Interaction between Continental Lithosphere and the Iceland Plume--Sr-Nd-Pb Isotope Geochemistry of Tertiary Basalts, NE Greenland

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Abstract
Volcanic rocks associated with Atlantic opening in northern east Greenland (73–76°N) form a ∼1-km thickness of basaltic lavas located on the coast some 400 km north of the major basalt accumulations of the Blosseville Coast (23. They are isotopically heterogeneous, with a basanite resembling Icelandic compositions, and an alkali basalt having much less radiogenic Pb and Nd. The bulk of the Upper Series tholeiites has a limited La/YbN range (4.7–7.3) but a wide range in isotope ratios, from almost Icelandic values to 87Sr/86Sr50=0.7100, 206Pb/204Pb50=18.7, and 143Nd/144Nd50=0.51247. This isotopic range is well correlated with SiO2, Ce/Pb, and K/Nb, in a manner suggesting crustal assimilation-fractional crystallization (AFC) relationships. The mantle-derived end-member of the Upper Series is displaced to slightly less radiogenic Nd than the Lower Series samples, perhaps through mixing with a small component from the subcontinental lithospheric mantle. A larger proportion of this melt was derived from garnet-facies mantle than for Lower Series samples, and melt fractions were smaller in both garnet and spinel stability fields. As isotopic compositions similar to those of Icelandic lavas are found in each of the three stratigraphic groups (Lower Series, basal enriched Upper Series, and normal Upper Series tholeiites), the Upper Series were derived from this mixed source, but still had a very dominant plume isotopic signature. The continued presence of a lithospheric ‘lid’ is indicated by the smaller melt fractions in both garnet and spinel facies (∼0.01 and 0.1, respectively) than those responsible for the Lower Series lavas. The thicker crust in the region allowed stagnation of the magmas in the plumbing system of a central volcano and consequent extensive accumulation, fractionation, and assimilation of crustal rocks.

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