Field relations, petrography and geochemistry of Archaean amphibolite dykes and malene supracrustal amphibolites, northwest Buksefjorden, southern West Greenland

Abstract

Abstract The Ameralik dyke swarm intrudes Amitsoq tonalitic—granitic gneisses, ca. 3750 Ma, and older supracrustal enclaves that are believed to have formed a basement to Malene supracrustal amphibolites and metasedimentary gneisses in northwest Buksefjorden. Eight varieties of Ameralik amphibolite dykes have been recognised on the basis of field relations and petrography. Malene metabasites, listed in order of decreasing abundance, are subdivided into (a) banded amphibolites derived by deformation of pillows, pillow breccias and agglomerates, (b) homogeneous amphibolites interpreted as lavas and sills, (c) leucocratic amphibolites that resemble many of the leucogabbros associated with the stratiform anorthosite complexes in southern West Greenland, (d) garnetiferous amphibolites regarded as sills, (e) ultramafic rocks forming discontinuous sheets and lenses regarded as picritic flows or sills, and (f) grey plagioclase—quartz rocks which may be related to oceanic plagiogranites. The Malene amphibolites and certain metasediments are intruded by two principal varieties of Malene amphibolite dykes that are similar to two main categories of Ameralik dyke. Rare preservation of primary contact phenomena of Ameralik dykes suggests intrusion into shear fractures, but conditions of intrusion of Malene dykes are unknown. REE patterns and variation of elements considered to have remained immobile during post-magmatic deformation and high-grade metamorphism show that Ameralik and Malene dykes had a similar magmatic history, having formed as low-K tholeiites with enrichment in Fe, Cr and Ni, like tholeiitic rocks in other Archaean terranes. Some Malene dykes are more evolved than Ameralik dykes. Malene homogeneous amphibolites are less evolved than most Ameralik and Malene dykes but lie on similar evolutionary trends. Homogeneous amphibolites have LREE-depleted patterns unlike most Ameralik and Malene dykes with slight LREE enrichment. Certain Ameralik and Malene dykes have LREE-depleted patterns and may have been feeders to parental lavas and sills of the homogeneous amphibolites. Malene garnetiferous amphibolites had an independent magmatic evolution. The age of the Ameralik and Malene dykes and Malene supracrustal amphibolites remains to be established isotopically. On the assumption that they are broadly contemporaneous and related to late Archaean processes, the dykes and supracrustal amphibolites can be integrated into a pattern of events linking the formation of the Malene and related amphibolites with the genesis of the enormous volumes of Nuk tonalitic—granitic gneisses and their equivalents, ca. 3000-2800 Ma, in the Archaean block of southern West Greenland.