The Longmenshan area is located at the western margin of Yangtze platform, where the Devonian is composed of clastic rocks, mixed clastic-carbonate rocks and carbonate rocks in ascending order, and was formed in the m...The Longmenshan area is located at the western margin of Yangtze platform, where the Devonian is composed of clastic rocks, mixed clastic-carbonate rocks and carbonate rocks in ascending order, and was formed in the marine environment from nearshore to outer shelf. Based on a study of six sections the Devonian in Longemenshan area is divided into 18 sequences and 5 sequence sets. The maximum transgressive high in the Devonian of this area occurred in Early Frasnian, corresponding to asymmetricus zone. The boundaries among the sequence sets are roughly corresponding to the bottom boundaries of Ⅰa',Ⅰc',Ⅱa, and Ⅱd in the Devonian of West Europe and North America, respectively. The frequencies of the relative sea level changes in this area vary in different periods, but can be correlated with those in the other regions of the world.展开更多
The Devonian is well developed in South China and has drawn a great attention from the geologists both at domestic and abroad. On the basis of study on the sequence stratigraphy in more than 10 sections in Guizhou, Gu...The Devonian is well developed in South China and has drawn a great attention from the geologists both at domestic and abroad. On the basis of study on the sequence stratigraphy in more than 10 sections in Guizhou, Guangxi, Longmenshan and Southern Qinling within South China we have identified 21 sequences and T-R cycles that correspond to third-order sea level changes. These sea level change cycles were controlled by autorhythm(Pragian,Eifelian), allorhythm(Lochkovian,Emsian,Givetian) and coupling rhythm mechanisms (frasian, Famennian and F-F boundary).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
文摘The Longmenshan area is located at the western margin of Yangtze platform, where the Devonian is composed of clastic rocks, mixed clastic-carbonate rocks and carbonate rocks in ascending order, and was formed in the marine environment from nearshore to outer shelf. Based on a study of six sections the Devonian in Longemenshan area is divided into 18 sequences and 5 sequence sets. The maximum transgressive high in the Devonian of this area occurred in Early Frasnian, corresponding to asymmetricus zone. The boundaries among the sequence sets are roughly corresponding to the bottom boundaries of Ⅰa',Ⅰc',Ⅱa, and Ⅱd in the Devonian of West Europe and North America, respectively. The frequencies of the relative sea level changes in this area vary in different periods, but can be correlated with those in the other regions of the world.
文摘The Devonian is well developed in South China and has drawn a great attention from the geologists both at domestic and abroad. On the basis of study on the sequence stratigraphy in more than 10 sections in Guizhou, Guangxi, Longmenshan and Southern Qinling within South China we have identified 21 sequences and T-R cycles that correspond to third-order sea level changes. These sea level change cycles were controlled by autorhythm(Pragian,Eifelian), allorhythm(Lochkovian,Emsian,Givetian) and coupling rhythm mechanisms (frasian, Famennian and F-F boundary).
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.
