云南梅树村前寒武系—寒武系界线层型候选剖面的古地磁研究

FURTHER STUDY ON PALAEOMAGNETISM OF THE PRECAMBRIAN-CAMBRIAN BOUNDARY CANDIDATE STRATOTYPE SECTION AT MEISHUCUN, YUNNAN, CHINA

梅树村剖面,已被推荐为国际前寒武系—寒武系界线层型候选剖面。本文论述了对该剖面进行的古地磁研究,在剖面的碳酸盐及磷酸盐台地相地层中,依次采集了159块古地磁样品。热退磁结果表明:剖面中的大多数岩石已被现今地磁场强烈重磁化,但仍有57块样品保存有呈互为对踱的剩磁方向(平均D/l=4.2°/7.1°,K=9,α_(95)=6.6。),并计算出古地磁极位置为68.8°N,270.7°E。这与华南年轻的显生宙岩石所得到的极位置皆不一样。该项研究的结果构成一极性带,至少有9次倒转。此极性带与苏联西伯利亚,澳大利亚及美国西部所获得的资料对比,都共同显示出以频繁的极性倒转为特征。

The Meishucun section near Kunming, southwestern China, has been recommended as a stratotype section of the Precambrian—Cambrian Boundary.A total of 159 samples were collected from the sequence of carbonate rocks and phosphorites at this section. Thermal demagnetization results indicate that a great majority of the rocks at the section have been strongly overprinted by the recent strong magnetic field, but 57 samples have preserved stable remanent magnetism that points to the north or south with antipodal directions (mean D/I=4.2°/7.1°, K=9, α95=6.6°). On calculation the location of the palaeomagnetic pole was at 68.8°N, 270.7°E, which is different from any palaeopoles obtained from younger Phanerozoic rocks in the South China platform.The results reveal a polarity zonation which includes 9 reversal events at least. A comparison of China's magnetostratigraphic records with those from Siberia, Australia and the western U.S.A. shows that all the sections are characterized by frequent polarity reversals.