ABSTUCT Many normal and a few reversed characteristic directions of magnetization have been obtained by predominantly thermal demagnetization from ten sites of the Late Permian Emeishan Basalts collected near Kunming, Yumran Province. The normal ma~etization directions pass a fold test at the 99% confidence level and yield d~ination/~clina~on = 26"/ -12", k = 46, qg = 6" and a paleopole at 50"N, 241"E. However, the reversed-polarity directions, with d~lination/inc~ation at 244"/+ 3" are not antipodal to the normal ones, which is also noted in other studies of the Emeishan Basalts of the Yangtze Paraplatform of the South China Block. Speculations about the cause of this lack of antipodaiity center on: (1) local relative rotations, (2) incomplete demagnetization, (3) unusually large non-dipole fields, secular variation or asymmetric reversals in the Late Permian, (4) errors in sample orientation, or (5) later remagnetization represented by one or the other polarity group. The first two causes are ruled out by our obsetvations, and the third cannot be tested with data from China alone but is unlikely because it has not been observed elsewhere. Errors in sample o~en~~n may be present in those studies that used a magnetic compass, because sample intensities of magnetization are on tire high side. However, for the normal-polarity directions, a solar compass has been used in some of the studies and ail normal-polarity directions observed in the area are in agreement. Thus, sample orientation errors can be invoked only for the reversed-polarity directions. In order to test cause (5), we have examined the magnetic carriers in eight samples with scanning electron microscopy. Titanium-poor to titanium-rich magnetite, commonly cruciform in crystal habit, indicates primary igneous crystallization in two samples that have NE and WSW declinations. In contrast, six no~~-~Iari~ samples with NNE declinations show pervrsive replacement of original titanom~etite by titanic-me magnetite and sphene, indicating a high degree of alteration. We ascribe this alteration to late hydrothermal circulation and argue that it has caused remagnetization in post-Permian times. This study suggests, therefore, that the NE-SW directions are more likely to be representative of the Late Permian paleomagnetic field than the NNE directions.

A paleomagnetic study of Devonian sedimentary sequences from western and eastern Yunnan, China, yields contrasting paleolatitudes for these two areas. They indicate an equatorial position for eastern Yunnan, as part of the Yangtze Paraplatform or South China Block, and a paleolatitude of about 40 ø for western Yunnan as part of the Shan State (or Shan-Thai) Block, which probably was part of the Gondwana supercontinent. The majority of samples is strongly overprinted by a present-day field magnetization, but thermal demagnetization has been successful in isolating ancient, dualpolarity characteristic directions of magnetization in rocks from eastern and western Yunnan, with results from the latter passing the fold test. The paleopole from eastern Yunnan is located at 8.9øN, 10.4øE (declination/inclination = 279.2ø/ +2.7 ø, k = 13, alpha 95 = 6.9 ø, N = 33 samples from 7 sites). The paleopole from western Yunnan is located at 66.5øN, 133.8øE (declination/inclination = 18.00/+62.0 ø, k = 21, alpha 95 = 5.8 ø, N = 31 samples from 7 sites), yielding a paleolatitude of 42 ø. The two blocks studied are separated by the Red (Yuan) River and Lancang (Mekong) River fault zones, one of which is inferred to be an ancient suture.

Three magnetic components have been isolated in Ordovician formations of the Yangtze Paraplatform (South China Block). Two of these (Daqing A and Hongshiya B components) yield paleopoles that conform to the Carboniferous to Triassic segment of the apparent polar wander path for South China, and are therefore interpreted as remagnetizations. The third component (declination/inclination = 301°/+66°, N = 5 sites, k = 21.4, α95 = 17°) passes the fold test and is interpreted as primary (late Early Ordovician). The paleopole, at 39°S, 236°E, and the paleolatitude of 48°S support an Ordovician position of South China adjacent to Gondwana.

The Meishucun secton has been recommended as an international candidate stratotype secton of the Precambrian-Cambrian boundary. The paper deals with the palaeomagnetic study on the section. A total of 159 palaeomagnetic samples were successively collected from the platform-facies sequence of carbonates and phosphates at the section. Thermal demagnetization results indicate a great majority of the rocks at the section have been strongly overprinted by recent magnetic field, but 57 samples have preserved remanent magnetization with antipodal directions (mean D / I = 4.2 / 7.1 , K= 9, ; = 6.6 0) Baaed on calculation, the location of the palaeomagnetic pole was at 68.8 0 N and 270.7 E, which is different from any palaeopoles obtained from younger Phanerozoic rocks in South China. The results reveal a polarity zonation which includes at least 9 reversaI events. 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. Meishucun (24 O 44 N and 102°34 / E), situated on the southwest margin of the Yangtze platform, 72 km southwest of Kunming, is easy of access by bus. There the Sinian and Lower Cambrian are extensively developed. The Meishucun section is present in the Kunyang phosphorite mining district. The strata, extending for 12 km, striking E-W, dipping 10°2 0 °s outh, and having an exposed thickness of 313 m, are a set of carbonate and phosphate sediments of platform or bay facies. Based on lithology and fossil assemblage, the section is divided into 2 formations and 6 members (Table 1). The section consists of fresh and unmetamorphosed rocks, has no magmatic intrusion nearby, and belongs to a sttatigraphically complete and structurally stable area. Baiyanshao Mcm. z2 98 Vol.4 No. 1 ACTA GEOLOGICA SINICA