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Kûngnât, revisited. A review of five decades of research into an alkaline complex in South Greenland, with new trace-element and Nd isotopic data

Published online by Cambridge University Press:  05 July 2018

B. G. J. Upton
Affiliation:
School of GeoSciences, University of Edinburgh, Edinburgh EH9 3JW, UK
R. Macdonald
Affiliation:
IGMP Faculty of Geology, University of Warsaw, al. Żwirky i Wigury 93, 02-089 Warsaw, Poland
N. Odling
Affiliation:
School of GeoSciences, University of Edinburgh, Edinburgh EH9 3JW, UK
O. T. Rämö
Affiliation:
Dept. Geosciences and Geography, University Helsinki, 00014 Helsinki, Finland
B. Bagiński
Affiliation:
IGMP Faculty of Geology, University of Warsaw, al. Żwirky i Wigury 93, 02-089 Warsaw, Poland

Abstract

The Kûngnât Complex (1275±1.8 Ma) in the Gardar Alkaline Province, South Greenland, cuts Archaean gneisses and comprises two intersecting syenitic stocks and a gabbroic ring-dyke. The magmas, with increasingly more primitive compositions, were emplaced successively by ring-faulting and roof stoping. The syenites are orthocumulates (cumulus alkali feldspar, olivine, pyroxene, titanomagnetite and apatite; intercumulus phases include alkali amphibole, biotite, quartz and calcite). In the well dissected earlier stock, a 2.2 km-thick layered sequence displays graded modal layering, feldspar lamination and cryptic layering. Modal layering in both stocks is directed mainly inwards at 35° – 50°. Heterogeneous nucleation of the cumulus assemblage, close to steep thermal boundary layers, is inferred. The modal layering is ascribed primarily to gravitational sorting aided by the large density differential between a) feldspar and b) Fe-rich silicates and oxides. Episodic collapse of cumulus + melt slurries contributed to inward-dipping crystal pediments on the chamber floors. The Ring-Dyke (up to 100 m wide) is nearly continuous through 360°. Kûngnât exhibits a compositional nearcontinuum from olivine gabbro through syenite intermediaries to alkali granite, ascribed to protracted assimilation/fractional crystallization processes. The most radiogenic Nd isotope data from Kûngnât (εNdi values between –3.3 and –1.0) point to a lithospheric mantle source, whereas the most unradiogenic values imply enrichment in LREE by crustal contamination of the magmas.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2013

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