Magnetostratigraphic investigations on an Emeishan basalt section in western Guizhou province, China

A magnetostratigraphic investigation has been conducted on a late Permian Emeishan basalt section at Duge in western Guizhou province, China. The characteristic remanent magnetization (ChRM) directions resolved at high temperatures from the basalts pass the fold test with dual polarity and define a normal magnetozone of five sixths of the total thickness in the lower part and a reversed magnetozone of one sixth of the thickness in the top part of the section. The ChRM directions from the basalt clasts of pyroclastic rocks fail the conglomerate test, but the direction and polarity of the ChRM of the clasts from the upper reversed zone are consistent with those of the basaltic lavas in the reversed zone and the normal directions also dominate the ChRM of the clasts taken from the lower normal zone. This suggests that either the original remanence of the clasts was reset by cementing basaltic lavas or the ChRM of the basalts was acquired late but not beyond a field reversal. The magnetostratigraphic pattern (N-R) observed at Duge is as simple as that observed from some major continental flood basalt provinces in the world, where very few magnetic polarity zones were observed from volcanic sequences of great thickness; the emplacement of the Emeishan basalts in western Guizhou is estimated to be less than two million years. The normal polarity magnetozone observed indicates that the extrusion of the Emeishan basalts in western Guizhou postdates the Permo-Carboniferous reversed superchron (PCRS). The apparent shortness of the reversed magnetozone explains why far fewer reverse directions than normal ones have been so far reported from the Emeishan basalts. (C) 1998 Elsevier Science B.V. All rights reserved.