The Zhongshan Station of China is located in the Larsemann Hills, East Antarctica. Low pressure granulite facies gneisses together with late granites are outcroped in the region. Three biotite samples from a garnet se...The Zhongshan Station of China is located in the Larsemann Hills, East Antarctica. Low pressure granulite facies gneisses together with late granites are outcroped in the region. Three biotite samples from a garnet segregation, a syenogranite and a granite-pegmetite were measured with 40Ar/39Ar incremental heating technique. Biotites from the garnet segregation give an 40Ar/39Ar plateau age of 504±1Ma. Biotites from the syenogranite yield an 40Ar/39Ar plateau age of 494 ±1 Ma. Biotites from the granite-pegmatite give an 40 Ar/39 Ar plateay age of 486±1Ma. They verify 500 Ma thermal event called 'Pan African event' by previous K-Ar and Rb-Sr data. They are cooling ages of the biotites when the paleogeotherm of the area droped to the K-Ar closure temperature for biotite.展开更多
The Silurian bituminous sandstones（SBS） in the Tarim Basin, China are important basinwide reservoirs with an estimated area of approximately 249000 km^2. We investigated the ages of authigenic illites in the SBS res...The Silurian bituminous sandstones（SBS） in the Tarim Basin, China are important basinwide reservoirs with an estimated area of approximately 249000 km^2. We investigated the ages of authigenic illites in the SBS reservoirs and constrained their formation timing by using the 40^Ar/39^Ar step wise heating method. The age spectra, 39^Ar recoil loss and their controlling factors were investigated systematically. The 40^Ar/39^Ar ages were compared with the conventional K/Ar ages of identical clay fractions. The clay in the SBS reservoirs is dominated by orderly mixed-layer illite/smectite（I/S） with 5%–30% smectite layers. The I/S minerals morphology comprises primarily honeycomb, short filamentous and curved-lath particles, characteristic of authigenic illites. The unencapsulated 40^Ar/39^Ar total gas ages（UTGA） of the authigenic illites range from 188.56 ± 6.20 Ma to 491.86 ± 27.68 Ma, which are 7% to 103% older than the corresponding K/Ar ages of 124.87 ± 1.11 Ma to 383.45 ± 2.80 Ma, respectively. The K-Ar ages indicate multistage accumulations with distinct distribution patterns in the Tarim Basin： older（late Caledonian-early Hercynian） around the basin margin, younger（late Hercynian） in the basin centre, and the youngest（middle to late Yanshanian） in the Ha-6 well-block, central area of the North Uplift. The age difference is believed to have been caused by the 39^Ar recoil loss during the irradiation process. Compared with the K/Ar ages, the estimated 39^Ar recoil losses in this study are in the range from 7% to 51%. The 39^Ar recoil loss appears to increase not only with the decreasing particle sizes of the I/S, but also with increasing percentage of smectite layers（IR） of the I/S, and smectite layer content（SLC） of the samples. We conclude that due to significant 39^Ar recoil losses, UTGA may not offer any meaningful geological ages of the authigenic illite formation in the SBS and thus can not be used to represent the hydrocarbon charge timing. 39^Ar recoil losses during 40^Ar/39^Ar dating can not be neglected when dating fine authigenic illite, especially when the ordered mixed-layer I/S containing small amount of smectite layers（IR30%） in the reservoir formations. Compared with the unencapsulated Ar-Ar method, the conventional K-Ar method is less complicated, more accurate and reliable in dating authigenic illites in petroleum reservoirs.展开更多
