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Kiglapait Geochemistry II: Petrography

1979; Oxford University Press; Volume: 20; Issue: 3 Linguagem: Inglês

10.1093/petrology/20.3.591

1460-2415

S. A. Morse,

Geochemistry and Geologic Mapping

The mode of the Kiglapait intrusion carries 73 per cent feldspar, but average rocks near the base of the Upper Zone contain as little as 48 per cent feldspar. Olivine remained stable throughout the crystallization, but was locally suppressed by abundant crystallization of augite and titano-magnetite. Red biotite occurs as rims on Fe−Ti oxide minerals and is probably fluorine oxybiotite; its frequently similar occurrence in troctolitic rocks may, perversely, indicate dry magmas rather than damp ones. Excluded modal components follow Rayleigh fractionation behaviour; their presence in trace amount permits estimation of residual porosity in the Lower Zone. This porosity diminishes directly with accumulation rate from 0.14 to 0.03 over the first 80 per cent of crystallization history. The saturation ratio of excluded modal components is a well-behaved function of fraction solidified, and implies that the decrease in porosity continues above the 80 per cent solidified level. The saturation ratio links with bulk composition, porosity, and FL, allowing one of these parameters to be estimated from the others. The cumulus arrival of apatite is abrupt, but the earlier arrivals of augite, oxide minerals, and sulfide each occur over an interval, followed by an interval of overproduction. This behaviour is attributed to feedback on concentration gradients generated by a long history of plagioclase + olivine extraction, in the absence of perfect stirring. Diffusion plays a role in the differentiation of large, slowly cooled magma systems because radial mixing by convection is inefficient. Inherited potential supersaturation is the inevitable result. This leads to modal irregularities, and to crystallization effectively on metastable extensions of field boundaries. The track of the liquid in the Lower Zone is closely parallel to that in the system Fo−Di−An, but offset from it by the combined effects of Ab and P (toward plagioclase) and Fa (away from plagioclase). The latter effect is important, with the result that the shedding of plagioclase by an ascending magma will be much less marked than predicted from iron-free experimental systems.