Megacrysts and Associated Xenoliths: Evidence for Migration of Geochemically Enriched Melts in the Upper Mantle beneath Scotland

Abstract

Megacrysts, principally anorthoclase, Fe-rich biotite, clinopyroxene, INTRODUCTION magnetite, zircon and apatite, occur in alkali basaltic hosts at a This paper describes the paragenesis of enigmatic menumber of Scottish localities. These minerals occur, not only as gacryst assemblages in alkaline basalts which appear to discrete, and composite megacrysts, but also as polycrystalline syenite be the crystallization products of salic alkaline magmas (anorthoclasite) xenoliths. Composite xenoliths provide evidence that within the upper mantle and/or within ultramafic facies the anorthoclasites may occur as (pegmatitic) veins traversing in the lowermost crust. The dominant mineral phase is pyroxenitic wall-rocks which may themselves be localized metaalkali feldspar (mainly anorthoclase) suggesting that the somatized peridotites within the shallow mantle. The anorthoclasites magmas from which they crystallized were trachytic in crystallized from highly trace element enriched melts which, in composition. Mineral composition data suggest that some the case of the most geochemically extreme samples, were also of the parental magmas were geochemically highly peraluminous. Ion microprobe analyses show that the peraluminous evolved; the most extreme compositions were per(corundum-bearing) anorthoclasites comprise light rare earth element aluminous, containing modal corundum. (LREE)-enriched alkali feldspars together with corundum and NbThere are some 70 localities in northern Britain and rich oxides (ilmenorutile, samarskite, yttro-niobate and columbite). Ireland at which basaltic magmas have brought up deepThe high contents of incompatible elements, together with oxygen seated xenoliths and megacrysts (Upton et al., 1983; isotope data, indicate crystallization of these syenitic facies from felsic Hunter & Upton, 1987). The host magmas, which asmelts, possibly originating through partial melting of metasomatized cended through crust varying from Archaean to Protmantle lithologies. The aluminous character may be explained in erozoic in age (Menzies & Halliday, 1988), ranged from terms of preferential loss of alkalis in fugitive carbonatitic fractions basanite to olivine nephelinite in composition. In Scotland separated from the felsic melts. the localities define a broad swath extending from NE