View source for Isua Greenstone Belt

{{short description|Archean greenstone belt in southwestern Greenland}}
{{Infobox mountain
| name = Isua Greenstone Belt
| country = [[Greenland]]
| subdivision2_type = Mainly mafics, tonalitic gneisses, metasedimentary rocks, [[BIFs]], pillow basalts, and pillow breccias.
| length_km = 35
| length_ref = <ref name="Vickie"/><ref name="Nutman 2007"/>
| area_km2 =
| elevation =
| geology = {{enum|[[tonalite]]|[[mafic rock]]s|[[metasedimentary rock]]s|[[banded iron formation]]s|[[granite]]|[[granodiorite]]}}
| age = Archean
| orogeny =
| map_image = Nuuk Location.jpg
| map_caption = The general location of the Isua Greenstone belt (Nuuk Region)
}}
The '''Isua Greenstone Belt''' is an [[Archean]] [[greenstone belt]] in southwestern [[Greenland]], aged between 3.7 and 3.8 billion years.<ref name="Nutman 2007">{{cite book|last1=Nutman|first1=Allen P.|url=http://ro.uow.edu.au/cgi/viewcontent.cgi?article=1927&context=scipapers|title=The Itsaq Gneiss Complex of Southern West Greenland and the Construction of Eorarchean Crust at Convergent Plate Boundaries|last2=Friend|first2=Clark R.L.|last3=Horie|first3=Kenji|last4=Hidaka|first4=Hiroshi|date=2007|isbn=9780444528100|series=Developments in Precambrian Geology|volume=15|pages=187–218|format=PDF|doi=10.1016/S0166-2635(07)15033-7|access-date=3 September 2016}}</ref> The belt contains variably [[Metamorphism|metamorphosed]] [[mafic]] [[Volcanic rock|volcanic]] and [[sedimentary rock]]s, and is the largest exposure of [[Eoarchean|Eoarchaean]] [[Supracrustal rock|supracrustal]] rocks on Earth.<ref name=":2">{{Cite journal|last1=Nutman|first1=Allen P.|last2=Friend|first2=Clark R. L.|date=2009-08-01|title=New 1:20,000 scale geological maps, synthesis and history of investigation of the Isua supracrustal belt and adjacent orthogneisses, southern West Greenland: A glimpse of Eoarchaean crust formation and orogeny|url=https://www.sciencedirect.com/science/article/pii/S030192680900076X|journal=Precambrian Research|language=en|volume=172|issue=3|pages=189–211|doi=10.1016/j.precamres.2009.03.017|bibcode=2009PreR..172..189N|issn=0301-9268}}</ref> Due to its age and low metamorphic grade <ref name=":3">{{Cite journal|last1=Ramírez-Salazar|first1=Anthony|last2=Müller|first2=Thomas|last3=Piazolo|first3=Sandra|last4=Webb|first4=A. Alexander G.|last5=Hauzenberger|first5=Christoph|last6=Zuo|first6=Jiawei|last7=Haproff|first7=Peter|last8=Harvey|first8=Jason|last9=Wong|first9=Tsz Kin|last10=Charlton|first10=Callum|date=2021|title=Tectonics of the Isua Supracrustal Belt 1: P-T-X-d Constraints of a Poly-Metamorphic Terrane|journal=Tectonics|language=en|volume=40|issue=3|pages=e2020TC006516|doi=10.1029/2020TC006516|bibcode=2021Tecto..4006516R|s2cid=234022542|issn=1944-9194|doi-access=free}}</ref> relative to many Eoarchaean rocks, the Isua Greenstone Belt has become a focus for investigations on the emergence of life<ref>{{Cite journal|last1=Nutman|first1=Allen P.|last2=Bennett|first2=Vickie C.|last3=Friend|first3=Clark R. L.|last4=Van Kranendonk|first4=Martin J.|last5=Chivas|first5=Allan R.|date=September 2016|title=Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures|url=https://www.nature.com/articles/nature19355|journal=Nature|language=en|volume=537|issue=7621|pages=535–538|doi=10.1038/nature19355|pmid=27580034|bibcode=2016Natur.537..535N|s2cid=205250494|issn=1476-4687}}</ref><ref name=":4">{{Cite journal|last1=Allwood|first1=Abigail C.|last2=Rosing|first2=Minik T.|last3=Flannery|first3=David T.|last4=Hurowitz|first4=Joel A.|last5=Heirwegh|first5=Christopher M.|date=November 2018|title=Reassessing evidence of life in 3,700-million-year-old rocks of Greenland|url=https://www.nature.com/articles/s41586-018-0610-4|journal=Nature|language=en|volume=563|issue=7730|pages=241–244|doi=10.1038/s41586-018-0610-4|pmid=30333621|bibcode=2018Natur.563..241A|s2cid=52987320|issn=1476-4687}}</ref> and the style of [[tectonics]] that operated on the early Earth.