THE PETROLOGY AND GEOCHEMISTRY OF CONE-SHEETS FROM THE CUILLIN IGNEOUS COMPLEX, ISLE-OF-SKYE - EVIDENCE FOR COMBINED ASSIMILATION AND FRACTIONAL CRYSTALLIZATION DURING LITHOSPHERIC EXTENSION

The origin and evolution of the magma(s) involved in the formation of the olivine tholeiite cone-sheets which invade the approximately 60-Ma Cullin Ingeous Complex, Isle of Skye, have been investigated using mineral chemistry data together with whole-rock major-and trace-element and isotope (Sr and Nd) geochemistry data. The most primitive compositions identified are almost identical to those of the basalts being erupted at the present day along the spreading centre in Iceland. Rare examples of lavas from the slightly older lava pile of west-central and north Skye, together with a large number of dykes from the axial zone of the Skye Main Swarm, are of similar composition and are referred to as being of the Preshal More Basalt type (spelling is according to current Ordnance Survey of Great Britian maps). The intra-suite compositional variation exhibited by the cone-sheets can be explained in terms of relatively low-pressure fractionation of the three-phase assemblage olivine + clinopyroxene + plagioclase in their cotectic proportions of 10:35:55. Modelling of major- and compatible trace-element data indicates that the most evolved composition may be derived by 60% crystallization of the least evolved composition. Incompatible trace-element modelling implies greater degrees of crystal-liquid fractionation: Y and Zr indicate 64 and 68% crystallization, respectively, whereas the rare earth elements (Eu, Yb, Gd, and Ce) indicate between 71 and 75% crystallization. This decoupled behaviour of compatible and incompatible elements is attributed to the cone-sheet magma(s) evolving in a subjacent magma chamber, before final emplacement in the overlying crust. Sr- and Nd-isotope data indicate that the cone-sheet magma(s) assimilated upper-crustal lithologies during fractional crystallization, most likely involving amphibolite facies gneiss of the Lewisian Gneiss Complex. This style of simultaneous assimilation and fractional crystallization (AFC) process in the upper crust for the Skye cone-sheet magma(s) is in direct contrast to the processes identified for the magmas which produced the bulk of the lava field of west-central and north Skye, where assimilation occurred after fractionation. The cone-sheet magmas were generated by relatively large degrees of partial melting of a depleted mantle source associated with significant lithospheric stretching. The cause of this depletion together with the temporal relationships between the cone-sheet magmas and the dominant transitional magmas of the Skye lava field and sill complex, are also discussed.