Abstract

Two anti-parallel remanence directions have been isolated from samples of the Umfraville Gabbro situated in the southern part of the Grenville Province, Ontario. The magnetization yields a pole position at 160°E, graphic, A95 = 9.0°. Because of the dispersion of remanence directions, however, individual site poles plot over much of the Grenville Track. Regardless of direction and polarity of remanence, all samples unblock within a narrow temperature range of 525–560 °C. It is inferred that the magnetization process has involved slow cooling with both TRM and TCRM components being generated. Although one sample of zircon extracted from a genetically related syenite probably provides an estimate of the time of emplacement of 1160 Ma, the magnetization may be more closely related to the K–Ar age. Whole rock K–Ar ages range from 873 Ma to 1130 Ma, but all data points lie close to a 911 Ma isochron, with an initial 40Ar/36Ar ratio of 805. The isochron age is in close agreement with ages of 36Ar-free samples.

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