The Zoned Plagioclase of the Skaergaard Intrusion, East Greenland |
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| Authors: | MAAL?E SVEN |
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| Institution: | Institute of General Geology ?stervoldgade 5 1350, Copenhagen K, Denmark |
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| Abstract: | The variation in zoning and An-content of plagioclases in theSkaergaard intrusion was investigated using microprobe analysisand optical methods on material from the surface of the intrusionas well as from a drill core from the hidden zone of the layeredseries. The plagioclases display cryptic variation and prominentvariations in type of zoning with structural height: oscillatoryzoned and resorbed plagioclase cores are predominant in thehidden zone, unzoned cores are typical of the lower zone, andplagioclases of the upper zone display skeletal zoning. Thereverse, oscillatory and skeletal zoning of plagioclase coresis ascribed to supercooled crystallization, and not to convectionor other petrogenetic factors. The variations in type of zoningand An-content of plagioclases from the hidden zone suggestonset of convection at the same time in the cooling historyof the intrusion as is indicated by the transition between thetranquil division and the banded division of the marginal bordergroup. A crystallization model is suggested, which accountsfor the observed variations. The variation in An-content ofplagioclases from the hidden zone suggests a limited extensionof some hundred metres of the intrusion at depth. |
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