Petrogenesis of the Paleoproterozoic Kangamiut dike swarm, West Greenland: Implications for the tectonic history of Northeast Laurentia and the evolution of Basaltic magmas

The 2.04 Ga Kangâmiut dike swarm in West Greenland is one of many Paleoproterozoic mafic dike swarms that intrude Archean terrains throughout Northeast Laurentia and is distinguished from the others because it retains igneous mineralogies and textures in its southern portion, whereas its northern portion was recrystallized and deformed during the ca. 1.85 Ga Nagssugtoqidian orogeny. Brittle deformation of host rocks during dike emplacement and geothermobarometry using clinopyroxene phenocrysts suggest dike emplacement at less than ∼10 km. Granulite facies metamorphism of dikes and their host rocks in the northern portion of the swarm indicate maximum metamorphic depths of ∼30 km. This requires ∼20 km of crustal thickening between the time of dike emplacement and peak metamorphism.; Melting experiments at 1 atm and 8 kbar show that the dikes are cosaturated with olivine + clinopyroxene + plagioclase at low pressures, although mass balance calculations indicate that the dikes major element covariation are not a product of isobaric fractionation of these three phases. Quantitative modeling of the swarm's fractionation history suggests that the parental magmas followed multiple polybaric fractionation paths that had the general crystallization sequence: olivine (from 17–4 kbar) → olivine + clinopyroxene (from 7–4 kbar) → ± olivine + clinopyroxene + plagioclase (from 7–5 kbar) → olivine + plagioclase (<3 kbar) → ± olivine + clinopyroxene + plagioclase (<3 kbar). Sr, Nd, and Pb isotopes show that the maximum amount of crust contamination during crystallization was less than 8%.; Rare earth element systematics show that the dikes' primary magma originated by ∼5% melting of a slightly depleted mantle source during decompression melting between ∼27 and 17 kbar. The upper pressure limit of 27 kbar is coincident with an estimated solidus of Paleoproterozoic mantle consistent with a potential temperature of ∼1440°C. Similarities between rare earth element characteristics from six additional Paleoproterozoic dike swarms in Northeast Laurentia and the Kangâmiut dikes suggest that passive rifting was a common mode of continental break-up during the Paleoproterozoic, and that passive rifting during the Paleoproterozoic resulted in voluminous volcanism. Mantle plume magmatism is not required to explain the origin of many Paleoproterozoic dike swarms.