Late Paleozoic geodynamic evolution of the Junggar Basin (Northwestern China) : insights from new paleomagnetic results

The continental or oceanic nature and the geographic extent of the Junggar basement remain controversial; nevertheless this area presents special interests in order to better understand the Paleozoic tectonic framework of Central Asia. A paleomagnetic study has been performed on Late Paleozoic rocks around Junggar Basin in order to check if it can be regarded as a rigid block at the end of the Paleozoic. 158 sedimentary and volcano-sedimentary rock cores have been collected on 25 sites. Among them, 11 sites located in the southern border of the basin, in Bogda Shan near Urumqi, consist of Permian red sandstone, and 14 sites of Early Carboniferous grey sandstone come from the West Junggar Mountains, near Karamay. Magnetite is the main carrier for magnetic signal in this collection. In Permian red beds of Bogda Shan area, the characteristic components of P1 and P2 show a solo reversed polarity and passed a positive regional fold test, the magnetization can be therefore considered as primary. Two new paleomagnetic poles have been consequently calculated for the periods of P1 at 79.5°N, 36.6°E and of P2 at 60.4°N, 4.7°E with a paleolatitude at ~50 and 40°N, respectively. The Early Carboniferous sites are characterized by reverse polarity, and show a negative fold test indicating post-folding secondary magnetization. Thus, according to the spatial distribution of the sampling sites with respect to Late Carboniferous-Early Permian granitic plutons and Late Permian mafic dykes, two subgroups can be distinguished. The remagnetized sites, related to Late Carboniferous - Early Permian granite emplacement, yield a paleomagnetic pole at 65.3°N, 329.8°E, and the sites located at the vicinity of Late Permian mafic dykes present a paleomagnetic pole at 64.8°N, 179.4°E with relatively lower paleolatitudes at ~30 to 35°N, respectively. Our paleomagnetic data from sections near Urumqi, and previous ones as well, show stationary and coherent poles for South Junggar area during the Carboniferous and Permian. These poles are close to the Present. However, paleomagnetic data from NW Junggar indicate a significant anticlockwise rotation between the Late Carboniferous-Early Permian and the Late Permian. It seems that paleolatitudes of NW Junggar rocks are similar to those of NE Kazakhstan, with meaningful differences in declination until the Late Permian, and thereafter become consistent. Thus, the South and West Junggar areas probably experienced contrasting tectonic evolutions, and the Present Junggar Basin cannot be considered as a rigid block during the Late Paleozoic. The meaningful differences in both latitude and rotation of Late Permian poles between South and West Junggar may argue for the importance of post-Permian tectonics (e.g. Triassic to Cenozoic) in Central Asia.