To study the thinning of cratonic lithosphere in North China has been the hot subject of basic research in the fields of solid earth science in China. This paper presents an overview on the formation and evolution of ...To study the thinning of cratonic lithosphere in North China has been the hot subject of basic research in the fields of solid earth science in China. This paper presents an overview on the formation and evolution of continental crust, and outlines the mechanisms of forming the lithospheric mantle. It is suggested that the thinning of cratonic lithosphere principally proceeds in two ways, one by subduc- tion erosion (e.g., North China), and the other by a combination of subduction erosion and underplating degistion (e.g., Yangtze).展开更多
Moho depth and crustal average Poisson＇s ratio for 823 stations are obtained by H-n： stacking of receiver functions. These, to- gether with topography and receiver function amplitude information, were used to study ...Moho depth and crustal average Poisson＇s ratio for 823 stations are obtained by H-n： stacking of receiver functions. These, to- gether with topography and receiver function amplitude information, were used to study the crustal structure beneath the North China Craton （NCC）. The results suggest that modified and preserved crust coexist beneath the craton with generally Airy-type isostatic equilibrium. The equilibrium is relatively low in the eastern NCC and some local areas in the central and western NCC, which correlates well with regional geology and tectonic features. Major differences in the crust were observed beneath the eastern, central, and western NCC, with average Moho depths of 33, 37, and 42 km and average Poisson＇s ratios of 0.268, 0.267 and 0.264, respectively. Abnormal Moho depths and Poisson＇s ratios are mainly present in the rift zones, the northern and southern edges of the central NCC, and tectonic boundaries. The crust beneath Ordos retains the characteristics of typical craton. Poisson＇s ratio increases roughly linearly as Moho depth decreases in all three parts of the NCC with different slopes. Receiver function amplitudes are relatively large in the northern edge of the eastern and central NCC, and small in and near the rifts. The Yanshan Mountains and southern part of the Shanxi rift show small-scale variations in the receiver-function ampli- tudes. These observations suggest that overall modification and thinning in the crust occurred in the eastern NCC, and local crustal modification occurred in the central and westem NCC. Different crustal structures in the eastern, central, and western NCC suggest different modification processes and mechanisms. The overall destruction of the crustal structure in the eastern NCC is probably due to the westward subduction of the Pacific Plate during the Meso-Cenozoic time; the local modifications of the crust in the central and western NCC may be due to repeated reactivations at zones with a heterogeneous structure by successive thermal-tec展开更多
From the 1960 s to 1970 s, North China has been hit by a series of large earthquakes. During the past half century,geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associa...From the 1960 s to 1970 s, North China has been hit by a series of large earthquakes. During the past half century,geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associated studies in North China.They have made significant progress on several key issues in the geosciences, such as the crustal and upper mantle structure and the seismogenic environment of strong earthquakes. Deep seismic profiling results indicate a complex tectonic setting in the strong earthquake areas of North China, where a listric normal fault and a low-angle detachment in the upper crust coexist with a high-angle deep fault passing through the lower crust to the Moho beneath the hypocenter. Seismic tomography images reveal that most of the large earthquakes occurred in the transition between the high-and low-velocity zones, and the Tangshan earthquake area is characterized by a low-velocity anomaly in the middle-lower crust. Comprehensive analysis of geophysical data identified that the deep seismogenic environment in the North China extensional tectonic region is generally characterized by a low-velocity anomalous belt beneath the hypocenter, inconsistency of the deep and shallow structures in the crust, a steep crustalal-scale fault,relative lower velocities in the uppermost mantle, and local Moho uplift, etc. This indicates that the lithospheric structure of North China has strong heterogeneities. Geologically, the North China region had been a stable craton named the North China Craton or in brief the NCC, containing crustal rocks as old as ～3.8 Ga. The present-day strong seismic activity and the lower velocity of the lower crust in the NCC are much different from typical stable cratons around the world. These findings provide significant evidence for the destruction of the NCC. Although deep seismic profiling and seismic tomography have greatly enhanced knowledge about the deep-seated structure and seismogenic environment, some fundamental issues still remain and require further work.展开更多
