THE TERTIARY PICRITES OF WEST GREENLAND - CONTRIBUTIONS FROM ICELANDIC AND OTHER SOURCES

New isotope and trace element data are presented for suites of tholeiitic picrites from Disko, Ubekendt Ejland and Svartenhuk Halvo of the West Greenland Tertiary Volcanic Province. Geographic and stratigraphic subgroups are identified by major element, trace element and Sr and Nd isotope systematics. Early picrites on Disko have epsilon(Nd) = +8 to +10 and epsilon(Sr) = -22 to -15, similar to subaerial Icelandic lavas, and overlap compositionally with North Atlantic MORB. Most other picrites of the province are enriched relative to MORB but still have epsilon(Nd) > +6 and epsilon(Sr) < -10. Crustal sources cannot explain the observed geochemical variation. Modelling of the major oxides Al2O3, CaO and TiO2 and FeO' suggests that the main part of the West Greenland picrites was derived from a source similar to the most enriched Icelandic picrites by rather high degrees of partial melting (high CaO/Al2O3) at relatively high pressures (high FeO'). These features and the high MgO (19 wt% in liquids) are in accordance with their derivation from very hot plume mantle below the continental lithosphere. In the same group of picrites, both major elements, incompatible elements and Sr and Nd isotopes are coupled and are thought to identify the Icelandic plume end member at ca. 63 Ma and today: epsilon(Sr) = -15 to -18, epsilon(Nd) = +7.5 to +8.5, Al2O3/TiO2 = 7-10 and Zr/Nb = 14-26, and a trace element pattern as in the picrites of the Ordlingassoq Member at Disko and Group 1 of the equivalent lower formation at Svartenhuk Halvo. Lithospheric influence in the picrites is only detected in the northern part of the province. The influence from MORB mantle seen in the older Naujanguit Member of Disko decreases with time. The results lead to discussion and revision of recent models for continental flood basalt volcanism and mantle plumes [1,2] with regard to the site of picrite production relative to the plume axis and the timing of continental flood basalt volcanism in West and East Greenland during the early Tertiary.