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Journal of Petrology Volume 41 Number 5 Pages 627-649 2000
© Oxford University Press 2000
The Plagioclase–Magma Density Paradox Re-examined and the Crystallization of Proterozoic Anorthosites
DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES (DSTE), UNIVERSITÉ LIBRE DE BRUXELLES, CP160/02, AVENUE F. D. ROOSEVELT 50, B-1050, BRUSSELS, BELGIUM
Intermediate-composition plagioclase (An40–60) is typically less dense than the relatively evolved basaltic magmas from which it crystallizes and the crystallization of plagioclase produces a dense residual liquid, thus plagioclase should have a tendency to float in these magmatic systems. There is, however, little direct evidence for plagioclase flotation cumulates either in layered intrusions or in Proterozoic anorthosite complexes. The layered series of the Poe Mountain anorthosite, southeast Wyoming, contains numerous anorthosite–leucogabbro blocks that constrain density relations during differentiation. All blocks are more mafic than their hosting anorthositic cumulates, their plagioclase compositions are more calcic, and each block is in strong Sr isotopic disequilibrium with its host cumulate. Associated structures—disrupted and deformed layering—indicate that (1) a floor was present during crystallization and that plagioclase was accumulating and/or crystallizing on the floor, (2) compositional layering and plagioclase lamination formed directly at the magma–crystal pile interface, and (3) the upper portions of the crystal pile contained significant amounts of interstitial melt. Liquid densities are calculated for proposed high-Al olivine gabbroic parental magmas and Fe-enriched ferrodioritic and monzodioritic residual magmas of the anorthosites taking into account pressure, oxygen fugacity, P2O5, estimated volatile contents, and variable temperatures of crystallization. For all reasonable conditions, calculated block densities are greater than those of the associated melt. The liquid densities, however, are greater than those for An40–60 plagioclase, which cannot have settled to the floor. Plagioclase must either have been carried to the floor in relatively dense packets of cooled liquid plus crystals or have crystallized in situ. A sloping floor, possibly produced by diapiric ascent of relatively light plagioclase-rich cumulates, is required to allow for draining and removal of the dense interstitial liquid produced in the crystal pile and may be a characteristic feature during the crystallization of many Proterozoic anorthosites and layered intrusions.
KEY WORDS: magma; density; Proterozoic anorthosites; blocks; plagioclase
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