Early Proterozoic mafic dykes in the North Atlantic and Baltic cratons: field setting and chemistry of distinctive dyke swarms

Abstract

Abstract Widespread swarms of basic dykes intruded the Archaean North Atlantic craton (NAC) and the NW Baltic shield between 2.5 and 1.9 Ga. The oldest Mg-rich dykes and sills are 2.4–2.5 Ga and are associated with rift controlled acid and basic magmatism. In SW Greenland the main high-Mg swarms are c. 2.2 Ga. The high-Mg dykes have high Si, LILE and LREE and are low in Ca, Al, Ti and Nb. Isotopic compositions (Sr, Nd and Pb) demonstrate a high degree of crustal contamination. Individual dykes are remarkably constant in composition but there are large variations between adjacent dykes. Tholeiitic swarms were emplaced at c. 2.2 Ga in the southern part of the craton in SW and SE Greenland and Labrador. The majority are normal moderately evolved tholeiites with no marked LILE or LREE enrichment, a few c. 2.2 Ga comparatively mafic tholeiitic dykes show a LILE and LREE enrichment. Dense swarms of distinctive Fe-enriched hornblende-bearing tholeiitic dykes (the c. 2.0 Ga Kangâmiut dykes) intruded the northern part of the NAC in Greenland. Many are composite with intermediate centres. The Kangâmiut dykes were emplaced at depth during shearing in a regional N-S compressional regime and developed metamorphic mineral assemblages at the time of dyke injection. The 1.83 Ga Avayalik dykes from the eastern foreland of the Torngat orogen, northern Labrador show comparable syn-tectonic emplacement and autometamorphic assemblages. Emplacement was controlled by regional sinistral shearing after the first calc-alkaline igneous activity had occurred within the orogen.