Paleomagnetism of the Paleoproterozoic hematitic breccia and paleosol at Ville-Marie, Quebec: further evidence for the low paleolatitude of Huronian glaciation

摘要

The paleomagnetism of a saprolitic paleosol developed on Archean granite and of an overlying hematitic breccia in the basal Lorrain Formation of the Cobalt Group (Huronian Supergroup, approx 2.4-2.3 Ga) near Ville-Marie, Quebec, has been investigated to further constrain the paleolatitude of the Lorrain Formation and the subjacent glaciogenic Gowganda Formation. Only the breccia, whose hematite may be a product of weathering prior to or shortly after deposition, yielded useable results with stepwise thermal demagnetization. A stable component A (maximum unblocking temperature, T_(ub), of approx 675 deg C) carried by hematite is directed shallowly to the east-northeast (D = 59.5 deg, I = 2.4 deg, #alpha#_(95) = 6.7 deg, n = 47, dip-corrected), and a less stable component B (maximum T_(ub) of approx 580 deg C) carried by magnetite is directed steeply down to the northeast (D = 52.1 deg, I = 70.3 deg, #alpha#_(95) = 2.8 deg, n = 75, dip-corrected). Component A is ascribed to a chemical remanent magnetization (CRM) acquired when the hematitic breccia was deposited or soon thereafter. Component B is interpreted as a thermochemical overprint related to the Penokean Orogeny at 1.9-1.8 Ga. Dip-corrected specimen mean directions give the pole positions at latitude = 57.1 deg N, longitude = 338.3 deg E (dp = 4.1 deg, dm = 4.8 deg) for component B, and latitude = 21.1 deg S, longitude = 213.1 deg E (dp = 3.3 deg, dm = 6.7 deg) for component A. The more easterly directions for components A and B relative to comparable early and overprint components previously determined for the Lorrain and Gowganda formations at Elliot Lake and Desbarats 260-360 km southwest of Ville-Marie imply relative rotation of approx 30 deg or more about a vertical axis within the Southern Province since the Penokean Orogeny. The results suggest that the hematitic breccia and the Ville-Marie paleosol formed at a paleolatitude of 1.2 deg +- 3.4 deg, in accord with our earlier inference of equatorial paleolatitudes for the Lorrain and Gowgands formations and Huronian glaciation. Paleomagnetic data from several continents suggest that ferruginous weathering horizons formed over a wide range of paleolatitudes in the Proterozoic.