Geochemistry of Gardar intrusions in the Ivigtut area, South Greenland

The Gardar Alkaline Igneous Province is a Proterozoic, rift-related, province in south-west Greenland. It consists of lavas, central complexes and dyke swarms, dating from between 1350 and 1150 million years ago. The majority of the intrusives were emplaced into a Proterozoic mobile belt which formed during the Ketilidian orogeny at -1850-1740 Ma, but the north-western part of the province transgressed onto the edge of the South Greenland Archaean craton, and it is this area which has been investigated in this work. The Gardar intrusives within the craton include three central complexes, Grønnedal-tka, Ivigtut and Küngnât, as well as many dykes. Grønnedal-IIka is the oldest of the complexes, and consists of nepheline syenites cut by a carbonatite plug. The oldest dykes in the area are lamprophyres; these were succeeded by various groups of basic and alkaline salic dykes, including a major set of olivine dolerites known as the "Brown Dykes". The Ivigtut stock is an alkali granite which, prior to mining, contained the world's largest deposit of cryolite, and the granite shows a distinctive pattern of alteration related to the fluids from which the ore deposit formed. The Küngnat Complex consists of saturated and over-saturated syenites with an alkali gabbro ring-dyke. The petrography of the different rock-types is described together with the mineralogy of the Ivigtut granite. The Gardar rocks of the subject area represent a wide spectrum of 5i0 2 contents, from 35 wt% in the lamprophyres to 75 wt% in the granites, indicating a range from magmas of primitive compositions to highly evolved granites. The lamprophyre dykes have normalised trace element patterns similar to those of Ocean Island Basalts (OIB), whereas the patterns for the basaltic dykes suggest interaction with crust or sub-continental lithospheric mantle (SCLM). The trace and rare earth element patterns for the salic rocks show evidence of evolution by fractionation from basic magmas. The altered rocks of the Ivigtut stock show large amounts of enrichment in highly incompatible elements (including the HREE). In general, all the granitic rocks have the characteristics of A-type granites. Selected samples were analysed for Nd-, Srand (in some cases) Pb-isotopes in order. to investigate the petrogénesis of the Gardar magmas. Ndand Srisotope data show that the basic magmas were derived from a mildly depleted mantle source; and support the conclusion that the salic rocks evolved by fractionation of these basic parents, with accompanying crustal contamination in the silica over-saturated examples. Pbisotope data provide further confirmation of these conclusions. Magmatic carbonates occur within each of the central complexes, as well as in the lamprophyric dykes; Cand 0isotope data imply a mantle source for the carbonates at Grønnedal-fka and Ivigtut, but indicate crustal contamination at Kflngnât. Comparison with data from Gardar rocks emplaced in the Ketilidian mobile zone to the south suggests that magma sources were similar and indicates that the magmas intruded through the Archaean craton were not sourced in a different, older component of lithospheric mantle. Geochemical differences between the intrusives within the craton and those to the south can be attributed to the nature of the crustal contaminant. It is concluded that the Gardar lamprophyre magmas formed as volatile-rich small-degree partial melts within the asthenosphere, which were intruded with minimal lithospheric contamination. Larger melt volumes were derived from lithospheric mantle which had been selectively enriched in incompatible elements, possibly through the action of fluids derived from a subducting slab at the close of the Ketilidian orogeny. These melts were intruded as primitive dykes, or evolved through fractionation and variable crustal contamination to produce the spectrum of Gardar magmas.