Biotites as Indicators of Fluorine Fugacities in Late-Stage Magmatic Fluids: the Gardar Province of South Greenland

Abstract

The major element chemistry and fluorine contents of biotites from the Gardar alkaline province of South Greenland provide an insight into the F contents of late-stage fluids associated with the magmatism. Biotites were taken from composite intrusions ranging from alkali gabbro to syenites, nepheline syenites and alkali granites. In each complex they show a large range of Fe/ (Fe + Mg) (from 02 to ~1) and exhibit strong Fe-F avoidance. There is considerable variation in Ffor any value of Fe/(Fe + Mg) but for each centre maximum F values plotted against Fe/(Fe + Mg) define a nearly straight line of characteristic slope towards low or zero F at pure annite. Micas in the SiO2 undersaturated centres have higher F contents than those from oversaturated centres. Cl is low (<0-69 wt%) except in the Kungnat intrusion, where it reaches 1-4 wt%. Phase equilibrium and textural considerations suggest that most or all biotites grew subsolidus in a pervasive deuteric fluid. 5i8O values suggest that these fluids were largely magmatic in character and that extensive reactions with envelope fluids did not occur. Metasomatic resetting of F in biotites in early intrusive units in the aureole of later units can be demonstrated. Experimental data of Munoz (Mineralogical Society of America, Reviews in Mineralogy 13, 469-494, 1984) were used to calculate families of curves showing variation in F with Fe/(Fe + Mg) for biotites in equilibrium with fluids of fixed fugacity ratio, i(HF)/i(H2O), at fixed P andT. The resulting curves cut sharply across the maximum fluorine lines observed in the natural examples. As it seems highly unlikely that changes in fluid composition andT, acting together, could produce the observed linear relationship, we conclude that the partitioning of F between fluid and mica in the plutonic environment is not well modelled by the experiments. Possible explanations are short-range order (SRO) of Fe and Mg on octahedral sites in biotite (Mason, Canadian Mineralogist 30, 343-354,1992) or effects resulting from different F speciation in alkaline fluids. If perfect Fe-F avoidance is assumed, Fe-Mg SRO can increase maximum F content. The F levels seen in the silica-saturated centres are broadly consistent with a cooling-rate related control. It is possible that the higher F in biotites in the undersaturated centres reflects different spedation in the fluid rather than higher F contents, with strongly bound SiF/ complexes more common.