The petrology and geochemistry of the Igaliko Dyke swarm, south Greenland.

Abstract

The dykes from the Igaliko Nepheline Syenite complex belong to at least 3 individual swarms (i) a Mid-Gardar swarm in the Østfjordsdal valley, (ii) a Late-Gardar, Si-oversaturated swarm associated with the Younger Giant Dykes of Tugtutôq and (iii) a Si-undersaturated swarm intimately associated with the Late Gardar Igaliko Nepheline Syenite Central Complexes. In addition Early Gardar activity is recorded by the presence of some ultramafic lamprophyres which predate the Motzfeldt centre, sparse trachytes which are truncated by intrusions within the Motzfeldt centre and a possible BD(_0) dolerite which is also cut by the Motzfeldt centre. Most dykes however are bracketed between the Early and Late Igdlerfigssalik syenite intrusions. The main oversaturated and undersaturated suites can be separated on their Zr/Nb ratios (≈6.4 and 3.9 respectively). In addition, the undersaturated basic rocks have smooth chondrite normalised incompatible element spidergrams whereas the oversaturated basic rocks are characterised by negative Nb and positive P anomalies. Evolution of both suites can be modelled in terms of fractional crystallisation of feldspar, clinopyroxene, olivine, apatite and opaques from basaltic parents to either phonolitic or rhyolitic minimum compositions. In each instance these evolved composi tions are extremely rich in incompatible trace elements (REE, Nb, Zr, Rb). In some cases a high CO(_2) content in the undersaturated rocks may lead to the formation (by liquid immiscibility) of late stage carbonatite magmas. High CO(_2) also produces high ƒo(_2) in these magmas and it is argued that in some cases this can suppress the development of negative Eu anomalies on feldspar fractionation. The undersaturated swarm may have evolved from lamprophyric parental magmas, eg. camptonites, which are relatively abundant basic dykes. Ultramafic lamprophyres, often early, may have formed as extremely small degree partial melts at the onset of Gardar rifting. In the Late Gardar, magma genesis is related to the different extensional tectonic regimes which were operative at that time. Mineralogical evolution follows paths similar to several other Gardar suites and records a higher ƒo(_2) in the undersaturated rocks. Zr becomes concentrated in interstitial residual liquids in benmoreites and substitutes into amphibole as the newly proposed end-member zirconian-arfvedsonite.