Alkaline Rocks and Carbonatites of the World

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Kungnat Fjeld

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Occurrence number: 
063-00-031
Country: 
Greenland
Location: 
Longitude: -48.03, Latitude: 61.02

Figs 101 and 102. Emplaced in Precambrian Ketilidian quartzofeldspathic gneisses, the Kungnat Fjeld intrusion, covering about 15 km2, is one of the smaller Gardar intrusions. The complex consists of three steep-sided syenite bodies, the centres of intrusion of which moved progressively eastwards, followed by an alkali gabbro ring intrusion (Upton, 1960). The earliest southwest marginal syenite contains gneiss and basic igneous xenoliths and is a quartz syenite of perthite, quartz, fayalite, aegirine-augite, sodic amphibole, biotite and accessories. The two succeeding western and eastern syenites are layered intrusions, the western body having inwardly-dipping rhythmic banding with cryptic variation, trough banding and feldspar lamination. The western syenite is divided by a central gneiss raft into an upper and lower layered series, in both of which the colour index increases downwards. The principal minerals are perthite, fayalite, clinopyroxenes, ranging generally from augite through ferroaugite and ferrohedenbergite to aegirine-augite, barkevikitic to hastingsitic amphibole, which is usually interstitial or fringing pyroxene, and some riebeckite, biotite, quartz, which is concentrated in the upper parts of both layered series, and Fe-Ti oxides which are concentrated in mafic bands low down in the layered sequence. Sheets and dykes of riebeckite granite, with astrophyllite, and aegirine microgranite cut the western syenites. The eastern syenites are divisible into a border group with steeply dipping fluxion structures and a layered series which is rather more basic than the western syenites. The trend to sodic enrichment in pyroxenes and amphiboles apparent in the western syenites is much reduced in the eastern syenites which are composed of alkali feldspar, fayalitic olivine, ferroaugite or hedenbergite, hornblende, biotite, ilmenomagnetite and apatite. The late gabbro intrusion forms a complete ring with vertical to outward dips as low as 20° and a thickness of 100 m. The gabbros vary from labradorite-olivine rocks to rocks of andesine, titanaugite-ferroaugite, olivine, biotite and opaques; some interstitial alkali feldspar may be present. The geochemistry and petrogenesis of this complex is discussed in some detail by Upton (1960) and further chemical data are presented in Upton et al. (1971). Compositions of ferromagnesian silicates are given by Stephenson and Upton (1982), K, F and H2O metasomatism of gabbro adjacent to peralkaline granite sheets is described by Macdonald et al. (1973), and McDowell and Wyllie (1971) conducted experiments on four syenite and one granite sample from Kungnat. Data on fluid inclusions in granite are given by Konnerup-Madsen et al. (1985).

Age: 
A Rb-Sr whole rock isochron gave an age of 1245±16 Ma (Breemen and Upton, 1972, p. 3385).
References: 
BREEMEN, O.van and UPTON, B.G.J. 1972. Age of some Gardar intrusive complexes, South Greenland. Bulletin of the Geological Society of America, 83: 3381-90. KONNERUP-MADSEN, J., DUBESSY, J. and ROSE-HANSEN, J. 1985. Combined raman microprobe spectrometry and microthermometry of fluid inclusions in minerals from igneous rocks of the Gardar province (south Greenland). Lithos, 18: 271-80. MACDONALD, R., UPTON, B.G.J. and THOMAS, J.E. 1973. Potassium- and fluorine-rich hydrous phase coexisting with peralkaline granite in South Greenland. Earth and Planetary Science Letters, 18: 217-22. MCDOWELL, S.D. and WYLLIE, P.J. 1971. Experimental studies of igneous rock series: the Kungnat syenite complex of southwest Greenland. Journal of Geology, 79: 173-94. STEPHENSON, D. and UPTON, B.G.J. 1982. Ferromagnesian silicates in a differentiated alkaline complex: Kungnat Fjeld, South Greenland. Mineralogical Magazine, 46: 283-300. UPTON, B.G.J. 1960. The alkaline igneous complex of Kungnat Fjeld, South Greenland. Meddelelser om Gronland, 123(4): 1-145. UPTON, B.G.J., THOMAS, J.E. and MACDONALD, R. 1971. Chemical variation within three alkaline complexes in South Greenland. Lithos, 4: 163-84
Fig. 101 Location of lamprophyric carbonatite dykes in the Iluilarssuk area (after Walton and Arnold, 1970, Fig. 1). Fig. 102 Kungnat Fjeld (after Upton, 1960, Plate 5).
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