Abstract

An ENE-WSW trending swarm of Gardar dykes, traversing Mellemlandet and G. E Holm Nunataq is principally composed of a 'main series' with compositions ranging from alkali olivine basalt to trachyte and rhyolite, and scarcer phonolitic trachyte associates. The most basic 'main series' magmas were emplaced as several giant dykes up to 650 m wide. Synformally layered gabbroic and anorthositic cumulates are locally developed within these. At Syenitknold internal differentiation within a giant dyke gave rise to syenogabbros, layered syenite cumulates and peralkaline nepheline syenite pegmatites. A large xenolithic mass of exotic feldspathic gabbro within the syenites is ascribed to the foundering of feldspar-rich roofing facies into the underlying magma chamber. Less extreme differentiation in the same giant dyke east of Syenitknold produced syenogabbroic cumulates containing evidence for vigorous convective flow having developed in the cooling intrusion. Smaller (< 40 m wide) and younger dykes are almost invariably of more differentiated character. The commonest dykes ( < 15 m wide) are of benmoreite and trachyte. Dykes with their interiors crowded with plagioclase xenocrysts and anorthositic inclusions are referred to as 'big feldspar dykes' (B.F.D.s). While all compositions from basalt to benmoreite may be involved in the B.F.D.s, the B.F.D. character is typical of the hawaiites and mugearites. Small (typically < 1 m), scarce dykes and sills of highly silica-undersaturated types range from ultramafic lamprophyres to carbonatites. These may be representative of a compositional continuum between 36 and 2 wt % SiO2,. The main swarm is so closely similar to that seen to the WSW, extending through Tugtutoq and the Narssaq and Qagssiarssuk areas, that it is thought to be merelya faulted continuation ofthe latter. Itso, this swarm, c. 15 km across, is at least 140 km long. The magnitude and extent of this alkaline swarm and its individual components, may well be unique: it differs from other swarms (e.g. that of the roughly contemporaneous Nunarssuit-Isortoq swarm) in the size and abundance of the salic dykes within it. It was almost certainly related to extensive fissure eruption of basic to salic lavas. A clockwise change of several degrees between the orientation of early giant dykes and later differentiated dykes is related to a change in the extensional stress direction during the development of the Gardar rift system.

Highlights

  • Small, scarce dykes and sills of highly silica-undersaturated types range from ultramafic lamprophyres to carbonatites

  • Salic dykes are relatively plentiful in the neighbourhood of the large central complexes. (e.g. Grønnedal-Ika and Igaliko areas; Emeleus & Upton, 1976), where they may be attributed to lateral injections from large sub-volcanic magma chambers

  • It is of critical importance because it involved a whole spectrum of magmas ranging from alkali olivine basalt to alkali rhyolites

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Summary

GRØNLANDS GEOLOGISKE UNDERSØGELSE

Gardar dykes north of the Igaliko Syenite Complex, southern Greenland by B. No 106 Report on the 1980 geological expedition IO the Pcary Land region. No 113 Petrography and geochemislry of amphibolites from the Nordre Strømfjord area in tbe central pan of tbe Nagssugtoqidian of Wcst Greenland. No 114 Processing and interpretation of aeromagnetic data in The Geological Survey of Greenland. No 116 Tertiary volcanic rocks from Bontekoe ø, East Greenland. No 117 Origin of quartzo.feldspathic supracrustal rocks from the central pan of the Nagssugtoqidian mobile helt of West Greenland. No 121 Biostratigraphic studies in western North Greenland; Operation Grant land 1965-1966. No 122 Aeromagnetic maps af parts of southem and central West Greenland: acquisition. No 126 Repon on the 1984 geological cxpedition to central and western onh Greenland.

Giant dykes
Petrography of giant dyke lithologies
Carbonatites and lamprophyres
Ile di co ol hy ml il ap
Cl P W norms
Findings
Concluding comments

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