Generation and Polybaric Differentiation of East Greenland Early Tertiary Flood Basalts

Abstract

Major element, trace element, isotope, and petrographic studies of the basal volcanics in the Kangerdlugssuaq Fjord region of the East Greenland margin provide constraints on the generation and subsequent differentiation and contamination histories of magmas during the early stage of continental rifting creating the North Atlantic ocean basin. Most primitive lavas of the succession are shown to have accumulated olivine. Corrections for crystal accumulation yield estimates for erupted primitive liquids with MgO contents between 9 and 13 wt %. The occurrence of suspended Fo88 olivine phenocrysts further requires the existence of parental magmas with MgO contents up to 17 wt % and FeO contents up to 14 wt %. Quantitative modeling of fractionation indicates that Kangerdlugssuaq magmas primarily differentiated at moderate pressures (∼8 kbar) to form dunitic and wehrlitic cumulates at depths of ∼25 km. Evolved East Greenland flood basalts from the Scoresby Sund region record a more complex fractionation history involving olivine ± clinopyroxene fractionation in the lower crust, as found for the Lower Basalts, followed by olivine + plagioclase (± clinopyroxene) fractionation at shallow crustal levels. Correlations between SiO2, incompatible elements, and Sr–Nd isotopic ratios for the Lower Basalts are consistent with bulk assimilation of up to 15–20% Archean leucogneiss with unradiogenic Sr and Nd isotopic ratios characteristic of Lewisian-type lower crust. Comparisons between inferred magma types for Lower Basalts and estimated primary liquids from elsewhere in the North Atlantic show complex relationships among FeO, MgO, TiO2 contents and CaO/Al2O3 ratios that relate to regional and temporal changes in mantle temperatures, lithospheric thickness, and involvement of at least two end-member mantle sources. We postulate that the Lower Basalts of East Greenland were derived from iron-poor mantle, depleted by prior melt extraction (and distinct from MORB source mantle), that was enriched in TiO2 and LREEs by a metasomatic event shortly before Early Tertiary magmatism. We associate this ‘depleted’ mantle source with the ancestral Iceland plume anomaly.