Picritic magmas and the Rum ultramafic complex, Scotland

Abstract

Three small picritic dykes, intruded at a late stage in the evolution of the Rum basic–ultra-basic complex, Inner Hebrides, shed new light on the nature of the magmas responsible for the main complex. The magmas are of transitional (mildly alkalic) type, generated by relatively small-fraction (6–7%) melting of a depleted mantle source. Melting is tentatively deduced to have commenced at ±100 km, straddling the garnet–spinel transition. Of the three samples, one (M9) is remarkable for the preservation of very primitive characteristics, with olivines of Fo₉₃ containing highly aluminous spinels; these appear unique within the British Tertiary Volcanic Province. Sr, Nd and Pb isotopes indicate only minor ([les ]4%) contamination with Precambrian crustal lithologies, reflecting the rapidity of ascent of the magma batches. The Rum picrites have ¹⁸⁷Os/¹⁸⁸Os ratios and trace element characteristics comparable to those of recent picrites from Iceland, suggesting minimal temporal change of the depleted parts of the Iceland plume over 60 Ma. Movements of the Long Loch Fault may have been instrumental in causing decompression melting of the spreading Iceland plume-head and facilitating ascent of the melts to near-surface levels.