Paleomagnetism of Late Precambrian Keweenawan igneous and baked contact rocks from Thunder Bay district, northern Lake Superior

Abstract

A paleomagnetic study was carried out on Late Precambrian (age ^ 1.1 Ga) Keweenawan igneous and baked contact rocks from the Thunder Bay region, Northern Lake Superior. Detailed af and thermal demagnetization studies of igneous and baked contact rocks demonstrate that the Middle Keweenawan reversal remains asymmetric throughout the full range of af coercivities and blocking temperatures. Baked contact tests are always positive and the site mean paleodirections do not show streaking. Moreover, the baked rocks yield systematically less scattered paleomagnetic directions than their igneous counterparts, which demonstrates the suitability of baked rocks for paleomagnetic studies. These results show that the asymmetry of the reversal is due to original TRMs acquired at the time of cooling and thus the asymmetry of remanence directions reflects either apparent polar wandering or non-dipole disturbances during Keweenawan time rather than secondary overprints.

Remagnetization in Keweenawan rocks. Part I: conglomerates

“…Both demagnetization procedures indicate single component behaviour and thus a stable remanence direction. Pebbles adjacent to the flow contact (within 1-2 cm) when subjected to AF and thermal demagnetization also show the same result both in degree of stability and in the remanence direction (Pesonen 1978). The same component also completely dominates the remanence in pebbles further from the contact out to a distance of 1 m, and is present in clasts 1-5 m from the contact (Fig.…”

“…(1) Baked contact tests for Keweenawan normal and reversed dikes showing stable, asymmetric directions are fully positive (Pesonen 1978(Pesonen , 1979.…”

“…For sites 1, 2, and 3A, the histograms are generally flat for samples "with no C R M thus indicating random directions, whereas for site 4 at Mamainse Point there is a definite bias toward low P values in samples supposedly uncontaminated by CRM on the basis of their stability. The reason for this behaviour (and also the general lack of 0 values above 5") is that of all the conglomerate samples those at site 4 were generally more resistant to AF demagnetization (Table 3) after AF treatment exhibited more pronounced swinging behaviour after thermal demagnetization (Pesonen 1978). The "thermal" swings were generally in the opposite sense to those produced by AF means and were directed away from the overprint direction.…”