The North China Craton(NCC) is an important part of eastern China. Recent studies have shown that the eastern NCC(ENCC) has undergone significant lithospheric thinning and destruction since the late Mesozoic. Destruct...The North China Craton(NCC) is an important part of eastern China. Recent studies have shown that the eastern NCC(ENCC) has undergone significant lithospheric thinning and destruction since the late Mesozoic. Destruction of the cratonic lithosphere is necessarily accompanied by crustal deformation. Therefore, a detailed crustal deformation model can provide basic observational constraints for understanding the process and mechanisms of the destruction of the NCC. In this study, we estimated the crustal azimuthal anisotropy beneath 198 broadband stations in the NCC with a joint analysis of Ps waves converted at the Moho from radial and transverse receiver function data. We also performed a harmonic analysis to test the reliability of the measured anisotropy. We obtained robust crustal azimuthal anisotropy beneath 23 stations that are mostly located on the western margin of the Bohai Bay Basin, Yin-Yan orogenic belt, and Taihang Mountains, which reflects the crustal deformation characteristics in those regions. The crustal shear wave splitting time was found to range from 0.05 s to 0.68 s, with an average value of 0.23 s, which reveals a distinct crustal anisotropy in the Trans-North China Orogen(TNCO) and its adjacent areas. Our analysis of the results suggests that the strong NW-SE tectonic extension in the late Mesozoic and Cenozoic played an important role in crustal anisotropy in this region. In addition, the E-W trending crustal anisotropy on the margin of the Bohai Bay Basin indicates an effect of the ENE-WSW trending horizontal principal compressive stress. The crustal anisotropy in the Yin-Yan orogenic belt may be an imprint of the multiple-phase shortening of a dominant N-S direction from the early-to-middle Jurassic to the Early Cretaceous. Stations in the Taihang Mountains show large splitting times and well-aligned NW-SE fast directions that correlate with those measured from SKS splitting and that are possibly related to the lithospheric modification and magmatic underplating from the Late Mesozoic to展开更多
Seismic tomography can provide both fine P-wave and S-wave velocity structures of the crust and upper mantle. In addition, with proper computation, Poisson＇s ratio images from the seismic velocities can be determined...Seismic tomography can provide both fine P-wave and S-wave velocity structures of the crust and upper mantle. In addition, with proper computation, Poisson＇s ratio images from the seismic velocities can be determined. However, it is unknown whether Poisson＇s ratio images have any advantages when compared with the P-wave and S-wave velocity images. For the purposes of this study, high- resolution seismic tomography under the eastern part of North China region was used to determine detailed 3-D crustal P- and S-wave seismic velocities structure, as well as Poisson＇s ratio images. Results of Poisson＇s ratio imaging show high Poisson＇s ratio （high-PR） anomalies located in the Hengshan-North Taihang-Zhangjiakou （H-NT-Z） region, demonstrating that Poisson＇s ratio imaging can provide new geophysical constraints for regional tectonic evolution. The H-NT-Z region shows a prominent and continuous high-PR anomaly in the upper crust. Based on Poisson＇s ratio images at different depths, we find that this high-PR anomaly is extending down to the middle crust with thickness up to about 26 kin. According to rock physical property measurements and other geological data, this crustal Poisson＇s ratio anomaly can be explained by Mesozoic partial melting of the upper mantle and basaltic magma underplating related to the lithospheric thinning of the North China craton.展开更多
