Mafic Silicates from the Nepheline Syenites of the Motzfeldt Centre, South Greenland

A. P. Jones*
The University of Chicago, Department of Geophysical Sciences, 5734 South Ellis Avenue, Chicago, Illinois, USA 60637
*Present address: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.

Abstract: At least five successive intrusions of nepheline-bearing magmas fractionated in situ and developed repeatedly overlapping, yet distinct chemical trends in the mafic silicates. Representative electron probe analyses are given for olivine, clinopyroxene, amphibole, biotite, and aenigmatite and their variations discussed. Increasing Na with fractionation produced aegirine-rich clinopyroxene and caused Na/K in amphibole to increase from 2.4 to 7.0. Siz/Alz increased in both amphibole and in the less abundant mica. Mn-enrichment is common to all of the mafic silicates, including the olivines and it is suggested that the parent magmas were initially Mn-rich.

Mineralogically the Motzfeldt centre is evolved and similar to the other centres in the Igaliko complex. However, the Motzfeldt centre contains the most fractionated rocks (lujavrites) and is in this respect similar to the peralkaline Ilimaussaq intrusion. The compositional ranges and trends of the mafic silicates in Motzfeldt thus provide links between the uncommon peralkaline plutons and the more abundant and typical nepheline syenites.

Mineralogical Magazine; March 1984 v. 48; no. 346; p. 1-12; DOI: 10.1180/minmag.1984.048.346.01
© 1984, The Mineralogical Society
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