Isua Greenstone Belt
The Isua gneiss is a rock formation in the Archean Canadian Shield north-east of Nuuk on the edge of the Greenland ice sheet. The rock formation contains the Isua greenstone belt approximately 3.8 billion years old rocks that were long considered the oldest rocks of the earth. In them there are indications that the geological processes at that time, about 700 million years after the formation of the Earth, resembled those of today. The existence of traces of organic processes in the Isua rocks as signs of early life is still controversial.
Location and geological framework
The rock formation of the Isua gneisses belongs geologically to the Nain craton, part of the North Atlantic Craton in eastern Canada and Greenland. The North Atlantic craton is continued in the Karelian Craton in Finland, from which it is separated in the emergence of the Atlantic since the Jurassic.
The Isua rocks are in Itsâq - gneiss complex, which stretches from Nuuk to Isua over a length of about 150 km. Perhaps the rocks of the nearly 100km northern Aasivik - Terrans are also part Itsâq - gneiss complex, a possible connection, however, is hidden under the ice of Greenland.
In the meso-to neoarchaischen Isua greenstone belt are early archaic rock complexes as small enclaves in the so-called Amîtsoq - gneiss. The largest and best preserved of these is known is that of Isua that incurred by their great age and the presence of the earth's surface ( suprakrustal ) sediments. It is an approximately 30 km long and 1-5 km wide rock body relatively little deformed mica slate, which takes place in a large, almost circular arc from the edge of the inland ice to the west.
The deposit is located about 20 km southeast of the locality of the same name, just north of the mountain Ataneq and about 10 km north of the glacier Qavdlunâtsiait, it is divided by a lake in an eastern and a western deposits. The nearest inhabited places - Neriunaq, Kapisillit, Uummannaq and Qoornoq - located about 60 km to the south and southwest and are small settlements with few residents.
Plattentektonisch considered it might be here represents the oldest ophiolite (postponed on continents oceanic crust ) of the earth.
History of exploration
Towards the end of the 1960s it was discovered from a larger magnetic anomaly on the edge of the inland ice at a geophysical survey of an area in west-central Greenland from the plane. Subsequent investigations revealed a large deposit of iron ore in a belt of highly metamorphic rocks, which was later named after a location in the vicinity of the Isua greenstone belt. In the 1970s and the early 1980s, the area of scientists of the Geological Survey of Denmark and Greenland ( GEUS ) has been extensively studied and mapped in detail. In 1988 the first geological map of the area was published.
As early as the 1970s drew attention to the similarity of the Amîtsoq gneisses in Nuuk with the Isua rocks recently found the geologist Vic McGregor. In the Amîtsoq gneisses he had small areas of older rocks found that could be attributed to a formation at the surface. He assumed that these rocks had termed Akilia follow-up were older than the Amîtsoq gneisses, and compared it with the much better digested in Isua rocks. First age dating had shown with 3.6 to 3.7 billion years ago, a great age for the Amîtsoq gneisses. So if was true equating the Akilia sequence with the Isua rocks, they had to be even older. This was confirmed on the basis of subsequent datings who committed the Isua rocks with an age from 3.7 to 3.8 Ga.
Since then, they have been studied by several research groups under different issues, and 1998-2001 were the subject of the research project Isua Multidisciplinary Research Project ( IMRP ).
Geological situation and rocks
The Isua greenstone belt can be divided into five structural units, of which one in the east and in the west consists of little deformed rocks in which supracrustal rocks are relatively well preserved.
The stretched in southeast-northwest western deposits is divided into three parts, two outer mica schist units, surrounded by geological faults a central core of chloritic amphibole - mica schists with remnants of volcanic pillow- lava structures, metamorphic siliceous shales, conglomerates and bands ores. This central core is deformed relatively little compared to the accompanying mica schists. The western deposits is elongated in a north-south direction and contains, among other Bändererze, carbonate rocks with magnetite and Sideritbändern and clastic sediments.
Geochemical analyzes of rocks yielded a distribution of the rare earths, which was well consistent with formation of acidic volcanics. The first datings were performed on Einzelzirkonen and gave an age of 3.761 to 3.782 Ga ( billion years).
The parent rock of gneiss and mica schist documents after deposition of a metamorphosis at 450-600 ° C and were intensely folded and verschuppt, has remained so probably nowhere get a real stratigraphic sequence. The archaic rocks are younger granitic to tonalitic gneisses as countless small as well as up to kilometer-sized clods of intense deformation and metamorphism largely spared, so that in some areas of lower stress sedimentary and volcanic features such as graded bedding, ignimbrite - stratification or pillow Lava have been preserved.
The entire geological sequence of events in the development of the Isua greenstone belt are as follows:
- Deposition of volcanic - sedimentary sequences from 3.8 Ga, intrusion of tonalite, Trondhjemiten and subordinate diorites and granodiorites of
- First metamorphosis by 3.8 Ga, associated with tectonic stacking with the participation of peridotites and gabbro - anorthosite rocks
- Several metamorphic events from 3.75 to 3.55 Ga
- Granitic and mafic / ultramafic intrusions from 3.65 to 3.55 Ga, last convolution
Both the granitic gneisses and the mica schist are penetrated by a swarm metadoleritischer Dykes that have arisen as a conclusion of geological evolution from about 3.5 Ga. Since then, the Isua gneisses are essentially unchanged.
In contrast to the igneous rocks of up to 4.03 Ga old Acasta gneisses in northern Canada, but similar to the possibly up to 4.3 Ga old rocks of the Nuvvuagittuq Greenstone Belt in the Hudson Bay, the Isua rocks were originally to the earth's surface deposited. Although the precise deciphering their geological history is not simple, they allow important conclusions on the young Earth. For instance, the interpretation of boulder- bearing rocks as a conglomerate due to the strong deformation difficult, but is now recognized by many geologists. However, such rocks need for development as well as the pillow- lavas the existence of a hydrosphere. Their presence is an indication that geological processes vonstattengingen like today.
This also applies to plate tectonic processes. The mapping work and datings have shown that the supracrustal rocks were pushed together into stacks of not originally contiguous units and may represent a duplex structure. The stacking not belong together rock units is a typical feature of younger orogens caused by plate tectonic processes. The study of fluid inclusions in minerals also allows conclusions to be drawn that at that time there were already hydrothermal systems as they exist today, such as near the mid-ocean ridges.
In addition, enter the Isua rocks clues to the early stages of the formation of the Earth. The end of the Great bombardment ( Late Heavy Bombardment, LHB ), a period for which a violent bombardment of the Earth is believed by meteorites, could be estimated at about 3.8 to 3.83 Ga by dating of lunar rocks. The Isua gneisses are therefore only about 30 to 60 million years younger, and so far no evidence of effects of the LHB were found, especially not to the destruction of an existing atmosphere or water shell. Since the original deposit date of the parent rocks of the Isua gneisses is not precisely known, it could be that they were not deposited during the end of this event.
Signs of early life?
Since the late 1970s were some of the agents of the view that in some rocks of the Isua gneisses vorkämen signs of carbon of organic origin, and that might have existed at this time of life. This assumption was based on abnormally low values of δ13C in graphite, which occur mainly in carbonate-rich rocks. This depletion of 13C is typical of biological processes, formed during material as in photosynthesis 12C enrichment of isotopes occurs.
Once it became clear that this is partly caused by metasomatic rocks, reported by Mark A. van Zuilen and his colleagues in a paper published in 2003 that at least probably comes a portion of the carbon from the thermal conversion of siderite. However, other studies did not rule out other resources, especially in the conglomeratic units in the west and the Bändererze in the east, an organic origin by methane production and photosynthesis.