Abstract

The Big Spruce alkaline complex has yielded a Rb–Sr isochron of 2111 ± 40 m.y. The rocks are unmetamorphosed, but the local geochronological evidence indicates that they have had a complicated thermal history. The complex has four magnetizations, which can be separated by a two-stage demagnetization procedure. The D magnetization (19 sites; 022, +88; α95 = 8°; pole 67 °N 113 °W) is considered to have been acquired at the time of emplacement because it generally has the highest blocking temperatures (500 to 650 °C). The X, Y, and Z magnetizations are believed to be later overprints, as they generally have lower blocking temperatures and have directions that are comparable to those found in younger rocks in the interval of approximately 2000 to 1400 m.y. Most of the overprints seem to have been acquired at times of mild reheating during this interval.

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