Acasta gneiss

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A piece of Acasta gneiss in the Natural History Museum Vienna

The Acasta-Gneiss or Acasta-Gneiss Complex (after the nearby Acasta River ) is a rock unit of the Archean in northwestern Canada . The unit, which consists mainly of gneiss , contains areas whose original rocks have been dated to an age of up to 4030  Ma and are therefore among the oldest known rocks on earth .

location

The rocks of the Acasta gneiss emerge around the Acasta River in the settlement area of ​​the Dogrib Indians, east of Great Bear Lake and north of Little Crapeau Lake , about 350 km north of Yellowknife , the capital of the Canadian Northwest Territories . They belong to the western part of the slave kraton , an archaic continental core in the Canadian shield .

History of exploration

Archaic rocks were first detected on the Acasta River as part of the mapping of the Wopmay orogen in 1984 and examined and mapped several times by the end of the 1980s. The dates known so far were made in an area of ​​about 20 km².

First, the gneisses were dated to an age of 3.48 Ga (billion years) by Samuel Bowring and William Randall Van Schmus . In 1989, a team of researchers led by SA Bowring, Ian S. Williams and William Compston obtained an age of slightly less than 4 billion years using SHRIMP dating with 3962 ± 3 Ma. The even older age was only recognized in the course of subsequent research by several research groups. Since then, the area of ​​the Acasta gneiss has been mapped and geologically processed in detail.

Geological framework

The Acasta Gneiss Complex is located in the far west of the Slave Craton, one of the four archaic cratons that make up much of Canada and Greenland. The geology of the slave province is characterized by the existence of a basement more than 2.8 billion years old , consisting of amphibolite facies granitic gneisses and intrusives , which occur together with quartzites , volcanites , conglomerates and ribbon ores . Rocks from the Yellowknife Supergroup lie on top of the basement and consist mainly of mafic to rockic volcanic rocks and turbiditic gray-wacke - claystone sequences. Granitic to tonalitic plutons penetrated into all of these rocks about 2.6 billion years ago and caused the deformation and metamorphosis of the previously formed sequences.

Geological situation and rock

In connection with the rocks of the western slave province, the gneiss deposits lie in a tectonic high altitude, in which the old rocks emerge under cover-like thrusts in an area about 50 km long and 30 km wide. The occurrence of the Acasta gneiss lies in the so-called Exmouth culmination in the south of this area. The raising of the respective layers probably dates back to the Wopmay - orogeny back as they both in the eastern foothills and in the internal metamorphic zone of this orogenic belt are.

The Acasta gneisses are divided into two parts by a disturbance . The rocks in the east of the fault are predominantly pink-colored, massive to layered granitic orthogneiss , in the west, on the other hand, in a complicated way interwoven, biotite and hornblende- bearing tonalitic and granitic gneisses. About 3.6 billion years ago, granitic plutons invaded the succession, which today have metamorphic banding, but in some areas still show remnants of the original igneous texture .

The sequence of metamorphic events that affected the rocks is complex and involves at least five overprints. Finally, between 1.9 and 1.26 Ga, syenitic and mafic dykes penetrated the sequence of layers during and after the Wopmay orogenesis .

The gneiss, the parent rock of which has been dated to an age of more than 4 billion years, occurs as inclusions of tonalitic and gabbroider composition in the Acasta gneiss. These are granitic and dioritic intrusions that have been strongly influenced by metamorphosis. Recent geochemical investigations on the rock show no interaction with the continental crust that was already present at the time. Presumably, the parent rocks were melted out of the then mantle or the rest of the crust.

meaning

More than 4,300 Ma old zircons have already been found in the Narryer Mountains in western Australia . In 2006, a single zircon from the Acasta gneisses was also dated to be 4.2 billion years old. While these crystals are only microscopic evidence of a disappeared earth's crust, the Acasta gneisses have been preserved as a whole in the continental shield on the Acasta River, which has been flattened by glaciers . The parent rocks of gneisses show no evidence of the impact of asteroids, which according to previous presentation during the Great bombings have fallen (Late Heavy Bombardment, LHB) in the period from 4.1 to 3.8 billion years ago to the inner planets of the solar system, including on earth.

The former sediments of the Isua gneiss in the Canadian Shield northeast of Nuuk on the edge of the Greenland Ice Sheet, dated to more than 3,800 mya , are of comparable importance . In contrast to the igneous rocks of the Acasta gneiss, the parent rocks of the Isua gneiss were partly deposited on the surface of the earth and give evidence that water already existed then, and that the deposit conditions were similar to today.

