Magnetism of South Mongolian Middle Paleozoic ophiolites

Abstract

This paper presents new paleomagnetic data from South Mongolia, Middle Paleozoic rocks were studied from three different tectonic zones within the South Mongolian fold belt. From north to south these are the Gobi-Altai zone (in terms of paleotectonic position, the ‘north’ shelf of the paleobasin), the Trans-Altai zone (ophiolite complexes) and the South Gobia zone (the ‘south’ shelf). Several paleomagnetic field tests, including conglomerate, baked contact and fold tests, were carried out to determine magnetic stability. In addition, principal component analysis of the stepwise thermal demagnetization data was used. The ophiolites are characterized by three ancient components. One magnetic component is pre-folding and directed to the south with a very gentle inclination (declination ( D) = 180–210°, inclination ( I) = −6 ⋯ +2°, K = 120). Two post-folding magnetic components were observed also. The first post-folding magnetic component has D = 282.4°, I = −36.4°, k = 30.3, and the second has D = 318.6°, I = 33.0°, k = 60.5. The final magnetic component is very close to the Permian paleomagnetic direction expected from the North China platform. The first post-folding component of magnetization may have been acquired during the Variscan accretion in Early Carboniferous time when the basin was closed. The South Mongolian paleo-ocean was located in equatorial latitudes near the Tarim block, the South China platform and probably the North China platform in the Devonian.