Metapelite is one of the predominant rock types in the high-pressure–ultrahigh-pressure（HP–UHP） metamorphic belt of western Tianshan, NW China; however, the spatial and temporal variations of this belt during meta...Metapelite is one of the predominant rock types in the high-pressure–ultrahigh-pressure（HP–UHP） metamorphic belt of western Tianshan, NW China; however, the spatial and temporal variations of this belt during metamorphism are poorly understood. In this study, we present comparative petrological studies and 40^Ar/39 ^Ar geochronology of HP and UHP pelitic schist exposed along the Habutengsu valley. The schist mainly comprises quartz, white mica, garnet, albite and bluish amphibole. In the Mn O–Na2O–Ca O–K2O–Fe O–Mg O–Al2O3–Si O2–H2O（Mn NCKFMASH） system, P–T pseudosections were constructed using THERMOCALC 333 for two representative pelitic schists. The results demonstrate that there was a break in the peak metamorphic pressures in the Habutengsu area. The northern schist has experienced UHP metamorphism, consistent with the presence of coesite in the same section, while the southern one formed at lower pressures that stabilized the quartz. This result supports the previous finding of a metamorphic gradient through the HP–UHP metamorphic belt of the Chinese western Tianshan by the authors. Additionally, phengite in the northern schist was modelled as having a Si content of 3.55–3.70（a.p.f.u.） at the peak stage, a value much higher than that of oriented matrix phengite（Si content 3.32–3.38 a.p.f.u.）. This indicates that the phengite flakes in the UHP schist were subjected to recrystallization during exhumation, which is consistent with the presence of phengite aggregates surrounding garnet porphyroblast. The 40^Ar/39^ Ar age spectra of white mica（dominantly phengite） from the two schists exhibit similar plateau ages of ca. 315 Ma, which is interpreted as the timing of a tectonometamorphic event that occurred during the exhumation of the HP–UHP metamorphic belt of the Chinese western Tianshan.展开更多
The Wutonggou iron deposit is located in the well-known iron metallogenic belt in the eastern Tianshan,NW China,and has been regarded as a sedimentary iron deposit.Although hydrothermal overprinting could play indispe...The Wutonggou iron deposit is located in the well-known iron metallogenic belt in the eastern Tianshan,NW China,and has been regarded as a sedimentary iron deposit.Although hydrothermal overprinting could play indispensable roles in the formation of high-grade iron ores in sedimentary iron deposits,previous studies mainly focused on sedimentary-related iron mineralization,while the nature and contribution of hydrothermal fluids are poorly constrained.Accordingly,an integrated study of ore geology,H-O-C isotopes and^(40)Ar-^(39)Ar dating,is conducted on the Wutonggou deposit,in order to reveal the features,source,and timing of hydrothermal mineralization.The studied deposit includes two mining sections namely the Jianshan and Wutonggou.Theδ^(18)O values of early magnetite from the Jianshan section range from+3.0‰to+5.8‰that nearly consistent with classic magmatic magnetite,while increase to 6.3‰-8.0‰in the late stage.Quartz from the two sections shows comparable H-O isotopic compositions and identical fractionation trends,and is plotted in or periphery to the primary magmatic water area.Calcites from the two sections are broadly similar in carbon and oxygen isotopic compositions,and siderite from the Wutonggou section is plotted in the same region.Thus,comparable stable isotopic compositions and evolution trends indicate similar magmatic fluids contributed hydrothermal iron mineralization in the two mining sections.Moreover,water-rock interactions of varying degrees generated distinct mineralization styles in the Jianshan and Wutonggou sections,and caused the isotopic fractionation in late stages.Biotite extracted from a hydrothermal siderite ore yielded a^(40)Ar-^(39)Ar plateau age of 299.5±2.0 Ma,indicates the timing of hydrothermal iron mineralization is corresponding to the emplacement of vicinity granitoids.Taken together,the hydrothermal mineralization in the Wutonggou iron deposit was the product of remobilization and upgrading of early sedimentary iron ores,and ore-forming fluids were most probably originated from regional granitic magmatism.展开更多