<ref name=":5">{{Cite journal|last=A. Alexander G. Webb, Thomas Müller, Jiawei Zuo, Peter J. Haproff, and Anthony Ramírez-Salazar|date=2020|title=A non–plate tectonic model for the Eoarchean Isua supracrustal belt|journal=Lithosphere|volume=12|issue=1|pages=166–179|doi=10.1130/L1130.1|bibcode=2020Lsphe..12..166W|s2cid=213886867|doi-access=free}}</ref><ref name=":9">{{Cite journal|last1=Furnes|first1=Harald|last2=de Wit|first2=Maarten|last3=Staudigel|first3=Hubert|last4=Rosing|first4=Minik|last5=Muehlenbachs|first5=Karlis|date=2007-03-23|title=A Vestige of Earth's Oldest Ophiolite|url=https://www.science.org/doi/10.1126/science.1139170|journal=Science|language=en|volume=315|issue=5819|pages=1704–1707|doi=10.1126/science.1139170|pmid=17379806|bibcode=2007Sci...315.1704F|s2cid=11691449|issn=0036-8075}}</ref>
==Overview==
The Isua Greenstone Belt, also known as the '''Isua supracrustal belt''' since it is composed primarily of [[supracrustal rock]]s, is located in southwestern [[Greenland]], in the Isukasia [[terrane]],<ref name="Vickie">{{cite book|last1=Bennett|first1=Vickie C.|last2=Nutman|first2=Allen P.|title=Encyclopedia of Scientific Dating Methods |chapter=Isua Supracrustal Belt, West Greenland: Geochronology |date=April 2014|isbn=978-94-007-6326-5|pages=1–4|doi=10.1007/978-94-007-6326-5_109-1}}</ref> near the [[Nuuk]] capital region.<ref name="Bennett">{{cite book|last1=Bennett|first1=Vickie C.|last2=Nutman|first2=Allen P.|title=Encyclopedia of Scientific Dating Methods |chapter=Isua Supracrustal Belt, West Greenland: Geochronology | date=2014| location=Australia|isbn=978-94-007-6326-5|page=1|doi=10.1007/978-94-007-6326-5_109-1}}</ref> It forms the largest supracrustal [[Enclave (geology)|enclave]] in the [[Itsaq Gneiss Complex]], which predominantly comprises 3850 - 3600 million year old (Ma) [[felsic]] [[Gneiss|orthogneisses]].<ref>{{Cite journal|last1=Nutman|first1=Allen P.|last2=Bennett|first2=Vickie C.|last3=Friend|first3=Clark R. L.|last4=Horie|first4=Kenji|last5=Hidaka|first5=Hiroshi|date=2007-08-21|title=~3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex|url=http://link.springer.com/10.1007/s00410-007-0199-3|journal=Contributions to Mineralogy and Petrology|language=en|volume=154|issue=4|pages=385–408|doi=10.1007/s00410-007-0199-3|bibcode=2007CoMP..154..385N|s2cid=52855243|issn=0010-7999}}</ref> The [[greenstone belt]] comprises two major sequences of metamorphosed mafic volcanic and sedimentary rocks, which were divided on 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.<ref name=":2" /> The younger southern terrane is further subdivided into two subterranes: one predominantly comprising [[boninite]]-like metavolcanic rocks, and the other comprising [[Tholeiitic magma series|tholeiitic]] and [[Picrite basalt|picritic]] metavolcanics.<ref name=":2" /> The Isua Greenstone Belt is bounded to the West by the Ivinnguit Fault, which divides the Eoarachaean Itsaq Gneiss Complex from younger ([[Mesoarchean|Mesoarchaean]]) rocks of the [[Akia terrane|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 the north of the belt.<ref name=":2" />
===Scientific methods===
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 [[Lithology|lithologies]] and units within the belt using a combination of [[Geologic map|geological mapping]] and [[U-Pb]] [[zircon]] dating, typically using [[sensitive high-resolution ion microprobe |sensitive high-resolution ion microprobe (SHRIMP)]], analyses;<ref name=":2" /> major and trace element chemistry;<ref name=":6">{{Cite journal|last=Rollinson|first=Hugh|title=No plate tectonics necessary to explain Eoarchean rocks at Isua (Greenland)|url=https://doi.org/10.1130/G49278.1|journal=Geology|year=2022|volume=50|issue=2|pages=147–151|doi=10.1130/G49278.1|bibcode=2022Geo....50..147R}}</ref><ref name=":7">{{Cite journal|last=Julian A. Pearce and Mark