Possible Liquid Immiscibility Textures in High-Magnesia Basalts from the Ventersdorp Supergroup, South Africa

Abstract

The lowermost succession of Javas in the Proterozoic Ventersdorp Supergroup contains Jight weathering ocelli up to 15 cm in diameter which occur in layers of a darker weathering volcanic material. Sorne 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 fiow caused spalling of large drops of the Jighter weathering material into the other. Severa) 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.

Critical phenomena originating magmatic rocks in western Sicily.

“…Accord ing to these authors, these ocelli are of an order of magnitude bigger than nearly all ocelli pre viously reported as evidence for liquid immis cibility. Taking into account the small dif ferences in composition between the coexisting pairs of liquids, CAWTHORN et al (1979) sug gested that the Ventersdorp Supergroup original magmas must have had alomost consolute com position. GELINAS et al (1976), studying pre eruption immiscibility in Archean tholeiites, also report lavas that contain varioles having compositions essentially the same as that of the matrix.…”

A mechanism forming silicic segregations from basaltic magma discovered in igneous rocks of Western Sicily

“…Philpotts & Hodgson, 1968;Ferguson & Currie, 1972;Gelinas, Brooks & Trzciensk, 1976;Philpotts, 1979;Furnes, Malm & Robins, 1981) suggested that coalescence has been involved in the silicate liquid immiscibility origin. Here, only two spectacular examples w»U be briefly recorded: (1) immiscible leucocratic ocelli in lamprophyres of Callander Bay (Canada) are often concentrated in more or less planar zones locally forming large planar segregations (Ferguson & Currie, 1971); (2) in the basaltic rocks of the Ventersdorp Supergroup (South Africa), coalescence of some immiscible ocelli on to the bottom of discrete light weathering layers gives rise to hemispherical forms which appear to merge completely with the same light layers (Cawthorn et al 1979). According to Cawthorn et al (1979), these light layers are the ultimate end-stage of the coalescence.…”

“…Here, only two spectacular examples w»U be briefly recorded: (1) immiscible leucocratic ocelli in lamprophyres of Callander Bay (Canada) are often concentrated in more or less planar zones locally forming large planar segregations (Ferguson & Currie, 1971); (2) in the basaltic rocks of the Ventersdorp Supergroup (South Africa), coalescence of some immiscible ocelli on to the bottom of discrete light weathering layers gives rise to hemispherical forms which appear to merge completely with the same light layers (Cawthorn et al 1979). According to Cawthorn et al (1979), these light layers are the ultimate end-stage of the coalescence. If the process occurs on a large scale, the ocelli may be eliminated because there will be sufficient time for the conjugate liquids to separate from each other (Roedder, 1978).…”