This work reports an important episode of extensional,mafic magmatism that impacted the North China Craton(NCC)during the Permo-Triassic and influenced the evolution of this Craton.We sampled 30 representative mafic d...This work reports an important episode of extensional,mafic magmatism that impacted the North China Craton(NCC)during the Permo-Triassic and influenced the evolution of this Craton.We sampled 30 representative mafic dykes cropping out in eastern NCC,within the Hebei Province,China.New zircon LA-ICP-MS U-Pb ages,geochemical,and Sr-Nd-Pb isotopic data for these mafic dykes identified them as typical dolerites formed between 211.9±1.3 and 263.9±2.6 Ma.The mafic rocks fall into the alkaline and calc-alkaline series.They are enriched in light rare earth elements,some large ion lithophile elements(e.g.,Rb.Ba,and Sr),K,and Pb,and depleted in Nb,Ta,and Ti.The dykes have high initial 87Sr/86Sr ratios(0.7034-0.7178),negative eNd(t)values(—2.2 to—5.9),and relatively constant initial Pb isotopic ratios,that are EMI-like:(206Pb/204Pb)i=16.39-16.74,(207pb/204pb)j=15.22-15.24,and(208Pb/204Pb)i_=36.66-36.86,respectively.Our results indicate that the Hebei Province dolerites were likely derived from magma gen erated through low-medium degree partial melting(3.0-20%)of an EMI-like garnet-lherzolite mantle source.The parental magmas fractionated olivine,clinopyroxene,and Ti-bearing phases with obvious crustal contamination during rapid ascent and dyke emplacement.Since the possible influence of subduction of the Yangtze Plate has been excluded,we propose a model for the origin of the investigated mafic rocks,involving the Pennian collision between the Siberian Block and the NCC.The mafic dykes of eastern NCC within Hebei Province formed during a period of crustal thinning in response to extension after the early Permian collision.展开更多
As the principal ore mineral in various tungsten(-gold)deposits,scheelite(CaWO_(4))plays an important role in directly dating the timing of ore formation,and in tracing associated material sources through the study of...As the principal ore mineral in various tungsten(-gold)deposits,scheelite(CaWO_(4))plays an important role in directly dating the timing of ore formation,and in tracing associated material sources through the study of its Sm-Nd geochronology and Nd isotopic characteristics.Since the retention of Sm-Nd systematics within scheelite is presently unconstrained,equivocal interpretations for isotopic data resulting from this method have occurred quite often in previous studies that apply these isotopic data.In order to better elucidate the closure of Sm-Nd in scheelite,the kinetics of Sm and Nd within this mineral lattice were investigated through calculation of diffusion constants presented herein.The following Arrhenius relations were obtained:D_(Nd)=4.00exp(-438 kJ·mol^(–1)/RT)cm^(2)/s D_(Sm)=1.85exp(-427 kJ·mol^(–1)/RT)cm^(2)/s showing diffusion rate of Nd is near identical to Sm in scheelite when at the same temperature.However,compared to other rare earth elements(REEs),which have markedly different atomic radii to either Nd or Sm,these are shown to exhibit a great variation in diffusivities.The observed trends in our data are in excellent agreement with the diffusion characteristics of REEs in other tetragonal ABO4 minerals,indicating that ionic radius is a key constraint to the diffusivity of REEs in the various crystal lattices.With this in mind,the same substitution mechanism and a very slight discrepancy in radii will allow us to infer that significant Sm/Nd diffusional fractionation in scheelite is unlikely to occur during most geological processes.Based upon the diffusion data determined herein,Sm and Nd closure temperatures and retention times in scheelite are discussed in terms of diffusion dynamics.Those results suggest that closure temperatures for Sm-Nd within this mineral are relatively high in contrast to the temperature ranges of ore-formation responsible for scheelite-related deposits,and any later thermal environments.It is likely,therefore,that relevant isotopic information could be easily retained under most geological conditions,since initial crystallization of the scheelite.In addition,comparison of this mineral-element pair over a range of temperatures with some other common minerals used as geochronometers(e.g.,zircon and apatite)indicates that Sm-Nd system has a slower diffusive rate in scheelite than for Sr in apatite or