A new fragment of the early earth crust: the Aasivik terrane of West Greenland |
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| Institution: | 1. Kobenhavns Universitet, Geologisk Museum, Øster Voldgade 5-7, DK-1350 Kobenhavn K, Denmark;2. Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Kobenhavn K, Denmark;3. RSES, The Australian National University, Canberra ACT 0200, Australia;1. Department of Geology and Geography, West Virginia University, 98 Beechurst Avenue, Brooks Hall, Morgantown, WV, 26506, USA;2. Department of Geology, Kent State University, 221 McGilvrey Hall, Kent, OH, 44242, USA;3. Via Alessandro Volta 16, 22070, Appiano Gentile, Como, Italy;4. Department of Earth and Biological Sciences, Loma Linda University, Loma Linda, CA, 92350, USA;1. LNEG – Laboratório Nacional de Energia e Geologia, Apartado 7586, 2610-999 Amadora, Portugal;2. CeGUL – Centro de Geologia, Universidade de Lisboa, 1749-016 Lisboa, Portugal;3. IEE-USP – Instituto de Energia e Ambiente, Universidade de São Paulo, Rua do Lago, 562, CEP: 05508-080 São Paulo, Brazil;1. Institute of Geoscience, Frankfurt University, Altenhöferallee 1, 60438 Frankfurt, Germany |
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| Abstract: | The Aasivik terrane is a ～1500 km2 complex of gneisses dominated by ～3600 Ma components, which has been discovered in the Archaean craton of West Greenland, ～20–50 km south of the Paleoproterozoic Nagssugtoqidian orogen. The Aasivik terrain comprises granulite facies tonalitic to granitic gneisses with bands of mafic granulite, which include disrupted mafic dykes. Four gneiss samples of the Aasivik terrain have given imprecise SHRIMP U–Pb zircon ages of 3550–3780 Ma with strong loss of radiogenic lead and new growth of zircon probably associated with a granulite facies metamorphic event(s) at ～2800–2700 Ma. To the Southeast, the Aasivik terrane is in tectonic contact with a late Archaean complex of granitic and metapelitic gneisses with apparently randomly distributed mafic and ultramafic units, here named the Ukaleq gneiss complex. Two granitic samples from the Ukaleq gneiss complex have U–Pb zircon ages of 2817 ± 10 and 2820 ± 12 Ma and tzircon εNd values of 2.3–5.4. Given their composition and positive εNd values, they probably represent melts of only slightly older juvenile crust. A reconnaissance SHRIMP U–Pb study of a sample of metasedimentary rock from the Ukaleq gneiss complex found ～2750–2900 Ma zircons of probable detrital origin and that two or more generations of 2700–2500 Ma metamorphic zircons are present. This gneiss complex is provisionally interpreted as a late Archaean accretionary wedge. A sample of banded granulite facies gneiss from a complex of banded gneisses south of the Aasivik terrain here named the Tasersiaq gneiss complex has yielded two zircon populations of 3212 ± 11 and 3127 ± 12 Ma. Contacts between the three gneiss complexes are mylonites which are locally cut by late-post-kinematic granite veins with SHRIMP U–Pb zircon ages of ～2700 Ma. The isotopic character and the relationships between the lithologies from the different gneiss complexes suggest the assembly of unrelated rocks along shear zones between 2800 and 2700 Ma. The collage of Archaean gneiss complexes were intruded by A-type granites, here named the Umiatsiaasat granites, at ～2700 Ma, later than the tectonic intercalation of the gneiss complexes. |
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