Precise U–Pb ages and geochemistry of Palaeoproterozoic mafic dykes from southern West Greenland: Linking the North Atlantic and the Dharwar cratons

Abstract

Palaeoproterozoic magmatism throughout the North Atlantic Craton (NAC) has produced dyke swarms of diverse orientations and emplacement ages. In southern West Greenland, previously identified swarms include the ca. 2500Ma Kilarsaarfik dykes, ca. 2215Ma boninitic norite (BN) dykes, and the abundant ca. 2040Ma Kangâmiut and coeval 2050–2030Ma MD3 dykes. Additional insight into the distribution of swarms is provided by new baddeleyite U–Pb ID-TIMS dates interpreted as emplacement ages for six dykes. The geochemistry for a total of 42 samples on 40 dykes (including the dated dykes) provides some constraints on their petrogenesis. Two E–W trending dykes yield ages of 2365±2Ma and 2374±4Ma, representing a hitherto unknown magmatic event. We propose the name Grædefjord dykes for dykes of this age in southern West Greenland. One NE–SW trending dyke is dated at 2209±5Ma, close to a previously published 2214±10Ma age for a N–S trending dyke (member of BN dykes). Three dykes yield MD3 ages of 2053±2Ma (E–W trending), 2049±6Ma (N–S trending) and 2042±2Ma (WNW–ESE trending). Ca. 2.21Ga old and predominantly NE–SE and NNE–SSE trending dolerites (some of which are high-Al basalts) were derived from a relatively depleted mantle and injected during a boninite–dolerite magmatic event, together with nearby BN dykes derived from a harzburgitic, yet highly LREE and LILE enriched, subcontinental lithospheric mantle (SCLM). Ca. 2.05Ga Kangâmiut–MD3 dolerites were derived from a similar ambient asthenospheric mantle and subsequently experienced crustal assimilation during fractional crystallisation.The ca. 2370Ma dykes in the NAC are coeval with the Dharwar Giant Dyke Swarm of the Dharwar Craton (DC) and based on this unique age match it is proposed that the Dharwar and NAC were neighbours in the Palaeoproterozoic. Furthermore, both blocks share 2.22–2.21Ga dykes, and matching the trends of 2.2–2.1 and 2.37Ga swarms between the two blocks allows several plausible reconstructions.