The lower crust beneath the North China Craton(NCC)was transformed during the craton destruction in the Mesozoic,however,the transformation processes are yet to be fully understood.Compositional and geochronological v...The lower crust beneath the North China Craton(NCC)was transformed during the craton destruction in the Mesozoic,however,the transformation processes are yet to be fully understood.Compositional and geochronological variations of granulite and pyroxenite xenoliths provided insights into the nature of the lower crust before and after the craton destruction.In this study,we summarized the latest results of geochemistry and zircon geochronology coupled with Hf-O isotopes from granulite and pyroxenite xenoliths hosted by Phanerozoic igneous rocks in NCC.Comparing previous studies on the granulite terranes and adakitic rocks of NCC,we aim to discuss the destruction processes of lower crust beneath the NCC.The granulite and pyroxenite xenoliths of NCC were divided into two and three groups,respectively,based on the differences of geochemical features.Group I granulite xenoliths from the NCC have silicic-basic compositions,with metamorphic ferrosilite.The Group I granulite xenoliths show relatively lower Mg#values of pyroxenes and whole-rock than that of the Group II granulite xenoliths,and enrichments of light rare earth elements and Sr-Nd isotopic compositions.Their zircons display Archean-Phanerozoic ages with three peaks of Neoarchean,Paleoproterozoic,and Mesozoic.Generally,Group I granulite xenoliths show close affinities to the granulite terranes of the NCC in terms of the major and trace elements and Sr-Nd isotopic compositions,with a consistent Archean-Proterozoic evolutionary history.However,Group I granulite xenoliths have abundant Phanerozoic zircons with variable Hf isotopic compositions from depleted to enriched,which could be formed by modifications of magma underplating.Therefore,Group I granulite xenoliths represent the modified ancient lower crust beneath the NCC.The Group II granulite and Group III pyroxenite xenoliths from the NCC have similar geochemical features and are basic in compositions,with metamorphic to magmatic orthopyroxenes.The Group II granulite and Group III pyroxenite xenoliths usually展开更多
The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution,especially during the Archean.Howe...The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution,especially during the Archean.However,the scarcity of exposure to these rocks,the complexity of lithological relationships,and the high degree of superimposed deformation,especially with long-lived magmatism,make it difficult to study ancient rocks.Despite this complexity,exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America’s primitive crust.Therefore,here we present field relationships,LA-ICP-SFMS zircon U-Pb ages,and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex.The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic,and felsic members ranging from TTG(Tonalite-Trondhjemite-Granodiorite)to granitic composition.Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex:Eoarchean(ca.3.65–3.60 Ga),early Paleoarchean(ca.3.55–3.52 Ga),middle-late Paleoarchean(ca.3.49–3.33 Ga)and Neoarchean(ca.2.74–2.58 Ga),with no records of Mesoarchean rocks.Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time.In the Eoarchean,rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources,whereas mantle depletion played a role in the Paleoarchean,followed by greater differentiation of the crust with thickening and recycling in the middle–late Paleoarchean.A different stage of crustal growth and recycling dominated the Neoarchean,probably owing to the thickening of the continental crust by collision,continental arc growth,and mantle differentiation.展开更多
Zircon U-Pb dating by both SHRIMP and LA-ICP-MS and geochemical study of theTiaojishan Formation and the Donglintai Formation from Xishan, Beijing, reveal that ages of upperlavas of Tiaojishan Formation and Middle of ...Zircon U-Pb dating by both SHRIMP and LA-ICP-MS and geochemical study of theTiaojishan Formation and the Donglintai Formation from Xishan, Beijing, reveal that ages of upperlavas of Tiaojishan Formation and Middle of Donglintai Formation are 137.1 ± 4.5 Ma(2σ) and 130—134 Ma, respectively. The fomer is slightly older than the latter and the age difference betweenthese two formations is less than 5 Ma. These lines of evidence prove that the two volcanoes eruptedwithin a short time. The age of the Tiaojishan Formation from Xishan, Beijing is distinctivelydifferent from that of the Chende Basin. This indicated that the ages of Tiaojishan lavas varied indifferent regions. The Tiaojishan Formation consists of typical adakite (SiO_2 = 56%, Na_2O =3.99—6.17, Na_2O/K_2O = 2.2 -3.1, Sr = 680-1074 x 10^(-6), Y = 13.2-16.3 x 10^(-6), Yb = 1.13-1.52x 10^(-6), Sr/Y = 43-66), high-Mg adakite and high-Mg andesite (Mg# = 54—55). Features ofcontinental crust of adakite from the Tiaojishan Formation and its syngeneric middle silicic vocanicrocks, such as typical Nd-Ta negative abnormality and Pb possive abnormality, indicate that theselavas are originated from partial melts of continental crust. These results suggest that the adakitefrom the Tiaojishan Formation of Xishan, Beijing derived from thickened eclogitic lower crust andlithosphere beneath the North China craton at mesozoic that was foundered into the aesthenosphere,and subsequenctly partially melted and interacted with mantle olivine during melts upward migration.The age of lavas from the Tiaojishan Formation restrained the foundation which should last at leastuntil 137 Ma. Lavas of the Donglintai Formation are rhyolith and andesite with normal Mg# and thusthey did not interact with the mantle. These lavas represent remobilized melts of lower crustmaterial caused by mantle aesthenosphere upwelling migration induced by foundation.展开更多
