Paleomagnetism and electron microscopy of the Emeishan Basalts, Yunnan, China

Abstract

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, Yunnan Province. The normal magnetization directions pass a fold test at the 99% confidence level and yield declination/inclination = 26[deg]/-12[deg], k = 46, [alpha]95 = 6[deg] and a paleopole at 50[deg]N, 241[deg]E. However, the reversed-polarity directions, with declination/inclination at 244[deg] / + 3[deg] 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 antipodality center on: 1. (1) local relative rotations.2. (2) incomplete demagnetization.3. (3) unusually large non-dipole fields, secular variation or asymmetric reversals in the Late Permian.4. (4) errors in sample orientation.5. (5) later remagnetization represented by one or the other polarity group. The first two causes are ruled out by our observations, and the third cannot be tested with data from China alone but is unlikely because it has not been observed elsewhere. Errors in sample orientation may be present in those studies that used a magnetic compass, because sample intensities of magnetization are on the high side. However, for the normal-polarity directions, a solar compass has been used in some of the studies and all 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 normal-polarity samples with NNE declinations show pervasive replacement of original titanomagnetite by titanium-free 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.