New Paleomagnetic Results From The Volcanic Rocks Of The Xinglonggou Formation In The Beipiao Basin Of The Yanshan Belt Of The North China And Their Tectonic Implications

The Lower Jurassic Xinglonggou Formation?Fm?volcanic rocks are widely exposed in the Beipiao basin of the Yanshan belt?YSB?of the northern margin of the North China Block?NCB?.The strata were well-dated?176.7±3.5 Ma?and its fresh volcanic rocks are amenable to paleomagnetic study.A series of new paleomagnetic study on well-dated?¹75 Ma?volcanic rocks of the Xinglonggou Fm in the Beipiao basin of the YSB have been carried out.113 samples from 13 sites in the two sections of the Beipiao area were collected.All of these samples were subjected to stepwise thermal demagnetization.After removed a low temperature component?<300°C?which was acquired in the recent geomagnetic field,a stable high temperature component?350°C-580°C?was isolated.The direction of high temperature component for Liangjiatun section is Dg=42.0°,Ig=75.4°,k=61.5,?₉₅=6.2° in situ,and Ds=355.0°,Is=47.4°,k=61.5,?₉₅=6.2° in tilt-corrected;the Sujiayingzi section is Dg=207.3°,Ig=-62.5°,k=973.6,?₉₅=4.0°in situ,and Ds=181.0°,Is=-44.2°,k=988.5,?₉₅=3.9° in tilt-corrected.The characteristics remanent directions of the two sections together can pass a reversal test and a fold test.Thus,it can be considered as primary paleomagnetic record.And our analyses demonstrate that the paleomagnetic directions obtained from two sections pass the secular variation model of the “A95-statistical reliability envelope”.The averaged site-level VGPs of Xinglonggou Fm?¹75 Ma?in this article is at 76.5°N,313.7°E?A95=5.9°?.In this article,we have obtained a well-dated paleomagnetic pole from volcanic rocks of the Xinglonggou Fm in the Beipiao basin of the YSB.It consistent with the paleopole from the sedimentary rocks of the North China craton,indicating that the Yanshan belt and the North China craton were a tectonic unit.Compared with the coeval pelaomagnetic pole of Siberia Block?SIB?,there is a significant divergence which shows the existence of Mongol-Okhotsk Ocean.The paleomagnetic poles of the NCB and the South China Block?SCB?demonstrate that the east part of the NCB and the SCB had already collided before Early Jurassic,however,there was a remnant Paleo-Qinling ocean basin with a hatch towarded west;it probably closed by the rotation of two blocks by Later Jurassic.After Early Jurassic,the influence of the diachronous collision of the SIB and the SCB made the NCB anticlockwise rotation according to other blocks surrounded.