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.
SHRIMP U–Pb zircon isotopic data have been obtained for four samples collected from granitoids and paragneisses in the Fraser Complex, a large composite metagabbroic body cropping out in the Mesoproterozoic Albany‐Fraser Orogen of Western Australia. The data are combined with the results of field mapping and petrographic analysis to revise a model for the geological evolution of the Fraser Complex. Three main phases of deformation are recognised in the Fraser Complex (D1–3) associated with two metamorphic events (M1–2), which involve four distinguishable episodes of recrystallisation. The first metamorphic event recognised (M1a/D1) reached granulite facies and is characterised by peak T ≥800°C and P = 600–700 MPa. A syn‐M1a/D1 charnockite has a U–Pb SHRIMP zircon age of 1301 ± 6 Ma, which also provides an estimate for the age of intrusion of Fraser Complex gabbroic rocks. Disequilibrium textures comprising randomly oriented minerals (M1b), consistent with approximately isobaric cooling, formed in various lithologies in the interval between D1 and D2. Post‐D1, pre‐D2 granites intruded at 1293 ± 8 Ma and were foliated during the D2 event, which culminated in the burial of the Fraser Complex to depths equivalent to 800–1000 MPa. Following burial, pyroxene granulites on the western boundary of the complex were pervasively retrogressed to garnet amphibolite (M2a). An igneous crystallisation age of 1288 ± 12 Ma from a syn‐M2a aplite dyke suggests that retrogression may have occurred only a few millions of years after the peak of granulite facies metamorphism. Exhumation to depths of less than ∼400 MPa occurred within ∼20–30 million years of the M2a pressure peak. Associated deformation (D3) is characterised by the development of mylonite and transitional greenschist/amphibolite facies disequilibrium textures (M2b). Keywords: Albany‐Fraser OrogenFraser ComplexgeochronologyMesoproterozoic
Abstract Archean basement inliers within the Northern Highland terrane (NHT), Scottish Caledonides, have been correlated with the Lewisian Gneiss Complex of the Laurentian foreland. New zircon U-Pb ages indicate that the NHT basement contains evidence for magmatism at 2823–2687 Ma and 1772–1655 Ma. The first group compares with crystallization ages of the foreland Archean gneisses. However, the second group, and a supracrustal unit, formed ∼100–250 m.y. after the youngest major phase of juvenile magmatism and sedimentation in the foreland. Also, there is no indication within the NHT basement of the Paleoproterozoic mafic and felsic intrusions common within the foreland, leading us to conclude that there is no firm basis for correlation of the two crustal blocks. The Caledonian Moine thrust, which separates the foreland and the NHT basement, is thought to have reworked a Grenvillian suture indicated by the presence of the ca. 1100–1000 Ma Eastern Glenelg eclogites. On the basis of the new isotopic data, we propose that the NHT basement was a fragment of Baltica that was emplaced onto Laurentia during the Grenvillian orogeny, representing a further example of basement terrane transfer in the circum–North Atlantic orogens.
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