Contrasting styles of tectonism and magmatism in the late Archaean crustal evolution of the northeastern part of the Ivisârtoq region, inner Godthåbsfjord, southern West Greenland

Abstract

Varieties of late Archaean gneisses recognized on the basis of field, petrographic and chemical data include multiphase tonalitic—granitic gneisses with abundant enclaves of amphibolite, granodioritic gneisses largely devoid of enclaves, leucocratic schlieric gneisses with pods of amphibolite and hornblendite and grey granitic gneisses with locally abundant enclaves of amphibolite. The bulk of the gneisses are correlated with the Nûk gneisses (c. 3000–2900 Ma) of outer Godthåbsfjord.Most of the gneisses were emplaced during a period of horizontal tectonic interleaving of thick slices of Amitsoq gneisses (c. 3700 Ma) and younger Malene supracrustal rocks (mainly amphibolites older than c. 2900 Ma). Interleaving took place on tectonic slides during regional, horizontal, heterogeneous simple shear movements. Flattening strains modified Ameralik amphibolite dykes in the Amitsoq gneisses and gave rise to S fabrics (banding and schistosity) in the Malene rocks, the S fabrics facilitating emplacement of voluminous Nûk gneisses mainly as broadly concordant sheets. Potash feldspar blastesis and localized partial melting occurred in some of the earlier phases of Nûk gneisses and possibly the Amitsoq gneisses, but the timing of these processes in relation to this period of emplacement is uncertain.High temperatures appear to have been sustained into the second period of deformation when large, mainly upright domes and synforms, some cuspate, were superimposed on the broadly horizontal pile of Amitsoq gneisses, Malene rocks and Nûk gneisses. Tectonic fabrics are dominated by mineral and ribbing lineations coaxial with folds of this second period. Magmatic emplacement of sheets of granitic gneisses and pegmatites continued, many of the sheets being injected coplanar with the second phase parasitic folds. Pegmatites and schlieric gneisses were generated by syntectonic and younger feldspar blastesis and partial melting. The close of late Archaean magmatism is marked by the post-tectonic Qôrqut granite and pegmatite complex (c. 2500 Ma).The horizontal tectonic movements, emplacement of gneisses, generation of domes and synforms and syntectonic injection of younger granitic gneisses appear to have taken place in the interval between c. 3000–2600 Ma ago. Similar relations between late Archaean tectonism and magmatism have been recognized elsewhere in the Archaean block of Greenland, but syntectonic injection of granitic gneisses and transformation of older gneisses to schlieric types appear to be more widespread in this part of the Archaean block in Ivisârtoq.