Cumulates from primitive rift-related East Greenland Paleogene magmas: Petrological and isotopic evidence from the ultramafic complexes at Kælvegletscher and near Kærven

Abstract

A number of Paleogene ultramafic intrusions occur in the Kærven–Kælvegletscher area of the Kangerlussuaq Fjord, central East Greenland. We present geological, geochemical and isotopic data from five of these: Kælvegletscher, Kartografvig, North Kærven, East Kærven and ‘Mordor’. The intrusions are emplaced in the Archaean basement and the northern part of the Kælvegletscher intrusion is truncated by the Kangerlussuaq intrusion and the North Kærven by the Kærven Gabbro. The ultramafic rocks range from dunite over wehrlite and lherzolite to olivine clinopyroxenite and display olivine and olivine–chromite cumulate textures ranging into olivine–chromite–clinopyroxene and occasional clinopyroxene–olivine cumulates. Dunite mineralogy consists of early formed olivines (Fo 86–89) ± chrome–spinels (Cr# 69–95) with occasional interstitial clinopyroxene (Mg# 89–97), while wehrlites and olivine clinopyroxenites retain more Fe-rich olivine (Fo 77–85) and range to less magnesian clinopyroxene (Mg# 77) and lower Cr# in chromite (Cr# 66). The ultramafic rocks are interpreted as cumulates formed by almost primary mantle-derived magmas, which in the Kælvegletscher complex were recurrently intruded into brittle rocks of previous intrusions. Clinopyroxene geochemistry indicates that the magmas were tholeiitic and had relatively low TiO 2 = 1–2 wt.%. Relative proportions of trapped liquids in the cumulates are modelled using trace elements, which suggest that contact rocks may contain 2–3 times as much trapped liquid as rocks from the interior part of intrusions. Lower than primordial mantle values of Ba/Nb, Rb/Nb and Ba/La ratios, which are high in the local crust, indicate little crustal contamination for most rocks. HREE modelling suggests melt extraction at around 3 GPa. Isotopic compositions range: ε Sr(55 Ma) = − 14–+ 34, ε Nd(55 Ma) = − 3.7–+ 3.8, 206Pb/ 204Pb = 15.44–17.83, 207Pb/ 204Pb = 14.87–15.39 and 208Pb/ 204Pb = 36.41–38.15. Except for a few samples that are strongly crustally contaminated, two components explain most of the isotopic variation. One is suggested to be a lithospheric mantle component, the Kærven end-member, with unradiogenic Pb, Sr and radiogenic Nd, but crustal contamination cannot be ruled out because of low Sr/Pb ratios and very low abundances of incompatible elements in the rocks. The other end-member approximates the most unradiogenic Pb in Icelandic basalts and is similar in composition to the NAEM [North Atlantic End-Member [Ellam, R.M., Stuart, F.M., 2000. The Sub-lithospheric Source of North Atlantic Basalts: evidence for, and Significance of, a Common End-member. J. Petrol. 41, 919–932]. There is no indication of the important IREM (Icelandic Radiogenic End-Member [Ellam, R.M., Stuart, F.M., 2000. The Sub-lithospheric Source of North Atlantic Basalts: evidence for, and Significance of, a Common End-member. J. Petrol. 41, 919–932] otherwise seen in lavas to the north and east of Kangerlussuaq. It is indicated that two components of the Icelandic mantle plume at continental break-up 55 Ma ago were isolated and adjacent under the Kangerlussuaq area. Paleogene igneous rocks in Central East Greenland dominated by NAEM and IREM, respectively, are suggested to trace the extent of these plume components.