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《ACTA GEOSICIENTIA SINICA》 1994-Z1
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THE GEOCHRonical STUDIES OF SIBAO GROUP IN THE SOUTHERN MARGIN OF JIANGNAN MASSIF

Han Fa Shen Jianzhong Nie Fengjun Ding Xiaoshi(Institute of Mineral Deposits,CAGS)Li Shichao(Team of Regional Geologic Survey,Guangxi Zhuang Autonomous Region)Yang Kaitai Yang Chen(Team of Peroleum Geologic Survey,Guangxi Zhuang Autonomous Region)(Instit  
The  ̄(207)Pb/ ̄(206)Pb-ages on single zircons by using kober's method,and Sm-Nd and Rb-Sr isotopicsystems have been studied for the mafic-ultramafic volcanic rocks from Sibao Group in Baotan districtalong the southem margin of Jiangnan massif.The ̄(207)Pb- ̄(206)Pb determinations on single zirconse separat-ed from pillowed basalt lava in the upper part of Wentong Formation show that two groupe of fine-grained primary zircons give the  ̄(207)Pb/ ̄(206)Pb ages of 1734±20 Ma and 1863±7 Ma,respectively.Eight samples of mafic and ultramafic volcanic rocks collected from Wentong,Yangjia,Shaping,andJiepei gives a Sm-Nd isochron age of 1782±82 Ma,with the initial Nd value of 0.51035,which isvery consistent with the  ̄(207)Pb/ ̄(206)Pb ages of fine-grained primary zircons,which indicate the formationof Sibao Group from the late Ealy Proterozoic to early Middle Proterozoic time.The ε_(Nd)(t) of 0.6 forthe analyzed samples implies the derivation of mafic-ultramafic magma from a depleted mantle source.The Rb-Sr isotopic systein has been strongly disturbed probably by the late tectonic movement,mag-matic intrusions, and metamorphism,and reveals the lack of correlation between  ̄(87)Sr/ ̄(86)Sr and ̄(87)Rb/ ̄(86)Sr,which can not give any meaning of rock-forming age.The delineation of the forming agefor Sibao Group causes our reconsidering the forming age of the corresponding metavolcano-sedimen-tary sequences in Jiangnan massif,and excludes the possibility of the relict ophiolite suites for thosemetavolcano-Sedimentary sequences.
【CateGory Index】: P597.3
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