Reports of the discovery of approximately 4,280 Ma old rocks in the Nuvvuagittuq greenstone belt in northern Quebec , Canada in 2008 are still the subject of research. The ages of these rocks published in scientific journals were initially only between 3,661 ± 4 to 3,817 ± 16 mya, but more recent work confirmed the age of approx. 4,300 Ma for the Nuvvuagittuq rocks.

Since 2003, a 4-ton stone from the Acasta Gneiss Complex that was brought to the National Museum of the American Indian by the Smithsonian Institute has been on display in Washington, DC .

See also

Individual evidence

  1. ^ A b c Allen P. Nutman, Clark RL Friend, Vickie C. Bennett: Review of the oldest (4400-3600 Ma) geological and mineralogical record: Glimpses of the beginning. Episodes, Vol. 24, No. 2, 2001 ( online  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. )@1@ 2Template: Dead Link / www.episodes.org  
  2. ^ SA Bowring, WR Van Schmus: U – Pb zircon Constraints on Evolution of Wopmay Orogen. NWT Geological Association of Canada / Mineralogical Association of Canada, p. 47 (Abstract 9), 1984
  3. ^ SA Bowring, IS Williams and W. Compston: 3.96 Ga gneisses from the Slave Province, Northwest Territories, Canada. Geology, Vol. 17, pp. 971-975, 1989 ( online abstract of the article )
  4. ^ SA Bowring, IS Williams: Priscoan (4.00–4.03Ga) orthogneisses from northwestern Canada. Contributions to Mineralogy and Petrology, Vol. 134, pp. 3-16, 1999, doi : 10.1007 / s004100050465
  5. ↑ a b Tsuyoshi Iizuka, Tsuyoshi Komiyaa, Yuichiro Uenoa, Ikuo Katayamaa, Yosuke Ueharaa, Shigenori Maruyama, Takafumi Hirata, Simon P. Johnson and Daniel J. Dunkley: Geology and zircon geochronology of the Acasta Gneiss Complex, northwestern on Canada: New constraints on Canada its tectonothermal history. Precambrian Research, Vol. 153, No. 3–4, March 1, 2007, pp. 179–208, doi : 10.1016 / j.precamres.2006.11.017 ( online version, pdf; 5.6 MB  ( page no longer retrievable , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. )@1@ 2Template: Toter Link / ea.cu-tokyo.ac.jp  
  6. ^ Gerhard H. Eisbacher: North America . In: Geology of the Earth . 1st edition. tape 2 . Ferdinand Enke Verlag, Stuttgart 1986, ISBN 3-432-96901-5 , p. 13 .
  7. ↑ JR Reimink, et al .: No evidence for Hadean continental crust within Earth's oldest rock unit evolved. Nature Geoscience. doi : 10.1038 / NGEO2786 ( online version, pdf; 1.3 MB )
  8. ↑ Tsuyoshi Iizuka, Kenji Horie, Tsuyoshi Komiya, Shigenori Maruyama, Takafumi Hirata, Hiroshi Hidaka and Brian F. Windley: 4.2 Ga zircon xenocryst in an Acasta gneiss from northwestern Canada: Evidence for early continental crust. Geology, Vol. 34, No. 4, pp. 245-248, April 2006, doi : 10.1130 / G22124.1
  9. ↑ Jonathan O'Neil, Richard W. Carlson, Don Francis, Ross K. Stevenson: Neodymium-142 Evidence for Hadean Mafic Crust. Science, Vol. 321, No. 5897, pp. 1828-1831, September 26, 2008, doi : 10.1126 / science.1161925
  10. ↑ Jean David, Laurent Godin, Ross Stevenson, Jonathan O'Neil and Don Francis: U-Pb ages (3.8-2.7 Ga) and Nd isotope data from the newly identified Eoarchean Nuvvuagittuq supracrustal belt, Superior Craton, Canada. GSA Bulletin, Vol. 121; No. 1-2; Pp. 150-163; January 2009, doi : 10.1130 / B26369.1
  11. ^ John Adam et al .: Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the Earth's early continental crust. Geology, v. 40, pp. 363-366, 2012

Web links

Coordinates: 65 ° 10 ′ 31 ″  N , 115 ° 33 ′ 29 ″  W.