Highly precise 40Ar-39Ar dating results demon-strate that the ages of potash-rich volcanic rocks in western Shandong Province are 114.7-124.3 Ma, and that of the lamprophyres is 119.6 Ma. The potash-rich volcanic rock...Highly precise 40Ar-39Ar dating results demon-strate that the ages of potash-rich volcanic rocks in western Shandong Province are 114.7-124.3 Ma, and that of the lamprophyres is 119.6 Ma. The potash-rich volcanic rocks have relatively high (87Sr/86Sr)i ratios ( 0.708715-0.711418) and distinctly negative εNd values ( -11.47--17.54), and are enriched in radiogenic lead (206Pb/204Pb=17.341-17.622, 207Pb/204Pb=15.525-15.538, 208Pb/204Pb=37.563-37.684). Similarly, the lamprophyres also have quite low εNd values (-11.57 - -19.64). Based on the fact that the Sr, Nd and Pb isotopic compositions of potash-rich volcanic rocks are very consistent with that of the clinopyroxene separates, and by integrating comprehensive analyses of their tectonic settings, and extensive comparisons of the Sr, Nd isotopic composi-tions with that of the related simultaneous rocks, it is con-cluded that the potash-rich volcanic rocks and lamprophyres in western Shandong Province were most possibly derived from the partial melting展开更多
Lead isotopic analytic data of 30 ores gathered from the Zhuanmiao boron deposit, Wengquangou boron (iron) deposit and its Dongtaizi Ore Member constitute three isochrons, the corresponding ages of which are 1902 ...Lead isotopic analytic data of 30 ores gathered from the Zhuanmiao boron deposit, Wengquangou boron (iron) deposit and its Dongtaizi Ore Member constitute three isochrons, the corresponding ages of which are 1902 ± 12 Ma, 1852 ± 9 Ma and 1917 ± 48 Ma. Lead isotopic analyses of marble from the Xiquegou Member of the Qingchenzi orefield yield a Pb-Pb isochron age of 1844 ± 27 Ma. 40Ar-39Ar quick neutron activation dating of phlogopites and microclines coexisting with ore minerals in the Wengquangou boron (iron) and Zhuanmiao boron deposits shows that: (1) the phlogopite from the Wengquangou has a plateau age of 1923 ± 1.5 Ma and an isochron age of 1924 ± 2.5 Ma; (2) the microcline from the Wengquangou has the plateau age of 1407 ± 5.4 Ma and 220 ± 12 Ma and an isochron age of 1403 ± 19 Ma; (3) the phlogopites from the Zhuanmiao yield a plateau age 1918 ± 1.3 Ma and an isochron age of 1918 ± 2.9 Ma; (4) the microclines from the Zhuanmiao yield the plateau age of 1420 ± 16 Ma and 250 ± 8 Ma and an isochron age of 1425 ± 19 Ma and 269 ± 16 Ma. These ages indicate that the eastern Liaoning area happened around 1900 Ma, an important tectonomagmatic event, which is consistent with the worldwide Mid-Proterozoic tectonomagmatic event. During this period, the Proterozoic Liaohe Group was folded and underwent strong normal metamorphism, and the (hydrothermal) sedimentary boron deposits (or source beds) formed earlier were strongly superimposed by mineralization, resulting in enrichment of boron; later regional geological processes made little contribution to the formation of the boron deposits. Lead isotopic components show that the U-Pb and Th-Pb isotopic system reached homogenization in the ores whereas only the U-Pb isotopic system reached homogenization in the marble from the Xiquegou district, which indicates that the boron deposits superimposed in the studied area endured a relatively strong process of hydrothermal migmatization during the end phase of early Proterozoic metamorphism.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
基金This project is supported by National Natural Science Foudation of ChinaState Antarctic Committee of China
文摘The Zhongshan Station of China is located in the Larsemann Hills, East Antarctica. Low pressure granulite facies gneisses together with late granites are outcroped in the region. Three biotite samples from a garnet segregation, a syenogranite and a granite-pegmetite were measured with 40Ar/39Ar incremental heating technique. Biotites from the garnet segregation give an 40Ar/39Ar plateau age of 504±1Ma. Biotites from the syenogranite yield an 40Ar/39Ar plateau age of 494 ±1 Ma. Biotites from the granite-pegmatite give an 40 Ar/39 Ar plateay age of 486±1Ma. They verify 500 Ma thermal event called 'Pan African event' by previous K-Ar and Rb-Sr data. They are cooling ages of the biotites when the paleogeotherm of the area droped to the K-Ar closure temperature for biotite.
