Using laser 40Ar/39Ar dating method,we have gotten the metamorphic ages of lawsonite blueschist and epidote blueschist from Jiuquan,northern Qilian Mountain,NW China.The high quality laser 40Ar/39Ar dating of glaucoph...Using laser 40Ar/39Ar dating method,we have gotten the metamorphic ages of lawsonite blueschist and epidote blueschist from Jiuquan,northern Qilian Mountain,NW China.The high quality laser 40Ar/39Ar dating of glaucophane from lawsonite blueschist gives an isochron age of 413±5 Ma.The isochron age obtained from phengite in epidote blueschist is 415±7 Ma.These data,combining with peak metamorphic P-T conditions and regional geological setting,allow us to infer that the lower limit of the ages of the prograde subduction metamorphism from lawsonite blueschist facies to epidote blueschist facies occurred at ca.413-415 Ma,which also suggests that the formation of lawsonite blueschist in the northern Qilian Mountain maybe resulted from the corner flow in the cold subduction zone.This study shows that the final closing time of the northern Qilian remnant oceanic basin is about 413-415 Ma,which also represents the convergent age between the North China Craton and the Qaidam block.展开更多
In some hydrothermal deposits no K-rich minerals can be selected, and it is very diffi cult to determine the minerogenetic age by the traditional K-Ar method. In this study, the paragenous muscovite and quartz from Lu...In some hydrothermal deposits no K-rich minerals can be selected, and it is very diffi cult to determine the minerogenetic age by the traditional K-Ar method. In this study, the paragenous muscovite and quartz from Lushui tungsten Deposit, western Yunnan have been analysed by the <sup>40</sup>Ar/<sup>39</sup> Ar technique. The aim of the work was to investigate the展开更多
Four international standards,Ga1550,MMhb-1,Lp-6,Bern 4M,and one domesticstandard BT-1 have been intercalibrated.The repeated measurements on MMhb-1 with different massdemonstrate that MMhb-1 is inhomogeneous in age an...Four international standards,Ga1550,MMhb-1,Lp-6,Bern 4M,and one domesticstandard BT-1 have been intercalibrated.The repeated measurements on MMhb-1 with different massdemonstrate that MMhb-1 is inhomogeneous in age and its average age is 519.8 Ma.The results of Bern4M and Lp-6 reflect that they have an invariable value of^(40)Ar*/^(39)Ar_(k)(F)and the ages weobtained are consensus with their K-Ar age:Lp-6=127.7Ma;Bern 4M=18.2 Ma.Analyses of BT-1 agespectra,Ca/K and Cl/K spectra as well as inverse isochrons indicate that the sample is homogeneousand invariable and keeps close chemically,with its trapped argon isotope composition close to theatmosphere.The dating results show that age values are reproducible and steady,total fusion age,step-heating age,plateau age and isochron age are in accord with each other within the error range(2σ).Therefore,we recommend 28.7 Ma as the calibrated age of BT-1.We also discuss the variationin neutron flux gradients of Beijing 49-2 reactor.It was found that the neutron flux gradientvaries considerably,and more monitors(standard samples)are needed to fix the trend of variation.The coefficient of the 49-2 reactor that transfers the ratio of production rate of^(37)Ar_(Ca)/^(39)Ar_(K) into Ca/K ratio is 1.78.This is different from that reported earlier,2.0,which may be caused by the reconstruction of the reactor.展开更多
The Jitang metamorphic complex is key to studying the tectonic evolution of the Northern Lancangjiang zone.Through structural-lithological mapping,structural analysis and laboratory testing,the composition of the Jita...The Jitang metamorphic complex is key to studying the tectonic evolution of the Northern Lancangjiang zone.Through structural-lithological mapping,structural analysis and laboratory testing,the composition of the Jitang metamorphic complex was determined.The macro-and microstructural analyses of the ductile detachment shear zone(Guoxuepu ductile shear zone,2–4 km wide)between the metamorphic complex and the overlying sedimentary cap show that the shear sense of the ductile shear zones is top-to-the-southeast.The presence of various deformation features and quartz C-axis electron backscatter diffraction(EBSD)fabric analysis suggests multiple deformation events occurring at different temperatures.The average stress is 25.68 MPa,with the strain rates(έ)ranging from 9.77×10^(−14)s^(−1)to 6.52×10^(−16)s^(−1).The finite strain of the Guoxuepu ductile shear zone indicates an elongated strain pattern.The average kinematic vorticity of the Guoxuepu ductile shear zone is 0.88,implying that the shear zone is dominated by simple shear.The muscovite selected from the protomylonite samples in the Guoxuepu ductile shear zone yields a 40Ar-39Ar age of 60.09±0.38 Ma.It is suggested that,coeval with the initial Indo–Eurasian collision,the development of strike-slip faults led to a weak and unstable crust,upwelling of lower crust magma,then induced the detachment of the Jitang metamorphic complex in the Eocene.展开更多
