Magnetostratigraphic Dating On The Emeishan Large Igneous Province,the Results From The Binchuan And Yanyuan Areas

The Emeishan basalts are widely distributed in Yunnan,Sichuan and Guizhou provinces of Southwest China,which are considered as a product of the Large Igneous Province(LIP)during the Permian.In order to constrain the eruptive duration of the Emeishan LIP,magnetostratigraphic study was carried out in Binchuan and Yanyuan areas in the inner zone of the igneous province.High-precision geochronological dating was carried out on the eruptive tuffs in Binchuan area.The dating results show a large dispersion.The tuff in the Daying section of Binchuan area yields an age at 259.3±3.8Ma,which is comparative with the available ages obtained from the mafic or ultramafic intrusive rocks.Theses results indicate that the Emeishan basalts were probably emplaced close to the Middle-Late Permian boundary.The Emeishan LIP eruption may be temporally linked to the end-Guadalupian mass extinctions.Our magnetostratigraphic results in Binchuan area are the first report in the inner zone of the Emeishan LIP.The normal magnetozone was found in the low-Ti(LT)basalts of the lower part of the section,and the reversal magnetozone was obtained in the middle and upper part of the section that composed of the LT and high-Ti(HT)basalts.The basaltic lava flows at Yanyuan area are geochemically classified into LT and HT types according to our new results.The Luomapu section can be compared with the LT basalts in the lower part of the sequences at Binchuan area.The Gongmushan section is correlative to the HT basalts.The magnetostratigraphic results also indicate a simple magnetostratigraphic normal and reversal polarity pattern from the lower and upper parts of the combined sections at Yanyuan,respectively.Our magnetostratigraphic results obtained from the sections in the inner zone of the Emeishan LIP are correlative with the available results from those obtained from the middle zone of the Emeishan LIP.By comparing with available magneostratigraphic and paleomagnetic data carried out in the Emeishan LIP,these results imply that the eruption of the basalts in the Emeishan LIP can be divided into early(normal polarity subchron)and late(reversal subchron)stages.The early stage included LT basalts in the inner zone and most of the HT basalts in the intermediate and outer zones,and the late stage contained a part of LT and HT basalts in the upper parts of the sections in the inner zone and a few in the intermediate zone of the Emeishan LIP.Correlating with available Middle-Late Permian magnetostratigraphy and Permian magnetic time scale(Ogg et al.,2008),the result suggests that the Emeishan LIP emplaced before the Capitanian-Wuchiapingian boundary,and maybe ended closed to the boundary.Our result also suggests that the Emeishan LIP was formed over a very short time interval,less than one million years.