文摘The Silurian bituminous sandstones（SBS） in the Tarim Basin, China are important basinwide reservoirs with an estimated area of approximately 249000 km^2. We investigated the ages of authigenic illites in the SBS reservoirs and constrained their formation timing by using the 40^Ar/39^Ar step wise heating method. The age spectra, 39^Ar recoil loss and their controlling factors were investigated systematically. The 40^Ar/39^Ar ages were compared with the conventional K/Ar ages of identical clay fractions. The clay in the SBS reservoirs is dominated by orderly mixed-layer illite/smectite（I/S） with 5%–30% smectite layers. The I/S minerals morphology comprises primarily honeycomb, short filamentous and curved-lath particles, characteristic of authigenic illites. The unencapsulated 40^Ar/39^Ar total gas ages（UTGA） of the authigenic illites range from 188.56 ± 6.20 Ma to 491.86 ± 27.68 Ma, which are 7% to 103% older than the corresponding K/Ar ages of 124.87 ± 1.11 Ma to 383.45 ± 2.80 Ma, respectively. The K-Ar ages indicate multistage accumulations with distinct distribution patterns in the Tarim Basin： older（late Caledonian-early Hercynian） around the basin margin, younger（late Hercynian） in the basin centre, and the youngest（middle to late Yanshanian） in the Ha-6 well-block, central area of the North Uplift. The age difference is believed to have been caused by the 39^Ar recoil loss during the irradiation process. Compared with the K/Ar ages, the estimated 39^Ar recoil losses in this study are in the range from 7% to 51%. The 39^Ar recoil loss appears to increase not only with the decreasing particle sizes of the I/S, but also with increasing percentage of smectite layers（IR） of the I/S, and smectite layer content（SLC） of the samples. We conclude that due to significant 39^Ar recoil losses, UTGA may not offer any meaningful geological ages of the authigenic illite formation in the SBS and thus can not be used to represent the hydrocarbon charge timing. 39^Ar recoil losses during 40^Ar/39^Ar dating can not be neglected when dating fine authigenic illite, especially when the ordered mixed-layer I/S containing small amount of smectite layers（IR30%） in the reservoir formations. Compared with the unencapsulated Ar-Ar method, the conventional K-Ar method is less complicated, more accurate and reliable in dating authigenic illites in petroleum reservoirs.
基金financially supported by National Natural Science Foundation of China(Nos 41372004,41330210,41121062)
文摘Metapelite is one of the predominant rock types in the high-pressure–ultrahigh-pressure（HP–UHP） metamorphic belt of western Tianshan, NW China; however, the spatial and temporal variations of this belt during metamorphism are poorly understood. In this study, we present comparative petrological studies and 40^Ar/39 ^Ar geochronology of HP and UHP pelitic schist exposed along the Habutengsu valley. The schist mainly comprises quartz, white mica, garnet, albite and bluish amphibole. In the Mn O–Na2O–Ca O–K2O–Fe O–Mg O–Al2O3–Si O2–H2O（Mn NCKFMASH） system, P–T pseudosections were constructed using THERMOCALC 333 for two representative pelitic schists. The results demonstrate that there was a break in the peak metamorphic pressures in the Habutengsu area. The northern schist has experienced UHP metamorphism, consistent with the presence of coesite in the same section, while the southern one formed at lower pressures that stabilized the quartz. This result supports the previous finding of a metamorphic gradient through the HP–UHP metamorphic belt of the Chinese western Tianshan by the authors. Additionally, phengite in the northern schist was modelled as having a Si content of 3.55–3.70（a.p.f.u.） at the peak stage, a value much higher than that of oriented matrix phengite（Si content 3.32–3.38 a.p.f.u.）. This indicates that the phengite flakes in the UHP schist were subjected to recrystallization during exhumation, which is consistent with the presence of phengite aggregates surrounding garnet porphyroblast. The 40^Ar/39^ Ar age spectra of white mica（dominantly phengite） from the two schists exhibit similar plateau ages of ca. 315 Ma, which is interpreted as the timing of a tectonometamorphic event that occurred during the exhumation of the HP–UHP metamorphic belt of the Chinese western Tianshan.
基金supported by the National Key R&D Program of China Ministry of Science and Technology(No.2018YFC0604005)the China Geological Survey through Geological Survey Project Grant(No.1212011085527).
文摘The Wutonggou iron deposit is located in the well-known iron metallogenic belt in the eastern Tianshan,NW China,and has been regarded as a sedimentary iron deposit.Although hydrothermal overprinting could play indispensable roles in the formation of high-grade iron ores in sedimentary iron deposits,previous studies mainly focused on sedimentary-related iron mineralization,while the nature and contribution of hydrothermal fluids are poorly constrained.Accordingly,an integrated study of ore geology,H-O-C isotopes and^(40)Ar-^(39)Ar dating,is conducted on the Wutonggou deposit,in order to reveal the features,source,and timing of hydrothermal mineralization.The studied deposit includes two mining sections namely the Jianshan and Wutonggou.Theδ^(18)O values of early magnetite from the Jianshan section range from+3.0‰to+5.8‰that nearly consistent with classic magmatic magnetite,while increase to 6.3‰-8.0‰in the late stage.Quartz from the two sections shows comparable H-O isotopic compositions and identical fractionation trends,and is plotted in or periphery to the primary magmatic water area.Calcites from the two sections are broadly similar in carbon and oxygen isotopic compositions,and siderite from the Wutonggou section is plotted in the same region.Thus,comparable stable isotopic compositions and evolution trends indicate similar magmatic fluids contributed hydrothermal iron mineralization in the two mining sections.Moreover,water-rock interactions of varying degrees generated distinct mineralization styles in the Jianshan and Wutonggou sections,and caused the isotopic fractionation in late stages.Biotite extracted from a hydrothermal siderite ore yielded a^(40)Ar-^(39)Ar plateau age of 299.5±2.0 Ma,indicates the timing of hydrothermal iron mineralization is corresponding to the emplacement of vicinity granitoids.Taken together,the hydrothermal mineralization in the Wutonggou iron deposit was the product of remobilization and upgrading of early sedimentary iron ores,and ore-forming fluids were most probably originated from regional granitic magmatism.
