Anatomy of an Early Archean gneiss complex: 3900 to 3600 Ma crustal evolution in southern West Greenland
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Research Article| May 01, 1993 Anatomy of an Early Archean gneiss complex: 3900 to 3600 Ma crustal evolution in southern West Greenland Allen P. Nutman; Allen P. Nutman 1Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 2601, Australia Search for other works by this author on: GSW Google Scholar Clark R. L. Friend; Clark R. L. Friend 2Department of Geology, Oxford Brookes University, Headington, Oxford OX3 OBP, England Search for other works by this author on: GSW Google Scholar Peter D. Kinny; Peter D. Kinny 3Grant Institute, University of Edinburgh, Edinburgh EH9 3JW, Scotland Search for other works by this author on: GSW Google Scholar Victor R. McGregor Victor R. McGregor 4Atammik, Maniitsoq 3912, Greenland Search for other works by this author on: GSW Google Scholar Geology (1993) 21 (5): 415–418. https://doi.org/10.1130/0091-7613(1993)021<0415:AOAEAG>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Allen P. Nutman, Clark R. L. Friend, Peter D. Kinny, Victor R. McGregor; Anatomy of an Early Archean gneiss complex: 3900 to 3600 Ma crustal evolution in southern West Greenland. Geology 1993;; 21 (5): 415–418. doi: https://doi.org/10.1130/0091-7613(1993)021<0415:AOAEAG>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Early Archean complex of southern West Greenland consists of polyphase, tonalitic-trondhjemitic-granodioritic (TTG) and granitic Amîtsoq gneisses with inclusions of volcanic and sedimentary rocks, gabbros, and ultramafic rocks. In this complex, rocks of similar appearance and composition were found to be of different ages by U-Pb zircon dating; the Amîtsoq gneisses comprise 3870,3820-3810, 3760, 3730, 3700, and 3625 Ma TTG and 3660-3650 and 3625 Ma granites, and their inclusions belong to several supracrustal sequences with a similar spread of ages. These results show that the complex grew by episodic addition of new TTG and welding together of rocks of different ages. A possible plate-tectonics scenario is as follows: Melting of subducted mafic (oceanic) crust formed ≥3700 Ma microcontinents consisting of TTG suites with predominantly mafic inclusions. At 3650 Ma, collision between microcontinents caused crustal thickening, high-grade metamorphism, and emplacement of leucogranites. At 3625 Ma,subduction at the edge of the >3625 Ma continental mass created a new crustal addition comprising both TTG and granite, while granites were emplaced into the >3625 Ma continental mass. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.Keywords:
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1. The early Archaean rocks in northern Labrador can be subdivided into the ea. 3.78 Ga Nulliak supracrustal association, the migmatitic Uivak I gneisses, the dominant phase of which was emplaced at ea. 3.73 Ga, and the Uivak II augen gneiss. Inherited low-U rounded inclusions within igneous zircons in the Uivak I gneisses have ages between 3.73 and 3.86 Ga and are more likely to have been derived from a pre-existing high-grade metamorphic gneiss complex than from the Nulliak association. In the early Archaean there were probably several rapid cycles of sedimentary deposition and volcanism followed by emplacement of major plutons. Mid Archaean gneisses are more abundant in northern Labrador than previously realised. The late Archaean metamorphic history of these gneisses is different from the history of the early Archaean gneisses. Whereas an important part of the mid Archaean suite was emplaced in granulite facies and retrogressed at the time of granitoid veining at ea. 2.99 Ga, the major part of the early Archaean rocks were reworked under granulite facies conditions in a sequence of closely spaced events between 2. 7 and 2.8 Ga. The two groups of gneisses had different metamorphic histories until ea. 2.7 Ga, but late and post-tectonic granites of 2.5- 2. 7 Ga age cut across both. It is suggested that the terrane model in southern West Greenland can be extended to Labrador and that tectonic intercalation of early and mid Archaean gneisses took place around 2.7 Ga. Correlation between the Maggo gneisses around Hopedale, mid Archaean gneisses in northernmost Labrador and gneisses from the Akia terrane in West Greenland is suggested. Like the Malene supracrustals in West Greenland the Upernavik supracrustals in Labrador are composite associations, the youngest of which are thought to have been deposited around 2. 7 Ga.
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The boundary between the Archaean gneiss block and the Nagssugtoqidian mobile belt (Escher et al., 1976) runs through the area south-east of the inner part of Søndre Strømfjord (fig. 42). The Archaean block here is mainly composed ofbrownish hypersthene gneisses cut by numerous basic dykes, the Kangåmiut dyke swarm. The Nagssugtoqidian mobile belt consists mainly of light grey biotite gneisses, formed by shearing and recrystallisation of gneisses from the Archaean block, together with numerous amphibolite layers, derived from the Kangâmiut dykes.
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Late Archean (3.0-2.5 Ga) greenstone belts are a major component of the Superior Province of the Canadian Shield where alternating, metavolcanic - rich and metasedimentary - rich subprovinces form a prominent central striped region bordered in part by high-grade gneiss subprovinces, the Pikiwitonei and Minto in the north, and the Minnesota River Valley in the south. The high-grade gneiss subprovinces are characterized by granulite facies gneiss of plutonic and supracrustal origin, and by abundant plutonic rocks. Minnesota River Valley has rocks older than 3.5 Ga; absolute ages of Pikiwitonei and Minto rocks are unknown but Minto does have north-south structural trends distinctive from the dominant east-west structures of Superior Province. A discussion follows.
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Gneisses which crop out along the southern coast of Ameralik between Kangimut-sangmissoq and Qasigianguit (K-s-Q) are the subject of long-standing controversy concerning their relationship to the early Archean Amitsoq gneisses of the Godthaab district. On the basis of field observations, it was argued that gneisses at Kangimut-sangmissoq and Qasigianguit are correlatives of the early Archean Amitsoq gneisses. The data were reexamined and it is concluded that the K-s-Q gneisses represent an addition of substantially juvenile mantle-derived material to the Archean craton of West Greenland during the late Archean times. Some of the parent magmas have undergone interaction with older crust, as indicated by Pb isotope evidence for contamination with Amitsoq-derived Pb. However, the positive epsilon Nd(I) value for the K-s-Q gneisses firmly rules out any significant material contribution from the Amitsoq gneisses to the K-s-Q gneisses.
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