Ilimaussaq intrusive complex

The Ilímaussaq alkaline complex (Fig.1) is one of a number of intrusive complexes in the Gardar igneous province, a mid-Proterozoic rift zone in South Greenland. The first detailed description, with a geological map and a discussion of the petrogenesis of the complex, was presented by Ussing (1912), who introduced the term "agpaitic nepheline syenite". Since the appearance of Ussing’s memoir numerous papers on the geology, mineralogy, petrology, geochemistry and economic geology of the complex have been published.
According to the recommendations of the IUGS Subcommission on the Nomenclature and Classification of Igneous Rocks (Le Maitre 1989), the term "agpaitic" should be restricted to peralkaline nepheline syenites having complex Zr-Ti silicate minerals such as eudialyte and rinkite instead of the more common minerals zircon, titanite and ilmenite.

ilimaussaq(map)2017(1).jpg

Fig.1: Simplified geological map of the Ilímaussaq complex. Modified from Hunt (2017).



The complex measures 17 × 8 km, and the exposed vertical thickness is about 1700 m. It is estimated that the complex was emplaced 3-4 km below the contemporary surface at the discontinuity between the Ketilidian crystalline basement (c.1800 Ma) and the overlying Eriksfjord Formation made up of continental sandstones and lavas of mainly basaltic composition.
The basement and the overlying sandstones and lavas are intruded by numerous mainly basaltic dykes. The Eriksfjord Formation is the surface expression of Gardar activity and is preserved only in down-faulted blocks. Gardar activities embrace the period 1350 to c. 1120 Ma; the Ilímaussaq complex (dated at 1143 +/- 21 Ma) is thus an expression of young Gardar activity.

The basalts in the part of the Gardar rift zone which contains the Ilímaussaq igneous complexes (Fig.1) are richer in alkalis, P, Ba, Sr, Nb and LREE than the basic rocks in other parts of the Gardar province. This indicates an origin in a mantle source enriched in incompatible elements, perhaps because of metasomatism. The agpaitic nepheline syenites of the Ilímaussaq complex are considered to be products of extended fractionation of transitional to alkali basaltic melts in deep magma chambers combined with some crustal contamination.

Ilimaussaq magmatic phases

Three intrusive phases may be distinguished in the formation of the Ilímaussaq complex (Figs.2-3):

1) The first phase is made up of augite syenite which is preserved only as a partial marginal shell and in the roof.
2) The second phase consists of alkali granite and quartz syenite which are found in the roof and as blocks engulfed by rocks of the third intrusive phase.
3) The third intrusive phase occupies the major part of the complex. It is made up of a roof series, a floor series, and an intermediate sequence.

ilimaussaq(3).jpg

Fig.2: Successive intrusion of three melt batches at Ilimaussaq




ilimaussaq(section).jpg

Fig.3: E-W Schematic section of the Ilímaussaq alkaline complex. From Jens Konnerup-Madse.





ilimaussaq2017.jpg

Fig.4: Schematic section through the c. 1700 m of exposed stratigraphy of the Ilímaussaq alkaline complex.



The Roof Series:
The roof series (Fig.4) crystallised from the top downwards, forming the succession pulaskite, foyaite, sodalite foyaite and naujaite. The contacts of these rocks grade into each other, but blocks of the uppermost rocks were loosened from the temporary roof of the magma chamber and engulfed by the underlying crystallising rocks. Larsen (1976) demonstrated that there is a gradual evolution in mineralogy from pulaskite to naujaite. The primary mineral association alkali feldspar, nepheline, fayalite, hedenbergite, apatite and titanomagnetite is substituted downwards by sodalite, nepheline, alkali feldspar, aegirine, arfvedsonite, eudialyte and aenigmatite.

The floor series:
The floor series (Fig.4) is made up of a layered and laminated series of kakortokite, that is an agpaitic nepheline syenite with the major minerals alkali feldspar, nepheline, aegirine, arfvedsonite and eudialyte. The bottom of the series is unknown. The lowermost visible part is made up of centimetre-thick layers with varying contents of mafic minerals, feldspar and eudialyte.
The layered series of kakortokite passes gradually upwards into a thin unit of transitional layered kakortokite which again passes gradually into the intermediate sequence of lujavrites.

Intermediate sequence:
The upper part of the intermediate sequence (Fig.4) is made up of black arfvedsonite- rich, fine-grained laminated rocks. Dykes and sheets of lujavrite intersect the rocks of the roof zone. On the Kvanefjeld plateau, in the northernmost part of the complex, lujavrites are in contact with the volcanic roof of the complex, which is strongly fenitised adjacent to the lujavrites. Bohse & Andersen (1981) distinguish a lower aegirine lujavrite I zone which gradually passes into the overlying aegirine lujavrite II, a transitional lujavrite zone and a arfvedsonite lujavrite zone.
There are several generations oflujavrite. For exemple, one of the latest phases consists of naujakasite lujavrite rich in steenstrupine (instead of eudialyte), this represents the hyper-agpaitic stage of development characterised by naujakasite, steenstrupine, ussingite, vitusite and other minerals.

