New paleomagnetic results from the Triassic sills in the Paleoproterozoic Tuanshanzi Formation in Kuancheng region of Yanshan belt, North China Block, and their tectonic implications

The Yinshan-Yanshan tectonic belt is located along the northern margin of the North China Block (NCB), whose tectonic trends are dominantly E-W, with some E-NE and NE trends in the east. Controversy still exits on how and when this deformation formed. Therefore, paleomagnetic research was carried out in Kuancheng area to study the deformation zone. 199 standard paleomagnetic core samples were collected from 16 Triassic diorite sills, whose U-Pb zircon age is 227 ± 2.8 Ma (Wang, 2012). They intruded into the Tuanshanzi (TSZ) Formation (332°∠35°), whose age is between ~1625 and ~1671 Ma (Li et al., 2011). The lower horizon of the TSZ Formation is greyish-black dolomite and the upper is purple silty micrite dolomite. Rock magnetic results indicate that the dominate remanence-carrier of the sills is magnetite, while that of the purple silty micrite dolomite is hematite. Stepwise thermal demagnetization on 101 samples was finished using ASC TD-48 furnace in 5-25°C steps up to 580°C or 680°C. All remanence measurements were finished on the 2G 755-4K superconducting magnetometer in Paleomagnetism and Environmental Magnetism Laboratory, China University of Geosciences (Beijing). Most samples could isolate a stable component that decayed towards the origin after heated above 350°C. A pair of character remanent magnetization (ChRM) direction, confronting with each other, has been isolated from the sill samples. On site mean, one group (Ds=293.5°, Is=-78.6°, N=6) is identified as reversed; while the other group (Ds=113.0°, Is=83.3°, N=3) is regarded as positive. The result passed the McFadden & McElhinny (1990) reversal test on B grade at 95% confidence level (γ0=0.5017 < γcritical=0.5341). In addition, the results of the sills and the adjacent rocks passed the baked contact tests. So the ChRM may be obtained during the cooling of the magma. The paleomagnetic pole of this study (28.2°N, 132.6°E, A95=5.3°) is different from that of the Triassic stratum (60.5°N, 4.1°E, A95=2.7°) of the NCB. This may be caused by tectonic movements. Before the intrusion, the attitude of the stratum was 150°∠54.5°, calculated from the direction of ChRM. This implies that a fold may have happened. This fold may extend in E-W direction, caused by an N-S thrust, because the direction of ChRM is perpendicular to the hinge. Then the intrusion started and the magnetic grains in the sills gained a stable thermal remanent magnetization. At this time, the study area was located at the southern wing of the fold, with a stratum attitude of 150°∠54.5°. But with the development of the thrust, the study area evolved to be the northern wing, with a stratum attitude of 332°∠35°. Further study is still needed to constrain the time of the tectonic deformation changing. References Li H K, Su W B, Zhou H Y, et al., 2011. Earth Sci. Front. 18(3): 108-120. McFadden P L and McElhinny M W, 1990. Geophys. J. Int. 103(3): 725-729. Wang Y, 2012. Terra Nova. in press.