Major and trace elements analysis has been carried out on the Late Ladinian Tabai basalts from Yunnan Province with the aim of studying their petrogenesis. Their SiO2 contents range from 43.63 wt.% to 48.23 wt.%. The ...Major and trace elements analysis has been carried out on the Late Ladinian Tabai basalts from Yunnan Province with the aim of studying their petrogenesis. Their SiO2 contents range from 43.63 wt.% to 48.23 wt.%. The basalts belong to the weakly alkaline(average total alkalis Na2 O+K2O=3.59 wt.%), high-Ti(3.21 wt.% to 4.32 wt.%) magma series. The basalts are characterized by OIB-like trace elements patterns, which are enriched in large ion lithosphile elements(LILE) including Rb and Ba, and display negative K, Zr and Hf anomalies as shown on the spider diagrams. The Tabai basalts display light rare-earth elements(LREE) enrichment and are depleted in heavy rare-earth elements(HREE) on the REE pattern. Those dates indicate that the parental magma of the Tabai basalts was derived from low-degree(1%–5%) partial melting of garnet peridotite. The magma underwent olivine fractional crystallization and minor crustal contamination during their ascent. The Tabai basalts were related to a relaxation event which had triggered the Emeishan fossil plume head re-melting in the Middle Triassic.展开更多
The Lower Cambrian on the Yangtze Platform in South China （internationally equivalent to the Nerreneuvian and 2nd series of the Cambrian） is valuable for understanding the early evolution of life, the global biogeoc...The Lower Cambrian on the Yangtze Platform in South China （internationally equivalent to the Nerreneuvian and 2nd series of the Cambrian） is valuable for understanding the early evolution of life, the global biogeochemical circles, and the major changes of the ocean. In particular, a precise radiometric calibration of the Lower Cambrian of this region is a critical realm in its multidisciplinary studies. A SHRIMP U-Pb geochronology study of the K-bentonite in the topmost Laobao Formation at the Pingyin section, Guizhou, South China yielded an age of 536±5 Ma （MSWD -- 0.75）, suggesting that the K-bentonite here can be correlated with the intensely studied K-bentonite within the middle Zhongyicun Member （Bed 5） of the Zhujiaqing For- mation at the Meishucun section in Yunnan. Such a regional correlation result reveals that the Bed 5 K-bentonite also extends into deeper-water environments rather than being deposited merely in the shallow platform interior of the Yangtze Platform. The age of the K-bentonite at the Pingyin section also implies that its overlying polymetallic Ni-Mo layer should be younger than 536e5 Ma. Hence the previous placement of the Precambrian/Cambrian boundary at this layer is inappropriate. Combined with the results of stratigraphic correlations, it is suggested that the K-bentonites in the middle Zhongyicun Member of the Zhujiaqing Formation （or the equivalents） and the base of the Shiyantou Formation （or its equivalents）, together with the polymetallic Ni-Mo layer, serve as three important marker beds. Their self-consistent radiometric ages have established an im- proved geochronologic framework for the Lower Cambrian in South China.展开更多
基金financially supported jointly by the Crisis Mines Continued Resources Exploration Project of the China Geological Survey(No.2008186)the 12th Five-Year Plan Project of the State Key Laboratory of Ore-deposit Geochemistry,Chinese Academy of Sciences(SKLODGZY125-02)the National Natural Science Foundation of China(No.41073032)
文摘Major and trace elements analysis has been carried out on the Late Ladinian Tabai basalts from Yunnan Province with the aim of studying their petrogenesis. Their SiO2 contents range from 43.63 wt.% to 48.23 wt.%. The basalts belong to the weakly alkaline(average total alkalis Na2 O+K2O=3.59 wt.%), high-Ti(3.21 wt.% to 4.32 wt.%) magma series. The basalts are characterized by OIB-like trace elements patterns, which are enriched in large ion lithosphile elements(LILE) including Rb and Ba, and display negative K, Zr and Hf anomalies as shown on the spider diagrams. The Tabai basalts display light rare-earth elements(LREE) enrichment and are depleted in heavy rare-earth elements(HREE) on the REE pattern. Those dates indicate that the parental magma of the Tabai basalts was derived from low-degree(1%–5%) partial melting of garnet peridotite. The magma underwent olivine fractional crystallization and minor crustal contamination during their ascent. The Tabai basalts were related to a relaxation event which had triggered the Emeishan fossil plume head re-melting in the Middle Triassic.
基金supported by National Natural Science Foundation of China(Grant Nos.4096300240672053&40803016)
文摘The Lower Cambrian on the Yangtze Platform in South China （internationally equivalent to the Nerreneuvian and 2nd series of the Cambrian） is valuable for understanding the early evolution of life, the global biogeochemical circles, and the major changes of the ocean. In particular, a precise radiometric calibration of the Lower Cambrian of this region is a critical realm in its multidisciplinary studies. A SHRIMP U-Pb geochronology study of the K-bentonite in the topmost Laobao Formation at the Pingyin section, Guizhou, South China yielded an age of 536±5 Ma （MSWD -- 0.75）, suggesting that the K-bentonite here can be correlated with the intensely studied K-bentonite within the middle Zhongyicun Member （Bed 5） of the Zhujiaqing For- mation at the Meishucun section in Yunnan. Such a regional correlation result reveals that the Bed 5 K-bentonite also extends into deeper-water environments rather than being deposited merely in the shallow platform interior of the Yangtze Platform. The age of the K-bentonite at the Pingyin section also implies that its overlying polymetallic Ni-Mo layer should be younger than 536e5 Ma. Hence the previous placement of the Precambrian/Cambrian boundary at this layer is inappropriate. Combined with the results of stratigraphic correlations, it is suggested that the K-bentonites in the middle Zhongyicun Member of the Zhujiaqing Formation （or the equivalents） and the base of the Shiyantou Formation （or its equivalents）, together with the polymetallic Ni-Mo layer, serve as three important marker beds. Their self-consistent radiometric ages have established an im- proved geochronologic framework for the Lower Cambrian in South China.
