Chevkinite-group minerals in UK Palaeogene granites: underestimated ree-bearing accessory phases

Chevkinite-group minerals were sought in 26 granitic bodies from the Palaeogene sequences of Scotland and Northern Ireland, ranging from biotite ± amphibole granites through fayalite-ferrohedenbergite granites to alkali amphibole granites. Chevkinite-(Ce) was found, and analyzed by electron microprobe, in 20 of the bodies, confirming that it is a ubiquitous accessory phase in these rocks. The abundance of the chevkinite is highest in the latter two granite types, sometimes forming more than 50 crystals per thin section. Textural evidence indicates that they crystallized from early- to late-magmatic stages. Compositional variation within and between crystals in the same rock may have resulted partly from fractional crystallization and/or magma mixing and partly from element mobility during metamictization. In some rocks, subsequent hydrothermal alteration affected the composition of the chevkinite-(Ce), in extreme cases leaving totally pseudomorphed material. Apart from mirroring a small enrichment of the LREE in the peralkaline host rocks, there are no systematic changes in chevkinite composition with different bulk-rock compositions.