Barium-, Cl- and Ti-rich biotite occurs together with garnet, plagioclase and amphibole within narrow shear zones in 1800 Ma old noritic granulites in the Flakstadoy Basic Complex, Lofoten, north Norway. The granulite facies assemblage, plagioclase, clinopyroxene, orthopyroxene, biotite and ilmenite, was replaced by an amphibolite facies mineral assemblage including Ba-, Cl- and Ti-rich biotite during ductile deformation. Biotite shows complex compositional variations with respect to the contents of Ba, K, Cl, Ti, Al, Fe, Mg and Si. There are correlations between Si, AlIV, K, Ba and Cl and between AlVI and Ti. Titanium and Cl are uncorrelated. The Fe and Mg are correlated to both Cl and Ti. Multivariate analysis shows that most of the compositional variation of biotite can be described by two exchange reactions. This indicates that most of the variation in biotite composition was controlled by two chemical variables of the system. The content of the first exchange component (Ti1.0 Fe0.6 Al -1.1 VI Mg-0.8) in biotite can be related to the original distribution of Ti-bearing minerals in the igneous protolith. The content of the second exchange component (Al 0.4 IV Fe0.8 Ba0.5 Cl1.0 Si-0.4 Mg-1.0 K-0.5 OH-1.0) is related to compositional variations of an externally derived Ba- and Cl-bearing fluid in equilibrium with biotite. The initially low Cl-content of the externally derived fluid was increasing during bioite forming reactions, because OH was preferentially incorporated, relative to Cl, into biotite. Continued hydration/chloridisation reactions resulted in a gradual consumption of the free fluid phase, resulting in local fluid-absent conditions. The composition of biotite reflects the composition of the last fluid in equilibrium with the mineral, i.e. the composition of the fluid immediately before the grain boundaries were fluid-undersaturated. Thus, the variations in biotite composition reflect how the fluid was gradually consumed throughout the shear zone rock. The correlations between Fe, Mg, Ba, K and Cl can be attributed to differences between the structure of the crystal lattices and the sizes of the cation sites of OH-phlogopite and Cl-annite. The dependency of the Fe/Mg ratios of biotite on the Cl-and Ti-content has a strong effect on the Fe−Mg partitioning between biotite and garnet. The relationship between lnKD, X Ti Bt and X Cl Bt can be expressed by the regression equation: lnK′ D =-1.82+2.60X Ti Bt +5.67X Cl Bt