The Naruo porphyry copper deposit containing more than 2 Mt of copper is located in the Duolong ore district in the west of the Bangongco–Nujiang belt in central Tibet.New zircon U-Pb,biotite^(40)Ar/^(39)Ar,zircon(U-...The Naruo porphyry copper deposit containing more than 2 Mt of copper is located in the Duolong ore district in the west of the Bangongco–Nujiang belt in central Tibet.New zircon U-Pb,biotite^(40)Ar/^(39)Ar,zircon(U-Th)/He ages,published age data together with thermal modeling were presented in this paper to investigate the thermal history of Naruo deposit.Thermal modeling reveals a prolonged magmatic-hydrothermal evolution firstly cooling from~700℃to~350℃at 120 Ma,then cooling to 230℃at 106 Ma and maintaining at 200℃from 106 to 90 Ma which is attributed to multiple magmatic events and thermal effect of strike-slip fault.Affected by thrust nappe structure,the sample was consistent with 120℃from 70 to 63 Ma.The Naruo deposit started to experience exhumation at a rate of~0.07 km/Myr since 60 Ma which is related to India-Asia collision.The prolonged magmatic-hydrothermal evolution process might have important influence on the Naruo deposit.The ore-related intrusions preserved in the foot walls of strike-slip fault and thrust nappe structure are the objects of future exploration in the Duolong ore district.展开更多
Ages determined with the 40Ar/39Ar isotopic system affirms the Early Cretaceous volcanic activity in the Barton Peninsula, King George Island, Antarctica. Two specimens of basaltic andesite collected from the lower-mo...Ages determined with the 40Ar/39Ar isotopic system affirms the Early Cretaceous volcanic activity in the Barton Peninsula, King George Island, Antarctica. Two specimens of basaltic andesite collected from the lower-most volcanic sequence of the peninsula were irradiated and analyzed in different experiments, yielding an identical age spectrum, and two magmatic thermal events of the Early Cretaceous (120.4±1.6 Ma, 119±1 Ma) and Early Tertiary (53.1±1.5 Ma, 52±1 Ma) are distinguished. The former is interpreted to represent the primary cooling age of basaltic andesite, whereas the latter is the thermal reset age caused by the intrusion of granitic pluton. These new ages clearly indicate that volcanism was active during the Early Cretaceous on the Barton Peninsula and that intensive hydrothermal alteration and mineralization of Mesozoic volcanic rocks resulted from Tertiary magmatism.展开更多
Radiogenic isotope dating of illitic clays has been widely used to reconstruct thermal and fluid flow events in siliciclastic sedimentary basins,the information of which is critical to investigate mechanisms of hydroc...Radiogenic isotope dating of illitic clays has been widely used to reconstruct thermal and fluid flow events in siliciclastic sedimentary basins,the information of which is critical to investigate mechanisms of hydrocarbon maturation.This study carried out Rb-Sr and^(40)Ar-^(39)Ar dating of authigenic illitic clay samples separated from the Palaeogene sandstone in the northern South China Sea.Our Rb-Sr data further confirm the previously reported three periods of fluid flow events(at 34.5±0.9,31.2±0.6,and 23.6±0.8 Ma,respectively)in the northern South China Sea,which are related to regional episodic tectonism.However,^(40)Ar-^(39)Ar ages of illite obtained in this study are significantly younger than the corresponding Rb-Sr ages.The significantly younger^(40)Ar-^(39)Ar ages were probably due to ^(40)Ar loss caused by later dry heating events on the Hainan Island that have not affected the Rb-Sr isotopic systematics.The inconsistency between Rb-Sr and^(40)Ar-^(39)Ar data should be attributed to different isotopic behaviors of K-Ar and Rb-Sr isotopic systematics in illite.Our results indicate that Rb-Sr isotopic dating method may be a preferential approach for clay dating in geological settings where exist younger dry heating events.展开更多