Ten volcanic samples at Zhangwu,western Liaoning Province,North China were selected for a sys-tematic geochemical,mineralogical and geochronological study,which provides an opportunity to ex-plore the interaction betw...Ten volcanic samples at Zhangwu,western Liaoning Province,North China were selected for a sys-tematic geochemical,mineralogical and geochronological study,which provides an opportunity to ex-plore the interaction between the continental crust and mantle beneath the north margin of the North China craton.Except one basalt sample(SiO2= 50.23%),the other nine samples are andesitic with SiO2 contents ranging from 53% to 59%.They have relatively high MgO(3.4%―6.1%,Mg#=50―64) and Ni and Cr contents(Ni 27×10?6―197×10?6,Cr 51×10?6―478×10?6).Other geochemical characteristics of Zhangwu high-Mg andesites(HMAs) include strong fractionation of light rare earth elements(LREE) from heavy rare earth elements(HREE),and Sr from Y,with La/Yb greater than 15,and high Sr/Y(34― 115).Zircons of andesite YX270 yield three age groups with no Precambrian age,which precludes ori-gin of the Zhangwu HMAs from the partial melting of the Precambrian crust.The oldest age group peaking at 253 Ma is interpreted to represent the collision of the Siberia block and the North China block,resulting in formation of the Central Asian orogenic belt by closure of the Mongol-Okhotsk Ocean.The intermediate age group corresponds to the basalt underplating which caused the wide-spread coeval granitoids in the North China craton with a peak 206Pb/238U age of 172 Ma.The youngest age group gives a 206Pb/238U age of 126±2 Ma,which is interpreted as the eruption age of the Zhangwu HMAs.The high 87Sr/86Sri(126 Ma)>0.706 and low εNd(t)= ?6.36―?13.99 of the Zhangwu HMAs are distinct from slab melts.The common presence of reversely zoned clinopyroxene phenocrysts in the Zhangwu HMAs argues against the origin of the Zhangwu HMAs either from melting of the water saturated mantle or melting of the lower crust.In light of the evidence mentioned above,the envisaged scenario for the formation of the Zhangwu HMAs is related to the basaltic underplating at the base of the crust,which led to the thickening of the lower crust and formation of lower crustal ec展开更多
Backscattered electron images, in situ Hf isotopes, U-Pb ages and trace elements of zircons in a banded granulite xenolith from Hannuoba basalt have been studied.The results show that the banded granulite is a sample ...Backscattered electron images, in situ Hf isotopes, U-Pb ages and trace elements of zircons in a banded granulite xenolith from Hannuoba basalt have been studied.The results show that the banded granulite is a sample derived from the early lower crust of the North China craton.It is difficult to explain the petrogenesis of the xenolith with a single process. Abundant information on several processes,however, is contained in the granulite. These processes include the addition of mantle material, crustal remelting,metamorphic differentiation and the delamination of early lower crust. About 80% of zircons studied yield ages of 1842±40Ma, except few ages of 3097-2824 Ma and 2489-2447Ma. The zircons with ages older than 2447Ma have high εHr(up to+18.3) and high Hf model age (2.5-2.6Ga), indicating that the primitive materials of the granulite were derived mainly from a depleted mantle source in late Archean. Most εHr of the zircons with early Proterozoic U-Pb age vary around zero, but two have high εHr up to+9.2-+10.2, indicating mantle contribution during the collision and assembly between the Eastern and Western blocks in the early Proterozoic that resulted in the amalgamation of the North China craton.展开更多
基金Supported by the Chinese Academy of Sciences (Grant No. KZCX2-YW-Q08)National Natural Science Foundation of China (Grant Nos. 90714009 and 40634019)
文摘To study the thinning of cratonic lithosphere in North China has been the hot subject of basic research in the fields of solid earth science in China. This paper presents an overview on the formation and evolution of continental crust, and outlines the mechanisms of forming the lithospheric mantle. It is suggested that the thinning of cratonic lithosphere principally proceeds in two ways, one by subduc- tion erosion (e.g., North China), and the other by a combination of subduction erosion and underplating degistion (e.g., Yangtze).
