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

摘要

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 (<70°N). The Lower Lava Series, which makes up the lower half of the sequence at Hold with Hope and all of that at Wollaston Forland, is composed of homogeneous quartz tholeiites (5–8% MgO). These are mildly light rare earth element (LREE) enriched (La/YbN 2.06±0.45, 1 S.D.) and show strong chemical and Pb-Nd-Sr isotopic similarities to Icelandic tholeiites. They are distinguished from Atlantic mid-ocean ridge basalt (MORB) in having less radiogenic Pb and Nd, higher Δ8/4 and lower Δ7/4, and depletion in K and Rb relative to other incompatible elements, and show no evidence of a MORB asthenosphere component in their source. A single nephelinite in the Lower Series has essentially similar isotopic characteristics and K, Rb depletion. The tholeiites were derived from the hot head of the Iceland plume, which had spread laterally within the upper mantle, and represent large melt fractions (15–20%) from spinel-facies mantle combined with small melt fractions (∼2.2%) from the garnet facies. Pb isotopic data indicate that the Iceland plume contains no MORB asthenospheric component, and is therefore most unlikely to arise from enriched streaks in the convecting upper mantle. The K, Rb depletion is shared with the HIMU ocean islands, and suggests a similar origin for the Iceland plume in subduction-processed oceanic crust. The relatively low 206Pb/204Pb ratios, and near-MORB Sr-Nd isotopes, suggest that Iceland overlies an immature HIMU plume. The conformably overlying upper half of the Hold with Hope sequence (the Upper Lava Series) is extremely heterogeneous, being mainly olivine and quartz tholeiites (4.5–9.5% MgO in inferred melt compositions, and up to 27% in accumulative lavas), with occasional undersaturated compositions. The latter are concentrated near the base of the Upper Series, and are associated with strongly incompatible-element-enriched tholeiites. These enriched samples have La/YbN from 7.3 to 28.5, with most tholeiites ∼13, and the undersaturated rocks >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.