PALEOMAGNETISM OF THE BAYAN-GOL FORMATION, WESTERN MONGOLIA

Oriented samples of the Lower Cambrian Bayan Gol Formation from Salaan y Gel, Mongolia, were collected at roughly 5 m stratigraphic intervals for palaeomagnetic analysis. Progressive alternating-field and therma l demagnetization isolated two magnetic components: a present-field ov erprint, typically removed by 10 mT fields and similar to 200 degrees C heating; and a high-coercivity, high-unblocking-temperature (550-600 degrees C), predominantly single-polarity component that was imparted to the rocks prior to early or middle Palaeozoic deformation. Single- polarity magnetization at Salaany Gol contrasts with results from Lowe r Cambrian rocks on the Siberian platform, previously considered corre lative with the Bayan Gol Formation, which show a prominent change in polarity bias near the top of the Tommotian Stage. Two hypotheses can explain this discrepancy. First, the entire Bayan Gol Formation may co rrelate with the predominantly reversely polarized, lower half of the Tommotian Stage in Siberia. This model is consistent with plausible in terpretations of delta(13)C profiles for the Zavkhan basin and the Sib erian platform. Alternatively, the characteristic magnetic direction f rom our samples may be a pre-fold overprint. If post-accretionary, the n comparison with Siberian palaeomagnetic results suggest a Silurian-D evonian remagnetization age, and existing bio-and chemostratigraphic c orrelations provide the most reliable spatial and temporal links betwe en the Zavkhan basin and the Siberian platform. If the observed magnet ic directions are primary or an immediate overprint then they may be u sed to constrain the early Cambrian palaeogeography of the Zavkhan bas in and the Palaeo-Pacific Ocean. Mean inclination of 62+/-4 degrees co rresponds to a palaeolatitude of 44+/-5 degrees, several thousand kilo metres from the equatorial Siberian craton.