Paleomagnetism, anisotropy of magnetic susceptibility, and argon–argon geochronology of the Clearwater anorthosite, Saskatchewan, Canada

Abstract

The Clearwater anorthosite in Saskatchewan is a major gabbro–anorthosite layered intrusion lying within the Archean Rae Province along the northwestern hinterland of the Paleoproterozoic Trans-Hudson Orogen (THO). The intrusion, which could be at least 2.4 Ga old, carries a well-defined remanent magnetization with a direction of Declination = 112° and Inclination = +38° ( k=316, α 95=3.1°, N=8). The corresponding paleomagnetic pole (Plat. = 6.5°N, Plong. = 48.2°W, dp = 2.2°, dm = 3.7°) is the first one obtained from the Archean Western Granulite Domain of the Rae Province. Field observations, supplemented by anisotropy of magnetic susceptibility data, show that the magnetization post-dates ductile deformation of the anorthosite. Argon–argon plateau dates of 1917±7 Ma for hornblende and 1839±5 Ma for biotite are interpreted as cooling ages, with the hornblende age approximating the time of remanence. The pole position, and others of comparable age from the northwestern margin of the THO form a loose cluster, the mean of which is about 90° of arc away from that of a broadly contemporaneous group of poles from the Superior Province. This relation supports previous interpretations of paleomagnetic data that, prior to the THO at about 1.9 Ga ago, a major ocean (the Manikewan ocean), at least 5000 km across, separated the Superior and Rae/Hearne Provinces.