Paleomagnetic Study Of The Permian Basalts In Southwest China And Its Geological Significance

The dating of volcanic eruptions is hampered by the geochronological methods,e.g.U-Pb zircon dating,its precision and uncertainty,because the eruption was occurred in a relatively short time intervals.In this paper,based on the hypothesis of geocentric axial dipole and geomagnetic secular variation,eruptive duration is estimated by the flow-by-flow analysis of paleomagnetic directions for the Emeishan basalts sampled from Dongchuan area of South China block.The correlation of previous magnetostratigraphic works was also used to determine the total duration of initial normal polarity chron for the Emeishan Large Igneous province(ELIP).The Dongchuan basalts covered 630-m-thick are thermally demagnetized to isolate the reliable characteristic remanent magnetization(ChRM),which all recorded normal polarity after bedding corrections and passed a positive fold test at the 99%confidence level(McFadden,1990).Magnetostratigraphic result from the Dongchuan section reveals a normal polarity that is correlative to other sections during the first episode in the ELIP.The flow-by-flow analysis of the Dongchuan basalts show that it erupted in the form of four directional groups and fourteen individual lava directions,showing a pure duration shorter than 3000 years if the eruptions were not punctuated.Magnetostratigraphic results and the hypothesis of geocentric axial dipole show that the total duration of the normal polarity chron in the ELIP could be roughly estimated within 105 years.Considering the pure duration of the normal polarity was quite short interval for the ELIP,the volatile emission must be hugely discharged in a large velocity,and the gases emitted by this velocity would last hundreds of years,indicating a significantly change on the environment.In consequence,the ELIP eruption may have contributed to the end-Guadalupian mass extinction.The Baoshan Terrane of southwestern China is considered to have been part of the Cimmerian block during the late Paleozoic;consequently,knowledge of its paleoposition and geological evolution can provide constraints on the Permian breakup of northern East Gondwana.Therefore,we conducted paleomagnetic studies on lower Permian basalts from four localities,and LA-ICP-MS U-Pb studies on the detrital zircon separated from the clastic sediments of the lower Permian Dingjiazhai Formation in two locations.The paleomagnetic results from 23 sites yield a single reversed polarity directed downwards to the southwest,giving a site-mean direction of Dg/Ig=156.7°/56.6°(Kg=8.0,?85=11.4°)before tilt correction,and Ds/Is=218.3°/60.1°(K=14.1,?95=8.4°)after tilt correction.The result passed the fold test.All available data were examined section-by-section using the angular dispersion(SB)of virtual geomagnetic poles(VGPs)to ensure that the GSV was completely averaged out.Because the dispersion in declinations is likely to have been affectedby subsequent tectonic deformation,the paleosecular variation(PSV)could not be evaluated from all the data amassed from different sections,and the PSV was able to be removed from only four(combined)sections.A small-circle fit of these VGPs gives an averaged paleocolatitude of 51.9°±3.7°(N=31 sites)centered on 24° N,99° E.The result indicates that the sampled area of the Baoshan Terrane was located at a latitude of 38°S±3.7° during the late early Permian.The detrital zircons from the Dingjiazhai Formation show a widely distributed from the Early Proterozoic to Early Paleozoic,a maximum peak value of?0.76Ga and several small peaks(?0.52Ga,?0.95Ga,?1.06Ga,?1.8Ga,?2.5Ga).The provenance of the detrital zircons age with?0.52Ga was mostly from the Baoshan Terrane and a minority from the Gondwanan blocks.The magmatic activities related with severalo ther age peaks had not yet been found in the Baoshan Terrane,which should be come from the nearby blocks/terranes.Combined on the previous detrital zircons studies from the Baoshan Terrane and other blocks/terranes,we compare their age spectras,showing a familiarity relationship between the Baoshan Terrane and Australia,India.A comparison of our result with early Permian data from Gondwanan blocks suggests the Baoshan Terrane was situated near the northern India and Western Australia in the Paleozoic,and broke away from the Gondwana after the early Permian.