The timing and duration of kimberlite and related magmatism in southern West Greenland have been investigated by determining precise U-Pb perovskite/pyrochlore and Rb-Sr phlogopite ages for 37 samples collected in the Sisimiut, Sarfartoq and Maniitsoq regions. A summary of these results indicate that this magmatism occurred during three main periods within a time span of more than 1 b.y.: 1) Mesoproterozoic (1284-1209 Ma), 2) Neoproterozoic (604-555 Ma), and 3) Jurassic (166-158 Ma). Rb-Sr phlogopite ages were determined for four Mesoproterozoic ultramafic lamprophyre samples collected ~ 50 km ESE to SSE of Sisimiut and they have broadly similar ages between 1284 and 1209 Ma. The most precise ages are between 1246 and 1209 Ma and are interpreted to be a distal manifestation of alkaline and ultrapotassic magmatism related to the ~ 1300-1130 Ma Gardar rift ~ 800 km further south. The majority of the new dates (28) are from dykes and sheets emplaced over an ~ 50 m.y. time span during the Neoproterozoic, a period dominated by ultramafic lamprophyres, some of which are transitional into kimberlites. Initiation of this alkaline magmatism occurred at 604 Ma, however the majority of dykes and sheets were emplaced in the period between 585-555 Ma with possible pulses at 578.7 ± 1.4 and 556.7 ± 1.2 Ma. Calcite kimberlites in the Maniitsoq region and the Sarfartoq carbonatite are constrained to the youngest period of magmatism at ~ 560 Ma. Many of the kimberlites and ultramafic lamprophyres in the Neoproterozoic West Greenland alkaline province were emplaced following the opening of the Iapetus Ocean at about 615 Ma and are interpreted to be linked to continental rifting. The youngest alkaline and ultrapotassic magmatism in West Greenland (Mesozoic to Paleogene) extends for more than 300 km along the coast and the ages obtained in this study for two Jurassic carbonatite complexes, the 165 Ma Qaqarssuk complex near Maniitsoq and the 158 Ma Tikiusaaq complex east of Nuuk, indicate that carbonatites are also an integral part of this Jurassic alkaline province. There is a peak in alkaline and ultrapotassic magmatism in West Greenland during the period 165-145 Ma, possibly indicating a period of increased extension during the opening of the North Atlantic Ocean.
- Programområde 4: Mineralske råstoffer