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.
Research Article| June 01, 1979 Structural evolution of the Archean rocks in Ivisârtoq and the neighboring inner Godthåbsfjord region, southern West Greenland R. P. Hall; R. P. Hall 1Department of Geology, Portsmouth Polytechnic, Burnaby Road, Portsmouth PO1 3QL, England Search for other works by this author on: GSW Google Scholar C.R.L. Friend C.R.L. Friend 2Department of Geology, Oxford Polytechnic, Headington, Oxford, England Search for other works by this author on: GSW Google Scholar Geology (1979) 7 (6): 311–315. https://doi.org/10.1130/0091-7613(1979)7<311:SEOTAR>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation R. P. Hall, C.R.L. Friend; Structural evolution of the Archean rocks in Ivisârtoq and the neighboring inner Godthåbsfjord region, southern West Greenland. Geology 1979;; 7 (6): 311–315. doi: https://doi.org/10.1130/0091-7613(1979)7<311:SEOTAR>2.0.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 Reconnaissance and some detailed mapping of Archean rocks in the inner Godthåbsfjord region has revealed a remarkable set of interference fold structures. These folds have amplitude and wavelength values on the order of 30 km. They are delineated by Malene metavolcanic, meta-sedimentary, and ultramafic bodies (3,100 m.y. old) and two generations of gneisses. In the Ivisârtoq area at the center of the region, evidence is preserved of early interleaving by thrusting of the Malene rocks with the ancient Amîtsoq gneiss (3,700 m.y. old). These two juxtaposed complexes then underwent two slightly oblique phases of major folding during which the younger Nûk gneiss (2,900 m.y.) was injected. 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.
from the Akia terrane, southern West Greenland, supported by Sm-Nd isotope geochemistry, document its middle Archaean accretional history and provide new evidence about the location of its northern boundary. Zircon populations in grey gneiss and inherited zircons in granite show that magmatic accretion of new continental crust, dominated by intrusion of tonalite sheets in a convergent island arc setting, occurred between c. 3050 and 3000 Ma, around and within a c. 3220 Ma continental core. In the central part of the terrane, tonalite sheets were intercalated with older supracrustal rocks of oceanic affinity by intrusion, thrusting and folding during the Midterhøj and Smalledal deformation phases of Berthelsen (1960). Continued tonalite injection led to a thermal maximum with granulite facies conditions at c. 2980 Ma, dated by metamorphic zircons in grey gneiss. The metamorphic maximum was contemporaneous with upright, angular folds of the Pâkitsoq deformation phase. Within a few million years followed high-grade retrogression and intrusion of two large dome-shaped tonalite-granodiorite complexes, granites s.l. derived from remobilisation of grey gneiss, and post-kinematic diorite plugs. Whereas the relative chronology of these events is firmly established from field observations, zircons from the post-granulite facies intrusions all yielded statistically indistinguishable emplacement ages of c. 2975 Ma. These results show that crustal growth occurred in several short-lived events starting at c. 3220 Ma, and that final maturation and stabilisation of new, thick continental crust took place rapidly (within c. 20 Ma) at c. 2975 Ma.
The complex range of orthogneisses has been subdivided on the basis of field characteristics into an old polyphase group, grey phlebitic gneisses, younger varieties of pink granitic gneisses that occur principally as extensive sheets, mixed orthogneisses with schlieric facies and undifferentiated gneisses with dioritic facies. Mafic sheets, now amphibolites, were emplaced at various stages in the evolution of the gneisses. Enclaves and sheets of supracrustal rocks include paragneisses, graphitic schists, marble, amphibolite and stratiform gabbroic complexes with anorthosite. Nappes with curvilinear hinge lines and belts of mylonite developed in high amphibolite conditions after emplacement of the sheets of pink granitic rocks (now variably deformed gneisses) into the old orthogneisses and supracrustal rocks. Principal displacements in the mylonites were shallow and N-directed. The nappes and mylonites were deformed by upright-inclined folds with north-westerly vergence. The nappes, mylonites and north-westerly verging folds are presumed to be Caledonian. N- and S-directed extensions indicated by small-scale ductile structures younger than the regional folding suggest that longitudinal collapse predominated in this part of the Caledonian belt in the south-west of Dove Bugt.
40 Ar/ 39 Ar and Rb–Sr mineral ages have been determined from various lithologies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb–Sr muscovite ages of c . 428, c . 421 and c . 413 Ma obtained from Moine Thrust Zone mylonites are interpreted to date closely regional thrusting during the Late Silurian to Early Devonian. 40 Ar/ 39 Ar muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb–Sr analyses of muscovites from the same samples; it is likely that this discrepancy results from a component of extraneous or ‘excess’ argon. 40 Ar/ 39 Ar hornblende ages and Rb–Sr and 40 Ar/ 39 Ar muscovite ages obtained from structurally higher metamorphic units in the Caledonian thrust nappes generally range between c . 440 Ma and c . 410 Ma. These ages are interpreted to date cooling during and following ‘D 2 ’ regional thrusting and folding within internal sectors of the nappe sequence. A possible tectonic model involves the Silurian collision of Baltica with Scottish segments of Laurentia resulting in the Scandian orogeny and broadly coeval Moine Thrust Zone. D 2 structures were superimposed on structures and metamorphic fabrics formed during a regional Mid-Ordovician tectonothermal event dated previously at c . 470–460 Ma. Syn-D 2 temperatures were generally >600°C and sufficient to achieve more or less complete thermal rejuvenation of Rb–Sr and 40 Ar/ 39 Ar isotopic systems in muscovite and hornblende, even in areas of low D 2 strain.
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