Two Generations of Clinopyroxene Phenocrysts in Ultrapotassic Lavas (Lamproites) of the Quaternary Subglacial Gaussberg Volcano (East Antarctica)

Study of the morphological features and chemical composition of the ultrapotassic lavas from the Gaussberg volcano (East Antarctica) revealed two generations of clinopyroxene phenocrysts. These two groups of clinopyroxenes show distinct genetical trends that could not appear during the fractional crystallization. The first group (I) consists of high magnesian (Mg# 79–92) diopside. The second group (II) consists of more ferrous clinopyroxene grains (Mg# 59–69) (“green cores”), which are usually partly resorbed and contain zones of different composition. The Group I differs from the Group II in its higher contents of Ti, Cr, Ni and lower contents of Fe and Al. The characteristics of the natural samples were compared to the run products of the crystallization experiments with the Gaussberg ultrapotassic melt.. Based on these observations, it is proposed that the Group I (magnesian) is in equilibrium with the lamproitic melts, whereas ferrous Group II may be the relicts of clinopyroxene grains formed from the different, probably earlier portions of magma. This suggests a multistage activation of melting and associated crystallization processes in the magmatic system of the Gaussberg volcano and possible heterogeneity of a magma source.