K. Reagan|date=2019|title=Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics|journal=Geosphere|volume=15|issue=4|pages=1008–1037|doi=10.1130/GES01661.1|bibcode=2019Geosp..15.1008P|s2cid=195803899|doi-access=free}}</ref> structural analyses;<ref name=":4" /><ref name=":5" /><ref name=":10">{{Cite journal|last1=Komiya|first1=Tsuyoshi|last2=Maruyama|first2=Shigenori|last3=Masuda|first3=Toshiaki|last4=Nohda|first4=Susumu|last5=Hayashi|first5=Mamoru|last6=Okamoto|first6=Kazuaki|date=1999-09-01|title=Plate Tectonics at 3.8–3.7 Ga: Field Evidence from the Isua Accretionary Complex, Southern West Greenland|url=https://www.journals.uchicago.edu/doi/10.1086/314371|journal=The Journal of Geology|volume=107|issue=5|pages=515–554|doi=10.1086/314371|pmid=10504134|bibcode=1999JG....107..515K|s2cid=24418938|issn=0022-1376}}</ref> [[geothermobarometry]] and metamorphic modelling using [[phase diagram]]s to determine metamorphic conditions;<ref name=":3" /><ref name="Rollinson">{{cite journal|last1=Rollinson|first1=Hugh|date=5 February 2003|title=Metamorphic history suggested by garnet-growth chronologies in the Isua Greenstone Belt, West Greenland|journal=Precambrian Research|volume=126|issue=3–4|pages=181–196|bibcode=2003PreR..126..181R|doi=10.1016/s0301-9268(03)00094-9}}</ref> and a wide range of [[Stable isotope analysis|stable]],<ref name="earlynaturelife" /><ref>{{Cite journal|last1=Kurzweil|first1=F.|last2=Münker|first2=C.|last3=Hoffmann|first3=J.E.|last4=Tusch|first4=J.|last5=Schoenberg|first5=R.|date=July 2020|title=Stable W isotope evidence for redistribution of homogeneous 182W anomalies in SW Greenland|url=http://www.geochemicalperspectivesletters.org/article2024|journal=Geochemical Perspectives Letters|volume=14|pages=53–57|doi=10.7185/geochemlet.2024|bibcode=2020GChPL..14...53K |s2cid=225595453|doi-access=free}}</ref> [[Radiogenic isotope|radiogenic]],<ref>{{Cite journal|last1=Hoffmann|first1=J. Elis|last2=Münker|first2=Carsten|last3=Polat|first3=Ali|last4=König|first4=Stephan|last5=Mezger|first5=Klaus|last6=Rosing|first6=Minik T.|date=December 2010|title=Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland|url=https://linkinghub.elsevier.com/retrieve/pii/S0016703710005405|journal=Geochimica et Cosmochimica Acta|language=en|volume=74|issue=24|pages=7236–7260|doi=10.1016/j.gca.2010.09.027|bibcode=2010GeCoA..74.7236H}}</ref> and short-lived [[isotope]] systems.<ref>{{Cite journal|last1=Willbold|first1=Matthias|last2=Elliott|first2=Tim|last3=Moorbath|first3=Stephen|date=September 2011|title=The tungsten isotopic composition of the Earth's mantle before the terminal bombardment|url=http://www.nature.com/articles/nature10399|journal=Nature|language=en|volume=477|issue=7363|pages=195–198|doi=10.1038/nature10399|pmid=21901010|bibcode=2011Natur.477..195W|s2cid=4419046|issn=0028-0836}}</ref>
==Lithologies==
[[File:Li 2.jpg|thumb|600x500px|right|Map showing the location of the Isua Greenstone Belt (ISB, top right) within the Itsaq Gneiss Complex. Also shown are the younger Akia Terrane to the northwest, and Tasiusasuaq Terrane to the south. Modified from Nutman et al., 2007 and Naerra et al., 2012.<ref name="Nutman 2007"/><ref name=Naerra>{{cite journal|last1=Naerra|first1=T.|last2=Schersten|first2=A.|last3=Rosing|first3=M. T.|last4=Kemp|first4=A. I. S.|last5=Hoffman|first5=J. E.|last6=Kokfelt|first6=T.F.|last7=Whitehouse|first7=M. J.|title=Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago|journal=Nature|date=May 2012|volume=485|issue=7400|doi=10.1038/nature11140|pages=627–630|pmid=22660324|bibcode=2012Natur.485..627N|s2cid=205228999}}</ref>]]
