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49-744: The Isua Greenstone Belt, also known as the Isua supracrustal belt since it is composed primarily of supracrustal rocks , is located in southwestern Greenland , in the Isukasia terrane , near the Nuuk capital region. It forms the largest supracrustal enclave in the Itsaq Gneiss Complex , which predominantly comprises 3850 - 3600 million year old (Ma) felsic orthogneisses . The greenstone belt comprises two major sequences of metamorphosed mafic volcanic and sedimentary rocks, which were divided on

98-553: A heat pipe or mantle plume origin for the belt. This forms part of a much broader debate about when plate tectonics emerged on Earth, and whether the Archaean Earth operated under a fundamentally different tectonic regime. Furnes et al. (2007) suggested that the presence of pillow lavas and closely spaced parallel dykes indicated that the Isua Greenstone Belt represented an ophiolite. The interpretation of

147-576: A few localities (e.g. Adak Island in Alaska and Mindanao in the Philippines), they argue that due to a higher mantle potential temperature of the Earth, a hotter and softer crust may have enabled intense adakite-type subduction during Archean time. TTGs packages were then generated in such settings, with large scale proto-continents formed by collisions at a later stage. However, other authors doubt

196-502: A heat-pipe model, rapid eruption of volcanic rocks and the corresponding removal of melt from the mantle below causes downward movement of the lithosphere and burial of mafic rocks. The buried mafic rocks eventually heat up and melt, producing the TTGs associated with the Isua Greenstone Belt. This model can account for the mafic composition of pelitic sediments at Isua, suggesting there was little felsic crust present during its formation, and

245-430: A little on pressure. Different groups of TTG should therefore have experienced distinct geothermal gradients , which corresponding to different geodynamic settings. The low pressure group has formed along geotherms around 20–30 °C/km, which are comparable to those during the underplating of plateau bases. Mantle upwellings add mafic basement to the crust and the pressure due to the cumulation thickness may reach

294-403: A pressure over 20 kbar, with the source rock containing garnet and rutile but no amphibole or plagioclase. The medium pressure group has transitional features between the other two groups, corresponding to melting under a pressure around 15 kbar with the source rock containing amphibole, much garnet, but little rutile and no plagioclase. Medium pressure TTGs are the most abundant among

343-605: A protracted event between ~2.9 and 2.6 Ga, followed by widespread retrogression of locally varying intensity. The effect of these two metamorphic and deformational events adds significant complexity to interpreting the primary geochemical compositions and geological structures present in the belt (e.g., see below). Because of its age, the Isua Greenbelt has long been the focus of studies seeking to identify signs of early terrestrial life. In 1996, geologist Steve Mojzsis and colleagues hypothesized that isotopically light carbon in

392-451: A small proportion), while Archean TTG rocks are major components of Archean cratons . The quartz percentage among felsic minerals in TTG rocks is usually larger than 20% but less than 60%. In tonalite and trondhjemite, more than 90% of the feldspars are plagioclase , while in granodiorite , this number is between 65% and 90%. Trondhjemite is a special kind of tonalite , with most of

441-498: A wide range of stable , radiogenic , and short-lived isotope systems. The Isua Greenstone Belt comprises many different lithologies. The most abundant rock types are mafic metavolcanic rocks with a range of compositions from boninite -like to tholeiites and picrites . Though boninitic amphibolites at Isua are often interpreted as evidence for the action of plate tectonics, these are not true boninites and non-plate tectonic models can also account for their formation. Texturally,

490-562: Is a stub . You can help Misplaced Pages by expanding it . Tonalite-Trondhjemite-Granodiorite Tonalite–trondhjemite–granodiorite ( TTG ) rocks are intrusive rocks with typical granitic composition ( quartz and feldspar ) but containing only a small portion of potassium feldspar . Tonalite , trondhjemite , and granodiorite often occur together in geological records , indicating similar petrogenetic processes. Post Archean (after 2.5 Ga) TTG rocks are present in arc -related batholiths , as well as in ophiolites (although of

