Trace-Element Evidence of Multistage Mantle Fusion and Polybaric Fractional Crystallization in the Palaeocene Lavas of Skye, NW Scotland

The Palaeocene magnesian transitional basalts of the Main Lava Series (SMLS) of Skye, NW Scotland, have concentration ranges of K, Ti, P, Rb, Sr, Ba, Nb, Ta, Zr, Hf, Th and light REE varying by a factor of up to two at a given value of (FeO + Fe2O3)/(FeO + Fe2O3 + MgO). Their chondrite-normalized REE patterns vary widely in slope and cuvature, with (Ce/Yb)N=2.2–4.7. The abundances of Ti, P, Zr, Hf, Eu, Gd and Tb correlate negatively with Si-saturation and are thought to be primary, reflecting variable localized partial melting (∼5 per cent) for each magma batch at about 60 km depth of a spinel-lherzolite upper mantle, leaving a lherzolitic residuum. Y and the heavy REE vary little with Si-saturation, due to their partial retention in residual mantle diopside. The large abundance ranges of Rb, Sr, Ba, Nb, Th, La, Ce and Nd in the SMLS basalts, uncorrelated with Si-saturation, may reflect local upper-mantle variability in the concentrations of the ultra-incompatible elements beneath Skye, caused by the pre-Palaeocene extraction of small quantities of alkalic, incompatible-element-rich magma, such as formed the Permian lamprophyre dykes of western Scotland. The trace element data confirm major-element, least-squares models, which show that fractional crystallization of SMLS magnesian basalt to less-magnesian basalt residua involved the separation of 10 per cent olivine and 4 per cent plagioclase, whilst the fractionation of SMLS less-magnesian basalt to hawaiite occurred at about 35 km depth by precipitation of 8 per cent olivine, 15 per cent plagioclase and 21 per cent aluminous sub-calcic augite. The variation of Nb and Ta abundances in hawaiites, mugearites and low-Fe intermediate lavas suggests that these elements partitioned strongly into liquidus titanomagnetite microphenocrysts. Zircon fractionation occurred during the final stages of evolution of benmoreites and trachytes, the latter representing the residuum of at least 90 per cent fractional crystallization of SMLS basalt magma. High-Ca, low-alkali olivine tholeiites of the Preshal Mhor magma type occur near the top of the present lava field erosional remnant and predominate in the dyke swarm transecting it. They have low incompatible trace-element abundances and REE patterns with (Ce/Yb)N ∼0.6, similar to those of many mid-ocean ridge basalts. Models attempting to explain the genesis and relations of the contrasting SMLS and Preshal Mhor basalts by postulating separate mantle sources, arranged in p