Figure 7
From: A large explosive silicic eruption in the British Palaeogene Igneous Province
Sr, Nd and Pb isotope ratios for the Sgùrr of Eigg and Òigh-sgeir pitchstones. (a,b) 143Nd/144Nd vs. 87Sr/86Sr, (c) 207Pb/204Pb vs. 206Pb/204Pb, and (d) 208Pb/204Pb vs. 206Pb/204Pb (at 59 Ma). The average crustal terrane compositions of Lewisian gneiss (granulite- and amphibolite-facies, LG-G and -A) and Moine meta-sedimentary rocks (M), as well as upper mantle proxies are given for reference. Fields for the major igneous centres in the region are shown for comparison: Skye granites (SG), Skye Main Lava Series (basalts-hawaiites-benmoreites; SMLS), Rum rhyodacites and microgranites (RRMG), and Ardnamurchan cone sheets (basalts-andesites-rhyolites, AC). An initial trend towards Lewisian granulite-facies material is implied by the Sr-Nd data and confirmed by the Pb isotopes. The OSSEP samples plot where the granulite-facies and amphibolite-facies Lewisian gneiss fields overlap, which is remote from the field for the relatively radiogenic Moine rocks and the Palaeogene mantle. To investigate the specific contamination history of the OSSEP samples, we can use the primitive picritic dyke from the Isle of Rum (‘sample M9’) or the average North Atlantic mantle End Member, which are both widely viewed as approximations of the isotope compositions of Palaeocene mantle-derived magmas. The OSSEP Sr and Nd data are consistent with a mixing model whereby the Sgùrr of Eigg and Òigh-sgeir magmas are derived from upper mantle melt that underwent 10–20% contamination by lower crustal granulite-facies-type Lewisian gneiss. This first contamination event was followed by up to 50% mixing/contamination with upper crustal amphibolite-facies type Lewisian Gneiss23,27,31,44. The felsic rocks of the Sgùrr of Eigg and Òigh-sgeir pitchstones thus record identical cumulative effects of contamination events in the deep and in the shallow crust. See Supplementary Table S6 for data sources.