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 [[Picrite basalt|picrites]]. Though boninitic [[amphibolite]]s at Isua are often interpreted as evidence for the action of plate tectonics,<ref name=":12">{{Cite journal|last1=Polat|first1=A|last2=Hofmann|first2=A. W|last3=Rosing|first3=M. T|date=2002-04-01|title=Boninite-like volcanic rocks in the 3.7–3.8 Ga Isua greenstone belt, West Greenland: geochemical evidence for intra-oceanic subduction zone processes in the early Earth|url=https://www.sciencedirect.com/science/article/pii/S0009254101003631|journal=Chemical Geology|language=en|volume=184|issue=3|pages=231–254|doi=10.1016/S0009-2541(01)00363-1|bibcode=2002ChGeo.184..231P|issn=0009-2541}}</ref> these are not true boninites <ref name=":7" /> and non-plate tectonic models can also account for their formation.<ref name=":6" /> Texturally, the mafic metavolcanics include [[pillow lavas]] and pillow [[breccia]]s, 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,<ref name=":2" /> and the only examples of potential [[andesite]] are significantly weathered.<ref name=":13">{{Cite journal|last1=Friend|first1=C. R. L.|last2=Nutman|first2=A. P.|date=2010-11-01|title=Eoarchean ophiolites? New evidence for the debate on the Isua supracrustal belt, southern West Greenland|journal=American Journal of Science|language=en|volume=310|issue=9|pages=826–861|doi=10.2475/09.2010.04|bibcode=2010AmJS..310..826F|s2cid=129049164|issn=0002-9599|doi-access=free}}</ref>
The mafic volcanic sequences contain abundant meta-ultramafic rocks, including amphibolites, serpentinites, carbonated-[[peridotite]]s and peridotite.<ref name=":8">{{Cite journal|last1=Szilas|first1=Kristoffer|last2=Kelemen|first2=Peter B.|last3=Rosing|first3=Minik T.|date=2015-09-01|title=The petrogenesis of ultramafic rocks in the >3.7Ga Isua supracrustal belt, southern West Greenland: Geochemical evidence for two distinct magmatic cumulate trends|url=https://www.sciencedirect.com/science/article/pii/S1342937X1400238X|journal=Gondwana Research|language=en|volume=28|issue=2|pages=565–580|doi=10.1016/j.gr.2014.07.010|bibcode=2015GondR..28..565S|issn=1342-937X}}</ref> The majority of these are widely accepted to be intrusive in origin, representing ultramafic cumulates.<ref name=":8" /> Some peridotite lenses have been interpreted as [[Obduction|obducted]] [[Mantle (geology)|mantle]] fragments,<ref name=":14">{{Cite journal|last1=Friend|first1=C.R.L.|last2=Nutman|first2=A.P.|date=July 2011|title=Dunites from Isua, Greenland: A ca. 3720 Ma window into subcrustal metasomatism of depleted mantle|url=http://pubs.geoscienceworld.org/geology/article/39/7/663/130617/Dunites-from-Isua-Greenland-A-ca-3720-Ma-window|journal=Geology|language=en|volume=39|issue=7|pages=663–666|doi=10.1130/G31904.1|bibcode=2011Geo....39..663F|issn=1943-2682}}</ref> and used as evidence to support 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.<ref name=":15">{{Cite journal|last1=Waterton|first1=P.|last2=Guotana|first2=J. M.|last3=Nishio|first3=I.|last4=Morishita|first4=T.|last5=Tani|first5=K.|last6=Woodland|first6=S.|last7=Legros|first7=H.|last8=Pearson|first8=D. G.|last9=Szilas|first9=K.|date=2022-02-01|title=No mantle residues in the Isua Supracrustal Belt|journal=Earth and Planetary Science Letters|language=en|volume=579|pages=117348|doi=10.1016/j.epsl.2021.117348|bibcode=2022E&PSL.57917348W|s2cid=245454643|issn=0012-821X|doi-access=free}}</ref>
Metasedimentary rocks include [[banded iron formation]] and detrital [[quartzite]], likely representing a metamorphosed [[siliciclastic]] sedimentary rock.<ref name=":2" /><ref>{{Cite journal|last1=Nutman|first1=Allen P.|last2=Friend|first2=Clark R.L.|last3=Paxton|first3=Shane|date=August 2009|title=Detrital zircon sedimentary provenance ages for the Eoarchaean Isua supracrustal belt southern West Greenland: Juxtaposition of an imbricated ca. 3700Ma juvenile arc against an older complex with 3920–3760Ma components|url=https://linkinghub.elsevier.com/retrieve/pii/S0301926809000795|journal=Precambrian Research|language=en|volume=172|issue=3–4|pages=212–233|doi=10.1016/j.precamres.2009.03.019|bibcode=2009PreR..172..212N}}</ref> Although they do not form part of the supracrustal belt itself, the belt is hosted in and in places intruded by [[Tonalite-Trondhjemite-Granodiorite|tonalite-trondhjemite-granodiorite]] (TTG) [[Gneiss#Composition|orthogneisses]].
