The Canadian Shield, the largest expanse of Precambrian rocks on Earth, is host to Earth’s oldest known rock – the Acasta Gneiss of the Slave Province of the Shield, located about 300 kilometres north of Yellowknife, Northwest Territories.Gneiss is a type of rock that has been metamorphosed (affected by intense heat and pressure) at depth in Earth’s crust. Most scientists assumed that this environment lasted for as long as 500 million years and aptly named it the Hadean (Greek for Hell) Era.Radiometric age-dating of the Acasta Gneiss, a rock found in Canada’s Northwest Territories and the Jack Hills Conglomerate from Western Australia has now changed our thinking about Earth’s beginnings.In contrast, the layered successions of rocks give only relative ages – older than, or younger than. Category: Rocks, Fossils, Minerals and Meteorites Decade: 1980s References Stern, R. Age of the World’s Oldest Rocks Refined Using Canada’s SHRIMP: The Acasta Gneiss Complex, Northwest Territories, Canada; Geoscience Canada, v. The Jack Hills Conglomerate, a 3,000 million year old sedimentary rock from which the oldest, at 4,200 million years, terrestrial minerals have been found. Very old rocks, those greater than 3,200 million years old occur on all the continents but in small blocks generally less than 500 km across and until recently the oldest rocks known were found at Isua in south-western Greenland.
The composition and texture of the Acasta Gneiss suggest that, before metamorphism, the gneiss was an igneous rock that formed about 4.03 billion years ago. Strongly deformed and locally migmatized gneisses occur at several places in the southern Eastern Desert of Egypt and in Sinai and have variously been interpreted as a basement to Pan-african (≈900 to 600 Ma) supracrustal and intrusive assemblages.A suite of grabbroic to granitic gneisses was investigated in the Hafafit area, which constitutes an I-type calc-alkaline intrusive assemblage whose chemistry suggests emplacement along an active continental margin and whose granitoid members can be correlated with the so-called ‘Older Granites’ of Egypt.Pb single zircon evaporation from three samples of the Hafafit gneisses yielded protolith emplacement ages between 677 ± 9 and 700 ± 12 Ma and document granitoid activity over a period of about 23 Ma.A migmatitic granitic gneiss from Wadi Bitan, south-west of Ras Banas, has a zircon age of 704 ± 8 Ma, and its protolith was apparently generated during the same intrusive event as the granitoids at Hafafit.