By using the SHRIMP U-Pb and single zircon stepwise evaporation methods, the authors have obtained some results for granitoids from eastern Hebei Province. The Yuhuzhai hyperthene tonalitic granite was formed 2550 Ma ...By using the SHRIMP U-Pb and single zircon stepwise evaporation methods, the authors have obtained some results for granitoids from eastern Hebei Province. The Yuhuzhai hyperthene tonalitic granite was formed 2550 Ma ago, the Qingyangshu gabbroic gneiss 2536 Ma, the Yinmahe granodioritic gneiss near Lücao, Lulong County, 2533 Ma, the gabbro-dioritic gneiss near Longwan, Qianxi County, 2518-2515 Ma, the Qiuhuayu trondjemitic gneiss at Zunhua 2515 Ma, the Xiaoguanzhuang tonalitic gneiss at Zunhua 2495 Ma, and the Cuizhangzi gneiss in Qianxi County 2492 Ma. These geochronilogical data demonstrate that, though diverse in composition, type and origin, the granitic gneisses in eastern Hebei Province were emplaced and crystallized in a rather short period of magmatic activity. The formation of such a great amount of gneisses in this small time gap suggests that it was a critical crust accretion stage at the end of Neoarchean. The fact that granitoids of various types occurred at the same time implies a large-scale underplating （mantle plume） activity, which was then responsible for the crust accretion.展开更多
Coesite inclusions, together with omphacite, jadeite, garnet and phengite inclusions, were identified in zircons separates from almost all gneissic core samples of pre-pilot drillhole CCSD-PP2 by the Laser Raman spect...Coesite inclusions, together with omphacite, jadeite, garnet and phengite inclusions, were identified in zircons separates from almost all gneissic core samples of pre-pilot drillhole CCSD-PP2 by the Laser Raman spectros-copy and the cathodoluminescence method. These data indicate that gneissic rocks consisting of paragneisses and or-thogneisses ubiquitously experienced UHP metamorphism. This research may be of great significance for an in-depth study of the subduction-exhumation mechanism of the Sulu UHP metamorphic belt and selecting the drilling site for the Chinese Continental Scientific Drilling Project.展开更多
Granitic gneiss(orthogneiss)and Himalayan leucogranite are widely distributed in the Himalayan orogen,but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear.In this st...Granitic gneiss(orthogneiss)and Himalayan leucogranite are widely distributed in the Himalayan orogen,but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear.In this study,we present the petrological,geochronological and geochemical results for orthogneisses and leucogranites from the Zhada area,Western Himalayas.Zhada orthogneiss is composed mainly of quartz,plagioclase,K-feldspar,biotite and muscovite,with accessory zircon and apatite.Orthogneiss zircon cathodoluminescence(CL)images show that most grains contain a core with oscillatory zoning,which indicates an igneous origin.Sensitive high-resolution ion microprobe(SHRIMP)U-Pb dating of the zircon cores in the orthogneiss shows a weighted ^(206)Pb/^(238)U age of 515±4 Ma(early Paleozoic),with sponge-like zircon rims of 17.9±0.5 Ma(Miocene).Zhada leucogranite shows^(206)Pb/^(238)U ages ranging from 19.0±0.4 Ma to 12.4±0.2 Ma,the weighted average age being 16.2±0.4 Ma.The leucogranites have a low Ca content(<1 wt%),FeOt content(<1 wt%),Rb content(67.0-402 ppm),Sr content(<56.6 ppm),Ba content(3.35-238 ppm)and Rb/Sr ratio(0.5-14.7),which are similar to the geochemical characteristics of the Himalayan leucogranite derived from muscovite dehydration partial melting of metasediments and representative of most Himalayan leucogranites.The highly variable Na_(2)O+K_(2)O(4.33 wt%-9.13 wt%),Al_(2)O_(3)(8.44 wt%-13.51 wt%),ΣREE(40.2-191.0 ppm),Rb(67.0-402 ppm)and Nb(8.23-26.4 ppm)contents,^(87)Sr/^(86)Sr(t)ratios(0.7445-0.8605)andεNd(t)values(−3.6 to−8.2)indicate that the leucogranite is derived from a heterogenetic source.The nonradiogenic Nd isotope values of the studied Zhada leucogranite and orthogneiss range from−8.2 to−3.6 and from−8.7 to−4.1,respectively.Therefore,the general mixing equation was used to perform the Sr and Nd isotope mixing calculations.The results indicate that the heterogenetic source was the Tethyan Himalayan Sequence(THS)/Higher Himalayan Crystalline(HHC)metasediments and Zhada 展开更多
Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block(NCB).These poorly foliated...Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block(NCB).These poorly foliated granites have previously been assumed to be Mesozoic in age and never dated,and so their significance has not been recognised until now.The first granite(AG1)is a porphyritic syenogranite with megacrystic K-feldspar,and the second(AG2)is a quartz syenite with perthitic megacryst.Zircons from the granites yield LA-ICP-MS U-Pb ages of 2499±10 Ma(AG1),and 2492±28 Ma(AG2),which are slightly younger than the granodioritic orthogneiss that they intrude with a crystallisation U-Pb age of 2537±34 Ma.The younger granites have higher assays for SiO_(2)(71.91%for AG1 and 73.22%for AG2)and K_(2)O(7.52%for AG1 and 8.37%for AG2),and much lower assays for their other major element than the granodioritic orthogneiss.All of the granodioritic orthogneiss and granite samples have similar trace element patterns,with depletion in Th,U,Nb,and Ti and enrichment in Rb,Ba,K,La,Ce,and P.This indicates that the granites are derived from the orthogneiss as partial melts.Although they exhibit a similar REE pattern,the granites have much lower total REE contents(30.97×10^(−6) for AG1,and 25.93×10^(−6) for AG2),but pronounced positive Eu anomalies(Eu/Eu^(*)=8.57 for AG1 and 27.04 for AG2).The granodioritic orthogneiss has an initial ^(87)Sr/^(86)Sr ratio of 0.70144,εNd(t)value of 3.5,and εHf(t)values ranging from−3.2 to+2.9.The orthogneiss is a product of fractional crystallisation from a dioritic magma,which was derived from a mantle source contaminated by melts derived from a felsic slab.By contrast,the AG1 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.6926 that is considered too low in value,εNd(t)value of 0.3,andεHf(t)values between+0.57 and+3.82;whereas the AG2 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.70152,εNd(t)value of 1.3,andεHf(t)values between+0.5 and+14.08.These assays indicate that a Sr-Nd展开更多
基金The paper is supported by the National Natural Science Foundation of China （Grant Nos. 40420120135 and 40072060）.
文摘By using the SHRIMP U-Pb and single zircon stepwise evaporation methods, the authors have obtained some results for granitoids from eastern Hebei Province. The Yuhuzhai hyperthene tonalitic granite was formed 2550 Ma ago, the Qingyangshu gabbroic gneiss 2536 Ma, the Yinmahe granodioritic gneiss near Lücao, Lulong County, 2533 Ma, the gabbro-dioritic gneiss near Longwan, Qianxi County, 2518-2515 Ma, the Qiuhuayu trondjemitic gneiss at Zunhua 2515 Ma, the Xiaoguanzhuang tonalitic gneiss at Zunhua 2495 Ma, and the Cuizhangzi gneiss in Qianxi County 2492 Ma. These geochronilogical data demonstrate that, though diverse in composition, type and origin, the granitic gneisses in eastern Hebei Province were emplaced and crystallized in a rather short period of magmatic activity. The formation of such a great amount of gneisses in this small time gap suggests that it was a critical crust accretion stage at the end of Neoarchean. The fact that granitoids of various types occurred at the same time implies a large-scale underplating （mantle plume） activity, which was then responsible for the crust accretion.
基金This work was supported by the Project "1 :250000 Regional Geological Survey of the Lianyungang Sheet (I50C002004) of China" and "Geological Research for Selecting the Drillhole in the Donghai Area of the Chinese Continental Scientific Drilling Project"
文摘Coesite inclusions, together with omphacite, jadeite, garnet and phengite inclusions, were identified in zircons separates from almost all gneissic core samples of pre-pilot drillhole CCSD-PP2 by the Laser Raman spectros-copy and the cathodoluminescence method. These data indicate that gneissic rocks consisting of paragneisses and or-thogneisses ubiquitously experienced UHP metamorphism. This research may be of great significance for an in-depth study of the subduction-exhumation mechanism of the Sulu UHP metamorphic belt and selecting the drilling site for the Chinese Continental Scientific Drilling Project.
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.42072114,41503006)the Deep Resources Exploration and Mining,the National Key R&D Program of China(Grant No.2018YFC0604106)+1 种基金the Chinese Geological Survey(Grant No.DD20190167)the China Scholarship Council(Grant No.201808110080).