Polymetamorphic units are important constituents of continent-continent collisional orogens,and rift metamorphic assemblages are often overprinted by subsequent metamorphism during subduction and collision.This study ...Polymetamorphic units are important constituents of continent-continent collisional orogens,and rift metamorphic assemblages are often overprinted by subsequent metamorphism during subduction and collision.This study reports the metamorphic conditions and evolution of the Dorud-Azna metamorphic units in the central part of the Sanandaj-Sirjan zone(SSZ),Iran.Here,new geothermobarometry results are integrated with ^(40)Ar/^(39)Ar mineral and Th-U-Pb monazite and thorite ages to provide new insight of polyphase metamorphism in the two different basement units of the SSZ,the lower Galeh-Doz orthogneiss and higher Amphibolite-Metagabbro units.In the Amphibolite-Metagabbro unit,staurolite micaschist underwent a prograde P-T evolution from 640±20℃/6.2±0.8 kbar in garnet cores(M1)to 680±20℃/7.2±1.0 kbar in garnet rims(M2).Three Th-U-Pb monazite ages of 306±5 Ma,322±28 Ma and 336±39 Ma from the garnet-micaschists testify the Carboniferous age of M1 metamorphism.In the same unit,the metagabbro records P-T conditions of 4.0±0.8 kbar and 580±50℃ in the(magmatic)amphibole core(Late Carboniferous intrusion)to 7.5±0.7 kbar and 700±20℃ in the amphibole rim indicating a prograde P-T path during subsequent burial(M1).New ^(40)Ar/^(39)Ar dating of white mica from the staurolite micaschist yielded a staircase pattern ranging from 36±12 Ma to 170±2 Ma.This implies polymetamorphism with a minimum Late Jurassic cooling age through the Ar retention temperature of ca.425±25℃ after M2 metamorphism and a Paleogene low-grade metamorphic overprint(M3),while ^(40)Ar/^(39)Ar white mica dating of garnet micaschist yielded a plateau age of 137.84±0.65 Ma.We therefore interpret the amphibolite-grade metamorphism M2 to have predated 170 Ma and is likely between 180 and 200 Ma.Furthermore,it is overprinted at about 36 Ma under retrogressive low-grade M3 metamorphism(at temperatures of~350-240℃)during final shortening and exhumation.In the underlying Galeh-Doz unit,the Panafrican granitic orthogneiss intruded at P-T condit展开更多
New geochronologic data from midcontinental Laurentia demonstrate that emplacement of the 1476-1470 Ma Wolf River granitic batholith was not an isolated igneous event,but was accompanied by regional metamorphism,defor...New geochronologic data from midcontinental Laurentia demonstrate that emplacement of the 1476-1470 Ma Wolf River granitic batholith was not an isolated igneous event,but was accompanied by regional metamorphism,deformation,and sedimentation.Evidence for such metamorphism and deformation is best seen in siliciclastic sedimentary rocks of the Baraboo Interval,which were deposited closely following the 1.65-1.63 Ga Mazatzal orogeny.In Baraboo Interval strata,muscovite parallel to slatey cleavage,in hydrothermal veins,in quartzite breccia,and in metamorphosed paleosol yielded ^(40)Ar/^(39)Ar plateau ages of 1493-1465 Ma.In addition,U-Th-total Pb dating of neoblastic overgrowths on detrital monazite gave an age of 1488±20 Ma,and recrystallized hematite in folded metapelite gave a mean U/Th-He age of 1411±39 Ma.Post-Baraboo,arkosic polymictic conglomerate,which contains detrital zircon with a minimum peak age of 1493 Ma,was intruded by a 1470 Ma granite porphyry at the northeastern margin of the Wolf River batholith.This episode of magmatism,regional deformation and metamorphism,and sedimentation,which is designated herein as the Baraboo orogeny,provides a midcontinental link between the Picuris orogeny to the southwest and the Pinware orogeny to the northeast,completing the extent of early Mesoproterozoic(Calymmian)orogenesis for 5000 km along the southern margin of Laurentia.This transcontinental orogen is unique among Precambrian orogenies for its great width(-1600 km),the predominance of ferroan granites derived from partial melting of lower continental crust,and the prevalence of regional high T-P metamorphism related to advective heating by granitic magmas emplaced in the middle to upper crust.展开更多
基金supported by the National Natural Science Foundation of China (40821002)the National Basic Research Pro-gram of China (2009CB825007)
文摘Using laser 40Ar/39Ar dating method,we have gotten the metamorphic ages of lawsonite blueschist and epidote blueschist from Jiuquan,northern Qilian Mountain,NW China.The high quality laser 40Ar/39Ar dating of glaucophane from lawsonite blueschist gives an isochron age of 413±5 Ma.The isochron age obtained from phengite in epidote blueschist is 415±7 Ma.These data,combining with peak metamorphic P-T conditions and regional geological setting,allow us to infer that the lower limit of the ages of the prograde subduction metamorphism from lawsonite blueschist facies to epidote blueschist facies occurred at ca.413-415 Ma,which also suggests that the formation of lawsonite blueschist in the northern Qilian Mountain maybe resulted from the corner flow in the cold subduction zone.This study shows that the final closing time of the northern Qilian remnant oceanic basin is about 413-415 Ma,which also represents the convergent age between the North China Craton and the Qaidam block.
