Rockmagnetism Of The Volcanic Rocks Of The Dashizhai Formation Of Lower Permian In Xilinhot Block,Inner Mongolia

The Early Permian Dashizhai Formation volcanic rocks are widely exposed in Inner Mongolia.Locations of the strata can be divided into northern,central,and southern zones.Some paleomagnetic investigations have been conducted on the Dashizhai Formation volcanic rocks in the Balinzuoqi,Xilinhot,Linxi and Xiwuqi areas,Inner Mongolia.The results,however,yielded significant conflict.Rock magnetism and petrography are the basis of paleomagnetism and also an effective method to verify the reliability of paleomagnetism data.Therefore,further rock magnetism and petrographic investigations are still required to resolve these problems.In this study,we carried out detailed rock magnetic and petrographical investigations in the Xilinhot and Xiwuqi areas(central zone),and Balinzuoqi and Linxi areas(southern zone)in the Xilinhot block.Our rock magnetic and petrographical results provided some good interpretations to the discrepancy of the Early Permian paleomagnetic data.A total of 208 oriented cores from 27 paleomagnetic sites were collected from the Dashizhai Formation volcanic rocks.The anisotropy of magnetic susceptibility was measured for all the samples.17 representative samples were subjected to the rock magnetic analyses,including isothermal remanent magnetization(IRM)acquisition curves,IRM component analysis,and thermal demagnetization of three-axis IRM.Eight representative samples were subjected to petrographic analyses.Our results indicate that the large discrepancy of paleomagnetic results can be attributed to different rock magnetic and petrographic characteristics in different zones in the Xilinhot block.For the Balinzuoqi and Linxi area(southern zone),rock magnetic results reveal that the lowcoercivity magnetic minerals were magnetite,and the high-coercivity magnetic minerals were hematite.AMS results show that magnetic foliation is widely developed.Petrographic observations suggest that the minerals have secondary growth and directional alignment,which may be caused by deformation and metamorphism.These feathers can suggest that these rocks may have suffered from remagnetization.For the Xilinhot and Xiwuqi areas(central zone),rock magnetic results reveal that both magnetite and hematite are present,but magnetite is the main magnetic carrier.Furthermore,the rocks may contain small amounts of pyrrhotite or coarser titanium-bearing magnetite grains.AMS and petrographic results indicate that no obvious deformation and alteration have occurred for these rocks and the AMS shows primary magnetic fabric.These feathers demonstrate that these areas are suitable to obtain the primary paleomagnetic records.