[[File:Timeline pic.jpg|500x400px|thumb|right|Cartoon of the age of various rock types formed during the evolution of the Isua Greenstone Belt. Note that the y-axis does not have a scale and vertical placement of the different lithologies has no significance.<ref name="Nutman 2007"/>]]
==Tectonics==
The tectonic setting in which the Isua Greenstone Belt formed remains contentious. Ideas can be broadly divided into [[Plate tectonics|plate tectonic]] models, in which the belt formed in one of several possible tectonic settings that exist on the modern day Earth,<ref name=":9" /><ref name=":11">{{Cite journal|last1=Friend|first1=Clark R. L.|last2=Nutman|first2=Allen P.|date=2010-11-01|title=Eoarchean ophiolites? New evidence for the debate on the Isua supracrustal belt, southern West Greenland|url=https://www.ajsonline.org/content/310/9/826|journal=American Journal of Science|language=en|volume=310|issue=9|pages=826–861|doi=10.2475/09.2010.04|bibcode=2010AmJS..310..826F|s2cid=129049164|issn=0002-9599|doi-access=free}}</ref> and non-plate tectonic or non-[[Uniformitarianism|uniformitarian]] models, in which the Isua Greenstone Belt formed in a tectonic regime that was different to the modern day Earth.<ref name=":5" /> Plate tectonic models can be further subdivided into those that argue that the Isua Greenstone Belt or parts of it represent an [[ophiolite]],<ref name=":9" /><ref name=":11" /> a sliver of [[Obduction|obducted]] [[oceanic crust]] and mantle, and those that argue that the belt represents an accretionary prism,<ref name=":10" /><ref name=":16">{{Cite journal|last1=Arai|first1=Tatsuyuki|last2=Omori|first2=Soichi|last3=Komiya|first3=Tsuyoshi|last4=Maruyama|first4=Shigenori|date=2015-11-01|title=Intermediate P/T-type regional metamorphism of the Isua Supracrustal Belt, southern west Greenland: The oldest Pacific-type orogenic belt?|url=https://www.sciencedirect.com/science/article/pii/S0040195115002875|journal=Tectonophysics|series=Special issue on Comparative tectonic and dynamic analysis of cratons, orogens, basins, and metallogeny|language=en|volume=662|pages=22–39|doi=10.1016/j.tecto.2015.05.020|bibcode=2015Tectp.662...22A|issn=0040-1951}}</ref> formed in a subduction zone. Non-plate tectonic models generally suggest a [[Heat-pipe tectonics|heat pipe]] or [[mantle plume]] origin for the belt.<ref name=":5" /><ref name=":6" /> This forms part of a much broader debate about when plate tectonics emerged on Earth, and whether the [[Archean subduction|Archaean Earth]] operated under a fundamentally different tectonic regime.
