Abstract

An extensive sheet of presumed middle Archaean supracrustal rocks within high-grade orthogneisses in inner Godthåbsfjord includes a section 3 km thick with exceptional preservation of pillow structures in low K tholeiitic amphibolites unambiguously indicating way up. The amphibolites are associated with pillow-structured and pyroclastic, komatiitic ultramafic schists, dunitic-harzburgitic sills, a stratiform gabbro, pyritous metacherts and quartzo-feldspathic gneisses and schists with field and chemical characteristics suggesting they formed as acid-intermediate tuffaceous sediments.

The sheet is tectonically overlain by older orthogneisses (Amîtsoq) with tracts and enclaves of the early Archaean Isua/Akilia supracrustal association. It is underlain by younger orthogneisses and granites with tracts of older, variably reworked Amîtsoq gneisses. Rocks of the higher level were emplaced on a tectonic slide which marks the upper boundary of the supracrustal sheet. The original lower boundary with Amîtsoq gneisses in the lower level may also have been a slide which has been obscured by emplacement of younger orthogneisses. Composite dome-in-dome structures with intervening synclinal cusps were superimposed on the sub-horizontal layer cake structure of the supracrustal sheet and the higher and lower levels of orthogneisses. Syntectonic granitic and amphibolite dykes (Kangilíngua dykes) were emplaced in parts of the synforms, whilst granite emplacement and feldspar blastesis took place in the cores of some domes. The dome and cusp system appears to be part of a regional series of late Archaean, steeply inclined to upright structures trending northeast in the Godthåbsfjord region. In inner Godthåbsfjord the system evolved in response to buoyancy forces generated by injection of basaltic magmas and partial melting of orthogneisses at depth within a regime of regional compression associated perhaps with transcurrent intraplate motions.

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