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Geological Society, London, Special Publications; 1987; v. 27; p. 261-275;
DOI: 10.1144/GSL.SP.1987.027.01.22
© 1987 Geological Society of London

Mid-Archaean basic magmatism of southern West Greenland

R. P. Hall & D. J. Hughes

Department of Geology, Portsmouth Polytechnic, Portsmouth PO1 3QL, UK

C. R. L. Friend

Department of Geology and Physical Sciences, Oxford Polytechnic, Oxford OX3 0BP, UK

Three groups of metamorphosed basic igneous rocks post-date the Amîtoq gneisses (c. 3700 Ma) and pre-date the Nûk gneisses (c. 2900 Ma) in the Archaean craton of southern West Greenland. These are the Malene metavolcanic amphibolites, the Ameralik dykes within the Amîtsoq gneisses and a series of large metagabbro-anorthosite complexes. The Malene rocks range from komatiitic to tholeiitic in chemical affinities and occur as extensive intermixed horizons throughout the craton. Metavolcanic amphibolites which are intercalated with the older Amîtsoq gneisses are chemically similar to those which are isolated from them. They probably all represent lavas erupted in an oceanic environment, some of which were subsequently tectonically intercalated with the older sialic crust. Significant geochemical differences between the Malene metavolcanic rocks and the Ameralik basic dykes, which intrude the Amîtsoq gneisses, suggest that these dykes could not have acted as feeders to the Malene lavas. The petrogenesis of the Malene rocks is considered in terms of normative behaviour and of the system CaO-MgO-Al2O3-SiO2 (CMAS). Both demonstrate the importance of olivine and clinopyroxene in the evolution of the Malene magmas and suggest that they were derived from relatively garnet-depleted and clinopyroxene-enriched mantle compared to average modern mantle. Evidence for a complementary aluminous portion of the Archaean mantle is provided by the abundance of metagabbro-anorthosite rocks and associated aluminous tholeiitic metavolcanic amphibolites.





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R. P. Hall and D. J. Hughes
Early Precambrian crustal development: changing styles of mafic magmatism
Geological Society, London, Memoirs, 1995; 16: 25 - 35.
[Abstract] [PDF]