基金supported by the National Basic Research Program of China(Grant No.2013CB733203)the National Natural Science Foundation of China(Grant Nos.41225016+1 种基金41125015)the National Science and Technology Major of China(Grant No.2011ZX05008-001)
文摘Moho depth and crustal average Poisson＇s ratio for 823 stations are obtained by H-n： stacking of receiver functions. These, to- gether with topography and receiver function amplitude information, were used to study the crustal structure beneath the North China Craton （NCC）. The results suggest that modified and preserved crust coexist beneath the craton with generally Airy-type isostatic equilibrium. The equilibrium is relatively low in the eastern NCC and some local areas in the central and western NCC, which correlates well with regional geology and tectonic features. Major differences in the crust were observed beneath the eastern, central, and western NCC, with average Moho depths of 33, 37, and 42 km and average Poisson＇s ratios of 0.268, 0.267 and 0.264, respectively. Abnormal Moho depths and Poisson＇s ratios are mainly present in the rift zones, the northern and southern edges of the central NCC, and tectonic boundaries. The crust beneath Ordos retains the characteristics of typical craton. Poisson＇s ratio increases roughly linearly as Moho depth decreases in all three parts of the NCC with different slopes. Receiver function amplitudes are relatively large in the northern edge of the eastern and central NCC, and small in and near the rifts. The Yanshan Mountains and southern part of the Shanxi rift show small-scale variations in the receiver-function ampli- tudes. These observations suggest that overall modification and thinning in the crust occurred in the eastern NCC, and local crustal modification occurred in the central and westem NCC. Different crustal structures in the eastern, central, and western NCC suggest different modification processes and mechanisms. The overall destruction of the crustal structure in the eastern NCC is probably due to the westward subduction of the Pacific Plate during the Meso-Cenozoic time; the local modifications of the crust in the central and western NCC may be due to repeated reactivations at zones with a heterogeneous structure by successive thermal-tec
基金supported by the National Natural Science Foundation of China (Grant Nos. 91014006, 90914005 & 41474073)
文摘From the 1960 s to 1970 s, North China has been hit by a series of large earthquakes. During the past half century,geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associated studies in North China.They have made significant progress on several key issues in the geosciences, such as the crustal and upper mantle structure and the seismogenic environment of strong earthquakes. Deep seismic profiling results indicate a complex tectonic setting in the strong earthquake areas of North China, where a listric normal fault and a low-angle detachment in the upper crust coexist with a high-angle deep fault passing through the lower crust to the Moho beneath the hypocenter. Seismic tomography images reveal that most of the large earthquakes occurred in the transition between the high-and low-velocity zones, and the Tangshan earthquake area is characterized by a low-velocity anomaly in the middle-lower crust. Comprehensive analysis of geophysical data identified that the deep seismogenic environment in the North China extensional tectonic region is generally characterized by a low-velocity anomalous belt beneath the hypocenter, inconsistency of the deep and shallow structures in the crust, a steep crustalal-scale fault,relative lower velocities in the uppermost mantle, and local Moho uplift, etc. This indicates that the lithospheric structure of North China has strong heterogeneities. Geologically, the North China region had been a stable craton named the North China Craton or in brief the NCC, containing crustal rocks as old as ～3.8 Ga. The present-day strong seismic activity and the lower velocity of the lower crust in the NCC are much different from typical stable cratons around the world. These findings provide significant evidence for the destruction of the NCC. Although deep seismic profiling and seismic tomography have greatly enhanced knowledge about the deep-seated structure and seismogenic environment, some fundamental issues still remain and require further work.
