Alkaline Rocks and Carbonatites of the World

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Ubekendt Ejland

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Occurrence number: 
063-00-023
Country: 
Greenland
Location: 
Longitude: -53.83, Latitude: 71.17

The Tertiary volcanic province of central West Greenland is dominated by basaltic flows of tholeiitic affinities, but in the upper part of the 4-5 km succession on Ubekendt Ejland are alkali basalts containing a little analcime, salite with a moderate proportion of aegirine molecule, biotite and a brown amphibole (Larsen, 1977, p. 11). On the western part of the same island a swarm of lamprophyric dykes includes kersantites, camptonites and monchiquites and Larsen (1981) described an olivine monchiquite containing analcime in the groundmass and also ocelli, some of which are thought possibly to have replaced leucite. The dyke contains numerous megacrysts and rock fragments, including nepheline syenites of alkali feldspar, nepheline, analcime and glass. Drever and Gribbon (1968) report the discovery of melilite and nephelinite among the monchiquite dykes but there appears to have been no subsequent description. At Quarsut on the north coast of Nugssuaq, an island lying south of Ubekendt Ejland, and only some 10 km west of Quigussaq (No. 24), a group of picritic sills occurs one of which is emplaced into Cretaceous sediments and is 50 m thick; it contains a 1 m thick horizontal sheet of alkaline dolerite with pegmatitic layers and lenses containing analcime, aegirine-augite, kaersutite, arfvedsonite, katophorite and biotite (Clarke and Pedersen, 1976, p. 380). A useful locality map will be found in Clarke and Pederson (1976, Fig. 1_324).

Age: 
Ages on three lamprophyres from Ubekendt Ejland determined by K-Ar were 30.6±5 Ma, 32.5±4.8 Ma and 39.5±9.6 Ma (Parrott and Reynolds, 1975).
References: 
CLARKE, D.B. and PEDERSEN, A.K. 1976. Tertiary volcanic province of West Greenland. In A. Escher and W.S. Watt (Eds). Geology of Greenland. Gronlands Geologiske Undersogelse: 365-85. DREVER, H.I. and GRIBBON, P.W.F. 1968. St Andrews University Greenland projects, 1938-67. Polar Record, 14: 187-213. LARSEN, J.G. 1977. Transition from low potassium olivine tholeiites to alkali basalts on Ubekendt Ejland. Meddelelser om Gronland, 200(1): 1-42. LARSEN, J.G. 1981. Medium pressure crystallization of a monchiquitic magma - evidence from megacrysts of Drever's block, Ubekendt Ejland, West Greenland. Lithos, 14: 241-62. PARROTT, R.J.E. and REYNOLDS, P.H. 1975. Argon-40/argon-39 geochronology: age determinations of basalts from the Labrador Sea area. Abstracts with Programs, Geological Society of America, 7: 835
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