Abstract

The remanent magnetism of anorthositic rocks from the Harp Lake Complex consists of two antiparallel components, M and H, defined on the basis of their coercivities. M has remanent coercivities generally in the range 100–1000 Oe (7.96 × 103–7.96 × 103 A/m) but sometimes exceeding 2000 Oe (159 × 103 A/m), and is carried by magnetite and hematite, the former being the predominant contributor. H has remanent coercivities in excess of 2000 Oe (159 × 103 A/m), and is carried by hematite probably containing some ilmenite in solid-solution. The mean direction, irrespective of sign, based on 110 samples from 24 collecting sites is 270°, + 01° (α95 = 6°) with a pole 02°N, 154°W (A95 = 4°). This is considered to be the average direction of the geomagnetic field during initial cooling following intrusion of the complex at about 1450 Ma. At this time the complex was at great depth. The Harp dikes, which intrude the complex, have steeper inclinations, (263°, + 44°, α95 = 7°, pole 19°N, 132°W). The country rocks in the thermal aureole have directions roughly similar to those of the complex, but deflected towards the directions in the dikes (272°, + 29°,α95 = 10°, pole 14°N, 144°W). It is suggested that the country rocks were magnetized during final uplift of the area but prior to the intrusion of the Harp dikes. These and other paleomagnetic poles for the interval 1300–1500 Ma fall in the central Pacific indicating that Laurentia was in low latitudes. Previous reviewers have connected these poles by a simple polar loop, but these new observations indicate that the polar path may be more complicated, and a doubly looped path is suggested as a means of reconciling the results. There are about 20 accurately determined pole positions for the interval 1500–1200 Ma from North America, but only 4 from the rest of the world, and it is not yet possible to determine from such an unbalanced body of data the positions of other Precambrian shields relative to North America during this time interval.

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