基金supported by the National Natural Science Foundation of China (Grant Nos. 41574034, 41688103, and 91414301)
文摘The North China Craton(NCC) is an important part of eastern China. Recent studies have shown that the eastern NCC(ENCC) has undergone significant lithospheric thinning and destruction since the late Mesozoic. Destruction of the cratonic lithosphere is necessarily accompanied by crustal deformation. Therefore, a detailed crustal deformation model can provide basic observational constraints for understanding the process and mechanisms of the destruction of the NCC. In this study, we estimated the crustal azimuthal anisotropy beneath 198 broadband stations in the NCC with a joint analysis of Ps waves converted at the Moho from radial and transverse receiver function data. We also performed a harmonic analysis to test the reliability of the measured anisotropy. We obtained robust crustal azimuthal anisotropy beneath 23 stations that are mostly located on the western margin of the Bohai Bay Basin, Yin-Yan orogenic belt, and Taihang Mountains, which reflects the crustal deformation characteristics in those regions. The crustal shear wave splitting time was found to range from 0.05 s to 0.68 s, with an average value of 0.23 s, which reveals a distinct crustal anisotropy in the Trans-North China Orogen(TNCO) and its adjacent areas. Our analysis of the results suggests that the strong NW-SE tectonic extension in the late Mesozoic and Cenozoic played an important role in crustal anisotropy in this region. In addition, the E-W trending crustal anisotropy on the margin of the Bohai Bay Basin indicates an effect of the ENE-WSW trending horizontal principal compressive stress. The crustal anisotropy in the Yin-Yan orogenic belt may be an imprint of the multiple-phase shortening of a dominant N-S direction from the early-to-middle Jurassic to the Early Cretaceous. Stations in the Taihang Mountains show large splitting times and well-aligned NW-SE fast directions that correlate with those measured from SKS splitting and that are possibly related to the lithospheric modification and magmatic underplating from the Late Mesozoic to
基金sponsored by Special National Science and Technology Project on "Scientific research of fault zone of Wenchuan Earthquake"(the 15th Subject)the director fund of Institute of Geology of Chinese Academy of Geological Sciences
文摘Seismic tomography can provide both fine P-wave and S-wave velocity structures of the crust and upper mantle. In addition, with proper computation, Poisson＇s ratio images from the seismic velocities can be determined. However, it is unknown whether Poisson＇s ratio images have any advantages when compared with the P-wave and S-wave velocity images. For the purposes of this study, high- resolution seismic tomography under the eastern part of North China region was used to determine detailed 3-D crustal P- and S-wave seismic velocities structure, as well as Poisson＇s ratio images. Results of Poisson＇s ratio imaging show high Poisson＇s ratio （high-PR） anomalies located in the Hengshan-North Taihang-Zhangjiakou （H-NT-Z） region, demonstrating that Poisson＇s ratio imaging can provide new geophysical constraints for regional tectonic evolution. The H-NT-Z region shows a prominent and continuous high-PR anomaly in the upper crust. Based on Poisson＇s ratio images at different depths, we find that this high-PR anomaly is extending down to the middle crust with thickness up to about 26 kin. According to rock physical property measurements and other geological data, this crustal Poisson＇s ratio anomaly can be explained by Mesozoic partial melting of the upper mantle and basaltic magma underplating related to the lithospheric thinning of the North China craton.