Ar in quartz,and only a little faster than for Pb in zircon.It should be noted,within most hydrothermal deposits where zircon has crystallized,its size is typically no more than 100μm,whereas scheelite commonly occurs as macroscopic grains.For this reason,the larger dimensions of scheelite would provide a robust Sm-Nd system more able to resist perturbations,relating to any later thermal process.As such Sm-Nd investigations of scheelite are akin to U-Pb within zircon samples used in isotopic dating.These observations indicate that Sm-Nd age and isotopic information can provide reliable data in all but the most extreme case,especially when data are extracted from macroscopic grains of scheelite that are chosen to be“pristine”(i.e.,free of surface alteration and/or fractures).展开更多
The Baiyangping Cu-Ag polymetallic ore district is located in the northern part of the Lanping-Simao foreland fold belt,between the Jinshajiang-Ailaoshan and Lancangjiang faults,and the deposit can be divided into eas...The Baiyangping Cu-Ag polymetallic ore district is located in the northern part of the Lanping-Simao foreland fold belt,between the Jinshajiang-Ailaoshan and Lancangjiang faults,and the deposit can be divided into eastern and western ore zones.Based upon microscope observation of ore minerals and analysis of zinc,copper,and strontium isotope composition,we conclude that:(1)the zinc isotopic compositions of sphalerite from the eastern and western ore belt of the Baiyangping polymetallic ore deposits are enriched in both the heavy(-0.09‰ to+0.15‰) and light(-0.19‰ to-0.01‰)zinc isotopes.Rayleigh fractionation is likely the additional factor controlling the observed temporal and spatial variations in zinc isotopes in the two studied ore zones.The zinc isotopic composition in the Baiyangping polymetallic Pb-Zn deposits may have the same fractionation as that of magmatic-hydrothermal,VHMS,SEDEX,and MVT deposits,as demonstrated by geological and other geochemical evidence;(2) the range of δ^(65)Cu in massive tetrahedrite is from-0.06‰ to+0.12 ‰ that relates to the early stages of ore-formation,which are higher than that of venial chalcopyrite(from-0.72‰ to-0.07‰)formed at a late ore-forming stage in the western ore belt.Different ore-forming stages and alteration or leaching processes are likely the main factors controlling the observed variations in copper isotopes in the western ore zone;(3) the ^(87)Sr/^(86)Sr value of hydrothermal calcite in eastern(0.7080-0.7093) and western(0.7085-0.7113) ore belt suggested that mineralization of early calcite,with^(87)Sr/^(86)Sr values much higher than in ancient Late Triassic seawater,may be related to recrystallization from a radiogenic Sr-rich or silicifying fluid,either from the strata that the ore-forming fluid flows through or from other fluids.展开更多
文摘This work reports an important episode of extensional,mafic magmatism that impacted the North China Craton(NCC)during the Permo-Triassic and influenced the evolution of this Craton.We sampled 30 representative mafic dykes cropping out in eastern NCC,within the Hebei Province,China.New zircon LA-ICP-MS U-Pb ages,geochemical,and Sr-Nd-Pb isotopic data for these mafic dykes identified them as typical dolerites formed between 211.9±1.3 and 263.9±2.6 Ma.The mafic rocks fall into the alkaline and calc-alkaline series.They are enriched in light rare earth elements,some large ion lithophile elements(e.g.,Rb.Ba,and Sr),K,and Pb,and depleted in Nb,Ta,and Ti.The dykes have high initial 87Sr/86Sr ratios(0.7034-0.7178),negative eNd(t)values(—2.2 to—5.9),and relatively constant initial Pb isotopic ratios,that are EMI-like:(206Pb/204Pb)i=16.39-16.74,(207pb/204pb)j=15.22-15.24,and(208Pb/204Pb)i_=36.66-36.86,respectively.Our results indicate that the Hebei Province dolerites were likely derived from magma gen erated through low-medium degree partial melting(3.0-20%)of an EMI-like garnet-lherzolite mantle source.The parental magmas fractionated olivine,clinopyroxene,and Ti-bearing phases with obvious crustal contamination during rapid ascent and dyke emplacement.Since the possible influence of subduction of the Yangtze Plate has been excluded,we propose a model for the origin of the investigated mafic rocks,involving the Pennian collision between the Siberian Block and the NCC.The mafic dykes of eastern NCC within Hebei Province formed during a period of crustal thinning in response to extension after the early Permian collision.
