EARLY ARCHAEAN BASIC MAGMATISM IN WEST GREENLAND - GEOCHEMISTRY OF THE AMERALIK DYKES

This paper describes the geochemistry of pre-3000 m.y. metadolerite (Ameralik) dykes from the well documented areas of Isua, Godthaabsfjord and Ameralik in West Greenland. The dykes fall conveniently into three divisions: (A) a more magnesian range of basaltic compositions having low K2O contents and flat REE patterns, (B) a less magnesian range of basaltic compositions with more fractionated trace element geochemistry, and (C) an 'Ugpik' group having compositions more consistent with the accumulation of pyroxenes in basaltic magma. Within Group A may be distinguished two minor variants, additional to the main group, one of which probably represents the influence of secondary processes.The secondary influx of elements by residence contamination has been examined in the undeformed dykes at Isua, and is shown to have a negligible effect on the dyke interiors. Comparison of the Isua dykes, all belonging to group A, with more deformed members of Group A in Godthaabsfjord allows constraints to be placed on element migration during deformation and metamorphism affecting the latter area. The original igneous geochemical affinity of the dykes of group B, which are not represented in the well preserved Isua enclave, can thereby be defined with greater confidence.Trace element fractionation tests in conjunction with a least-squares petrographic mixing model are consistent with groups A and B belonging to a single igneous fractionation series.The Ameralik dykes are shown to have much in common geochemically with Archaean basalt lavas present in most greenstone belts, but the dykes do not appear to have supplied a typical greenstone belt assemblage. It is argued that the Archaean tholeiite association has no exact geochemical analogue among recent examples of low-K tholeiite, and that geochemical comparisons therefore provide little information on the tectonic setting in which greenstone belts comparisons therefore provde little information on the teactonic setting in which greenstons beltsdeveloped. The occurrence of low-K tholetitic dykes in the interior of a continental mass is without parallel in recent geological history (except where the continent has subsequently rifted apart), and implies an Archaean sub-sialic mantle structure different from the present-day subcontinental mantle. © 1979 Oxford University Press.