The form and internal structure of the alkaline Kangerdlugssuaq intrusion, East Greenland

Abstract

Summary. The Kangerdlugssuaqintrusion (68' 2r N. 32' 30" W., roughly circular and 33 km across) is of Tertiary age, like the nearby Skaergaard intrusion. At gh~ present level of erosion the outer part is of nordmarkite and inwards there are successive rings of transitional pulaskite (without quartz or nepheline), main pulaskite and, in the centre, foyaite. The different rock types are due to gradual ehanges in the amounts of cer~aia of the minerals and there are no sharp junctions such as would be expected to result from successive injections.Zones of inclusions of basalt, fallen from above, dip inwards at 30 to 60 ~ and, in places, there is a platy parallelism of the tabular alkali feldspars dipping similarly inwards. The three-dimensional form of the intrusion is pictured as resembling a pile of saucers of decreasing size, the secluence of formation being from the outer saucer of nordmarkite to the inner, and smaller saucers of increa~ingly nephelinerich syenites. Bottom accumulation of crystals is postulated to explain the disposition of the rock types, but the cause of the succession from quartz-bearing to felspathoidal syenites remains uncertain. N EPHELINE-I:earing syenites were found by the writer durirg the Watkins Expedition of 1930, on the west side of Kangerdlugssu~q fjord in east Greenland, about 20 miles from the layered basic complex which has since been named the Skaergaard intrusion. In 1932, accompanied by his brother, a further examination of the region was made and it was established that the nepheline-bearing rocks formed part of a large intrusion exhibiting a sequence of syenites from nordmarkites to pulaskites. In 1935-36, with the collaboration of W. A. Deer, the intrusion was mapped (1937, pp. 401 and 408) and shown to be a series of rings of considerable regularity, the outer being of nordmarkite which passes gradually into syenite, free from quartz, and then into syenite with increasing amounts of felspathoids, until an innermost foyaite is reached. Because of the gra.dualness of these changes, and the disposition of the various rock types, it was considered, at that time, that the change from a quartz syenite to an undersaturated nephelinesodalite syenite was probably the result of crystal fraetionation.Much preliminary loJboratory work wr~s done in the 1940's by the present writer and by Professor W. A. Deer who analysed some twenty

6: Keweenawan intrusive igneous rocks

“…The same magma compositions are found in the Keweenawan rocks in the Kapuskasing structural zone, the Coldwell Alkaline Province, and the quartz tholeiite and olivine tholeiite dikes and sills in the Thunder Bay-Grand Portage area. The evolution of the present rift system in the North Atlantic may be viewed as a progression involving the emplacement of the alkaline magma in the Kangerdlugssuaq intrusion in east Greenland (Wager, 1965), outpouring of high-FeTi quartz tholeiite flows of the Thulean and Faeroe Island plateau (Noe-Nygaard, 1966;Brooks, 1973;Brooks and Jakobsson, 1974), followed by the development of Iceland over the Mid-Atlantic rift system with its current outpouring of high-Al olivine tholeiite (Schilling and NoeNygaard, 1974). These parallelisms ( Fig.…”

Compositional variation in the pyroxenes and amphiboles of the Kangerdlugssuaq intrusion, East Greenland: further evidence for the crustal contamination of syenite magma

“…These results are not favourable to the idea of a crystal fractionation model for the intrusion and we suggest that the foyaite is closest to the original magma which has hybridized with the enclosing gneisses and basalts to produce the over-saturated rocks. Such a model is consistent with the existing isotopic data.T n E Kangerdlugssuaq alkaline intrusion, described first by Wager (1965), who regarded it as having formed by gravitational sinking of early formed crystals, has been regarded as a typical example of a syenite intrusion in which a range of rock types has developed by simple differentiation in situ (Sorensen, 1974, p. 38). In surface outcrop it is a large circular body, 30 km in diameter, having a volume estimated to be about 6500 km 3.…”

“…T n E Kangerdlugssuaq alkaline intrusion, described first by Wager (1965), who regarded it as having formed by gravitational sinking of early formed crystals, has been regarded as a typical example of a syenite intrusion in which a range of rock types has developed by simple differentiation in situ (Sorensen, 1974, p. 38). In surface outcrop it is a large circular body, 30 km in diameter, having a volume estimated to be about 6500 km 3.…”

U–Pb geochronology of the Eocene Kærven intrusive complex, East Greenland: constraints on the Iceland hotspot track during the rift-to-drift transition

“…If the single-intrusion model (e.g. Wager 1965; Riishuus et al 2008) is correct, then the age of the nordmarkite can be considered representative of the entire Kangerlussuaq Syenite. If not (e.g.…”

“…The nordmarkite zone, which comprises c . 90% of the intrusion (Wager, 1965), has been dated to 53±6 Ma by whole-rock Rb–Sr dating (Holm, 1991). The pulaskite zone comprises c .…”