BIOTITES AS INDICATORS OF FLUORINE FUGACITIES IN LATE-STAGE MAGMATIC FLUIDS - THE GARDAR PROVINCE OF SOUTH GREENLAND

The major element chemistry and fluorine contents of biotites from the Gardar alkaline province of South Greenland provide an insight into t he F contents of late-stage fluids associated with the magmatism. Biot ites were taken from composite intrusions ranging from alkali gabbro t o syenites, nepheline syenites and alkali granites. In each complex th ey show a large range of Fe/(Fe + Mg) (from 0.2 to similar to 1) and e xhibit strong Fe-F avoidance. There is considerable variation in F for any value of Fe/(Fe + Mg) but for each centre maximum F values plotte d against Fe/(Fe + Mg) define a nearly straight line of characteristic slope towards low or zero F at pure annite. Micas in the SiO2 undersa turated centres have higher F contents than those from oversaturated c entres. CI is low (<0.69 wt %) except in the Kungnat intrusion, where it reaches 1.4 wt %. Phase equilibrium and textural considerations sug gest that most or all biotites grew subsolidus in a pervasive deuteric fluid. delta(18)O values suggest that these fluids were largely magma tic 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 d ata of Munoz (Mineralogical Society of America, Reviews in Mineralogy 13, 469-494, 1984) were used to calculate families of curves showing v ariation in F with Fe/(Fe + Mg) for biotites in equilibrium with fluid s of fixed fugacity ratio, f(HF)/f(H2O), at fixed P and T. The resulti ng curves cut sharply across the maximum fluorine lines observed in th e natural examples. As it seems highly unlikely that changes in fluid composition and T, 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 experime nts. 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 f luids. If perfect Fe-F avoidance is assumed Fe-Mg SRO can increase max imum 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 speciation in the fluid rather than higher F contents, with strongly bound SiF4 degrees complexes more common.