Isotope, major, minor and trace element geochemistry of late-magmatic fluids in the peralkaline llimaussaq intrusion, South Greenland

The persodic, 1.16 Ga old llimaussaq 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 (theta O-18(qtz)=8.5%) and country rock-influenced (theta O-18(qtz) similar to 10 parts per thousand) 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 perdominantly secondary brine inclusions with upto 29.7wt.%NaClequiv or CH4-H2O-NaCl mixtures.These fluids are interpreted to reflect fluids in equilibrium with late-stage melts at llimaussaq at <= 450 degrees C. The carbon- and hydrogen-isotope composition of CH4 in ussingite (delta C-13=-6 to -3 parts per thousand, theta D=similar to-121 parts per thousand) is indicative of a magmatic abiogenic origin. However, the isotopic composition of CH,l in fluid inclusions in quartz veins resembles the signature of thermogenic CH4 (theta C-13=-43 to -23 parts per thousand, theta D= -176 to - 121 parts per thousand), but the higher hydrocarbons are mostly C-13-depleted in relation to CH4 (LIP 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 front which these alkaline melts are derived. (C) 2008 Elsevier B.V. All rights reserved.