Petrology of the Northern East Greenland Tertiary Flood Basalts: Evidence from Hold with Hope and Wollaston Forland

Predominantly silica-oversaturated tholeiitic basalts spread widely across the northern East Greenland continental margin (73–76° N) at the onset of Tertiary magmatism. These have been studied in two of the principal outcrop areas: Hold with Hope and Wollaston Forland. In Hold with Hope very minor eruption of nephelinitic magma preceded the tholeiitic phase. The early tholeiites of northern East Greenland tend to be more siliceous and aluminous and less iron and titanium rich than their more voluminous counterparts in the main East Greenland province south of 70° N. Some, notably those of Wollaston Forland, approach ‘enriched MORB’ compositions. The early flood basalts (distinguished in the Hold with Hope region as the Lower Plateau Lava Series, LPLS) show limited compositional range. Such variation as is shown is attributed to low-pressure plagioclase—olivine—augite fractionation. Intrusion of quartz dolerite sills within and beneath the LPLS may have constituted a separate but closely allied magmatic event. The sills are slightly more enriched in incompatible elements, have higher initial Sr isotopic ratios and show more pronounced Fe-enrichment trends. Conformably overlying the LPLS in the Hold with Hope area is a succession of incompatible element enriched basalts composing the Upper Plateau Lava Series (UPLS). The more extremely enriched lavas (anomalous UPLS) tend to define the base of the UPLS and range from olivine tholeiites to analcime basanites. Together with the more normal UPLS, these range from nearly aphyric to strongly porphyritic varieties; the latter include highly magnesian oceanites and ankaramites. The normal UPLS ranges from picritic olivine tholeiites to quartz tholeiites. While orthopyroxene phenocrysts occur among the more differentiated basalts, pigeonite appears wholly absent from the UPLS. Compositional variation among the normal UPLS is ascribed to olivine + spinel fractionation joined by clinopyroxene in compositions with less than 8 per cent MgO. The UPLS basalts have incompatible element levels and elemental ratios (e.g. Zr/Nb, Zr/Y, La/Y and Ti/V) that distinguish them from the earlier, more depleted tholeiites. Initial Sr isotope ratios vary widely, with high values indicative of crustal interaction. A genetic model is proposed whereby the more depleted tholeiites, as exemplified by those of Wollaston Forland and the earlier volcanic products on Hold with Hope, arose from large-scale melting of relatively infertile asthenosphere prior to the initiation of the Mohn's Ridge and continental separation, while the UPLS of the Hold with Hope region and isolated inland occurrences of nephelinitic and basanitic magmatism were produced by melting of K−Ti−P enriched facies of the heterogeneous mantle presumed to have formed the lower part of the sub-Greenland continental lithosphere. Basalts of the Vøring Plateau on the Norwegian continental shelf, may represent analogues of the UPLS on the eastern Atlantic margins.