Strontium Isotope Geochemistry and Petrogenesis of the Early Tertiary Lava Pile of the Isle of Skye, Scotland, and other Basic Rocks of the British Tertiary Province: an Example of Magma--Crust Interaction

Most of the flows in the Palaeocene lava pile remnant of Skyeare members of the Skye Main Lava Series (SMLS), comprisingtransitional basalts and two associated suites of evolved lavas.The first suite evolves through Fe-rich hawaiites and mugearitesto benmoreites, and the second suite evolves through Fe-poorintermediates to trachytes. Ca-rich, alkali-poor olivine tholeiites(the Preshal Mhor magma type) occur as sparse flows in the stratigraphicallyhighest parts of the lava pile remnant and are abundant in thedyke swarm transecting it. Initial ⁸⁷Sr/⁸⁶Sr ratios rangingfrom 0.70308 to 0.70571 in 45 SMLS samples show no significantcorrelation with degree of zeolitization (H₂O⁺), silica saturation,or ⁸⁷Rb/⁸⁶Sr. A moderately good negative correlation with totalSr confirms published Pb-isotope evidence of interaction withancient, sialic crust. Details of the (⁸⁷Sr/⁸⁶Sr)_l versus Srpattern are consistent with previous hypotheses that the SMLSbasalt-benmoreite suite evolved at a depth near the Moho, whilstthe low-Fe trend to trachyte resulted from near-surface basaltfractionation. (⁸⁷Sr/⁸⁶Sr)_l values ranging from 0.70307 to 0.70621 for PreshalMhor basalts show a strong positive correlation with total Sr,consistent with a model of extensive fractionation within theupper crust of a mantle-derived low ⁸⁷Sr/⁸⁶Sr-low Sr magma,which became progressively contaminated with comparatively radiogeniccrustal Sr. The lowest measured (⁸⁷Sr/⁸⁶Sr)_l values of 0.70307and 0.70308, for a Preshal Mhor basalt and for an SMLS basaltrespectively, are consistent with the hypothesis that thesetwo magma types were produced by successive phases of partialmelting from a single volume of upper mantle. (⁸⁷Sr/⁸⁶Sr)_l values for additional miscellaneous basaltic lavas,dykes and major intrusives from Skye and from nearby Isle ofMull exhibit considerable variability within the range 0.7038to 0.7072, whilst three basaltic dykes from Northern Englandare in the range 0.7089 to 0.7123. The latter values overlapwith published (⁸⁷Sr/⁸⁶Sr)_l values for some of the granitesin the Tertiary Province of northwest Scotland and indirectlyremove objections based on Sr-isotopic arguments to the genesisof the granites by fractionation of basalt contaminated withcrustal Sr, but neither prove this nor disprove large-scalecrustal partial fusion.