Magnetostratigraphic constraints on the Gondwanan origin of North China: Cambrian/Ordovician boundary results

Abstract

A significant gap in the middle Palaeozoic apparent polar wander (APW) path precludes polarity definition of the early Palaeozoic palaeopole for North China. This problem can in part be resolved by the intercontinental correlation of magnetic polarity patterns across small time intervals. A magnetostratigraphic study was carried out on upper Cambrian to lower Ordovician sediments near Zhaogezhuang (long. 118.5°E, lat. 39.7°N), North China. After stepwise thermal or thermal and alternating field demagnetizations, a characteristic magnetic component with normal and antipodal reversed directions was identified. These data, drawn from 49 samples, yield a north palaeopole at long. 294.6°E, lat. 32.9°N (dp = 3.0°, dm = 5.3°). A concordant magnetic polarity pattern around the Cambrian-Ordovician boundary and lowest Ordovician obtained from different continents favours a Southern Hemisphere origin (∼17°) formation site. Using the Cambrian-Ordovician APW paths between North China and Gondwana, we suggest that the North China block (NCB) was part of Gondwana during the Cambro-lowest Ordovician, and started breaking away from Gondwana in the lower Ordovician. This finding is contrary to some palaeomagnetic models where the NCB was quite separate from Gondwana in the late Proterozoic, and was attached to the 'Pacific' side of Antarctica.