Generation and Polybaric Differentiation of East Greenland Early Tertiary Flood Basalts

Abstract

Major element, trace element, isotope, and petrographic studies of magma diVerentiation; mantle melting the basal volcanics in the Kangerdlugssuaq Fjord region of the East Greenland margin provide constraints on the generation and subsequent diVerentiation 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 INTRODUCTION shown to have accumulated olivine. Corrections for crystal ac- The distinctive geochemical features of continental flood cumulation yield estimates for erupted primitive liquids with MgO basalts (CFBs) are strongly controlled by the composition contents between 9 and 13 wt %. The occurrence of suspended and structure of the continental lithosphere and underFo88 olivine phenocrysts further requires the existence of parental lying mantle. Significant controversy remains concerning magmas with MgO contents up to 17 wt % and FeO contents up the balance of contributions from the lithosphere and to 14 wt %. Quantitative modeling of fractionation indicates the mantle, and the mechanisms by which the various that Kangerdlugssuaq magmas primarily diVerentiated at moderate source components contribute to basalt petrogenesis. pressures (~8 kbar) to form dunitic and wehrlitic cumulates at How are basalt compositions influenced by generation depths of ~25 km. Evolved East Greenland flood basalts from the in anomalous regions in the deep mantle (plumes), by Scoresby Sund region record a more complex fractionation history interaction between rising magmas and the continental involving olivine ‐ clinopyroxene fractionation in the lower crust, lithospheric mantle, and by contamination with conas found for the Lower Basalts, followed by olivine + plagioclase tinental crust? How are the diVerentiation, con(‐ clinopyroxene) fractionation at shallow crustal levels. Cor- tamination, magma mixing, and storage histories of CFBs relations between SiO2, incompatible elements, and Sr‐Nd isotopic controlled by the structure of the continental lithosphere? ratios for the Lower Basalts are consistent with bulk assimilation And how do the thermal and chemical features of the of up to 15‐20% Archean leucogneiss with unradiogenic Sr and mantle and the extensional history of the continental Nd isotopic ratios characteristic of Lewisian-type lower crust. lithosphere aVect mantle melting and compositions of Comparisons between inferred magma types for Lower Basalts and primary liquids? estimated primary liquids from elsewhere in the North Atlantic These questions are best addressed in the context of show complex relationships among FeO, MgO, TiO2 contents a geologically and geochemically well-constrained CFB and CaO/Al2O3 ratios that relate to regional and temporal changes province, such as the North Atlantic Tertiary Province. in mantle temperatures, lithospheric thickness, and involvement of Separation of East Greenland from Northern Europe in at least two end-member mantle sources. We postulate that the the Early Tertiary at ~60 Myr ago was accompanied by Lower Basalts of East Greenland were derived from iron-poor voluminous volcanic activity. The Blosseville Coast of mantle, depleted by prior melt extraction (and distinct from MORB East Greenland, extending from Kangerdlugssuaq Fjord source mantle), that was enriched in TiO2 and LREEs by a to Scoresby Sund (Fig. 1), contains spectacularly exposed metasomatic event shortly before Early Tertiary magmatism. We sections of lavas. These sections begin at the basal contact associate this ‘depleted’ mantle source with the ancestral Iceland of the lavas with underlying Cretaceous‐Tertiary sediments and Archean basement, and continue to the plume anomaly.