文摘In some hydrothermal deposits no K-rich minerals can be selected, and it is very diffi cult to determine the minerogenetic age by the traditional K-Ar method. In this study, the paragenous muscovite and quartz from Lushui tungsten Deposit, western Yunnan have been analysed by the <sup>40</sup>Ar/<sup>39</sup> Ar technique. The aim of the work was to investigate the
基金supported by the National Natural Science Foundation of China(Grant No.40473031).
文摘Four international standards,Ga1550,MMhb-1,Lp-6,Bern 4M,and one domesticstandard BT-1 have been intercalibrated.The repeated measurements on MMhb-1 with different massdemonstrate that MMhb-1 is inhomogeneous in age and its average age is 519.8 Ma.The results of Bern4M and Lp-6 reflect that they have an invariable value of^(40)Ar*/^(39)Ar_(k)(F)and the ages weobtained are consensus with their K-Ar age:Lp-6=127.7Ma;Bern 4M=18.2 Ma.Analyses of BT-1 agespectra,Ca/K and Cl/K spectra as well as inverse isochrons indicate that the sample is homogeneousand invariable and keeps close chemically,with its trapped argon isotope composition close to theatmosphere.The dating results show that age values are reproducible and steady,total fusion age,step-heating age,plateau age and isochron age are in accord with each other within the error range(2σ).Therefore,we recommend 28.7 Ma as the calibrated age of BT-1.We also discuss the variationin neutron flux gradients of Beijing 49-2 reactor.It was found that the neutron flux gradientvaries considerably,and more monitors(standard samples)are needed to fix the trend of variation.The coefficient of the 49-2 reactor that transfers the ratio of production rate of^(37)Ar_(Ca)/^(39)Ar_(K) into Ca/K ratio is 1.78.This is different from that reported earlier,2.0,which may be caused by the reconstruction of the reactor.
基金financially supported by the China Geological Survey Scientific Research Project(Grant Nos.DD20190167 and DD20190053)the National Natural Science Foundation of China(Grant No.42172259).
文摘The Jitang metamorphic complex is key to studying the tectonic evolution of the Northern Lancangjiang zone.Through structural-lithological mapping,structural analysis and laboratory testing,the composition of the Jitang metamorphic complex was determined.The macro-and microstructural analyses of the ductile detachment shear zone(Guoxuepu ductile shear zone,2–4 km wide)between the metamorphic complex and the overlying sedimentary cap show that the shear sense of the ductile shear zones is top-to-the-southeast.The presence of various deformation features and quartz C-axis electron backscatter diffraction(EBSD)fabric analysis suggests multiple deformation events occurring at different temperatures.The average stress is 25.68 MPa,with the strain rates(έ)ranging from 9.77×10^(−14)s^(−1)to 6.52×10^(−16)s^(−1).The finite strain of the Guoxuepu ductile shear zone indicates an elongated strain pattern.The average kinematic vorticity of the Guoxuepu ductile shear zone is 0.88,implying that the shear zone is dominated by simple shear.The muscovite selected from the protomylonite samples in the Guoxuepu ductile shear zone yields a 40Ar-39Ar age of 60.09±0.38 Ma.It is suggested that,coeval with the initial Indo–Eurasian collision,the development of strike-slip faults led to a weak and unstable crust,upwelling of lower crust magma,then induced the detachment of the Jitang metamorphic complex in the Eocene.
