SHRIMP zircon age for a K-bentonite in the top of the Laobao Formation at the Pingyin section,Guizhou,South China  

作　　者：ZHOU MingZhong LUO TaiYi LIU ShiRong QIAN ZhiKuan XING LeCai 

机构地区：[1]School of Geographical and Environmental Sciences,Guizhou Normal University [2]State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences [3]Graduate University of Chinese Academy of Sciences

出　　处：《Science China Earth Sciences》2013年第10期1677-1687,共11页中国科学（地球科学英文版）

基　　金：supported by National Natural Science Foundation of China(Grant Nos.40963002;40672053&40803016)

摘　　要：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.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 Formation 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 536±5 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 improved geochronologic framework for the Lower Cambrian in South China.

关 键 词：Laobao Formation K-BENTONITE U-Pb geochronology Niutitang Formation polymetallic Ni-Mo layer 

分 类 号：P619.211[天文地球—矿床学] P597.3[天文地球—地质学]