Petrochemical and petrographic analyses of lamprophyric dykes of the Ards Peninsula, Ireland

Abstract

A petrochemical and petrographical comparative study of the lamprophyric dykes of the Ards Peninsula was undertaken to investigate geochemical variation in the series. The lamprophyres are divided into two compositional groups on the basis of the dominant femic minerals and further partitioned on the grounds of their phenocrystic phase assemblages, groundmass constituents and degrees of alteration. Most are clinopyroxene- and/or phlogopite-bearing minettes, a few are hornblende porphyrites and spessartites. Fifteen major-, minor- and trace-element analyses and petrographic studies were performed. The dykes are characterized by unusually low K20, P205 and compatibles and enrichments in A1203, FeO*, Na20 and volatiles. Petrogenetic modelIing indicates a possible crystal olivine + or - clinopyroxene + or - phlogopite fractionation accompanied by low degrees of partial melting with an influx of an external, volatile-rich, aqueous fluid depleted in K and P prior to intrusion. High, but variable, volatile components are partly primary and partially a reflection of post-magmatic chloritization and carbonatization.