Abstract
Numerous dyke intrusions are found in the Narssarssuaq area of the Gardar province, a Mid-Proterozoic intracontinental rift system. Ten to fifteen percent of these dykes, which range in composition from trachybasalt to phonolite and rhyolite, contain significant proportions of feldspar megacrysts and occasionally anorthosite xenoliths. Two groups of dykes are distinguished; the older group is more alkaline, richer in incompatible elements and contains more anorthosite xenoliths than the younger. It is probable that the main reason for the differences is variation in magma production through time and from one area to another. Chemical zonation in the dykes reflects compositional gradients in the feeding magma reservoirs; the magma reservoirs acting as open systems in which crystal fractionation was an important controlling process. The anorthosite xenoliths are not strictly cognate with their hosts, but were derived from comparable alkaline magmas with a composition roughly corresponding to the most primitive of the dykes. The plagioclase megacrysts were presumably formed at an early stage of the development of the magma chambers. Rb-Sr dating of one of the dykes from the older group of dykes gives an age of 1206 ± 20 Ma and an initial 87Sr/86Sr ratio of 0.7028 ± 0.0001 supporting a low degree of contamination with upper crustal Sr.
Highlights
Numerous dyke intrusions are found in the Narssarssuaq area of the Gardar province, a Mid-Proterozoic intracontinental rift system
Dykes were forrned during periods of high tension and high magma production, while more alkaline stocks, plugs and central intrusions forrned during periods of less active tension (Upton & Emeleus, 1987)
In some of the central complexes and in many different types of dykes abundant plagioclase megacrysts and anorthosite xenoliths are found. Dykes bearing such material have been terrned 'Big Feldspar Dykes' (BFDs) or just 'Feldspathic Dykes' (Ayrton, 1963; Bridgwater, 1967; Bridgwater & Harry, 1968) and they have been described in detail by Bridgwater &
Summary
All the BFDs in the Narssarssuaq area fall in two well distinguished groups (Table 1). Forrned in the same rifting system the two groups of dykes differ in age and main geographical areas. One of the two groups of dykes crosses Tugtut6q and the Narssaq peninsula and extends north and north-east of Narssarssuaq and G. F. Holm Nunataq (Upton & Fitton, 1985). Holm Nunataq (Upton & Fitton, 1985) It has been terrned the Main Swarm of the Tugtutoq-Ilimaussaq Dyke Swarm (Martin, 1985). The other group of dykes runs from the area east of Narssarssuaq and the Motzfeldt centre to Igaliko and further SSW (Fig. 1) It is referred to as the Southeast Swarm. ",k l":::::::,::::j Gardar central complex Southeast Swarm dyke with sample locality
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