Magmatic Evolution of the Skye Igneous Centre, Western Scotland: Modelling of Assimilation, Recharge and Fractional Crystallization

The Skye igneous centre, forming part of the British Tertiarymagmatic province, developed over a 7 Myr period (61–54Ma) and is characterized by a complex suite of lavas, hypabyssaland intrusive rocks of picritic to granitic composition. Theintrusion of magma from mantle to crust at 2x10^–3km³/yr(6 Mt/yr) advected magmatic heat of roughly 0·2 GW averagedover the period of magmatism supporting an ‘excess’heat flux of about 130 mW/m², or about twice the present-dayaverage continental heat flow. The volume of new crust generatedat Skye (15000 km³) spread over the present-day area of Skyecorresponds to 9 km of new crust. The geochemical evolutionof the Skye magmatic system is constrained using the Energy-ConstrainedRecharge, Assimilation, and Fractional Crystallization (EC-RAFC)model to understand variations in the Sr- and Pb-isotopic andSr trace-element composition of the exposed magmatic rocks withtime. The character (composition and specific enthalpy) of bothassimilant and recharge magma appears to change systematicallyup-section, suggesting that the magma reservoirs migrated toprogressively shallower levels as the system matured. The modelof the magma transport system that emerges is one in which magmabatches are stored initially at lower-crustal levels, wherethey undergo RAFC evolution. Residual magma from this stagethen migrates to shallower levels, where mid-crustal wall rockis assimilated; the recharge magma at this level is characterizedby an increasingly crustal signature. For some of the stratigraphicallyyoungest rocks, the data suggest that the magma reservoirs ascendedinto, and interacted with, upper-crustal Torridonian metasediments. KEY WORDS: assimilation; EC-RAFC model; geochemical modelling; magma recharge; Skye magmatism