基金supported by the National Natural Science Foundation of China(Grant Nos.41688103&42003026)。
文摘The lower crust beneath the North China Craton(NCC)was transformed during the craton destruction in the Mesozoic,however,the transformation processes are yet to be fully understood.Compositional and geochronological variations of granulite and pyroxenite xenoliths provided insights into the nature of the lower crust before and after the craton destruction.In this study,we summarized the latest results of geochemistry and zircon geochronology coupled with Hf-O isotopes from granulite and pyroxenite xenoliths hosted by Phanerozoic igneous rocks in NCC.Comparing previous studies on the granulite terranes and adakitic rocks of NCC,we aim to discuss the destruction processes of lower crust beneath the NCC.The granulite and pyroxenite xenoliths of NCC were divided into two and three groups,respectively,based on the differences of geochemical features.Group I granulite xenoliths from the NCC have silicic-basic compositions,with metamorphic ferrosilite.The Group I granulite xenoliths show relatively lower Mg#values of pyroxenes and whole-rock than that of the Group II granulite xenoliths,and enrichments of light rare earth elements and Sr-Nd isotopic compositions.Their zircons display Archean-Phanerozoic ages with three peaks of Neoarchean,Paleoproterozoic,and Mesozoic.Generally,Group I granulite xenoliths show close affinities to the granulite terranes of the NCC in terms of the major and trace elements and Sr-Nd isotopic compositions,with a consistent Archean-Proterozoic evolutionary history.However,Group I granulite xenoliths have abundant Phanerozoic zircons with variable Hf isotopic compositions from depleted to enriched,which could be formed by modifications of magma underplating.Therefore,Group I granulite xenoliths represent the modified ancient lower crust beneath the NCC.The Group II granulite and Group III pyroxenite xenoliths from the NCC have similar geochemical features and are basic in compositions,with metamorphic to magmatic orthopyroxenes.The Group II granulite and Group III pyroxenite xenoliths usually
基金funded by the Fundação de AmparoàPesquisa do Estado de São Paulo(FAPESP)Grant(#2012/15824-6and#2018/25465-0 to EPO)the Conselho Nacional de Desenvolvimento Científico e Tecnológico(CNPq)Grant(#305099/2019-1 to EPO)the Institute of Geosciences of the University of Campinas and by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior(CAPES)PhD Scholarship(#001)to the senior author。
文摘The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution,especially during the Archean.However,the scarcity of exposure to these rocks,the complexity of lithological relationships,and the high degree of superimposed deformation,especially with long-lived magmatism,make it difficult to study ancient rocks.Despite this complexity,exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America’s primitive crust.Therefore,here we present field relationships,LA-ICP-SFMS zircon U-Pb ages,and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex.The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic,and felsic members ranging from TTG(Tonalite-Trondhjemite-Granodiorite)to granitic composition.Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex:Eoarchean(ca.3.65–3.60 Ga),early Paleoarchean(ca.3.55–3.52 Ga),middle-late Paleoarchean(ca.3.49–3.33 Ga)and Neoarchean(ca.2.74–2.58 Ga),with no records of Mesoarchean rocks.Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time.In the Eoarchean,rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources,whereas mantle depletion played a role in the Paleoarchean,followed by greater differentiation of the crust with thickening and recycling in the middle–late Paleoarchean.A different stage of crustal growth and recycling dominated the Neoarchean,probably owing to the thickening of the continental crust by collision,continental arc growth,and mantle differentiation.
基金the National Natural Science Foundation of China(Grant Nos.40302015,40472099 and 40373013)the Program for Changjiang Scholars and Innovative Research Team in University.
文摘Zircon U-Pb dating by both SHRIMP and LA-ICP-MS and geochemical study of theTiaojishan Formation and the Donglintai Formation from Xishan, Beijing, reveal that ages of upperlavas of Tiaojishan Formation and Middle of Donglintai Formation are 137.1 ± 4.5 Ma(2σ) and 130—134 Ma, respectively. The fomer is slightly older than the latter and the age difference betweenthese two formations is less than 5 Ma. These lines of evidence prove that the two volcanoes eruptedwithin a short time. The age of the Tiaojishan Formation from Xishan, Beijing is distinctivelydifferent from that of the Chende Basin. This indicated that the ages of Tiaojishan lavas varied indifferent regions. The Tiaojishan Formation consists of typical adakite (SiO_2 = 56%, Na_2O =3.99—6.17, Na_2O/K_2O = 2.2 -3.1, Sr = 680-1074 x 10^(-6), Y = 13.2-16.3 x 10^(-6), Yb = 1.13-1.52x 10^(-6), Sr/Y = 43-66), high-Mg adakite and high-Mg andesite (Mg# = 54—55). Features ofcontinental crust of adakite from the Tiaojishan Formation and its syngeneric middle silicic vocanicrocks, such as typical Nd-Ta negative abnormality and Pb possive abnormality, indicate that theselavas are originated from partial melts of continental crust. These results suggest that the adakitefrom the Tiaojishan Formation of Xishan, Beijing derived from thickened eclogitic lower crust andlithosphere beneath the North China craton at mesozoic that was foundered into the aesthenosphere,and subsequenctly partially melted and interacted with mantle olivine during melts upward migration.The age of lavas from the Tiaojishan Formation restrained the foundation which should last at leastuntil 137 Ma. Lavas of the Donglintai Formation are rhyolith and andesite with normal Mg# and thusthey did not interact with the mantle. These lavas represent remobilized melts of lower crustmaterial caused by mantle aesthenosphere upwelling migration induced by foundation.
