Original paper

Chlorine-rich amphiboles: interplay between amphibole composition and an evolving fluid

Kullerud, Kåre

European Journal of Mineralogy Volume 8 Number 2 (1996), p. 355 - 370

27 references

published: Apr 26, 1996
manuscript accepted: Dec 15, 1995
manuscript received: Mar 23, 1995

DOI: 10.1127/ejm/8/2/0355

BibTeX file

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Abstract Cl-rich amphibole occurs together with garnet, plagioclase and Ba-, Ti- and Cl-rich biotite in narrow shear zones in 1.8 Ga old noritic granulites in the Flakstad0y Basic Complex, Lofoten, north Norway. The granulite-facies assemblage containing plagioclase, clinopyroxene, orthopyroxene, biotite and ilmenite was partially replaced by an amphibolite-facies assemblage including Cl-rich amphibole. The amphibolite-facies assemblage was formed during shear deformation associated with the infiltration of a Cl- and Ba-rich fluid. The Cl-content of amphibole varies between 0.1 and 1.0 atom per formula unit. There are strong correlations between Cl and K, Fe, Mg, AlVI, AlIV and Si. Multivariate analysis shows that the compositional variation of amphibole can be described by one Cl-free additive component (K0.1Na0.6)A(Ca19Mg2.9Fe17Ti09A04)VI (Al1.2Si6.8)IV(OH)2 and one complexly coupled exchange component (ECA1 = Si-1.6AlIV1.6AlVI0.9Fe1.3Mg-2.1K0.4Cl1OH-1). It is inferred that the compositional variations of amphibole along the exchange vector were caused by variations in only one chemical variable of the system, namely the activity ratio afluidHCl / afluidH2O of the grainboundary fluid. One model that would account for the variations involves a fluid with an initially low activity ratio afluidHCl / afluidH2O . Formation of amphibole and biotite during interaction between the primary igneous minerals and the fluid resulted in an increase in the Cl-content of the fluid, since OH is preferentially incorporated, relative to Cl, into these minerals. When the supply of the externally derived fluid stopped, the free fluid phase was gradually consumed as a consequence of continued hydration/chloridisation reactions. This resulted in local fluid-absent conditions. The fluid-absent domains expanded until all of the fluid was consumed. The composition of amphibole 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 amphibole composition provide a pattern for how the fluid phase gradually disappeared through the rock


chlorinefluidamphibolebiotitemultivariate analysis