=== Ophiolite controversy ===
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.<ref name=":9" /> The interpretation of 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 the belt.<ref name=":13" /><ref>{{Cite journal|last1=Nutman|first1=Allen P.|last2=Friend|first2=Clark R. L.|date=2007-11-02|title=Comment on "A Vestige of Earth's Oldest Ophiolite"|url=https://www.science.org/doi/10.1126/science.1144148|journal=Science|language=en|volume=318|issue=5851|pages=746|doi=10.1126/science.1144148|pmid=17975049|bibcode=2007Sci...318..746N|s2cid=38347037|issn=0036-8075}}</ref> Other objections related to the composition of the dykes, which are unlike those found in modern-day ophiolites.<ref>{{Cite journal|last=Hamilton|first=Warren B.|date=2007-11-02|title=Comment on "A Vestige of Earth's Oldest Ophiolite"|url=https://www.science.org/doi/abs/10.1126/science.1144931|journal=Science|volume=318|issue=5851|page=746|language=EN|doi=10.1126/science.1144931|pmid=17975050|bibcode=2007Sci...318..746H|s2cid=3136755}}</ref>
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,<ref name=":11" /><ref>{{Cite journal|last1=Polat|first1=A|last2=Hofmann|first2=A. W|date=2003-10-10|title=Alteration and geochemical patterns in the 3.7–3.8 Ga Isua greenstone belt, West Greenland|url=https://www.sciencedirect.com/science/article/pii/S0301926803000950|journal=Precambrian Research|series=Early Archaean Processes and the Isua Geenstone Belt, West Greenland|language=en|volume=126|issue=3|pages=197–218|doi=10.1016/S0301-9268(03)00095-0|bibcode=2003PreR..126..197P|issn=0301-9268}}</ref><ref name=":17">{{Cite journal|last1=Furnes|first1=Harald|last2=Rosing|first2=Minik|last3=Dilek|first3=Yildirim|last4=de Wit|first4=Maarten|date=November 2009|title=Isua supracrustal belt (Greenland)—A vestige of a 3.8 Ga suprasubduction zone ophiolite, and the implications for Archean geology|url=https://linkinghub.elsevier.com/retrieve/pii/S0024493709001170|journal=Lithos|language=en|volume=113|issue=1–2|pages=115–132|doi=10.1016/j.lithos.2009.03.043|bibcode=2009Litho.113..115F}}</ref> and boninite-like amphibolites have been interpreted to represent metamorphosed boninites.<ref name=":12" /><ref name=":11" /><ref name=":17" /> However, subsequent studies have pointed out that the boninite-like amphibolites are in fact low-titanium basalts, with too little silica to classify as boninites,<ref name=":7" /> and recent geochemical modelling suggests that the entire volcanic compositional range at Isua can be explained without requiring a plate tectonic setting.<ref name=":6" />
A further line of evidence used to invoke an ophiolite origin for the Isua Greenstone Belt is the presence of [[peridotite]] lenses in the volcanic sequence, particularly two [[dunite]] lenses referred to as 'lens A' and 'lens B'.<ref name=":14" /> These were argued to represent mantle rocks on the basis of their geochemistry, textures,<ref>{{Cite journal|last1=Kaczmarek|first1=Mary-Alix|last2=Reddy|first2=Steven M.|last3=Nutman|first3=Allen P.|last4=Friend|first4=Clark R. L.|last5=Bennett|first5=Vickie C.|date=April 2016|title=Earth's oldest mantle fabrics indicate Eoarchaean subduction|journal=Nature Communications|language=en|volume=7|issue=1|pages=10665|doi=10.1038/ncomms10665|issn=2041-1723|pmc=4757760|pmid=26879892|bibcode=2016NatCo...710665K}}</ref> and the presence of apparently high-pressure minerals.<ref name=":14" /> 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 the belt.<ref name=":11" /> 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 [[Cumulate rock|cumulates]] formed in magma chambers that fed the eruption of volcanic rocks in the Isua Greenstone Belt.<ref name=":15" /> 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 the Isua Greenstone Belt is an ophiolite.
=== Accretionary wedge models ===
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.<ref name=":10" /> 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.<ref name=":16" /> However, this interpretation has been strongly contested on the basis that rock types and strain are extremely consistent across the various faults in the proposed accretionary wedge,<ref name=":5" /> and that peak metamorphic grades are consistent across the entire belt.<ref name=":3" />
