Halogen and trace-element chemistry in the Gardar Province, South Greenland: Subduction-related mantle metasomatism and fluid exsolution from alkalic melts

XRF analyses of 152 magmatic dyke samples from a broad area (150x60 km) of the Gardar Province in Southern Greenland span the time 1280 to 1163 Ma and represent a wide compositional range (transitional olivine basalts to trachytes, alkalinity index of 0.3 to 1.5). Among those, 16 dyke samples were additionally analysed for Cl and Br. Generally, the dykes represent a continuous fractionation trend from relatively unfractionated basalts to more highly fractionated trachytes. Dykes from different areas exhibit a diverse geochemistry suggesting a heterogeneous and metasomatised mantle source. Enrichment in LILE, LREE and Sr and depletion in HFSE, Nb and Ti suggest that some of the metasomatism may have been associated with subduction processes predating Gardar activity by some 600 Ma (Ketilidian orogeny). The dykes are characterised by high F contents up to 1.2 wt.%, particularly in the vicinity of the Ivigtut fluoride deposit. F was probably derived from partial melting of lithospheric mantle enriched in F-apatite and F-phlogopite. High Cl/Br (>500) and low Cl/F (<1) ratios of the dykes point to a fluid degassing/separation process which is supported by mineral/rock and fluid inclusion data from the Ilimaussaq and Ivigtut intrusions. There, the analysed rocks and minerals generally show high Cl/Br (>300) and low Cl/F (<1) weight ratios whereas the fluid inclusions have complementary low ratios (Cl/Br +/- 100; Cl/F>10). Our investigations are in accordance with experimental data which show that F is preferentially enriched in the melt whereas Cl and especially Br are lost with the fluid phase. Accordingly, the halogens show an increase in incompatible behaviour in the magma in the order of F