| Summary: | The interaction between the Mid Atlantic Ridge and Iceland plume produced an uninterrupted sequence of tholeiite lava flows between NW Iceland and the Snaefellsnes Peninsula from 15 Ma until 5.2 Ma, when the spreading centre relocated to its present position on the Reykjanes Peninsula. Widespread glaciation, at the end of the Tertiary (3.3 Ma), eroded the lava flows in the extinct spreading centre. Renewed volcanism along the peninsula, during the glacial period, resulted in a sequence of off-axis, Quaternary flows that rest unconformably on the Tertiary rift-axis tholeiites. Basalt samples collected from the Snaefellsnes Peninsula, in the region of the extinct central volcano, Setberg, were used to determine both the length-scale of mantle heterogeneity beneath the peninsula and the time-scale during which the distinct domains of heterogeneity were sampled. In the Setberg region the basalt composition changed with time from tholeiitic in the Tertiary, through transitional, to mildly alkaline by 0.7 Ma. The change in major-element composition is coupled with an increase in LREE/HREE, a progressive increase in the ratios of highly to moderately incompatible trace elements (e.g. Ce/Y, Nb/Zr), increasing <sup>87</sup>Sr/<sup>86</sup>Sr, and decreasing <sup>145</sup>Nd/<sup>144</sup>Nd. The Pb-isotope ratios of the Quaternary basalts are generally more radiogenic than those of the Tertiary flows, but these younger basalts display a larger spread of Pb-isotope ratios. The oxygen-isotope ratios of the basalts from the Setberg region (Sigmarsson <i>et al.</i>, 1992; Hardarson, 1993) suggest that the geochemical signatures of the basalts are representative of the mantle source from which they were derived. The increasing fractionation of the REE and the change in major-element composition suggests that progressively deeper parts of the melt column were sampled following the cessation of spreading along the rift. The increasing values of the ratios Nb/Zr, Nb/Y, Zr/Y, and Ce/Y, after the cessation of spreading, and the positive correlations between these ratios indicate that the Quaternary basalts were produced by lower degrees of partial melting.
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