=== Non-plate tectonic models ===
Non-plate tectonic models include heat-pipe and mantle plume models,<ref name=":5" /><ref name=":6" /> both of which suggest that the volcanic sequences at Isua formed through eruption of mantle derived magmas with minimal crustal input. In a heat-pipe model,<ref>{{Cite journal|last1=Moore|first1=William B.|last2=Webb|first2=A. Alexander G.|date=September 2013|title=Heat-pipe Earth|url=http://www.nature.com/articles/nature12473|journal=Nature|language=en|volume=501|issue=7468|pages=501–505|doi=10.1038/nature12473|pmid=24067709|bibcode=2013Natur.501..501M|s2cid=4391599|issn=0028-0836}}</ref> 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.<ref name=":5" /> This model can account for the mafic composition of pelitic sediments at Isua, suggesting there was little felsic crust present during its formation,<ref name=":6" /> and the relatively simple deformation and uniform metamorphic grade observed across the belt.<ref name=":3" /><ref name=":5" /> 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.<ref>{{Cite journal|last1=Nutman|first1=Allen P.|last2=Friend|first2=Clark R.L.|last3=Bennett|first3=Vickie C.|last4=Kranendonk|first4=Martin van|last5=Chivas|first5=Allan R.|date=June 2021|title=Geodynamic environment of the ca. 3800 Ma Outer Arc Group, Isua (Greenland)|url=http://www.ajsonline.org/lookup/doi/10.2475/06.2021.01|journal=American Journal of Science|language=en|volume=321|issue=6|pages=643–679|doi=10.2475/06.2021.01|bibcode=2021AmJS..321..643N|s2cid=238412385|issn=0002-9599}}</ref>
== Metamorphism ==
Following its formation, the Isua Greenstone belt has undergone two major metamorphic episodes. The first predates the formation of the <3.5 Ga Ameralik dykes <ref name=":2" /> and is associated with the Eoarchaean deformation at Isua. [[Amphibolite Facies|Amphibolite-facies]] conditions were reached across the belt between ~3.7 and 3.6 Ga.<ref name=":3" /><ref name="Rollinson" /><ref name=":18">{{Cite journal|last=Rollinson|first=Hugh|date=2002|title=The metamorphic history of the Isua Greenstone Belt, West Greenland|url=https://doi.org/10.1144/GSL.SP.2002.199.01.16|journal=Geological Society, London, Special Publications|volume=199|issue=1|pages=329–350|doi=10.1144/gsl.sp.2002.199.01.16|bibcode=2002GSLSP.199..329R|s2cid=129110356|issn=0305-8719}}</ref><ref name=":19">{{Cite journal|last1=Nutman|first1=Allen P.|last2=Bennett|first2=Vickie C.|last3=Friend|first3=Clark R. L.|last4=Yi|first4=Keewook|date=2020-07-15|title=Eoarchean contrasting ultra-high-pressure to low-pressure metamorphisms (1000°C/GPa) explained by tectonic plate convergence in deep time|url=https://www.sciencedirect.com/science/article/pii/S030192682030125X|journal=Precambrian Research|language=en|volume=344|pages=105770|doi=10.1016/j.precamres.2020.105770|s2cid=218921084|issn=0301-9268|hdl=1885/224133|hdl-access=free}}</ref><ref name=":20">{{Cite journal|last1=Frei|first1=Robert|last2=Rosing|first2=Minik T|last3=Waight|first3=Tod E|last4=Ulfbeck|first4=David G|date=2002-02-01|title=Hydrothermal-metasomatic and tectono-metamorphic processes in the Isua supracrustal belt (West Greenland): a multi-isotopic investigation of their effects on the earth's oldest oceanic crustal sequence|url=https://www.sciencedirect.com/science/article/pii/S0016703701007815|journal=Geochimica et Cosmochimica Acta|language=en|volume=66|issue=3|pages=467–486|doi=10.1016/S0016-7037(01)00781-5|bibcode=2002GeCoA..66..467F|issn=0016-7037}}</ref> Though higher pressure conditions have been suggested locally on the basis of Ti-humite group minerals in peridotites,<ref name=":14" /><ref name=":19" /> the reliability of these minerals to document high pressure processes has been questioned.<ref name=":15" /> The second event also reached amphibolite-facies conditions, and appears to have been a protracted event between ~2.9 and 2.6 Ga, followed by widespread retrogression of locally varying intensity.<ref name=":3" /><ref name="Rollinson" /><ref name=":18" /><ref name=":20" /> 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).
