Megacrysts and associated xenoliths: Evidence for migration of geochemically enriched melts in the upper mantle beneath Scotland

Megacrysts, principally anorthoclase, Fe-rich biotite, clinopyroxene, magne tite, zircon and apatite, occur in alkali basaltic hosts at a number of Sco ttish localities. These minerals occur, not only as discrete, and composite megacrysts, but also as polycrystalline syenite (anorthoclasite) xenoliths . Composite xenoliths provide evidence that the anorthoclasites may occur a s (pegmatitic) veins traversing pyroxenitic wall-rocks which may themselves be localized metasomatized peridotites within the shallow mantle. The anor thoclasites crystallized from highly trace element enriched melts which, in the case of the most geochemically extreme samples, were also peraluminous . Ion microprobe analyses show that the peraluminous (corundum-bearing) ano rthoclasites comprise light rare earth element (LREE)-enriched alkali felds pars together with corundum and Nb-rich oxides (ilmenorutile, samarskite, y ttro-niobate and columbite). The high contents of incompatible elements, to gether with oxygen isotope data, indicate crystallization of these syenitic facies from felsic melts, possibly originating through partial melting of metasomatized mantle lithologies. The aluminous character may be explained in terms of preferential loss of alkalis in fugitive carbonatitic fractions separated from the felsic melts.