The Zoned Plagioclase of the Skaergaard Intrusion, East Greenland

• Published in: Journal of Petrology
• Main Author: MAALØE, SVEN
• Format: Text
• Language: English
• Published: Oxford University Press 1976
• Subjects: Articles; Greenland; East Greenland
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/17/3/398; https://doi.org/10.1093/petrology/17.3.398

The variation in zoning and An-content of plagioclases in the Skaergaard intrusion was investigated using microprobe analysis and optical methods on material from the surface of the intrusion as well as from a drill core from the hidden zone of the layered series. The plagioclases display cryptic variation and prominent variations in type of zoning with structural height: oscillatory zoned and resorbed plagioclase cores are predominant in the hidden zone, unzoned cores are typical of the lower zone, and plagioclases of the upper zone display skeletal zoning. The reverse, oscillatory and skeletal zoning of plagioclase cores is ascribed to supercooled crystallization, and not to convection or other petrogenetic factors. The variations in type of zoning and An-content of plagioclases from the hidden zone suggest onset of convection at the same time in the cooling history of the intrusion as is indicated by the transition between the tranquil division and the banded division of the marginal border group. A crystallization model is suggested, which accounts for the observed variations. The variation in An-content of plagioclases from the hidden zone suggests a limited extension of some hundred metres of the intrusion at depth.