== Possible signs of very early life ==
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 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."<ref name="earlynaturelife">{{cite journal |vauthors=Mojzsis SJ, Arrhenius G, McKeegan KD, Harrison TM, Nutman AP, Friend CR |date=1996 |title=Evidence for life on Earth before 3,800 million years ago. |pmid=8900275 |journal=Nature |volume=384 |issue=6604 |pages=55–59 |doi=10.1038/384055a0 |bibcode=1996Natur.384...55M |hdl=2060/19980037618 |s2cid=4342620 |hdl-access=free }}</ref>
In August 2016, an Australia-based research team presented evidence that the Isua Greenstone Belt contains the remains of [[stromatolite]] microbial colonies that formed [[Eoarchean|approximately 3.7 billion years ago]].<ref name="naturelife">{{cite journal |last1=Nutman |first1=Allen |last2=Bennett |first2=Vickie |last3=Friend |first3=Clark |last4=Van Kranendonk |first4=Martin |last5=Chivas |first5=Allan |title=Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures |journal=Nature |date=2016 |doi=10.1038/nature19355 |pmid=27580034 |volume=537 |issue=7621 |pages=535–538|bibcode=2016Natur.537..535N |s2cid=205250494 |url=https://ro.uow.edu.au/cgi/viewcontent.cgi?article=5181&context=smhpapers }}</ref><ref name="bbcpease">{{cite news |last1=Pease |first1=Roland |title=Wavy Greenland rock features 'are oldest fossils' |url=https://www.bbc.com/news/science-environment-37235447 |access-date=31 August 2016 |publisher=BBC |date=31 August 2016}}</ref> However, their interpretations are controversial.<ref name="naturelife" /><ref name=":0">{{cite journal|last1=Allwood|first1=Abigail C.|date=2018|title=Reassessing evidence of life in 3,700-million-year-old rocks of Greenland|journal=Nature|volume=563|issue=7730|pages=241–244|bibcode=2018Natur.563..241A|doi=10.1038/s41586-018-0610-4|pmid=30333621|s2cid=52987320}}</ref><ref name=":1">{{Cite journal|last1=Nutman|first1=Allen P.|last2=Bennett|first2=Vickie C.|last3=Friend|first3=Clark R. L.|last4=Van Kranendonk|first4=Martin J.|last5=Rothacker|first5=Leo|last6=Chivas|first6=Allan R.|date=2019-09-01|title=Cross-examining Earth's oldest stromatolites: Seeing through the effects of heterogeneous deformation, metamorphism and metasomatism affecting Isua (Greenland) ~3700 Ma sedimentary rocks|journal=Precambrian Research|volume=331|pages=105347|doi=10.1016/j.precamres.2019.105347|s2cid=182575508|issn=0301-9268|url=https://ro.uow.edu.au/smhpapers1/777}}</ref> If these structures are stromatolites, they predate the oldest previously known stromatolites, found in the [[Dresser Formation]] in western Australia, by 220 million years.<ref name="naturelife" />
The complexity of the stromatolites found at Isua, if they are indeed stromatolites, suggest that life on Earth 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.<ref name="naturelife" /> This conclusion is supported in part by the instability of Earth's surface conditions 3.7 billion years ago, which included intense asteroid bombardment.<ref name="allwood" /> The possible formation and preservation of fossils from this period indicate that life may have evolved early and prolifically in Earth's history.<ref name="allwood" />
The stromatolite fossils appear wavy and dome-shaped, are typically {{convert|1|-|4|cm|in|1|abbr=on}} high, and were found in [[iron]]- and [[magnesium]]-rich [[Dolomite (rock)|dolomite]]s that had recently been exposed by melting snow.<ref name="bbcpease" /> The surrounding rocks suggest that the stromatolites may have been deposited in a shallow marine environment.<ref name="naturelife" /> While most rocks in the Isua Greenstone Belt are too [[metamorphic rock|metamorphically]] altered to preserve fossils, the area of stromatolite discovery may have preserved original sedimentary rocks and the fossils inside them.<ref name="allwood">{{cite news |last1=Allwood |first1=Abigail |title=Geology: Evidence of life in Earth's oldest rocks  |work=News and Views |publisher=Nature |date=31 August 2016}}</ref> However, some geologists interpret the structures as the result of deformation and alteration of the original rock.<ref name=":0" />
The ISB sedimentary layers containing 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]] [[zircon]]s dated to {{nowrap |3.695 ± 0.4 billion}} years old. All layers, including those bordering the stromatolites, experienced metamorphism and deformation after deposition, and temperatures not exceeding {{convert|550|C|F|-2|abbr=on}}.<ref name="naturelife" /><ref name=":0" />
The identity of the ISB features as stromatolites is controversial, because similar features may form through non-biological processes.<ref name="allwood" /><ref name=":0" /> 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.<ref name=":1" /><ref name="naturelife" /><ref name="allwood" /> However, others suggest that the rocks are so altered that any sedimentary interpretations are inappropriate.<ref name=":0" />
In 2016, geologist and [[Geology of Mars|areologist]] [[Abigail Allwood]] stated that the discovery of Isua stromatolites makes the emergence of [[extraterrestrial life|life on other planets]], including [[Mars]] early after its formation, more probable.<ref name="allwood" /> However, in 2018, she and a team of additional geologists published a paper that raises significant questions as to the origin of the structures, interpreting them as arising from deformation.<ref name=":0" /> Thus, the ISB stromatolites remain a subject of ongoing investigation.<ref name="bbcpease" />
== See also ==
*[[Archean subduction]]
*[[Nuvvuagittuq Greenstone Belt]]
==References==
{{reflist|30em}}
{{Greenstone belts}}
[[Category:Greenstone belts]]
[[Category:Eoarchean volcanism]]
[[Category:Volcanoes of Denmark]]
[[Category:Landforms of Greenland]]
[[Category:Volcanoes of North America]]