Paleomagnetic Results From Sills In Luanshanzi Fonnation In Kuancheng Luojiagou Area, Hebei, And Their Tectonic Implications

The Yinshan-Yanshan tectonic belt is located near the northern margin of theNorth China block tectonic trends in the belt are dominantly E-W, but structures in theeastern portion of the belt trend E-NE and NE. Kuancheng situated at the transferzone of the east part of Yanshan belt. There is still controversial that how and whenthis transfer zone was formed.Several diorite sills intruded into the Tuanshanzi formation along the layersurface. The lower horizon of this formation is greyish-black dolomite and the upperhorizon is purple silty micrite dolomite. The attitude is332°∠35°.Precise U-Pbzircon ages of227±2.8Ma are obtained for these sills by Wang (2012,inpress).Theresult of rock magnetic and paleomagnetic studies on samples drilled from16of thesills, indicated that the main remanent magnetization carrier of sills is magnetite,while in the purple silty micrite dolomite is mainly hematite with high coercivity. Apair of character remanent magnetization (ChRM) direction confrontation with eachother have been isolated from the sills samples. One group(Ds=293.5°,Is=-78.6°,N=6)is identified for reversed polarity, and the other group (Ds=113,0°,Is=83.3°,N=3)is regarded as positive polarity. The result can pass the McFadden&McElhinny(1990) reversal test at95%confidence level, B grade (γ0=0.5017＜γcritical=0.5341)Bycomparing the ChRM of sills and the adjacent rocks ofdifferentdistance, it can be supposed to have passed generalized baked contact tests.The paleomagnetic pole of this study(28.2°N,132.6°E,α95=5.3°) is quitedifferent from the mean position of the Triassic pole (60.5°N,4,1°E,α95=2.7°)of North China Platform, we presume that was caused by tectonic movement. Duringthe time of the intrusion happens, the stratum attitude was150°∠54.5°,by inversionreckoning of the ChRM, this imply fold deformation happened before that time.TheChRM just perpendicular to the hinge of fold, which means the fold extended ineast-west direction. Combined with former paleomagnetic result, we know the studyarea lived through a counterclockwise rotation of30-35°. Further study is needed toconstrain the time of change of the tectonic trend.