Figure 5
From: A large explosive silicic eruption in the British Palaeogene Igneous Province
Geochemical data for the Òigh-sgeir and Sgùrr of Eigg pitchstones and minerals. (a) Total alkalis vs. silica diagram of the ‘OSSEP’ whole rock, glass and groundmass compositions. The Òigh-sgeir and Sgùrr of Eigg pitchstones show a narrow compositional cluster for whole rocks and another narrow compositional group for groundmass (glass) compositions. The Canna Lava Formation on Rum and Canna belongs to the Skye central complex, while the Eigg lavas pre-date Rum and Skye (see Supplementary Information S1 and Supplementary Table S6 for data sources and selection). Notably the combined OSSEP data overlap with the mixed-magma Marsco gabbro-granite suite of Skye and plot close to known trachyte lavas from the Skye Lava Group, while differing markedly from the rhyodacites and microgranites exposed on Rum. (b) Feldspar compositional triangles (An-Ab-Or) for Òigh-sgeir “free-floating” feldspar in the pitchstone (bottom left), feldspar in Òigh-sgeir plutonic inclusions (second from left), free-floating feldspar in Sgùrr of Eigg pitchstone (third from left), and from plutonic inclusions in the Sgùrr of Eigg pitchstone (top right). Note the compositional ranges are virtually identical. The plutonic inclusions in both pitchstone groups show some sparse K-rich compositions that are not present in the free-floating feldspar populations. This indicates that K-rich feldspar that was liberated from the plutonic inclusions was dissolved in the pitchstone melt (cf. ref.37).
