GENERATION AND POLYBARIC DIFFERENTIATION OF EAST GREENLAND EARLY TERTIARY FLOOD BASALTS

Major element, trace element, isotope, and petrographic studies of the basal volcanics in the Kangerdlugssuaq Fjord region of the East Green land margin provide constraints on the generation and subsequent diffe rentiation and contamination histories of magmas during the early stag e of continental rifting creating the North Atlantic ocean basin. Most primitive lavas of the succession are shown to have accumulated olivi ne. Corrections for crystal accumulation yield estimates for erupted p rimitive liquids with MgO contents between 9 and 13 wt%. The occurrenc e of suspended Fo(88) olivine phenocrysts further requires the existen ce of suspended magmas with MgO contents up to 17 wt% and FeO contents up to 14 wt%. Quantitative modeling of fractionation indicates that K angerdlugssuaq magmas primarily differentiated at moderate pressures ( similar to 8 kbar) to form dunitic and wehrlitic cumulates at depths o f similar to 25 km. Evolved East Greenland flood basalts from the Scor esby 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 (+/- clinopyr oxene) fractionation at shallow crustal levels. Correlations between S iO2, incompatible elements, and Sr-Nd isotopic ratios for the Lower Ba salts are consistent with bulk assimilation of up to 15-20% Archean le ucogneiss with unradiogenic Sr and Nd isotopic ratios characteristic o f 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 content s and CaO/Al2O3 ratios that relate to regional and temporal changes in mantle temperatures, lithospheric thickness, and involvement of at le ast two end-member mantle sources. We postulate that the Lower Basalts of East Greenland were derived from iron-poor mantle, depleted by pri or melt extraction (and distinct from MORB source mantle), that was en riched in TiO2 and LREEs by a metasomatic event shortly before Early T ertiary magmatism. We associate this 'depleted' mantle source with the ancestral Iceland plume anomaly.