Evidence for constitutional supercooling from orthopyroxene - pigeonite - plagioclase relations in the Borgmassivet suite of western Dronning Maud Land, Antarctica

Abstract

The Kullen-type sills of western Dronning Maud land are up to 400 m thick and form part of the Meso- to Neoproterozoic Borgmassivet suite. They consist of a Basal, Cumulate, Central and Upper zone. The petrography of the Cumulate zone, with special emphasis on the occurrences at Hammer Heads in the Annandagstoppane area, is described. Microstructures, such as delicate chains of orthopyroxene primocrysts and an increase in grain size of plagioclase chadacrysts from the cores to the margins of postcumulus clinopyroxene oikocrysts, indicatein situ crystallization. Composite grains with orthopyroxene at the core, which is mantled, in turn, by inverted pigeonite, a second generation orthopyroxene and, on the outer margin, clinopyroxene in a reaction relation with the orthopyroxene, are sporadically developed. The relation between the composite structures and plagioclase nucleation and growth is investigated by the study of plagioclase grain size distributions, nucleation density and An compositions of plagioclase cores, which indicate that two periods of plagioclase nucleation occurred. The first period of nucleation, represented by plagioclase chadacrysts enclosed in primocrysts of orthopyroxene, occurred during supercooling following intrusion. Release of latent heat of crystallization resulted in an increase in magma temperature and a normal sequence of fractional crystallization in which orthopyroxene crystallization was succeeded by pigeonite. The second period of plagioclase nucleation depleted the interface liquid in the zone of crystallization of calcium. The composition of the interface liquid was thus changed from the compositional field from which pigeonite crystallized to that from which orthopyroxene crystallized (cf the liquidus boundaries in the system CaMgSi2O6-Mg2SiO4SiO2 ofKushiro, 1972). This constitutional supercooling (Chalmers, 1964;Tiller, 1991) resulted in crystallization of orthopyroxene and the reverse crystallization sequence En-rich orthopyroxene -pigeonite - En-rich orthopyroxene, prior to reaction of the last-formed orthopyroxene with the liquid to form clinopyroxene and subsequent inversion of pigeonite. A possible crystallization sequence is discussed by making use of thespeculative Fo-Di-An-SiO2 (forsterite-diopside-anorthite-silica) system ofMorse (1980).