High-pressure Fluid–Rock Reactions involving Cl-bearing Fluids in Lower-crustal Ductile Shear Zones of the Flakstadøy Basic Complex, Lofoten, Norway

Abstract

Rocks occurring in narrow shear zones (<4 m wide) in a gabbro–anorthosite near Nusfjord, Flakstadøy, Lofoten, Norway, include Cl-enriched mineral assemblages, Cl-free mineral assemblages and eclogite-facies assemblages. Mineral equilibria calculations suggest that the different mineral assemblages formed under similar pressure and temperature conditions, at P = 11–14 kbar and T = 650–700°C. One reason for the mineralogical variations of the shear zones is that the rocks evolved from three distinctly different protolith types. Interactions between the rocks and an externally derived Cl-bearing hydrous fluid during shear zone formation resulted in a strong fractionation of the hydrous fluid, and extreme compositional variations of the hydrous mineral phases that formed in equilibrium with the fluid. Parts of the shear zone rocks experienced multiple infiltrations of fluids of different compositions because of local fluctuations of the fluid phase during the fluid–rock interactions. During the deformation, the externally derived fluid was introduced through the transiently highly permeable central parts of the shear zones. The fraction of the fluid that did not escape was rapidly consumed during subsequent hydration reactions.