EVIDENCE FOR CONSTITUTIONAL SUPERCOOLING FROM ORTHO-PYROXENE-PIGEONITE-PLAGIOCLASE RELATIONS IN THE BORGMASSIVET SUITE OF WESTERN DRONNING-MAUD LAND, ANTARCTICA

The Kullen-type sills of western Dronning Maud land are up to 400 m th ick and form part of the Meso- to Neoproterozoic Borgmassivet suite. T hey consist of a Basal, Cumulate, Central and Upper zone. The petrogra phy of the Cumulate zone, with special emphasis on the occurrences at Hammer Heads in the Annandagstoppane area, is described. Microstructur es, such as delicate chains of orthopyroxene primocrysts and an increa se in grain size of plagioclase chadacrysts from the cores to the marg ins of postcumulus clinopyroxene oikocrysts, indicate in situ crystall ization. Composite grains with orthopyroxene at the core, which is man tled, in turn, by inverted pigeonite, a second generation orthopyroxen e and, on the outer margin, clinopyroxene in a reaction relation with the orthopyroxene, are sporadically developed. The relation between th e composite structures and plagioclase nucleation and growth is invest igated by the study of plagioclase grain size distributions, nucleatio n density and An compositions of plagioclase cores, which indicate tha t two periods of plagioclase nucleation occurred. The first period of nucleation, represented by plagioclase chadacrysts enclosed in primocr ysts of orthopyroxene, occurred during supercooling following intrusio n. Release of latent heat of crystallization resulted in an increase i n magma temperature and a normal sequence of fractional crystallizatio n in which orthopyroxene crystallization was succeeded by pigeonite. T he second period of plagioclase nucleation depleted the interface liqu id in the zone of crystallization of calcium. The composition of the i nterface liquid was thus changed from the compositional field from whi ch pigeonite crystallized to that from which orthopyroxene crystallize d (cf the liquidus boundaries in the system CaMgSi2O6-Mg2SiO4-SiO2 of Kushiro, 1972). This constitutional supercooling (Chalmers, 1964; Till er, 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 pigeonit e. A possible crystallization sequence is discussed by making use of t he speculative Fo-Di-An-SiO2 (forsterite-diopside-anorthite-silica) sy stem of Morse (1980).