Ilimaussaq

The best known of the Gardar intrusions and one of the most remarkable alkaline complexes in the world, with its spectacular layered rocks of unparalleled mineralogy, Ilimaussaq is oval in plan, measuring 16 by 7 km and is separated into two parts by the Tunugdliarfik Fjord. In the northwest it cuts across the Dyrnaes-Narssaq complex and is also younger than the regional swarm of trachyte and trachydolerite dykes. As a result of faulting lower parts of the complex are exposed in the southeast, where the contacts are against the basement Julianehab granite, while in the northwest a partial roof of basalts and sandstones of the Eriksfjord Formation is preserved. Geological and geochemical evidence indicates that the complex was probably emplaced in three stages involving augite syenite, peralkaline acid rocks and agpaitic rocks. The early augite syenite is preserved along the western and southern margins and against the roof in the north. It has an outer chilled margin and locally steeply inward-dipping layering. The mineralogy is alkali feldspar, fayalitic olivine and ferrosalite with accessory titanomagnetite, apatite, Ti-rich hastingsite and biotite. The peralkaline acid rocks are represented by two sheets of granite and quartz syenite within and below the augite syenite of the roof zone in the north. The petrogenetic relationships of these rocks are not fully understood. They comprise alkali feldspar, quartz, aegirine and alkali amphibole with accessory aenigmatite, eucolite, neptunite, ilmenite and pyrochlore.

The third and principal stage of injection involved a peralkaline, silica-undersaturated magma which underwent a complex differentiation history. The term 'agpaitic' was introduced by Ussing (1912), after the locality of Agpat on the eastern margin of the complex, for these rocks. Initial crystallization led to both the floating and sinking of crystals. Against the roof pulaskites and foyaites are considered to represent early fractions, and comprise alkali feldspar, hedenbergite, katophorite, nepheline, fayalite and alkali pyroxene with accessories including aenigmatite, biotite, fluorite and eudialyte. There is then a transition through foyaite with interstitial sodalite to sodalite-rich rocks, named naujaite by Ussing, which have a thickness of about 600 m and develop some layering. The naujaites contain about 35-45% sodalite, together with poikilitic alkali feldspar, aegirine, arfvedsonite and eudialyte, with accessory rinkite, zeolites, aenigmatite and nepheline. Contemporaneously with accumulation against the roof there was probably bottom accumulation but any such rocks are below the present level of exposure. In the southeast of the complex laminated and layered kakortokites occur, and are considered to be younger than the roof accumulates and may represent the products of a new pulse of agpaitic magma (Bailey et al., 1981, p. 6). The kakortokites comprise a sequence of layered rocks at least 285 m in thickness within which 29 layered units, each of about 7 m thickness, can be distinguished. An ideal unit grades upwards from a black, arfvedsonite-rich base, through a red layer rich in eudialyte, to a white feldspathic layer. Aegirine and nepheline are also present together with accessory aenigmatite, rinkite and zeolites. Small-scale rhythmic layering and slump structures may also be observed. The kakortokites pass conformably upwards into 200-350 m of lujavrites which consist of microcline, albite, nepheline, sodalite, eudialyte and aegirine in the lowermost parts, passing upwards into arfvedsonite-rich types; these are considered to be cumulus phases. Accessories include sphalerite, steenstrupine, monazite, zeolites and Li-mica. The lujavrites are somewhat schistose and contain deformation structures considered to be indicative of deformation during the closing stages of the evolution of the complex. The arfvedsonite lujavrites intrude and brecciate the overlying naujaites, the augite syenite and the country rocks, and there are numerous pegmatites and hydrothermal veins. The contents of F, REE, P, Th, U, Sn, Li, Be and Ga reached their highest levels during the crystallization of these rocks, and has stimulated considerable research on their geochemistry and mineralogy. There is some fenitization around the complex (e. g. Hamilton, 1964) and it has been suggested that uranium-rich rocks near the roof of the complex may be fenitized and mobilized basic volcanics. (Sorensen et al., 1969 and 1974). Very many papers on all aspects of the complex have been published and lists of many of them will be found in Sorensen (1967) and Bailey et al. (1981, p. 125). A brief review of research on Ilimaussaq is given by Bailey et al. (1981), together with a number of papers on recent work. Sorensen et al. (1981) list all the minerals identified at Ilimaussaq up to 1979, together with a bibliography of 62 papers in the series 'Contributions to the Mineralogy of Ilimaussaq'. A 1:20000 scale coloured geological map and account of the general geology is given by Ferguson (1964) and Hamilton (1964) presents modal and chemical data on the main rock types, while papers on various aspects of the geochemistry are briefly reviewed by Bailey et al. (1981, p. 12). Compositions of pyroxenes, amphiboles and other mafic rock-forming minerals are given by Larsen (1976), and an account of fluid inclusion work by Konnerup-Madsen and Rose-Hansen (1981) and Konnerup-Madsen et al. (1985). A discussion of a closed system crystal-fractionation petrogenetic model is given by Engell (1973) but many other papers discuss petrogenesis.