The following abstract is from a talk presented at the 31st Yellowknife Geoscience Forum November 2003.

The Duck Lake Intrusive Sheet (Gibbins, 1987), or "Duck Lake sill", is a prominent, shallowdipping, mafic intrusion in folded Burwash Formation strata to the east of Yellowknife Bay. It can be followed from the Yellowknife River to Duck Lake, in places forming a prominent westfacing escarpment. Similar mafic intrusive rocks may extend further south to Ruth Island. Crude columnar jointing and a weak parting in basal mafic-ultramafic rocks, as well as the dip of the upper contact, indicate that the sill is dipping shallowly to the east at about 05° to 35°.

The outcrop location of the sill along the Ingraham Trail and its moderate easterly dip suggest that the intrusion can be projected into the subsurface as the "Y1" bright reflector on the SNORCLE seismic reflection profile (Cook et al., 1999). Indeed, impedance contrasts between the gabbroic rocks of the sill and surrounding metaturbidites are among the highest measured between rock units along the SNORCLE profile. The "Y1" reflector corresponds to a positive seismic velocity anomaly (D. Snyer, pers. comm.), further strengthening correlation with the Duck Lake sill rather than with a putative shallow dipping fault for which there is no independent evidence (cf. Cook et al., 1999). This case study prompts the question whether some other bright reflections on the SNORCLE profile may also represent mafic intrusions.

The sill is differentiated with a melanocratic gabbro base and a leucogabbro upper zone (see also Gibbins, 1987, and references therein). Interstitial quartz is present in the leucogabbro along the upper contact of the sill and is also present in fine-grained granophyric intergrowths between larger pyroxene and plagioclase crystals. Abundant brown baddeleyite and lesser amounts of skeletal zircon were recovered from this coarse-grained leucogabbro near the upper contact of the sill.

Two zircon and four baddeleyite single crystals were analyzed. A best-fit line through all the data points yields an upper intercept age of 2186+4/-2 Ma (probability of fit: 2%). This age suggests a temporal relationship with the ca. 2188 Ma Dogrib dyke swarm (Lecheminant et al., 1997). However, a closer inspection of the data suggests that the Duck Lake sill may be somewhat younger and that its age is best estimated by the weighted mean 207Pb/206Pb age of the five, near-concordant data points (4 baddeleyites and 1 zircon) at 2181+2 Ma (MSWD: 3.4). The latter age estimate includes two abraded, concordant, baddeleyite analyses with a weighted mean age 2180.0+1.1 (MSWD: 0.35).

The preferred age of the Duck Lake sill (2181+2 Ma) is similar to, or indistinguishable from, the age of several alkaline complexes in the southwestern Slave, e.g. at Blatchford Lake, Big Spruce Lake, and Squalus Lake. The Squalus Lake intrusion, in particular, has been dated at 2181+2 Ma (M. Villeneuve, unpubl. Data). The 2181+2 Ma Duck Lake sill, the somewhat older ca. 2188 Ma Dogrib dyke swarm, and the alkaline complexes probably relate to the progressive break-out of the Slave craton from its parental landmass, the late Archean Sclavia supercraton (Bleeker, 2003). In general, multiple precisely-dated mafic dyke swarms and short-lived magmatic events provide a "bar code" (i.e., in the sense of a time line punctuated by precisely-dated events) that characterize fragments of ancient crust such as the Slave craton (e.g., Bleeker, 2003). Establishing such "bar codes" for each of the ca. 35 cratonic fragments around the globe, combined with high-precision paleomagnetic data, is the most efficient way to identify correlations among the preserved Archean cratons.

Bleeker, W., 2003. The late Archean record: a puzzle in ca. 35 pieces. Lithos, in press.

Cook, F.A., van der Velden, A.J., Hall, K.W., and Roberts, B.J., 1999. Frozen subduction in Canada's Northwest Territories: Lithoprobe deep lithospheric reflection profiling of the western Canadian Shield. Tectonics, vol. 18, p. 1-24.

Gibbins, W., 1987. The Duck Lake Intrusive Sheet. In: W.A. Padgham (editor), Yellowknife Guide Book, Geological Association of Canada, p. 135-137.

LeCheminant, A.N., Buchan, K.L., and van Breemen, O., 1997. Paleoproterozoic continental break-up and reassembly: evidence from 2.19 Ga diabase dyke swarms in the Slave and western Churchill provinces, Canada. Geological Association of Canada, Abrstract Volume 22, p. A86.