Highly potassic mafic dykes from Antarctica

Abstract

Abstract Two exceptionally potassium‐rich alkali mela‐syenite dykes from widely separated localities in Antarctica consist of microcline, phlogopite, alkali amphibole (K‐richterite and K‐arfvedsonite), apatite, and minor quartz, anatase, rutile, sphene, barite, zircon, and opaque minerals; they represent the more deep‐seated equivalents of the potassium‐rich mafic volcanic and subvolcanic rock suite. Both are characterised by extremely high TiO2, K2O, P2O5, F, Rb, Sr, Zr, Nb, Ba, La, Ce, Pb, Th, and U, but relatively low Al2O3, CaO, and Na2O. The dyke from Priestley Peak, Enderby Land is relatively magnesian (mg 0.67–0.71) and probably represents a near‐primary magma; its compositional variation may be mainly explained in terms of phlogopite fractionation. The dyke from Mount Bayliss, MacRobertson Land is more evolved (mg 0.54–0.58), and a similar, more rapidly cooled specimen from nearby moraine shows textural evidence that early leucite reacted with liquid to give K‐feldspar. It also shows evidence, in the form of felsic ocelli, for liquid immisci‐bility, although it is not clear whether this was petrogenetically important or merely a late‐magmatic feature.