基金Co-supported by the National Natural Science Foundation of China (40521001,40673019)Ministry of Education of China (B07039,IRT0441) the Key Labo-ratory of Orogenic Belts and Crustal Evolution
文摘Ten volcanic samples at Zhangwu,western Liaoning Province,North China were selected for a sys-tematic geochemical,mineralogical and geochronological study,which provides an opportunity to ex-plore the interaction between the continental crust and mantle beneath the north margin of the North China craton.Except one basalt sample(SiO2= 50.23%),the other nine samples are andesitic with SiO2 contents ranging from 53% to 59%.They have relatively high MgO(3.4%―6.1%,Mg#=50―64) and Ni and Cr contents(Ni 27×10?6―197×10?6,Cr 51×10?6―478×10?6).Other geochemical characteristics of Zhangwu high-Mg andesites(HMAs) include strong fractionation of light rare earth elements(LREE) from heavy rare earth elements(HREE),and Sr from Y,with La/Yb greater than 15,and high Sr/Y(34― 115).Zircons of andesite YX270 yield three age groups with no Precambrian age,which precludes ori-gin of the Zhangwu HMAs from the partial melting of the Precambrian crust.The oldest age group peaking at 253 Ma is interpreted to represent the collision of the Siberia block and the North China block,resulting in formation of the Central Asian orogenic belt by closure of the Mongol-Okhotsk Ocean.The intermediate age group corresponds to the basalt underplating which caused the wide-spread coeval granitoids in the North China craton with a peak 206Pb/238U age of 172 Ma.The youngest age group gives a 206Pb/238U age of 126±2 Ma,which is interpreted as the eruption age of the Zhangwu HMAs.The high 87Sr/86Sri(126 Ma)>0.706 and low εNd(t)= ?6.36―?13.99 of the Zhangwu HMAs are distinct from slab melts.The common presence of reversely zoned clinopyroxene phenocrysts in the Zhangwu HMAs argues against the origin of the Zhangwu HMAs either from melting of the water saturated mantle or melting of the lower crust.In light of the evidence mentioned above,the envisaged scenario for the formation of the Zhangwu HMAs is related to the basaltic underplating at the base of the crust,which led to the thickening of the lower crust and formation of lower crustal ec
文摘Backscattered electron images, in situ Hf isotopes, U-Pb ages and trace elements of zircons in a banded granulite xenolith from Hannuoba basalt have been studied.The results show that the banded granulite is a sample derived from the early lower crust of the North China craton.It is difficult to explain the petrogenesis of the xenolith with a single process. Abundant information on several processes,however, is contained in the granulite. These processes include the addition of mantle material, crustal remelting,metamorphic differentiation and the delamination of early lower crust. About 80% of zircons studied yield ages of 1842±40Ma, except few ages of 3097-2824 Ma and 2489-2447Ma. The zircons with ages older than 2447Ma have high εHr(up to+18.3) and high Hf model age (2.5-2.6Ga), indicating that the primitive materials of the granulite were derived mainly from a depleted mantle source in late Archean. Most εHr of the zircons with early Proterozoic U-Pb age vary around zero, but two have high εHr up to+9.2-+10.2, indicating mantle contribution during the collision and assembly between the Eastern and Western blocks in the early Proterozoic that resulted in the amalgamation of the North China craton.
