U-Pb zircon study of tectonically bounded blocks of 2940-2840 Ma crust with different metamorphic histories, Paamiut region, South-West Greenland: implications for the tectonic assembly of the North Atlantic craton

New fieldwork, map interpretation, petrography and single zircon U-Pb geoch ronology has allowed the identification of different crustal blocks in the Paamiut region, in the southern portion of the West Greenland Archaean Crat on. Changes of metamorphic grade from only amphibolite facies to granulite facies (some subsequently retrogressed) corresponds with zones of Archaean high strain ductile deformation +/- mylonites. U-Pb zircon dates are presen ted for the TTG (tonalite, trondhjemite, granodiorite) protoliths from each block in the Paamiut region, and the southern portion of the previously id entified Tasiusarsuaq terrane lying to the north. The southern part of the Tasiusarsuaq terrane contains 2880-2860 Ma TTG rocks and underwent amphibol ite facies metamorphism. Structurally underneath the Tasiusarsuaq terrane t o the: south is the Sioraq block containing 2870-2830 Ma TTG rocks partly r etrogressed from granulite facies. Structurally underneath and to the south is the Paamiut block, dominated by 2850-2770 Ma granodioritic rocks that h ave only undergone amphibolite facies metamorphism. Also structurally overl ying the Paamiut block. but cropping out separately from the Sioraq block, is the Neria block. This appears to be dominated by 2940-2920 Ma gneisses t hat have been totally retrogressed from granulite facies and strongly defor med. In the southernmost part of the region the Neria block overlies the gr eenschist to lowermost amphibolite facies Sermiligaarsuk block that contain s the greater than or equal to 2945 Ma Tartoq Group. Rocks from all the blo cks record ancient loss of Pb from zircons and some new zircon growth at 28 20 Ma, interpreted to indicate a high grade metamorphic event at that time, including granulite facies metamorphism in the Sioraq and Neria blocks. Th e blocks of different metamorphic grade are interpreted to have moved to th eir current positions after the 2820 Ma metamorphism, explaining the change in metamorphic history across some mylonites and ductile shear zones which deform and retrogress granulite facies textures. The juxtaposed blocks and their contacts were subsequently folded under amphibolite facies condition s. The contacts are cut by undeformed Palaeoproterozoic dolerite dykes whic h post-date amphibolite facies metamorphism. These results, together with p reviously published data from the Godthabsfjord region (north of Paamiut) s hows that the North Atlantic Craton in West Greenland from Ivittuut in the south to Maniitsoq in the north ( similar to 550 km) consists of a mosaic o f ductile fault-bounded packages that attained their present relative posit ions in the late Archaean. (C) 2001 Elsevier Science B.V. All rights reserv ed.