基金granted by National Natural Science Foundation of China(Grant Nos.42002103,42102109)the Basal Research Fund of Chinese Academy of Geological Sciences(Grant No.KK2017)the Technical Service for Quality Monitoring of Mineral Exploration in Tiegelongnan Mining Area,Duolong Ore district,Gaize County,Tibet(Grant No.XZJL-JS-2021-004).
文摘The Naruo porphyry copper deposit containing more than 2 Mt of copper is located in the Duolong ore district in the west of the Bangongco–Nujiang belt in central Tibet.New zircon U-Pb,biotite^(40)Ar/^(39)Ar,zircon(U-Th)/He ages,published age data together with thermal modeling were presented in this paper to investigate the thermal history of Naruo deposit.Thermal modeling reveals a prolonged magmatic-hydrothermal evolution firstly cooling from~700℃to~350℃at 120 Ma,then cooling to 230℃at 106 Ma and maintaining at 200℃from 106 to 90 Ma which is attributed to multiple magmatic events and thermal effect of strike-slip fault.Affected by thrust nappe structure,the sample was consistent with 120℃from 70 to 63 Ma.The Naruo deposit started to experience exhumation at a rate of~0.07 km/Myr since 60 Ma which is related to India-Asia collision.The prolonged magmatic-hydrothermal evolution process might have important influence on the Naruo deposit.The ore-related intrusions preserved in the foot walls of strike-slip fault and thrust nappe structure are the objects of future exploration in the Duolong ore district.
基金This work was financially supported by the National Natural Science Foundation of China(49672107)CAS(KZ951-A1-205-02)+2 种基金the Ministry of Science and Technology of China(98-927-01-06-05)This work was also supported by the Ministry of Maritime Affairs and Fisheries,Korea through KORDI(PP98001 04)The 9th Korean Antarctic Re-search Program and the King Sejong Station headedby Dr.Yeadong Kim provided logistic support duringthe expedition.
文摘Ages determined with the 40Ar/39Ar isotopic system affirms the Early Cretaceous volcanic activity in the Barton Peninsula, King George Island, Antarctica. Two specimens of basaltic andesite collected from the lower-most volcanic sequence of the peninsula were irradiated and analyzed in different experiments, yielding an identical age spectrum, and two magmatic thermal events of the Early Cretaceous (120.4±1.6 Ma, 119±1 Ma) and Early Tertiary (53.1±1.5 Ma, 52±1 Ma) are distinguished. The former is interpreted to represent the primary cooling age of basaltic andesite, whereas the latter is the thermal reset age caused by the intrusion of granitic pluton. These new ages clearly indicate that volcanism was active during the Early Cretaceous on the Barton Peninsula and that intensive hydrothermal alteration and mineralization of Mesozoic volcanic rocks resulted from Tertiary magmatism.
基金supported by the National Natural Science Foundation of China(Nos.42072142,41702121,U19B2007)。
文摘Radiogenic isotope dating of illitic clays has been widely used to reconstruct thermal and fluid flow events in siliciclastic sedimentary basins,the information of which is critical to investigate mechanisms of hydrocarbon maturation.This study carried out Rb-Sr and^(40)Ar-^(39)Ar dating of authigenic illitic clay samples separated from the Palaeogene sandstone in the northern South China Sea.Our Rb-Sr data further confirm the previously reported three periods of fluid flow events(at 34.5±0.9,31.2±0.6,and 23.6±0.8 Ma,respectively)in the northern South China Sea,which are related to regional episodic tectonism.However,^(40)Ar-^(39)Ar ages of illite obtained in this study are significantly younger than the corresponding Rb-Sr ages.The significantly younger^(40)Ar-^(39)Ar ages were probably due to ^(40)Ar loss caused by later dry heating events on the Hainan Island that have not affected the Rb-Sr isotopic systematics.The inconsistency between Rb-Sr and^(40)Ar-^(39)Ar data should be attributed to different isotopic behaviors of K-Ar and Rb-Sr isotopic systematics in illite.Our results indicate that Rb-Sr isotopic dating method may be a preferential approach for clay dating in geological settings where exist younger dry heating events.
