Paleomagnetical Research On Lhasa Block Of Early Cretaceous Of Langjiu Formation

Lhasa block has always been critically vital and significant to the study of the evolution, dynamics and other problems of the Tibet Plateau. It is located in the most southern edge of Eurasian land, with Qiangtang Massif on its north and India plate on its south, the special spatial location of which makes Lhasa block a key to researching the scale of Neo-Tethys ocean between the Indian plate and Eurasian plate; the evolution of collision, amalgamation of the two blocks and the amount of Intra-continental crustal shortening of Eurasian block. There are fewer Cretaceous data available for the Lhasa block, especially scarce in the west, also the published data exists considerable disagreements and divergences.In this paper, systematical rock magnetic and paleomagnetic investigations of Cretaceous specimens of Langjiu formation in Lhasa terrain are carried out. By studying curves of isothermal remanent magnetization and demagnetization of three-orthogonal isothermal remanent magnetization, we find that hematite and magnetite are the main magnetic carrier minerals of high-temperature remanence. Eventually, we obtain that the average direction of high-temperature component of Langjiu formation in geography coordinate system Is D_g=193°,I_g=-20.5°, K_g=11.6, ?₉₅=20.5; in stratigraphic coordinate system is:D_s=181°, I_s=-30.2°, K_S=42, ?₉₅=10.5, and the corresponding paleomagnetic pole position calculated is: ?_p=76°, ?_p=256.5°, dp/dm= 11.7/6.5 ??_p refers to longitude,?_p refers to latitude?; the paleolatitude is:?= 16.2°±8.7°N. In the course of paleomagnetic research, specimens are separated into two groups, one group of specimen experiment is conducted in the institute of Geophysics, Chinese Academy of Sciences in Beijing, the other group is experimented in the institute of Earth Environment, Chinese Academy of Sciences in Xi'an. Experimental data analysis methods which are adopted mainly include the method of major component analysis and Fisher statistics. Contrasting the analysis results derived from Beijing institute and Xi'an institute, we find they are almost identical, which shows that the data obtained in the experiment process is more reliable and the accuracy is high. In addition, high-temperature characteristic remanence passed through fold test of Waston and Enkin ?1993? at the 95% confidence level, which suggests these high-temperature components isolated from Cretaceous Langjiu formation could be regarded as primary characteristic remanent magnetization.By collating and analyzing Cretaceous paleomagnetic data in Lhasa block, we find the palaeolatitude?reference point 32.2°N,80.5°N? in Lhasa block had been relatively stable, mainly revolving around 15°N,which indicates that there is no obvious north-south movement.