Alkaline Rocks and Carbonatites of the World

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Nunarssuit

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Occurrence number: 
063-00-034
Country: 
Greenland
Location: 
Longitude: -48, Latitude: 60.07

Figs 101 and 105. At 45x25 km the Nunarssuit complex is the most extensive of the Gardar plutons. It cuts Precambrian Julianehab granites and truncates the Gardar dykes, principally dolerites, which are numerous in this vicinity. Rock types found within the complex include gabbro, syenite and granite, not all of which has an alkaline character; mineral layering is well developed in parts of the complex and in all rock types (Harry and Pulvertaft, 1963, p. 82). The early Alangorssuaq gabbro is olivine-rich and contains a little mica and interstitial alkali feldspar. The extensive Helene granite in the north of the complex consists of perthite, hornblende, sometimes with thin borders and radiating acicular crystals of sodic amphibole, aegirine-augite, confined to about one third of studied samples, sporadic fayalite and a little biotite and fluorite. Most of the large Kitsigsut syenite lies beneath the sea but it forms numerous skerries to the west of the complex and consists of perthite, a pale green or brown clinopyroxene and fayalite. A biotite granite lying north of the Helene granite is also partly covered by the sea but is not peralkaline. The Nunarssuit syenite, covering about 300 km2, is the largest unit of the complex and is uniform in texture and mineralogy apart from mafic banding, which is occasionally spectacularly developed (see, for instance, the dust jacket on 'The Alkaline Rocks' edited by H. Sorenson). This syenite comprises perthite and albite-oligoclase, augite, often zoned to a pale green rim, hornblende and occasional probably sodic amphibole, fayalite and a little biotite; quartz occurs in some two thirds of examined rocks and aenigmatite was identified in one specimen. The late Kitdlavat granite is laccolithic in form, has weak layering and consists of quartz, perthite, amphibole with deep blue rims, an interstitial blue amphibole, aegirine-augite and sometimes aenigmatite. Small bodies of more strongly peralkaline granite occur within the Nunarssuit syenite in the southeast, one containing aenigmatite, aegirine, sodic amphibole and accessory astrophyllite and fluorite, while the Malenefjeld granite in the southeastern corner of the complex and extending an unknown distance beneath the sea is an aegirine-sodic amphibole granite and the most extensive of the strongly peralkaline granites of the complex. Dykes and pegmatites of similar mineralogy to these granites are widespread over the southern part of the complex, bastnaesite being abundant in pegmatites within the Malenefjeld granite.

Age: 
Rb-Sr whole rock isochrons for the Alangorssuaq gabbro, Helene granite, Nunarssuit biotite granite and Nunarssuit syenite gave ages of 1143±48, 1149±31, 1162±21 and 1154±14 Ma (Blaxland et al., 1978, p. 238).
References: 
BLAXLAND, A.B., BREEMEN, O.van, EMELEUS, C.H. and ANDERSON, J.G. 1978. Age and origin of the major syenite centres in the Gardar province of South Greenland: Rb-Sr studies. Bulletin of the Geological Society of America, 89: 231-244. HARRY, W.T. and PULVERTAFT, T.C.R. 1963. The Nunarssuit intrusive complex, South Greenland. Part 1. General description. Meddelelser om Gronland, 169(1): 1-136
Fig. 101 The Gardar Province, Southwest Greenland. Fig. 105 Nunarssuit (after Harry and Pulvertaft, 1963, Plate 3).
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