The Archean Slave craton in northwestern Canada is an ideal natural
laboratory for investigating Archean lithospheric formation and subsequent
evolution, and has become an international focus of broad geoscientific
investigation following the discovery of economic diamondiferous kimberlite
pipes in the centre of the craton during the early 1990s. The first
diamond mine, BHP's Ekati mine, went into production in October 1998, and
Diavik's kimberlites on their property are expected to be mined by 2003,
closely followed by DeBeers' Snap Lake kimberlite dyke. Given such intensity
of geoscientific scrutiny, the Slave craton was chosen for more detailed
geophysical imaging using Polaris both to contribute to studies of Archean
lithospheric formation and to obtain baseline geophysical data against
which other prospective diamond-producing areas could be compared.
A reconnaisance teleseismic study by Bank et al. (Tectonophysics, 2000)
shows tantalizing differences in SKS directions between the northern, central,
and southern sites. Also, there are differences in slowness of the lithospheric
mantle, with a fast "core" beneath the cental Slave region enveloped by
a slow region. Deep-probing magnetotelluric surveys conducted since 1996
revealed an unexpected and remarkable anomaly in electrical conductivity,
collocated with the Lac de Gras kimberlite field, which is modeled as a
spatially-confined upper-mantle region of extremely low resistivity (<30
?.m) at depths of 80-120 km. This anomalous upper mantle region is spatially
coincident with the above fast seismic core and a geochemically-defined
ultra-depleted harzburgitic layer interpreted as oceanic or arc-related
lithosphere emplaced during early tectonism.
These mantle images obtained from two geophysical techniques imply
a north-south division of the Slave's lithospheric mantle into three roughly
NE-SW-striking domains, consist with Grutter et al.'s (1999) proposal based
on G10 garnet populations. They are not consistent with the dominent east-west
division of the Slave's crust into an eastern arc terrane and a western
proto-Slave basement complex, and thus are likely unrelated to the ca.
2690 Ma north-south suturing of these two. The tectonic processes that
emplaced this geophysical-geochemical body are more likely related to the
subduction of a craton of unknown provenance to the south during 2630-2620
Ma. The Polaris array will provide high resolution geophysical images of
the region beneath the Lac de Gras kimberlite field and the mantle domains
to the north and south.
Bank, C.-G., Bostock, M.G., Ellis, R.M., and Cassidy, J.F., 2000. A
reconnaissance teleseismic study of the upper mantle and transition zone
beneath the Archean Slave craton in NW Canada: Tectonophysics, v. 319,
151-166.
Grütter, H.S., D.B. Apter and J. Kong, 1999. Crust-mantle coupling;
evidence from mantle-derived xenocrystic garnets. Contributed paper at:
The 7th International Kimberlite Conference Proceeding, J.B. Dawson Volume,
1, 307-31..
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