KeywordsGeochronology Oldest rocks Zircon
The Acasta Gneisses are the oldest known rocks on the surface of the Earth. They are exposed in northern Canada, north of Great Slave Lake, east of Great Bear Lake with the approximate position of 65° 10′ N and 115° 30′ W. They have a composition close to granitic and are interpreted to have formed, at least in part, from even older rocks that may be as old as 4.2 Ga.
The Acasta Gneisses are the oldest dated rocks on Earth. They are exposed in northwestern Canada (65° 10′ N and 115° 30′ W) along the western margin of the Archean Slave craton (>2.5 Ga), in the core of a north-trending fold in the foreland of the Wopmay orogen, a 2.02–1.84 Ga-old orogenic belt. The Acasta Gneisses range in age from 4.03 Ga to ca. 3.6 Ga with distinct groupings at 4.03–3.94 Ga, 3.74–3.72 Ga, and 3.66–3.58 Ga. Rocks from these three distinct groups are compositionally diverse and range from granite to quartz diorite to tonalite. Rocks have been deformed several times resulting in well-developed foliations (planar fabric present in metamorphic rocks and produced by reorientation of minerals). Lens-shaped boudins (cylinder-like structures making up a layer in a deformed rock) of serpentinized ultramafic rocks, up to several hundred meters long, occur throughout the gneisses. No ca. 4.0–3.6 Ga metasedimentary rocks have been discovered although sparse outcrops of locally tightly folded quartzite, iron formation, and pelite are found in the older gneisses. Weakly deformed, ca. 3.6 Ga-old granitic dikes cut many outcrops. During the 1.88 Ga Calderian orogeny to the west, sheets of 1.9–2.5 Ga-old rocks were thrust over the western edge of the Slave craton, resulting in a set of north-trending folds and metamorphism of underlying Archean rocks. Ar-Ar biotite and U-Pb apatite dates record complex reheating during this event at ca. 1.77 Ga.
The protoliths of the Acasta Gneiss range from granite to tonalite/diorite in composition. Their U-Pb zircon dates indicate that the oldest igneous crystallization ages are 4.03–3.96 Ga. Many zircons from all rock types contain older cores with the oldest at 4.06 and 4.2 Ga, which is consistent with the involvement of even older crust in their generation by partial melting or assimilation. In general, the geochemistry of the Acasta Gneisses is not different from other Archean and younger rocks: they are on average enriched in light rare earth elements with variable depletion in heavy rare earth elements, features that are thought to reflect the presence of garnet in the source area. Radiogenic isotope systematics in whole rocks (Sm-Nd) and zircon (Lu-Hf) are also consistent with the involvement of older continental crust. Many of the rocks have zircons with thin overgrowths likely related to metamorphism at ca 3.65 Ga, 3.6 Ga, and 3.4 Ga.
The formation and preservation of continental crust early in Earth’s history is of broad interest to Earth scientists because the oldest continental crust provides a record of magma formation and the role of water in generating granitic magmas over 4 billion years ago. The ca. 4 Ga granitoids are very similar to those formed much later in Earth’s history by plate-tectonic processes. No evidence of the late heavy bombardment is preserved in the Acasta Gneisses.
References and Further Reading
- Bowring SA, Housh TB, Isachsen CE (1990) The Acasta gneisses: remnant of Earth’s early crust. Origin of the earth. Oxford University Press, New YorkGoogle Scholar