539-738: Is bounded to the West by the Ivinnguit Fault, which divides the Eoarachaean Itsaq Gneiss Complex from younger ( Mesoarchaean ) rocks of the Akia Terrane . Elsewhere, it is bounded by felsic orthogneisses of the Itsaq Gneiss Complex. These show a similar age division to the supracrustal rocks of the Isua Greenstone Belt itself, with 3800 Ma gneisses to the south of the belt, and 3700 Ma gneisses to

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588-405: Is interpreted to be the partial melting of young and hot subducting oceanic slabs with minor interaction with surrounding mantle wedges, rather than mantle wedge melts like other arc-granitoids. Based on geochemical features (e.g. Mg , Ni , and Cr contents), adakites can be further divided into two groups, namely high SiO 2 adakites (HSA) and low SiO 2 adakites (LSA). It was then noted that

637-653: Is one of the major rock types in Archean cratons . In terms of trace element characteristics, Archean TTGs exhibit high light rare earth element (LREE) content yet low heavy rare earth element (HREE) content. However, they do not show Eu and Sr anomalies . These features indicate the presence of garnet and amphibole , but no plagioclase in the residual phase during partial melting or precipitation phase during fractional crystallization . Confirmed by geochemical modelling, TTG type magma can be generated through partial melting of hydrated meta- mafic rocks . To produce

686-737: Is petrogenetically similar to that of subduction , both of which involves deep burial of mafic rocks into the mantle. Post Archean TTG rocks are commonly found in arc settings , especially in continental arcs . Ophiolite also contains a small amount of TTG rocks. Continental arc TTG rocks are often associated with gabbro , diorite , and granite , which forms a plutonic sequence in batholiths . They are formed by hundred of plutons that directly related to subduction . For example, Coastal Batholith of Peru consists of 7–16% gabbro and diorite, 48–60% tonalite (including trondhjemite), and 20–30% granodiorite, with 1–4% granite. These TTG rocks in continental arc batholiths may partially originate from

735-583: The Yilgarn Craton in the Mesoarchaean (3.2–2.8 Ga) Jack Hills , Western Australia dated to up to 4.4 Ga, meaning that granitic continental crust and probably supracrustal rocks formed during the Hadean , within 200 million years of Earth accretion. Most Hadean rocks were probably recycled into the mantle before the end of the eon, however, and such pre-4.0 Ga mineral inclusions,

784-462: The magma differentiation (i.e. fractional crystallisation ) of the subduction induced mantle wedge melt at depth. However, the large volume of such TTG rocks infer their major generation mechanism is by the crustal thickening induced partial melting of the former gabbroic underplate at the base of the continental crust. Tonalitic composition rock crystallised first before the magma differentiated to granodioritic and later granitic composition at

833-467: The Archean TTGs were geochemically almost identical to high silica adakites (HSA), but slightly different from low silica adakites (LSA). This geochemical similarity let some researchers infer that the geodynamic setting of Archean TTGs was analogous to that of modern adakites. They think that Archean TTGs were generated by hot subduction as well. Although modern adakites are rare and only found in

882-449: The ISB features as stromatolites is controversial, because similar features may form through non-biological processes. Some geologists interpret the textures above the putative stromatolites as sand accumulation against their sides during their formation, suggesting that the features arose during the sedimentary process, and not through later, metamorphic deformation. However, others suggest that

931-536: The Isua Greenstone Belt contains the remains of stromatolite microbial colonies that formed approximately 3.7 billion years ago . However, their interpretations are controversial. If these structures are stromatolites, they predate the oldest previously known stromatolites, found in the Dresser Formation in western Australia, by 220 million years. The complexity of the stromatolites found at Isua, if they are indeed stromatolites, suggest that life on Earth

980-430: The Isua Greenstone Belt formed in a tectonic regime that was different to the modern day Earth. Plate tectonic models can be further subdivided into those that argue that the Isua Greenstone Belt or parts of it represent an ophiolite , a sliver of obducted oceanic crust and mantle, and those that argue that the belt represents an accretionary prism, formed in a subduction zone. Non-plate tectonic models generally suggest

