Abstract
The lavas of the Zig-Zag Dal Formation of eastern North Greenland constitute a Mesoproterozoic tholeiitic flood basalt succession up to 1,350 m thick, extending >10,000 km2, and underlain by a sill complex. U–Pb dating on baddeleyite from one of the sills thought to be contemporaneous with the lava extrusion, gives an age of 1,382±2 Ma. The lavas, subdivided from oldest to youngest into Basal, Aphyric and Porphyritic units, are dominantly basaltic (>6 wt.% MgO), with more evolved lavas occurring within the Aphyric unit. The most magnesian lavas occur in the Basal unit and the basaltic lavas exhibit a generalised upward decrease in Mg number (MgO/(MgO + Fe2O T3 )) through the succession. All of the lavas are regarded as products of variable degrees of olivine, augite and plagioclase fractionation and to be residual after generation of cumulates in the deep crust. The basaltic lavas display an up-section fall in the ratio of light to heavy rare-earth elements (LREE/HREE) but an up-section rise in Zr/Nb, Sc, Y and HREE. The older lavas (Basal and Aphyric units) are characterised by low ɛNd and ɛHf in contrast to higher values in the younger (Porphyritic unit) lavas. The Porphyritic Unit basalts are characterised by a notable enrichment in Fe and Ti. The Zig-Zag Dal succession is inferred to reflect an increase in melt fraction in the sub-lithospheric mantle, with melting commencing in garnet–lherzolite facies peridotites and subsequently involving spinel-facies mantle at increasingly shallow depths. Melting is deduced to have occurred beneath an attenuating continental lithosphere in conjunction with ascent of a mantle plume. Lithospheric contamination of primitive melts is inferred to have diminished with time with the Porphyritic unit basalts being products of essentially uncontaminated plume-source magmas. The high iron signature may reflect a relatively iron-rich plume source.
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Acknowledgements
Our thanks go to J.G. Fitton and D.E. James for whole-rock analyses and to J. N. Walsh for ICP-MS spectrometric analyses of rare-earth elements. BGJU gratefully acknowledges support from the Carnegie Trust for Scottish Universities and from the Geological Survey of Denmark and Greenland. OTR acknowledges help from the staff of the unit for Isotope Geology, Geological Survey of Finland, while making the Nd isotopic analyses and JBT thanks the French Institut National des Sciences de l’Univers for financial support. We are particularly grateful to S.A. Gibson, A.C. Kerr for very helpful reviews and our thanks go also to W.R. Baragar, L.-M. Larsen, J. G. Fitton and M. F. Thirlwall for constructive criticisms. Publication of this paper was authorised by the Geological Survey of Denmark and Greenland.
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Upton, B.G.J., Rämö, O.T., Heaman, L.M. et al. The Mesoproterozoic Zig-Zag Dal basalts and associated intrusions of eastern North Greenland: mantle plume–lithosphere interaction. Contrib Mineral Petrol 149, 40–56 (2005). https://doi.org/10.1007/s00410-004-0634-7
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DOI: https://doi.org/10.1007/s00410-004-0634-7