The island of Tugtutoq and the numerous small islands and skerries adjacent to it are composed of Precambrian granites and granite gneisses, which are cut by intensive swarms of dykes trending predominantly north-northeast-south-southwest. In places 10-20% of the area is occupied by dykes which are petrographically very variable and include a number of giant dykes up to 800 m thick. In the centre of Tugtutoq a ring complex of granites and syenites developed later in the igneous history of the area. The Tugtutoq dykes trend north-northeast directly towards the Narssaq-Dyrnaes and Ilimaussaq complexes which are only some 5-6 km away. The earliest olivine dolerite dykes were followed by emplacement of the Older Giant Dyke Complex (formerly known as the Hviddal giant dyke) (Upton et al., 1985), which extends for 20 km, averages 5-600 m in thickness, and is composite with gabbroic margins and a centre which varies from slightly silica-undersaturated ferroaugite syenite in the west-southwest to nepheline syenite in the east-northeast. The more westerly rocks comprise perthite, olivine, ferroaugite, barkevikite, biotite, ilmenomagnetite and apatite. Westwards biotite and olivine disappear, the zoned pyroxenes change to aegirine-augite and aegirine, and a green amphibole, which commonly envelops pyroxene, appears. Nepheline, zeolite, sodalite and analcime appear and increase in volume eastwards. A description of this dyke, with analyses of rocks and minerals, will be found in Upton (1964b, 53-78), and a more detailed mineralogical account in Upton et al. (1985). The Hviddal dyke was followed by emplacement of giant dykes of olivine gabbro (Upton, 1962 and 1964b; Upton and Thomas, 1980) of which the more northerly at the east end of Tugtutoq contains central masses of syenite, the Assorutit syenite. This consists of perthite, sometimes quartz, fayalite, pyroxene zoned to green rims and riebeckitic-arfvedsonitic amphiboles, as well as syenogabbro containing abundant biotite and pyroxenes zoned to green rims. There followed emplacement of the intense 'Mid-Gardar' swarm of dykes (Upton, 1962, p. 31 and 1964a, p. 12), which range from trachydolerites through trachytes to riebeckite and aegirine microgranites (Macdonald, 1969). The next major event was the emplacement of the Central ring complex involving six ring intrusions of syenite and granite followed by a stock of perthositic syenite (Upton, 1962, p. 41 and 1964a, p. 25). These rocks consist of varying proportions of perthite, quartz, fayalite, hedenbergite-aegirine-augite-aegirine, hornblende-riebeckite, biotite and accessories including astrophyllite, aenigmatite and fluorite. A further set of east-northeast-trending dolerite dykes was intruded after emplacement of the Central complex. The petrochemistry of the Central complex and of the dyke swarm is considered by Upton et al. (1971).
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.
MACDONALD, R. 1969. The petrology of alkaline dykes from the Tugtutoq area, South Greenland. Bulletin of the Geological Society of Denmark, 19: 257-82.
UPTON, B.G.J. 1962. Geology of Tugtutoq and neighbouring islands, South Greenland. Part I. Meddelelser om Gronland, 169: 1-60.
UPTON, B.G.J. 1964a. The geology of Tugtutoq and neighbouring islands, South Greenland. Part II. Nordmarkitic syenites and related alkaline rocks. Meddelelser om Gronland, 169(2): 1-62.
UPTON, B.G.J. 1964b. The geology of Tugtutoq and neighbouring islands, South Greenland. Part III. Olivine gabbros, syeno-gabbros and anorthosites of the Hviddal composite dyke. Meddelelser om Gronland, 169(3): 1-80.
UPTON, B.G.J. and THOMAS, J.E. 1980. The Tugtutoq younger giant dyke complex, South Greenland: fractional crystallization of transitional olivine basalt magma. Journal of Petrology, 21: 167-98.
UPTON, B.G.J., THOMAS, J.E. and MACDONALD, R. 1971. Chemical variation within three alkaline complexes in South Greenland. Lithos, 4: 163-84.
UPTON, B.G.J., STEPHENSON, D. and MARTIN, A.R. 1985. The Tugtutoq Older Giant Dyke Complex: mineralogy and geochemistry of an alkali gabbro-augite-syenite-foyaite association in the Gardar province of South Greenland. Mineralogical Magazine, 49: 623-42.