1029-503: The Isua Greenstone Belt is an ophiolite. The northeastern part of the Isua Greenstone Belt has been interpreted as part of an accretionary wedge on the basis of numerous small faults and apparent repetitions of the supracrustal sequence, with similarities to modern accretionary wedges. This was further supported by apparent metamorphic gradients in the same part of the belt, that are similar to those observed in modern subduction zones. However, this interpretation has been strongly contested on

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1078-433: The basis of zircon uranium-lead dating . These sequences are the 'southern terrane', which has an age of approximately 3800 Ma, and the 'northern terrane', which has an age of approximately 3700 Ma. The younger southern terrane is further subdivided into two subterranes: one predominantly comprising boninite -like metavolcanic rocks, and the other comprising tholeiitic and picritic metavolcanics. The Isua Greenstone Belt

1127-404: The basis that rock types and strain are extremely consistent across the various faults in the proposed accretionary wedge, and that peak metamorphic grades are consistent across the entire belt. Non-plate tectonic models include heat-pipe and mantle plume models, both of which suggest that the volcanic sequences at Isua formed through eruption of mantle derived magmas with minimal crustal input. In

1176-442: The belt. However, more recent work disputes a mantle origin for these rocks, and suggests that all features of the dunite lenses can be explained by them representing ultramafic cumulates formed in magma chambers that fed the eruption of volcanic rocks in the Isua Greenstone Belt. If this is the case, then no thrusting is required to bring them into contact with the supracrustal rocks, and the dunite lenses do not provide evidence that

1225-605: The belt. Other objections related to the composition of the dykes, which are unlike those found in modern-day ophiolites. Despite the disagreement on the presence of a sheeted dyke complex at Isua, alternative lines of evidence have been proposed in support of an ophiolite origin for the belt. These are primarily based on the geochemistry of the volcanic rocks in the belt: tholeiitic amphibolites have been interpreted as metamorphosed island-arc tholeiites, and boninite-like amphibolites have been interpreted to represent metamorphosed boninites. However, subsequent studies have pointed out that

1274-404: The boninite-like amphibolites are in fact low-titanium basalts, with too little silica to classify as boninites, and recent geochemical modelling suggests that the entire volcanic compositional range at Isua can be explained without requiring a plate tectonic setting. A further line of evidence used to invoke an ophiolite origin for the Isua Greenstone Belt is the presence of peridotite lenses in

1323-633: The existence of Archean subduction by pointing out the absence of major plate tectonic indicators during most of the Archean Eon. It is also noted that Archean TTGs were intrusive rocks while the modern adakite is extrusive in nature, thus their magma should differ in composition, especially in water content. Various evidence has shown that Archean TTG rocks were directly derived from preexisting mafic materials. The melting temperature of meta-mafic rocks (generally between 700 °C and 1000 °C) depends primarily on their water content but only

1372-650: The first abiogenic steps of life on Earth. No Hadean-aged zircon grains have been identified in the Nuvvuagittuq Supracrustal Belt in northern Quebec, but amphibolites have been dated to 4.28 Ga while 3.75 Ga-old banded iron formations indicate the minimum age of the belt. The Napier Mountains in Antarctica are of similar age. The Isua greenstone belt contains the oldest, well-preserved, supracrustals, dated at 3.8–3.7 Ga . This metamorphic rock -related article

1421-547: The formation of the <3.5 Ga Ameralik dykes and is associated with the Eoarchaean deformation at Isua. Amphibolite-facies conditions were reached across the belt between ~3.7 and 3.6 Ga. Though higher pressure conditions have been suggested locally on the basis of Ti-humite group minerals in peridotites, the reliability of these minerals to document high pressure processes has been questioned. The second event also reached amphibolite-facies conditions, and appears to have been

1470-546: The geotherms for the most abundant TTG subseries, medium pressure group, are between 12 and 20 °C/km. Other than hot subduction, such geotherms may also be possible during the delamination of mafic crustal base. The delamination may be attributed to mantle downwelling or an increase in density of the mafic crustal base due to metamorphism or partial melt extraction. Those delaminated meta-mafic bodies then sink down, melt, and interact with surrounding mantle to generate TTGs. Such delamination induced TTG generation process