基金support by the Afro-Asiatisches Institut,Salzburg for her Ph D thesis at the Salzburg University。
文摘Polymetamorphic units are important constituents of continent-continent collisional orogens,and rift metamorphic assemblages are often overprinted by subsequent metamorphism during subduction and collision.This study reports the metamorphic conditions and evolution of the Dorud-Azna metamorphic units in the central part of the Sanandaj-Sirjan zone(SSZ),Iran.Here,new geothermobarometry results are integrated with ^(40)Ar/^(39)Ar mineral and Th-U-Pb monazite and thorite ages to provide new insight of polyphase metamorphism in the two different basement units of the SSZ,the lower Galeh-Doz orthogneiss and higher Amphibolite-Metagabbro units.In the Amphibolite-Metagabbro unit,staurolite micaschist underwent a prograde P-T evolution from 640±20℃/6.2±0.8 kbar in garnet cores(M1)to 680±20℃/7.2±1.0 kbar in garnet rims(M2).Three Th-U-Pb monazite ages of 306±5 Ma,322±28 Ma and 336±39 Ma from the garnet-micaschists testify the Carboniferous age of M1 metamorphism.In the same unit,the metagabbro records P-T conditions of 4.0±0.8 kbar and 580±50℃ in the(magmatic)amphibole core(Late Carboniferous intrusion)to 7.5±0.7 kbar and 700±20℃ in the amphibole rim indicating a prograde P-T path during subsequent burial(M1).New ^(40)Ar/^(39)Ar dating of white mica from the staurolite micaschist yielded a staircase pattern ranging from 36±12 Ma to 170±2 Ma.This implies polymetamorphism with a minimum Late Jurassic cooling age through the Ar retention temperature of ca.425±25℃ after M2 metamorphism and a Paleogene low-grade metamorphic overprint(M3),while ^(40)Ar/^(39)Ar white mica dating of garnet micaschist yielded a plateau age of 137.84±0.65 Ma.We therefore interpret the amphibolite-grade metamorphism M2 to have predated 170 Ma and is likely between 180 and 200 Ma.Furthermore,it is overprinted at about 36 Ma under retrogressive low-grade M3 metamorphism(at temperatures of~350-240℃)during final shortening and exhumation.In the underlying Galeh-Doz unit,the Panafrican granitic orthogneiss intruded at P-T condit
基金Detrital zircon analyses were funded in part by the USGS National Cooperative Geologic Mapping Program under award G16AC00143(2016)to EKS.Support for the Arizona LaserChron Center,where detrital zircon analyses of samples 07ES15 and 05ES15 were performed,was provided by NSFEAR 1649254.Support for monazite geochronology was partially provided by an Institute for Lake Superior Geology student research grant and NSF-EAR 0620101 to AVL.
文摘New geochronologic data from midcontinental Laurentia demonstrate that emplacement of the 1476-1470 Ma Wolf River granitic batholith was not an isolated igneous event,but was accompanied by regional metamorphism,deformation,and sedimentation.Evidence for such metamorphism and deformation is best seen in siliciclastic sedimentary rocks of the Baraboo Interval,which were deposited closely following the 1.65-1.63 Ga Mazatzal orogeny.In Baraboo Interval strata,muscovite parallel to slatey cleavage,in hydrothermal veins,in quartzite breccia,and in metamorphosed paleosol yielded ^(40)Ar/^(39)Ar plateau ages of 1493-1465 Ma.In addition,U-Th-total Pb dating of neoblastic overgrowths on detrital monazite gave an age of 1488±20 Ma,and recrystallized hematite in folded metapelite gave a mean U/Th-He age of 1411±39 Ma.Post-Baraboo,arkosic polymictic conglomerate,which contains detrital zircon with a minimum peak age of 1493 Ma,was intruded by a 1470 Ma granite porphyry at the northeastern margin of the Wolf River batholith.This episode of magmatism,regional deformation and metamorphism,and sedimentation,which is designated herein as the Baraboo orogeny,provides a midcontinental link between the Picuris orogeny to the southwest and the Pinware orogeny to the northeast,completing the extent of early Mesoproterozoic(Calymmian)orogenesis for 5000 km along the southern margin of Laurentia.This transcontinental orogen is unique among Precambrian orogenies for its great width(-1600 km),the predominance of ferroan granites derived from partial melting of lower continental crust,and the prevalence of regional high T-P metamorphism related to advective heating by granitic magmas emplaced in the middle to upper crust.