文摘Granitic gneiss(orthogneiss)and Himalayan leucogranite are widely distributed in the Himalayan orogen,but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear.In this study,we present the petrological,geochronological and geochemical results for orthogneisses and leucogranites from the Zhada area,Western Himalayas.Zhada orthogneiss is composed mainly of quartz,plagioclase,K-feldspar,biotite and muscovite,with accessory zircon and apatite.Orthogneiss zircon cathodoluminescence(CL)images show that most grains contain a core with oscillatory zoning,which indicates an igneous origin.Sensitive high-resolution ion microprobe(SHRIMP)U-Pb dating of the zircon cores in the orthogneiss shows a weighted ^(206)Pb/^(238)U age of 515±4 Ma(early Paleozoic),with sponge-like zircon rims of 17.9±0.5 Ma(Miocene).Zhada leucogranite shows^(206)Pb/^(238)U ages ranging from 19.0±0.4 Ma to 12.4±0.2 Ma,the weighted average age being 16.2±0.4 Ma.The leucogranites have a low Ca content(<1 wt%),FeOt content(<1 wt%),Rb content(67.0-402 ppm),Sr content(<56.6 ppm),Ba content(3.35-238 ppm)and Rb/Sr ratio(0.5-14.7),which are similar to the geochemical characteristics of the Himalayan leucogranite derived from muscovite dehydration partial melting of metasediments and representative of most Himalayan leucogranites.The highly variable Na_(2)O+K_(2)O(4.33 wt%-9.13 wt%),Al_(2)O_(3)(8.44 wt%-13.51 wt%),ΣREE(40.2-191.0 ppm),Rb(67.0-402 ppm)and Nb(8.23-26.4 ppm)contents,^(87)Sr/^(86)Sr(t)ratios(0.7445-0.8605)andεNd(t)values(−3.6 to−8.2)indicate that the leucogranite is derived from a heterogenetic source.The nonradiogenic Nd isotope values of the studied Zhada leucogranite and orthogneiss range from−8.2 to−3.6 and from−8.7 to−4.1,respectively.Therefore,the general mixing equation was used to perform the Sr and Nd isotope mixing calculations.The results indicate that the heterogenetic source was the Tethyan Himalayan Sequence(THS)/Higher Himalayan Crystalline(HHC)metasediments and Zhada
基金The CAGS Research Fund(YYWF201715,YK1608)the project of China Geological Survey(DD20190437)financially support this study and contribution.
文摘Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block(NCB).These poorly foliated granites have previously been assumed to be Mesozoic in age and never dated,and so their significance has not been recognised until now.The first granite(AG1)is a porphyritic syenogranite with megacrystic K-feldspar,and the second(AG2)is a quartz syenite with perthitic megacryst.Zircons from the granites yield LA-ICP-MS U-Pb ages of 2499±10 Ma(AG1),and 2492±28 Ma(AG2),which are slightly younger than the granodioritic orthogneiss that they intrude with a crystallisation U-Pb age of 2537±34 Ma.The younger granites have higher assays for SiO_(2)(71.91%for AG1 and 73.22%for AG2)and K_(2)O(7.52%for AG1 and 8.37%for AG2),and much lower assays for their other major element than the granodioritic orthogneiss.All of the granodioritic orthogneiss and granite samples have similar trace element patterns,with depletion in Th,U,Nb,and Ti and enrichment in Rb,Ba,K,La,Ce,and P.This indicates that the granites are derived from the orthogneiss as partial melts.Although they exhibit a similar REE pattern,the granites have much lower total REE contents(30.97×10^(−6) for AG1,and 25.93×10^(−6) for AG2),but pronounced positive Eu anomalies(Eu/Eu^(*)=8.57 for AG1 and 27.04 for AG2).The granodioritic orthogneiss has an initial ^(87)Sr/^(86)Sr ratio of 0.70144,εNd(t)value of 3.5,and εHf(t)values ranging from−3.2 to+2.9.The orthogneiss is a product of fractional crystallisation from a dioritic magma,which was derived from a mantle source contaminated by melts derived from a felsic slab.By contrast,the AG1 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.6926 that is considered too low in value,εNd(t)value of 0.3,andεHf(t)values between+0.57 and+3.82;whereas the AG2 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.70152,εNd(t)value of 1.3,andεHf(t)values between+0.5 and+14.08.These assays indicate that a Sr-Nd