Petrography of Ilímaussaq alkaline rocks

Augite syenite
The augite syenite shows a xenomorphic texture with grain size varying between 2 and 20 mm. The main mineral are: strongly exsolved perthitic alkali feldspar, olivine, clinopyroxene and Fe–Ti oxides.

Sodalite foyaite
This rock type is typically medium to coarse grained with grain sizes up to 20 mm. The main mineral are: euhedral perthitic alkali feldpar, nepheline, sodalite, olivine and resorbed relics of augite, sector-zoned Na-rich clinopyroxene, aenigmatite, fluorite, rare eudialyte and zoned ferrorichterite. Analcime appears to occur as a late liquidus phase, but most analcime forms together with secondary sodalite by subsolidus replacement of primary sodalite and nepheline.

Naujaite
Euhedral sodalite and nepheline are cumulate phases in this rock type. Their grain size reaches >1 cm. Alkali feldspar occurs as large tabular crystals with frequent inclusions of sodalite and is interpreted to have crystallized as the first of the interstitial minerals, which additionally comprise aegirine, arfvedsonitic amphibole, eudialyte and again nepheline.

Kakortokite
The kakortokite occurs as a white, a red and a black variety. The differences are caused by variations of the modal content of arfvedsonitic amphibole (black), eudialyte (red) and feldspar (white). The white and red kakortokite contain eudialyte and feldspar as early, mostly euhedral liquidus phases, whereas the black variety shows amphibole and in places pyroxene as early phases.

Lujavrite
Many varieties of this rock type show flow textures. Euhedral nepheline, eudialyte, sodalite and clinopyroxene are enclosed in a mixture of albite, microcline, texturally later aegirine pyroxene, and amphibole, which are mostly aligned. Black lujavrite is distinguished from a green variety depending on the modal amount of either arfvedsonite (black) or aegirine (green). In some of the samples, rare minerals such as ussingite, naujakasite, steenstrupine, villiaumite occur, in some special varieties even as major constituents.

ilimaussaq2017(2).jpg

View from Tunudliarfik fjord looking north at white xenoliths of naujaite suspended in darker lujavrite. Vertical height approx 400m.



ilimaussaq2017(6).jpg

The layered kakortokites from north slopes of Kangerdluarssuk.



kakortokite(layered)1.jpg

The layered kakortokites from north slopes of Kangerdluarssuk.



ilimaussaq2017(3).jpg

Xenoliths of augite syenite within lujavrite. Note the flow banding in the lujavrite and the metasomatic alteration of the augite syenite. Lens cap 45 mm diameter.



ilimaussaq2017(4).jpg

Tugtupite-albite vein surface against host augite syenite.n Chisel is 15 mm wide at top.



eudyalite(pegmatite).jpg

Aegirine-eudialyte syenite pegmatite in naujaite. From Hunterian Museum Geology Collections




ilimaussaq2017(5).jpg

Polished surface of typical naujaite. Note pink eudialites poililitically enclose alkali feldspar on a large scale and sodalite on a small scale.



naujaite2017.jpg

Naujaite sample. Sodalite (gray), microcline (white), arfvedsonite (black), eudialyte (red). From Hunterian Museum Geology Collections




kakortokite(polished).jpg

Kakortokite sample. microcline (white) and eudialyte (red).




luajvrite(analcime).jpg

Analcime-rich lujavrite. From Hunterian Museum Geology Collections




lujavrite(steenstrupine).jpg

Steenstrupine (dark, big crystals) in a lujavrite. From Friends of Minerals Forum.




luajvrite(villiaumite).jpg

Naujakasite-rich lujavrite. Arfvedsonite (black), villiaumite (red) and naujakasite (lozenge-shaped).



Bibliography



• G.Markl; M.Marks; G.Schwinn e H.Sommer (2001): Phase Equilibrium Constraints on Intensive Crystallization Parameters of the Ilímaussaq Complex, South Greenland. Journal of Petrology. Num.12; vol. 42; pg: 2231–2258.
• Markl, G., Marks, M., Schwinn, G., & Sommer, H. (2001). Phase equilibrium constraints on intensive crystallization parameters of the Ilímaussaq Complex, South Greenland. Journal of Petrology, 42(12), 2231-2257.
• Sørensen, H. (Ed.). (2001). The Ilímaussaq alkaline complex, South Greenland: status of mineralogical research with new results (No. 301). Geological Survey of Denmark and Greenland, Ministry of Environment and Energy.
• S. Karup-Møller; J. Rose-Hansen (2013): New data on eudialyte decomposition minerals from kakortokites and associated pegmatites of the Ilimaussaq complex, South Greenland. In: Geological Society of Denmark. Bulletin, Vol. 61, 2013, p. 47-70.
• Hunt, E. J., Finch, A. A., & Donaldson, C. H. (2017). Layering in peralkaline magmas, Ilímaussaq Complex, S Greenland. Lithos, 268, 1-15.