基金This work was supported by the Major State Basic Research Program (Grant No. G1999043202) the National Natural Science Foundation of China (Grant No. 49872028).
文摘Highly precise 40Ar-39Ar dating results demon-strate that the ages of potash-rich volcanic rocks in western Shandong Province are 114.7-124.3 Ma, and that of the lamprophyres is 119.6 Ma. The potash-rich volcanic rocks have relatively high (87Sr/86Sr)i ratios ( 0.708715-0.711418) and distinctly negative εNd values ( -11.47--17.54), and are enriched in radiogenic lead (206Pb/204Pb=17.341-17.622, 207Pb/204Pb=15.525-15.538, 208Pb/204Pb=37.563-37.684). Similarly, the lamprophyres also have quite low εNd values (-11.57 - -19.64). Based on the fact that the Sr, Nd and Pb isotopic compositions of potash-rich volcanic rocks are very consistent with that of the clinopyroxene separates, and by integrating comprehensive analyses of their tectonic settings, and extensive comparisons of the Sr, Nd isotopic composi-tions with that of the related simultaneous rocks, it is con-cluded that the potash-rich volcanic rocks and lamprophyres in western Shandong Province were most possibly derived from the partial melting
基金supported by the National 973 Project of China(G1999043201)
文摘Lead isotopic analytic data of 30 ores gathered from the Zhuanmiao boron deposit, Wengquangou boron (iron) deposit and its Dongtaizi Ore Member constitute three isochrons, the corresponding ages of which are 1902 ± 12 Ma, 1852 ± 9 Ma and 1917 ± 48 Ma. Lead isotopic analyses of marble from the Xiquegou Member of the Qingchenzi orefield yield a Pb-Pb isochron age of 1844 ± 27 Ma. 40Ar-39Ar quick neutron activation dating of phlogopites and microclines coexisting with ore minerals in the Wengquangou boron (iron) and Zhuanmiao boron deposits shows that: (1) the phlogopite from the Wengquangou has a plateau age of 1923 ± 1.5 Ma and an isochron age of 1924 ± 2.5 Ma; (2) the microcline from the Wengquangou has the plateau age of 1407 ± 5.4 Ma and 220 ± 12 Ma and an isochron age of 1403 ± 19 Ma; (3) the phlogopites from the Zhuanmiao yield a plateau age 1918 ± 1.3 Ma and an isochron age of 1918 ± 2.9 Ma; (4) the microclines from the Zhuanmiao yield the plateau age of 1420 ± 16 Ma and 250 ± 8 Ma and an isochron age of 1425 ± 19 Ma and 269 ± 16 Ma. These ages indicate that the eastern Liaoning area happened around 1900 Ma, an important tectonomagmatic event, which is consistent with the worldwide Mid-Proterozoic tectonomagmatic event. During this period, the Proterozoic Liaohe Group was folded and underwent strong normal metamorphism, and the (hydrothermal) sedimentary boron deposits (or source beds) formed earlier were strongly superimposed by mineralization, resulting in enrichment of boron; later regional geological processes made little contribution to the formation of the boron deposits. Lead isotopic components show that the U-Pb and Th-Pb isotopic system reached homogenization in the ores whereas only the U-Pb isotopic system reached homogenization in the marble from the Xiquegou district, which indicates that the boron deposits superimposed in the studied area endured a relatively strong process of hydrothermal migmatization during the end phase of early Proterozoic metamorphism.
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.
