Acasta Gneiss Complex

Unit Name: Acasta Gneiss Complex
Unit Type: Lithodemic
Rank: Complex
Status: Informal
Usage: Currently in use
Age Interval: Hadean - Archean (4567.17 - 2500 ma)
Age Justification: Geochronology and stratigraphic relations. The penetrative deformation of these various plutonic rocks is believed to be related to Late Archean events. Zircon crystals from tonalite gneisses of the Acasta Gneiss Complex have been dated using the SHRIMP and yield ages up to 4.03 Ga, making them the oldest known terrestrial rocks (Bowring and Williams, 1999; Bowring et al., 1989; Stern and Bleeker, 1998). They are in turn cut by intrusive rocks at 3.75, 3.6, 3.4, and 2.9 Ga (Bleeker and Stern, 1997) (Rayner et al., 2005). Mapping of the Central Slave Cover Group into the Acasta River area, with the fuchsitic quartzite unconformably overlying the Acasta gneisses, firmly places Earth's oldest rocks within the old cratonic nucleus of the Slave Province prior to deposition of the ca. 2.8 Ga quartzite units (Bleeker et al., 2000).
Province/Territory: Northwest Territories

Originator: Bowring et al., 1989.

Distribution:

The Acasta Gneiss Complex (AGC) comprises basment rocks in the westernmost Slave Province (Sanborn et al., 2000). It lies at the western margin of the Archean Slave Province, Canada, about 300 km north of Yellowknife. This area of Archean rocks, several hundred square kilometres in extent, is exposed in a set of structural culminations within the foreland fold and thrust belt of the Paleoproterozoic Wopmay Orogen (King, 1986; St-Onge et al., 1988) (Stern and Bleeker, 1998).

Lithology:

The Acasta Gneiss Complex comprises a heterogeneous assemblage of foliated to gneissic tonalites, granodiorites, and granites, together with minor quartz-diorites, diorites, gabbros, and ultramafic rocks (Bowring et al., 1990) (Iizuka et al., 2006) with each lithology representing a separate phase of plutonism (Rayner et al., 2005).

Relationship:

The oldest gneiss units of the Acasta Gneiss Complex are cut by several generations of pegmatite and granite dykes (Sanborn et al., 2000). Quartzite and banded iron-formation of the Slave Cover Group unconformably overlay the Acasta Gneiss Complex (Bleeker et al., 2000).

History:

The Acasta Gneiss Complex (King, 1985) was first recognized during a regional mapping program carried out by the Geological Survey of Canada (King, 1986; St-Onge et al., 1988) (Stern and Bleeker, 1998). It was informally termed the Acasta gneisses in Bowring et al. (1989) before being renamed the Acasta Gneiss Complex.

References:

Bleeker, W. and Stern, R.A., 1997. The Acasta gneisses: an imperfect sample of the Earth¿s oldest crust. In: Cook, F., Erdmer, P. (eds), Slave-Northern Cordillera lithospheric evolution (SNORCLE) transect and cordilleran tectonics workshop meeting, 7-9 March; Lithoprobe Report, vol 56, Calgary, pp 32-35.

Bleeker, W., Stern, R., and Sircombe, K., 2000. Why the Slave Province, Northwest Territories, got a little bigger; Geological Survey of Canada, Current Research (Online), no. 2000-C2, 9 pages.

Bowring S.A. and Williams I.S., 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada; Contributions to Mineralogy and Petrology, vol. 134, p. 3-16.

Bowring, S.A., Housh, T.B., and Isachsen, C.E., 1990. The Acasta gneisses: Remnant of Earth's early crust, in Newsom, H.E., and Jones, J.H., eds., Origin of the Earth: New York, Oxford University Press, p. 319-343.

Bowring, S.A., Williams, I.S., and Compston, W., 1989. 3.96 Ga gneisses from the Slave province, Northwest Territories, Canada; Geology, v. 17, p. 971-975.

Iizuka, T., Horie, K., Komiya, T., Maruyama, S., Hirata, T., Hidaka, H., and Windley, B.F. 2006. 4.2 Ga zircon xenocryst in an Acasta gneiss from northwestern Canada: Evidence for early continental crust; Geology, v. 34, p. 245-248.

King, J.E., 1985. Structure of the metamophic-internal zone, Northern Wopmay Orogen, Northwest Territories, Canada; Queens University, Kingston, ON, PhD thesis, 208 p.

King, J.E., 1986. The metamporhic internal zone of Wopmay Orogen (Early Proterozoic), Canada: 30 km of structural relief in a composite section based on plunge projection; Tectonics, v. 5, p. 973-994.

Rayner, N., Stern, R.A., and Carr, S.D., 2005. Grain-scale variations in trace element composition of fluid-altered zircon, Acasta Gneiss Complex, northwestern Canada; Contributions To Mineralogy and Petrology, vol. 148, no. 6, p. 721-734.

Sanborn, N., Stern, R., Desgreniers, S., and Botton, G.A., 2000. Microstructure of Neoarchean zircon from the Acasta gneiss complex, Northwest Territories; Geological Survey of Canada, Current Research 2000-F3; Radiogenic Age and Isotopic Studie: Report 13, 12 p.

St-Onge, M.R., King, J.E., and Lalonde, A.E., 1988. Geology, east-central Wopmay Orogen, District of Mackenzie, Northwest Territories; Geological Survey of Canada, Open File Report 1923, 3 sheets, scale 1:125,000.

Stern R.A. and Bleeker W., 1998. Age of the world's oldest rocks refined using Canada's SHRIMP: the Acasta Gneiss Complex, Northwest Territories, Canada; Geoscience Canada, vol. 25, no. 1, p. 27-31.

Source: GEOLOGICAL SURVEY OF CANADA
Contributor: Michael Pashulka
Entry Reviewed: No
Name Set: Lithostratigraphic Lexicon
LastChange: 29 Nov 2010