Ferromagnesian Silicates in a Differentiated Alkaline Complex: Kûngnât Fjeld, South Greenland

D. Stephenson and B. G. J. Upton
Institute of Geological Sciences, Murchison House, Edinburgh EH9 3LA
Grant Institute of Geology, University of Edinburgh, Edinburgh EH9 3JW

Abstract: Kûngnât Fjeld is an intrusive complex resulting from three intrusive events involving trachyte, mafic trachyte and hawaiite magmas tapped successively from a single magmatic source. In situ differentiation produced gabbroic, syeno-gabbroic, syenitic, and quartz syenitic cumulates showing strong cryptic variation. Layered sequences of syenitic rocks total c. 3500 m. The compositional range is extended by late-stage minor intrusions of microsyenite and peralkaline granite. The principal cumulus minerals are feldspar (Ab40An60 to Or45Ab51An4); olivine (Fo56 to Fo1) and clinopyroxene (Di70Hd27Ac3 to Di6Hd90Ac4 followed by an abrupt change of trend towards Di3Hd78Ac19 in the layered series and nearly pure aegirine in some peralkaline residues). Intercumulus phases are amphibole (hastingsite to ferro-edenite to ferro-actinolite in the layered series with riebeckite to arfvedsonite in some associated granites) and biotite (annite31 to annite98.5).

Mineralogical Magazine; September 1982 v. 46; no. 340; p. 283-300; DOI: 10.1180/minmag.1982.046.340.02
© 1982, The Mineralogical Society
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