Abstract
The coarse-grained Upper Border Series rocks of the Skaergaard intrusion contain abundant skeletal crystals of magnetite and ilmenite, skeletal and hopper crystals of apatite, and less abundant sector-zoned augite crystals and hopper zircon crystals. In addition, the melanogranophyres which occur as pods and lenses in the lower part of the Upper Border Series and the upper part of the Layered Series are characterized by very coarse-grained dendritic ferrohedenbergite crystals. Skeletal, hopper, and sectorzoned crystals are not present in the Layered Series gabbros. The development of these unusual crystal morphologies in the Upper Border Series requires that the roof-zone magma was intermittently supersaturated and indicates that the Skaergaard magma chamber was compositionally zoned and that heat loss through the roof maintained a temperature gradient in the magma that was greater than the adiabatic gradient. It is suggested that supersaturation developed in the roof-zone of the intrusion as a result of convective overturn and magma mixing during the early stages of crystallization, and as a result of sudden volatile loss during the later stages of crystallization when the Upper Border Series rocks became rigid enough to fracture.
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Naslund, H.R. Supersaturation and crystal growth in the roof-zone of the Skaergaard magma chamber. Contr. Mineral. and Petrol. 86, 89–93 (1984). https://doi.org/10.1007/BF00373713
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DOI: https://doi.org/10.1007/BF00373713