云南梅树村前寒武系—寒武系界线剖面的硅同位素研究

SILICON ISOTOPE STUDY ON THE MEISHUCUN PRECAMBRIAN─CAMBRIAN BOUNDARY SECTION,YUNNAN PROVINCE

云南梅树村前寒武系—寒武系界线剖面是研究前寒武系—寒武系界线层型最重要的国际候选剖面之一。本文首次系统地研究了硅同位素在剖面中的变化规律；指出在“Ｂ点”—“Ｃ点”之间有海底喷气活动存在，“界线粘土层”为火山成因。“Ｃ点”附近点δ￣（１３）Ｃ、δ￣（１８）Ｏ负异常与大量生物活动有关，而非“灾变”所致。

The Meishucun section has been commonly favored as the best candidate for the global stratotypeof the Precambrian─Cambrian boundary. Silicon isotope investigation of the section is first undertakenin this study,and two positive and one nagative anomalies of δ￣(30)Si have been found.The positiveanomaly of the δ￣(30)Si is shown in siliceous bands and siliceous concretion,which is the characteristics ofthe siliceous rocks in shallow-Sea cabonate rock sequence. The nagative anomaly is found in the sectorbetween Marker B and C,where the δ￣(30)Si values of dolomitic phosphatic beds from beds 7 range from─0.5‰ to─0.2‰, averaging─0.4‰,the δ￣(30)Si value of siliceous dolomite from Dahai Member is─2‰, and the δ￣(30)Si of siliceus bands in same atrata are─0.8‰ and 0.0‰ .Ther δ￣(30)Si values areobviously lower than same rocks in undelaying beds and from other area, these results may beexplained by their genesis of sea floor exhalation The δ￣(30)Si values (0.0‰ to 0.1‰)of“boundaryclay band”near Marker C is obviously different from those of sedimentary clay, but close to those ofgranitoid.It is su88ested that“the boundary clay band”may be a fast alteration product of acidic tuffin sea+water environment.Two similar big nagative anomalies of δ￣(13)C and δ￣(18)O are found inphosphatic beds near Marker B and C,they may be caused by biological action and have nothing to dowith cataclysm event.