Subject index

Abstract

The persodic, 1.16 Ga old Ilímaussaq intrusion consists of alkali granites, syenites, and agpaitic nepheline syenites. These rocks are cut by late-magmatic veins and lenses consisting of albite, aegirine, ussingite (Na2AlSi3O8(OH)), fluorite, and occasionally quartz. Oxygen-isotope compositions of quartz indicates both orthomagmatic (δ18Oqtz = 8.5‰) and country rock-influenced (δ18Oqtz ~ 10‰) origin. While albite and aegirine do not contain fluid inclusions suitable for investigations, ussingite contains secondary hydrocarbon fluid inclusions, fluorite contains brine inclusions of primary and secondary origin with up to 26.3 wt.% NaClequiv., and quartz contains predominantly secondary brine inclusions with up to 29.7 wt.% NaClequiv. or CH4–H2O–NaCl mixtures. These fluids are interpreted to reflect fluids in equilibrium with late-stage melts at Ilímaussaq at ≤ 450 °C.The carbon- and hydrogen-isotope composition of CH4 in ussingite (δ13C = − 6 to − 3‰, δD = ~− 121‰) is indicative of a magmatic abiogenic origin. However, the isotopic composition of CH4 in fluid inclusions in quartz veins resembles the signature of thermogenic CH4 (δ13C = − 43 to − 23‰, δD = − 176 to − 121‰), but the higher hydrocarbons are mostly 13C-depleted in relation to CH4 (up to 7.3‰), which is typical of abiogenically-derived hydrocarbons.Ion-chromatography of fluid inclusion leachates from the late-stage veins reveals Cl/Br ratios of ~ 100. As such values seem to be typical of peralkaline magmatic rocks, at least in the Gardar Province of South Greenland, it is suggested that this ratio is typical of Gardar magmatic fluids and may be characteristic of the Cl/Br ratio of the lithospheric mantle from which these alkaline melts are derived.