Paleomagnetism of Devonian sedimentary rocks and mafic sills from the Meguma Terrane, Nova Scotia.

Abstract

Paleomagnetic data were obtained from the Lower-Middle Devonian Torbrook Formation (7 sites) which consists of fossiliferous shale, siltstone and quartzite and mafic intrusions (10 sites) composed mainly of gabbro sills. Some but not all the sills intrude the Torbrook Formation. Thus, some of these intrusives are of Silurian age while others are Late Devonian. The natural remanent magnetization site-mean directions are scattered but discordant from the present Earth's field for both the sedimentary rocks and the sills. Stepwise alternating field demagnetization up to 100mT and thermal treatment up to 625°C for the Torbrook Formation revealed the presence of a high unblocking temperature (400°-550°C) and an intermediate coercive force (20-50mT) direction (tilt corrected) at 342°, -10° with a corresponding paleopole at 38°N, 138°E. An additional direction (in situ) is observed at 26°, -72° with lower or overlapping unblocking temperature and coercivity spectra. This appears to be a post-folding magnetization and the corresponding ill-defined pole is 14°S, 101°E. The mafic sills yielded the following directions of magnetization with corresponding poles: a) 6 of the 10 sites gave 323°, -65°[139°E, 7°S], b) 4 sites with a component at 96°, -25°[150°W, 13°N], c) 9 sites with a component at 322°, -37°[152°E, 16°N] and with closely overlapping unblocking temperature and coercivity spectra. The age of intrusion of six sills is probably Early Silurian; their site-mean tilt corrected direction and paleopole are 320°, +17°[175°E, 38°N]. The age of intrusion of the other sills ranges from Silurian to Late Devonian. The pole positions are compared with those of the same age from Laurentia, Gondwana and the Avalon Terrane. During the Silurian period, the Meguma and Avalon zones were apparently not part of the same landmass. By Early-Middle Devonian time, both the Meguma Terrane and Avalon Zone had docked unto the North American craton. The dissimilarity in the apparent polar wander paths and paleolatitudes of Africa and the Meguma Terrane does not support a North African provenance for the Meguma Zone.