1519-757: The mafic metavolcanics include pillow lavas and pillow breccias , which indicate that the lavas erupted subaqueously, and requires the presence of surface water during the Eoarchaean. More felsic volcanic compositions have been observed, but it is not clear whether these represent volcanic or sedimentary rocks, and the only examples of potential andesite are significantly weathered. The mafic volcanic sequences contain abundant meta-ultramafic rocks, including amphibolites, serpentinites, carbonated- peridotites and peridotite. The majority of these are widely accepted to be intrusive in origin, representing ultramafic cumulates. Some peridotite lenses have been interpreted as obducted mantle fragments, and used as evidence to support

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1568-536: The north of the belt. A large number of geological and geochemical methods have been applied to the rocks of the Isua Greenstone Belt. These include subdivision of the various lithologies and units within the belt using a combination of geological mapping and U-Pb zircon dating, typically using sensitive high-resolution ion microprobe (SHRIMP) , analyses; major and trace element chemistry; structural analyses; geothermobarometry and metamorphic modelling using phase diagrams to determine metamorphic conditions; and

1617-480: The only traces from the earliest rock formation on Earth, are rare. The Acasta orthogneisses of the Slave Craton , Canada, are regarded to be the oldest rocks on Earth, dated to 4.06 Ga, include protoliths such as TTGs , amphibolite , gabbro , granite , and diorite . It is possible that such early Hadean TTGs were a source of phosphorus for the first oceans and therefore contributed nutrients to

1666-464: The operation of plate tectonics during the formation of the Isua Greenstone Belt. However, this interpretation is contested, and some studies suggest that all peridotites at Isua are cumulates, representing shallow level magma chambers and conduits with the volcanic sequences. Metasedimentary rocks include banded iron formation and detrital quartzite , likely representing a metamorphosed siliciclastic sedimentary rock. Although they do not form part of

1715-478: The origin of the structures, interpreting them as arising from deformation. Thus, the ISB stromatolites remain a subject of ongoing investigation. Supracrustal rock Supracrustal rocks ( supra (Latin for "above")) are rocks that were deposited on the existing basement rocks of the crust , hence the name. They may be further metamorphosed from both sedimentary and volcanic rocks . The oldest minerals on Earth are detrital zircon grains from

1764-444: The parallel dykes as a sheeted dyke complex was particularly important as sheeted dyke complexes are diagnostic of oceanic crust in ophiolites on the modern Earth. However, this interpretation was strongly contested on the basis that the sheeted dykes proposed by Furnes et al. were in fact a much younger generation of dykes, the ~3.5 billion year old (Ga) Ameralik dykes, and therefore unrelated to pillow lavas and other volcanic rocks of

1813-761: The plagioclase in the rock being oligoclase . The major accessory minerals of TTG rocks include biotite , amphiboles (e.g. hornblende ), epidote , and zircon . Geochemically , TTG rocks often have a high silica (SiO 2 ) content (commonly over 70 percent SiO 2 ), high sodium oxide (Na 2 O) content (with low K 2 O/Na 2 O ratio) compared to other plutonic rocks , and low ferromagnesian element content (the weight percentage of iron oxide , magnesium oxide , manganese dioxide , and titanium dioxide added together commonly smaller than 5%). Archean TTG rocks appear to be strongly deformed grey gneiss , showing banding, lineation, and other metamorphic structures , whose protoliths were intrusive rocks . TTG rock

1862-416: The possible stromatolites overlay volcanic rocks that are dated to 3.709 billion years old and are capped by dolomite and banded iron formations with thorium - uranium zircons dated to 3.695 ± 0.4 billion years old. All layers, including those bordering the stromatolites, experienced metamorphism and deformation after deposition, and temperatures not exceeding 550 °C (1,000 °F). The identity of

1911-525: The relatively simple deformation and uniform metamorphic grade observed across the belt. However, it has been criticised on a number of grounds, including the fact that there is no evidence that the 3.7 Ga volcanic rocks or TTGs ascended through the 3.8 Ga sequence, as would be expected for vertically stacked volcanism in a heat pipe model. Following its formation, the Isua Greenstone belt has undergone two major metamorphic episodes. The first predates

