A new fragment of the early earth crust: the Aasivik terrane of West Greenland

The Aasivik terrane is a similar to 1500 km(2) complex of gneisses dominate d by similar to 3600 Ma components, which has been discovered in the Archae an craton of West Greenland. similar to 20-50 km south of the Paleoproteroz oic 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 giv en imprecise SHRIMP U-Pb zircon ages of 3550-3780 Ma with strong loss of ra diogenic lead and new growth of zircon probably associated with a granulite facies metamorphic event(s) at similar to 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 mafi c 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 t(zircon) epsilon (Nd) values of 2.3-5.4. Given the ir composition and positive epsilon (Nd) values, they probably represent me lts of only slightly older juvenile crust. A reconnaissance SHRIMP U-Pb stu dy of a sample of metasedimentary rock from the Ukaleq gneiss complex found similar to 2750-2900 Ma zircons of probable detrital origin and that two o r more generations of 2700-2500 Ma metamorphic zircons are present. This gn eiss complex is provisionally interpreted as a late Archaean accretionary w edge. A sample of banded granulite facies gneiss from a complex of banded g neisses south of the Aasivik terrain here named the Tasersiaq gneiss comple x has yielded two zircon populations of 3212 +/- 11 and 3127 +/- 12 Ma. Con tacts between the three gneiss complexes are mylonites which are locally cu t by late-post-kinematic granite veins with SHRIMP U-Pb zircon ages of simi lar to 2700 Ma. The isotopic character and the relationships between the li thologies from the different gneiss complexes suggest the assembly of unrel ated rocks along shear zones between 2800 and 2700 Ma. The collage of Archa ean gneiss complexes were intruded by A-type granites, here named the Umiat siaasat granites, at similar to 2700 Ma, later than the tectonic intercalat ion of the gneiss complexes. (C) 2001 Elsevier Science B.V. All rights rese rved.