Acasta Gneiss

The Acasta - gneiss or Acasta gneiss complex ( after the nearby river Acasta River ) is a unit of Archean rocks in northwestern Canada. The predominantly composed of gneiss unit includes areas whose parent rocks were dated at up to 4030 Ma and are therefore among the oldest known rocks on earth.

Location

The rocks of the gneiss Acasta - emerge around the Acasta River in the settlement area of ​​the Dogrib Indians, east of the Great Bear Lake and north of Little Crapeau Lake, about 350 km north of Yellowknife, capital of Canada's Northwest Territories. They belong to the western part of the Slave Craton, an archaic Kontinentalkern in the Canadian Shield.

History of exploration

Rocks archaic age were detected on Acasta River as part of the mapping of the orogen - Wopmay the first time in 1984 and examined repeatedly until the late 1980s and mapped. The previously known datings were made in an area of ​​about 20 km ².

First, the gneisses were S. A. Bowring and W. R. Van Schmus an age of 3.48 Ga ( billion years) dated. In 1989, a research team was awarded to S. A. Bowring, individual section Williams and W. Compston by SHRIMP dating of 3962 ± 3 Ma with an age of a little less than 4 billion years. Only in the course of the subsequent research of several research groups that still higher age was detected. Since then, the field of Acasta gneisses has been mapped in detail and edited geologically.

Geological framework

The Acasta - gneiss complex is located in the extreme west of the Slave craton, one of the four archaic cratons that form a large part of Canada and Greenland. The geology of the Slave Province is characterized by the existence of more than 2.8 billion years old basement, which consists of granitic gneiss and amphibolite intrusions that occur together with quartzites, volcanic rocks, conglomerates and bands ores. The basement rocks are on the Yellowknife Supergroup, which consist mainly of mafic to felsic volcanic rocks and turbiditic greywacke - mudstone sequences. In all these rocks penetrated about 2.6 billion years granitic plutons to tonalitische and caused the deformation and metamorphism of the sequences previously formed.

Geological situation and rock

The gneiss deposits are associated with the rocks of the western Slave Province in a tectonic high position, in which occur the old days under rocks to blanket-like thrusts on an area of about 50 km long and 30 km wide. The occurrence of Acasta gneisses lies in the so-called Exmouth culmination south of the area. The uplift of the respective layers is probably due to the Wopmay - orogeny, as they are both in the eastern foothills and in the internal metamorphic zone of this orogen.

The Acasta gneisses are divided into two parts by a fault. The rocks in the east of the fault are predominantly pink, massive to layered granitic gneiss, on the other hand intertwined in a complicated way in the West, biotite and hornblende granitic gneisses and leading tonalitische. In the sequence granitic plutons are intruded about 3.6 billion years ago, which have now a metamorphic banding in some areas, however, still remains of the original igneous texture have.

The sequence of metamorphic events that have affected the rocks is complex and includes at least five over embossing. Last penetrated 1.9 to 1.26 Ga during and after Wopmay orogeny syenitic and mafic dykes in the layer sequence a.

The gneisses whose parent rocks were dated to an age of more than 4 billion years ago, occur as inclusions of tonalitischer and gabbroic composition in Acasta - gneiss. These are granitic and dioritic intrusions that were strongly overprinted by metamorphism.

Importance

While it is more than 4,300 Ma old zircons have been found in the Narryer - hills in western Australia. In 2006, even the single zircon from the Acasta gneisses has been dated to an age of 4.2 billion years. However, while these crystals are only microscopic evidence of a missing earth's crust, the Acasta gneisses have been preserved in the flat ground by glaciers continental plate on Acasta River as a whole. The parent rocks of gneisses show no evidence of the impact of asteroids that have been dropped, according to previous idea during the Great bombardment ( Late Heavy Bombardment, LHB ) in the period between 4.1 to 3.8 billion years ago to the inner planets of the solar system, including to the ground.

Of comparable importance to the dated more than 3,800 mya, former sediments of the Isua gneisses are in the Canadian Shield north-east of Nuuk on the edge of the Greenland ice sheet. In contrast to the igneous rocks of gneisses Acasta the parent rocks of the Isua gneisses were deposited partly on the surface, and give evidence that even then there was water, and the deposition conditions were similar to today.

Reports of the discovery of about 4,280 Ma old rocks in Nuvvuagittuq greenstone belt in northern Quebec in Canada in 2008 are still under investigation. Published in scientific journals age dating of these rocks were initially only 3,661 ± 4-3817 ± 16 mya, but recent work confirmed the age of about 4300 Ma for the Nuvvuagittuq - rocks.

Since 2003, a 4 -ton stone from the Acasta - gneiss complex, which was brought from the Smithsonian Institute to the National Museum of the American Indian in Washington, DC issued.