Possible liquid immiscibility textures in high-magnesia basalts from the Ventersdorp Supergroup, South Africa

The lowermost succession of lavas in the Proterozoic Ventersdorp Supergroup contains light weathering ocelli up to 15 cm in diameter which occur in layers of a darker weathering volcanic material. Some ocelli appear to merge, and discrete light weathering layers may be the ultimate end-stage of this coalescence. Alternatively, coexisting magmas in the neck of the volcano may have been erupted in varying proportions, and turbulence during flow caused spalling of large drops of the lighter weathering material into the other. Several lines of field evidence suggest that two distinct liquids coexisted and were rapidly quenched after eruption. Chemical data for ocelli and matrix are consistent with the hypothesis of liquid immiscibility. The differences in compositions between the coexisting pairs of liquids are small and it is suggested that the original magmas must have been close to the consulute composition