As one of the micro-blocks dispersed in the South China Sea(SCS), the basement of the Xisha Islands has rarely been drilled because of the thick overlying Cenozoic sediments, which has led to a confused understanding ...As one of the micro-blocks dispersed in the South China Sea(SCS), the basement of the Xisha Islands has rarely been drilled because of the thick overlying Cenozoic sediments, which has led to a confused understanding of the pre-Cenozoic basement of the Xisha Islands. Well CK-1, a kilometer-scale major scientific drill in the Xisha Islands in the northwestern SCS, penetrated thick reefal limestone(0–888.4 m) and the underlying basement rocks(888.4–901.4 m). In this study, we present the zircon U-Pb ages of basement basaltic pyroclastic rocks from Well CK-1 in the Xisha Islands of the northwestern SCS to investigate the basement nature of the Xisha microblock. The basement of Well CK-1 consists of basaltic pyroclastic rocks on the seamount. The zircon grains yielded apparent ages ranging from ca. 2 138.9 Ma to ca. 36 Ma. The old group of zircon grains from Well CK-1 was considered to be inherited zircons. Two Cenozoic zircons gave a weighted mean 206Pb/238U age of(36.3 ± 1.1) Ma,Mean Squared Weighted Deviations(MSWD) = 1.2, which may represent the maximum age of the volcano eruption. The Yanshanian inherited zircons(116.9–105.7 Ma and 146.1–130.2 Ma) from Well CK-1 are consistent with the zircons from Well XK-1, indicating that the basement of Chenhang Island may be similar to that of Well XK-1. We propose that the Xisha micro-block may have developed on a uniform Late Jurassic metamorphic crystalline basement, intruded by Cretaceous granitic magma.展开更多
Seafloor extension and associated rifting in the North Atlantic Area, which started in Early Paleogene (from −62 Ma), resulted in a few micro-continents being isolated and submerged below sea levels. Published isotopi...Seafloor extension and associated rifting in the North Atlantic Area, which started in Early Paleogene (from −62 Ma), resulted in a few micro-continents being isolated and submerged below sea levels. Published isotopic data for sunken offshore continental materials (basement) in the North Atlantic area are quite sparse, but a few do exist for the Rockall Plateau, or more precisely the Rockall Bank. Isotopic data for Early Paleogene basaltic materials, covering basement rocks of the Rockall Plateau, have hitherto only been publicised for the NW margin of the Hatton bank. The Early Paleogene basaltic archipelago of the Faroe Islands, on which some isotopic data do exist, rests on an ancient sunken continental crust of unknown geochemical and isotopic compositions. The objective of this contribution is to assess potential lead isotopic relationships between the Rockall Plateau and the Faroese sub-basaltic basement, based on the sparse available isotopic data existing for the former and using available isotopic data for slightly contaminated Faroese basaltic rocks. The results reached in this contribution point to a likely association between the Faroese sub-basaltic basement and the basement of the Rockall Bank and hence the Rockall Plateau and potentially also between Faroese basaltic rocks and contemporaneous counterparts from the Hatton Bank.展开更多
Holocene basaltic rocks of the Jingpohu area are located in the “Crater Forest” and Hamatang districts to the northwest of the Jingpohu Lake. Although there is only a distance of 15 km between the two districts, the...Holocene basaltic rocks of the Jingpohu area are located in the “Crater Forest” and Hamatang districts to the northwest of the Jingpohu Lake. Although there is only a distance of 15 km between the two districts, their petrological characteristics are very different: alkaline olivine basalt without any megacrysts in the former, and leucite tephrite with Ti-amphibole, phlogopite and anorthoclasite megacrysts in the latter. On the basis of their geochemical characteristics, the two types of basaltic rocks should belong to weakly sodian alkaline basalts. But leucite tephrite is characterized by higher Al2O3, Na2O and K2O, higher enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE), lower MgO and CaO, compatible elements and moderately compatible elements and lower Mg*values and Na/K ratios in comparison with alkaline olivine basalt. However, the two types of basaltic rocks have similar Sr, Nd, Pb isotopic compositions, which suggests that the mantle beneath the Jingpohu area was homogeneous before undergoing some geological processes about 3490 years ago. As the activity of the mantle plume led to different degrees of metasomatism, extreme mantle source heterogeneities occurred beneath the Jingpohu area. In comparison with alkaline olivine basalt, the leucite tephrite was derived from the more enriched mantle source region and resulted from strong metasomatism.展开更多
In the Chang＇e-3 mission, the Active Particle-induced X-ray Spectrometer（APXS） on the Yutu rover is used to analyze the chemical composition of lunar soil and rock samples. APXS data are only valid are only if the ...In the Chang＇e-3 mission, the Active Particle-induced X-ray Spectrometer（APXS） on the Yutu rover is used to analyze the chemical composition of lunar soil and rock samples. APXS data are only valid are only if the sensor head gets close to the target and integration time lasts long enough. Therefore, working distance and integration time are the dominant factors that affect APXS results. This study confirms the ability of APXS to detect elements and investigates the effects of distance and time on the measurements. We make use of a backup APXS instrument to determine the chemical composition of both powder and bulk samples under the conditions of different working distances and integration times. The results indicate that APXS can detect seven major elements, including Mg, Al, Si, K, Ca, Ti and Fe under the condition that the working distance is less than 30 mm and having an integration time of 30 min. The statistical deviation is smaller than 15%. This demonstrates the instrument＇s ability to detect major elements in the sample. Our measurements also indicate the increase of integration time could reduce the measurement error of peak area, which is useful for detecting the elements Mg, Al and Si. However, an increase in working distance can result in larger errors in measurement, which significantly affects the detection of the element Mg.展开更多
