Paleomagnetic Study Of The Upper Devonian And Lower Carboniferous Carbonates Near JiangYou, Sichuan,South China Block

We have obtained new late Devonain paleomagnetic pole for Yangze Block. A total of 472 paleomagnetic samples were collected from the upper Devonain carbonates from the Jiangyou section in Sichuan province.During thermal cleaning ,intensity of most samples increased progressively(below 300℃) and decayed rapidly (beyond 300℃) till to the noise level of magnetometer at about 500℃.Two magnetic components can be separated from most of samples.The low temperature component(100～300℃) is consistent with the present geomagnetic field. The middle temperature component(300～480℃) is characterized by deep reversed inclination(D=210.20,I=-80.40,κs=27.5,α95=7.40) The high temperature(420℃～520℃) component(D=49.60,I=44.40,κs=33.3,α95=8.00)isolated from 62 samples passes fold test and reverse test,yielding a pole position(46.50N,188.30E,dp=6.30,dm=10.10)different from the other poles for Yangze Block.It is therefore suggested that the high temperature was probaly primary magnetization and the Yangze Block was situated at middle latitudes in the northern Hemisphere in the late Devonian.The paleomagnetic investigation has been carried out on the Lower carboniferous limestone in the hope of identifying the early carboniferous paleomagnetic pole. In general, two magnetic components can be isolated after the removal of a soft viscous overprint by temperature step 100℃ from all samples studied.The low temperature (100~300℃) magnetic component is close to the present day Earth field and fails fold test , which corresponds to a recent overprint. The reversed middle temperature (300~480℃) component (Ds=216.10,Is=-75.50, κs=31.8, α95=4.70) passes fold test and yields a pole position(309.90E,51.20S, dp=7.9,dm=8.6, paleolatitude:62.7),which is completely consistent with MTC revealed from the Upper Devonian limestone in the aera.On the basis of paleomagnetic data and geological evidences , we suggest that the reversed middle temperature component is probably acquired at the time of the Early Jurassic folding in the west part of South China Block.Rock magnetic studies suggest magnetite presenting as pseudo-single,multidomain and little single domain,carrying the middle(300℃～420℃)and high(420℃～530℃) temperature components respectively, is the main magnetic mineral in the upper Devonian limestone.However,the middle temperature(300℃～480℃) is carried by the mixture of pseudo-single and multidomain magnetite which is also the main magnetic carrier in the lower Carboniferous limestone. Based on the principle that the depositional remanent magnetization(DRM) is identical to the direction of primary geomagnetic field, reading confirmed polar data from paleomagnetic database, the spatial occurence of buried layers can be calculated.