging to an upper continental crust.The relatively narrow range of variation observed in initial^(143)Nd/^(144)Nd ratios supports a similar source for the Menorcan slates(0.51165–0.51182).However,a marked trend is observed in these isotope ratios,from lower values at the base of the stratigraphic column(minimum value of 0.511941)to higher values at the top(maximum value of 0.512131).The^(147)Sm/^(144)Nd ratios vary between 0.1074 and 0.1238,within the range defined for siliciclastic rocks with felsic crustal provenance.The Nd model ages define a narrow range between 1496 Ma and 1754 Ma(Late Paleoproterozoic–Early Mesoproterozoic),and they are consistently younger up-section.These data rule out a provenance from the erosion of the West Africa Craton,as they are more compatible with a provenance from regions located in the Trans-Saharan Belt or Sahara Metacraton.The characteristics of the Menorcan Devonian sequence are compatible with its deposition in a narrow and deep peri-Gondwanan transtensional basin,generated to the south of an advancing Variscan orogenic wedge.Systematic variations in the Nd isotopic composition indicate the progressive and continuous denudation of increasingly more easterly North African sectors in a collisional context between Laurussia and Gondwana with a marked dextral component.These data must be interpreted in the sense that there was not a large oceanic domain during Devonian times to the south of Iberia,able to block the arrival of detrital material from North Africa.A large tract of the Paleo-Tethys Ocean would therefore not have existed during the Devonian south of Iberia.This ocean must therefore have had limited extent in this period towards the westernmost sectors.The Devonian peri-Gondwanan shelf was apparently continuous around Iberia.This platform was progressively affected by Variscan deformation advancing from north to south and incorporated into the Variscan orogenic wedge with the same vergence.展开更多
基金support has been provided by the Spanish project CGL2012-34618(Ministerio de Economía y Competitividad).
文摘In the Galicia Region of the NW Iberian Massif several allochthonous complexes(Cabo Ortegal,Órdenes and Malpica-Tui)contain a rootless Variscan suture that can be traced along the belt,from Iberia to the Bohemian Massif in Central Europe.Within these allochthonous complexes are several ophiolite zones bounded by two different continental terranes.There exist in NW Iberia two different ophiolite groups with different chemical compositions,isotopic signatures and structural positions.The Bazar and Vila de Cruces ophiolites,characterized by c.500 Ma protolith ages,represent the Lower Group,whereas the Careón,Purrido and Moeche ophiolites containing 395 Ma maficultramafic sequences represent the Upper Group.This younger group constitutes the most widespread ophiolites in the Variscan Belt.A thick serpentinite mélange(Somozas Mélange)occurring at the base of the Cabo Ortegal Complex also belongs to the ophiolite zones of the Variscan suture.In this paper we describe the Galician ophiolites of the Variscan suture and discuss their tectonic setting of formation.We interpret the generation of the Galician ophiolites within the geodynamic and paleogeographic evolution of the Rheic Ocean and the Pangea supercontinent.
基金support has been provided by the Spanish projects PID2020-112489 GBC21 and PID2020-112489 GB-C22 funded by MCIN/AEI.
文摘The c.1000-m-thick pre-orogenic Devonian mainly metapelitic sequence of North Menorca Island shows a fairly complete stratigraphic succession.The rocks of this sequence indicate gradually increasing deeper marine conditions of sedimentation towards its uppermost levels.Furthermore,the obtained sedimentary characteristics resemble those related to a deep and narrow basin-associated deposit.Thin sills of Ti-augite-bearing alkaline gabbros occur within the Devonian sequence.The intensity of Variscan deformation increases downwards through the structure.According to the characteristics of the Devonian sequence and its location within the Variscan Orogen,a correlation with similar units located in the southern sectors of the Central Iberian Zone(Iberian Massif)is suggested.The Devonian metapelitic rocks have geochemical characteristics suggesting that they represent moderately recycled mature siliciclastic sediments,generated from erosion of distant source areas belonging to an upper continental crust.The relatively narrow range of variation observed in initial^(143)Nd/^(144)Nd ratios supports a similar source for the Menorcan slates(0.51165–0.51182).However,a marked trend is observed in these isotope ratios,from lower values at the base of the stratigraphic column(minimum value of 0.511941)to higher values at the top(maximum value of 0.512131).The^(147)Sm/^(144)Nd ratios vary between 0.1074 and 0.1238,within the range defined for siliciclastic rocks with felsic crustal provenance.The Nd model ages define a narrow range between 1496 Ma and 1754 Ma(Late Paleoproterozoic–Early Mesoproterozoic),and they are consistently younger up-section.These data rule out a provenance from the erosion of the West Africa Craton,as they are more compatible with a provenance from regions located in the Trans-Saharan Belt or Sahara Metacraton.The characteristics of the Menorcan Devonian sequence are compatible with its deposition in a narrow and deep peri-Gondwanan transtensional basin,generated to the south of an advancing Variscan orogenic wedge.Systematic variations in the Nd isotopic composition indicate the progressive and continuous denudation of increasingly more easterly North African sectors in a collisional context between Laurussia and Gondwana with a marked dextral component.These data must be interpreted in the sense that there was not a large oceanic domain during Devonian times to the south of Iberia,able to block the arrival of detrital material from North Africa.A large tract of the Paleo-Tethys Ocean would therefore not have existed during the Devonian south of Iberia.This ocean must therefore have had limited extent in this period towards the westernmost sectors.The Devonian peri-Gondwanan shelf was apparently continuous around Iberia.This platform was progressively affected by Variscan deformation advancing from north to south and incorporated into the Variscan orogenic wedge with the same vergence.