基金financially supported by the National Natural Science Foundation of China(Grant No.41403035)the National Basic Research Program of China(Grant No.2014CB440901)。
文摘As the principal ore mineral in various tungsten(-gold)deposits,scheelite(CaWO_(4))plays an important role in directly dating the timing of ore formation,and in tracing associated material sources through the study of its Sm-Nd geochronology and Nd isotopic characteristics.Since the retention of Sm-Nd systematics within scheelite is presently unconstrained,equivocal interpretations for isotopic data resulting from this method have occurred quite often in previous studies that apply these isotopic data.In order to better elucidate the closure of Sm-Nd in scheelite,the kinetics of Sm and Nd within this mineral lattice were investigated through calculation of diffusion constants presented herein.The following Arrhenius relations were obtained:D_(Nd)=4.00exp(-438 kJ·mol^(–1)/RT)cm^(2)/s D_(Sm)=1.85exp(-427 kJ·mol^(–1)/RT)cm^(2)/s showing diffusion rate of Nd is near identical to Sm in scheelite when at the same temperature.However,compared to other rare earth elements(REEs),which have markedly different atomic radii to either Nd or Sm,these are shown to exhibit a great variation in diffusivities.The observed trends in our data are in excellent agreement with the diffusion characteristics of REEs in other tetragonal ABO4 minerals,indicating that ionic radius is a key constraint to the diffusivity of REEs in the various crystal lattices.With this in mind,the same substitution mechanism and a very slight discrepancy in radii will allow us to infer that significant Sm/Nd diffusional fractionation in scheelite is unlikely to occur during most geological processes.Based upon the diffusion data determined herein,Sm and Nd closure temperatures and retention times in scheelite are discussed in terms of diffusion dynamics.Those results suggest that closure temperatures for Sm-Nd within this mineral are relatively high in contrast to the temperature ranges of ore-formation responsible for scheelite-related deposits,and any later thermal environments.It is likely,therefore,that relevant isotopic information could be easily retained under most geological conditions,since initial crystallization of the scheelite.In addition,comparison of this mineral-element pair over a range of temperatures with some other common minerals used as geochronometers(e.g.,zircon and apatite)indicates that Sm-Nd system has a slower diffusive rate in scheelite than for Sr in apatite or Ar in quartz,and only a little faster than for Pb in zircon.It should be noted,within most hydrothermal deposits where zircon has crystallized,its size is typically no more than 100μm,whereas scheelite commonly occurs as macroscopic grains.For this reason,the larger dimensions of scheelite would provide a robust Sm-Nd system more able to resist perturbations,relating to any later thermal process.As such Sm-Nd investigations of scheelite are akin to U-Pb within zircon samples used in isotopic dating.These observations indicate that Sm-Nd age and isotopic information can provide reliable data in all but the most extreme case,especially when data are extracted from macroscopic grains of scheelite that are chosen to be“pristine”(i.e.,free of surface alteration and/or fractures).
基金financially supported by General Project of Natural Science Foundation of Shaanxi Province (2020JM-423)。
文摘The Baiyangping Cu-Ag polymetallic ore district is located in the northern part of the Lanping-Simao foreland fold belt,between the Jinshajiang-Ailaoshan and Lancangjiang faults,and the deposit can be divided into eastern and western ore zones.Based upon microscope observation of ore minerals and analysis of zinc,copper,and strontium isotope composition,we conclude that:(1)the zinc isotopic compositions of sphalerite from the eastern and western ore belt of the Baiyangping polymetallic ore deposits are enriched in both the heavy(-0.09‰ to+0.15‰) and light(-0.19‰ to-0.01‰)zinc isotopes.Rayleigh fractionation is likely the additional factor controlling the observed temporal and spatial variations in zinc isotopes in the two studied ore zones.The zinc isotopic composition in the Baiyangping polymetallic Pb-Zn deposits may have the same fractionation as that of magmatic-hydrothermal,VHMS,SEDEX,and MVT deposits,as demonstrated by geological and other geochemical evidence;(2) the range of δ^(65)Cu in massive tetrahedrite is from-0.06‰ to+0.12 ‰ that relates to the early stages of ore-formation,which are higher than that of venial chalcopyrite(from-0.72‰ to-0.07‰)formed at a late ore-forming stage in the western ore belt.Different ore-forming stages and alteration or leaching processes are likely the main factors controlling the observed variations in copper isotopes in the western ore zone;(3) the ^(87)Sr/^(86)Sr value of hydrothermal calcite in eastern(0.7080-0.7093) and western(0.7085-0.7113) ore belt suggested that mineralization of early calcite,with^(87)Sr/^(86)Sr values much higher than in ancient Late Triassic seawater,may be related to recrystallization from a radiogenic Sr-rich or silicifying fluid,either from the strata that the ore-forming fluid flows through or from other fluids.
