Paleogeology

Paleogeology, Paleoclimate, in relation to Evolution of Life on Earth

Acasta Gneiss Complex

Cleaned exposures of the Acasta gneisses at their discovery site. Ancient tonalites (4.03 Ga) occur on left side of the picture, and are intruded by highly deformed younger granite sheets and mafic dykes. Courtesy of Natural Resources Canada The Acasta Gneiss Complex is located in Canada's northwestern Slave Province, NWT. (more images at Geomorphology, lithology, geological history of the Slave craton.)

Dating from the Hadean at least 4.03-4.055 Ga, is Earth's oldest known crustal rock outcrop. The complex comprises mostly Gray gneiss (granodioritic gneiss), White gneiss (tonalitic to granitic gneiss), foliated granite (3.6 Ga), and many aplite and basaltic intrusions.


The gneisses are embedded in a large Mesoarchean-Hadean basement complex, which lies beneath the west-central Slave craton. This craton is a complex containing ancient collisional orogenic structures and accreted fragments. Its rock formations comprise ancient crust, juvenile arcs, mature arcs, and intervening accretionary prism material. All tectonostratigraphic units are cut by strike-slip faults and later-emplaced granitic plutons, and many strata show evidence of late extensional collapse structures. Most rock types are derived from basaltic protoliths. The basement complex is overlain by Neoarchean supracrustal sequences has plutonic intrusions dating from 2.72-2.58 Ga. (Synvolcanic plutons date from 2.72-2.67 Ga and granitic batholiths date from 2.59-2.58 Ga (late-orogenic).)

Zircon geochronology has demonstrated that at least four magmatic or metamorphic events occurred in the Acasta Gneiss Complex: 4.0~3.95 Ga, 3.75 Ga, 3.6 Ga and 3.4 Ga. Zircon from a White gneiss has revealed an age of 4,203 +/- 28Myr, implying that granitic continental crust was more widespread than previously thought, and that it was reworked into Early Archean continental crust.[r]

Weathering of the Acasta Gneisses indicates that they have been exposed by erosion for a considerable time. In other areas, quartzite, banded iron formations, and volcanic rocks sit unconformably atop the gneisses.

The Archean gneisses were united at the core of the Slave protocontinent, at least 2.9 Ga. In the nucleus of the old continent, juxtaposed rocks sometimes differ by a billion years in age, probably indicating episodic volcanic eruptions, fed by broad plumes of rock ascending periodically from the deep mantle, rather than having resulted from gradual tectonic accretion of crust at plate boundaries. Such lava flows presumably gradually built up the continental nuclei as a part of mantle mafic-ultramafic and crustal acid magmatism.

The Archean rocks subsequently experienced uplift, possibly because of a hot mantle plume. Elevation of the continent caused erosion, creating the unconformity. Two volcanic layers above the unconformity date to a little over 2.8 Ga. Subsequently the plume dissipated, and the region sank beneath an ancient ocean, accumulating sediments: quartz-rich sandstone, and then the banded iron formations (Yellowknife Supergroup). The volcanics accumulated as the Slave protocontinent was rifted apart about 2.8-2.7 Ga.

links: formation: Acasta Gneisses, 2; Acasta Gneisses; AG complex; hand-specimen: Acasta Gneiss, 1, 2, 3; Tonalite gneiss; Acasta Gneiss, 2; close-up: Acasta Gneiss; sem: Acasta Gneiss - sem; article (1999); Acasta gneiss and another old zircon; abstract, 2 pdf; article; news; Microstructure of Neoarchean zircon from the Acasta gneiss complex ... (pdf):

image of Acasta Gneisses courtesy of Natural Resources Canada (source website, Figures)

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