1960-421: The requirement of low pressure TTG production. The partial melting of the plateau base (which can be induced by further mantle upwelling) would then lead to low pressure TTG generation. The high pressure TTGs have experienced geotherms lower than 10 °C/km, which are close to modern hot subduction geotherms experienced by young slabs (but around 3 °C/km hotter than other modern subduction zones), whilst

2009-407: The rocks are so altered that any sedimentary interpretations are inappropriate. In 2016, geologist and areologist Abigail Allwood stated that the discovery of Isua stromatolites makes the emergence of life on other planets , including Mars early after its formation, more probable. However, in 2018, she and a team of additional geologists published a paper that raises significant questions as to

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2058-440: The stromatolites may have been deposited in a shallow marine environment. While most rocks in the Isua Greenstone Belt are too metamorphically altered to preserve fossils, the area of stromatolite discovery may have preserved original sedimentary rocks and the fossils inside them. However, some geologists interpret the structures as the result of deformation and alteration of the original rock. The ISB sedimentary layers containing

2107-434: The structure's carbon-rich layers was suggestive of biological activity having occurred there. "Unless some unknown abiotic process exists which is able both to create such isotopically light carbon and then selectively incorporate it into apatite grains, our results provide evidence for the emergence of life on Earth by at least 3,800 Myr before present." In August 2016, an Australia-based research team presented evidence that

2156-451: The supracrustal belt itself, the belt is hosted in and in places intruded by tonalite-trondhjemite-granodiorite (TTG) orthogneisses . The tectonic setting in which the Isua Greenstone Belt formed remains contentious. Ideas can be broadly divided into plate tectonic models, in which the belt formed in one of several possible tectonic settings that exist on the modern day Earth, and non-plate tectonic or non- uniformitarian models, in which

2205-424: The three groups form a continuous series. The low pressure group shows relatively low Al 2 O 3 , Na 2 O, Sr content and relatively high Y , Yb , Ta , and Nb content, corresponding to melting under 10–12  kbar with the source rock mineral assembly of plagioclase, pyroxene and possibly amphibole or garnet. The high pressure group shows the opposite geochemical features, corresponding to melting at

2254-589: The three groups. The geodynamic setting of the Archean TTG rock generation is currently not well understood. Competing hypotheses include subduction related generation involving plate tectonics and other non-plate tectonic models. Geochemical similarity shared between TTGs and adakites  have long been noted by researchers. Adakites are one type of modern arc lavas, which differ from common arc lavas (mostly granitoids) in their felsic and sodic nature with high LREE but low HREE content. Their production

2303-528: The very low HREE pattern, the melting should be conducted under a garnet-stable pressure-temperature field. Given that garnet stability rises dramatically with increasing pressure, strongly HREE-depleted TTG melts are expected to form under relatively high pressure. Besides the source composition and the pressure, the degree of melting and temperature also influence the melt composition. Archean TTGs are classified into three groups based on geochemical features, that are low, medium, and high pressure TTGs, although

2352-415: The volcanic sequence, particularly two dunite lenses referred to as 'lens A' and 'lens B'. These were argued to represent mantle rocks on the basis of their geochemistry, textures, and the presence of apparently high-pressure minerals. If true, the presence of mantle rocks within the supracrustal sequence at Isua would require that these rocks had been thrust to the surface, supporting an ophiolite origin for

2401-694: Was already sophisticated and robust by the time of their formation, and that the earliest life on Earth likely evolved over 4 billion years ago. This conclusion is supported in part by the instability of Earth's surface conditions 3.7 billion years ago, which included intense asteroid bombardment. The possible formation and preservation of fossils from this period indicate that life may have evolved early and prolifically in Earth's history. The stromatolite fossils appear wavy and dome-shaped, are typically 1–4 cm (0.4–1.6 in) high, and were found in iron - and magnesium -rich dolomites that had recently been exposed by melting snow. The surrounding rocks suggest that

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