基金The National Natural Science Foundation of China under contract Nos 42030502, 42090041 and 42166003the Guangxi Scientific Projects under contract Nos AD17129063 and AA17204074+1 种基金the Guangxi Youth Science Fund Project under contract 2019GXNSFBA185016the Ph.D. Research Start-up Foundation of Guangxi University under contract No. XBZ170339。
文摘As one of the micro-blocks dispersed in the South China Sea(SCS), the basement of the Xisha Islands has rarely been drilled because of the thick overlying Cenozoic sediments, which has led to a confused understanding of the pre-Cenozoic basement of the Xisha Islands. Well CK-1, a kilometer-scale major scientific drill in the Xisha Islands in the northwestern SCS, penetrated thick reefal limestone(0–888.4 m) and the underlying basement rocks(888.4–901.4 m). In this study, we present the zircon U-Pb ages of basement basaltic pyroclastic rocks from Well CK-1 in the Xisha Islands of the northwestern SCS to investigate the basement nature of the Xisha microblock. The basement of Well CK-1 consists of basaltic pyroclastic rocks on the seamount. The zircon grains yielded apparent ages ranging from ca. 2 138.9 Ma to ca. 36 Ma. The old group of zircon grains from Well CK-1 was considered to be inherited zircons. Two Cenozoic zircons gave a weighted mean 206Pb/238U age of(36.3 ± 1.1) Ma,Mean Squared Weighted Deviations(MSWD) = 1.2, which may represent the maximum age of the volcano eruption. The Yanshanian inherited zircons(116.9–105.7 Ma and 146.1–130.2 Ma) from Well CK-1 are consistent with the zircons from Well XK-1, indicating that the basement of Chenhang Island may be similar to that of Well XK-1. We propose that the Xisha micro-block may have developed on a uniform Late Jurassic metamorphic crystalline basement, intruded by Cretaceous granitic magma.
文摘Seafloor extension and associated rifting in the North Atlantic Area, which started in Early Paleogene (from −62 Ma), resulted in a few micro-continents being isolated and submerged below sea levels. Published isotopic data for sunken offshore continental materials (basement) in the North Atlantic area are quite sparse, but a few do exist for the Rockall Plateau, or more precisely the Rockall Bank. Isotopic data for Early Paleogene basaltic materials, covering basement rocks of the Rockall Plateau, have hitherto only been publicised for the NW margin of the Hatton bank. The Early Paleogene basaltic archipelago of the Faroe Islands, on which some isotopic data do exist, rests on an ancient sunken continental crust of unknown geochemical and isotopic compositions. The objective of this contribution is to assess potential lead isotopic relationships between the Rockall Plateau and the Faroese sub-basaltic basement, based on the sparse available isotopic data existing for the former and using available isotopic data for slightly contaminated Faroese basaltic rocks. The results reached in this contribution point to a likely association between the Faroese sub-basaltic basement and the basement of the Rockall Bank and hence the Rockall Plateau and potentially also between Faroese basaltic rocks and contemporaneous counterparts from the Hatton Bank.
文摘Holocene basaltic rocks of the Jingpohu area are located in the “Crater Forest” and Hamatang districts to the northwest of the Jingpohu Lake. Although there is only a distance of 15 km between the two districts, their petrological characteristics are very different: alkaline olivine basalt without any megacrysts in the former, and leucite tephrite with Ti-amphibole, phlogopite and anorthoclasite megacrysts in the latter. On the basis of their geochemical characteristics, the two types of basaltic rocks should belong to weakly sodian alkaline basalts. But leucite tephrite is characterized by higher Al2O3, Na2O and K2O, higher enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE), lower MgO and CaO, compatible elements and moderately compatible elements and lower Mg*values and Na/K ratios in comparison with alkaline olivine basalt. However, the two types of basaltic rocks have similar Sr, Nd, Pb isotopic compositions, which suggests that the mantle beneath the Jingpohu area was homogeneous before undergoing some geological processes about 3490 years ago. As the activity of the mantle plume led to different degrees of metasomatism, extreme mantle source heterogeneities occurred beneath the Jingpohu area. In comparison with alkaline olivine basalt, the leucite tephrite was derived from the more enriched mantle source region and resulted from strong metasomatism.
文摘In the Chang＇e-3 mission, the Active Particle-induced X-ray Spectrometer（APXS） on the Yutu rover is used to analyze the chemical composition of lunar soil and rock samples. APXS data are only valid are only if the sensor head gets close to the target and integration time lasts long enough. Therefore, working distance and integration time are the dominant factors that affect APXS results. This study confirms the ability of APXS to detect elements and investigates the effects of distance and time on the measurements. We make use of a backup APXS instrument to determine the chemical composition of both powder and bulk samples under the conditions of different working distances and integration times. The results indicate that APXS can detect seven major elements, including Mg, Al, Si, K, Ca, Ti and Fe under the condition that the working distance is less than 30 mm and having an integration time of 30 min. The statistical deviation is smaller than 15%. This demonstrates the instrument＇s ability to detect major elements in the sample. Our measurements also indicate the increase of integration time could reduce the measurement error of peak area, which is useful for detecting the elements Mg, Al and Si. However, an increase in working distance can result in larger errors in measurement, which significantly affects the detection of the element Mg.
