Cl-scapolite, Cl-amphibole, and plagioclase equilibria in ductile shear zones at Nusfjord, Lofoten, Norway: implications for fluid compositional evolution during fluid-mineral interaction in the deep crust

Abstract

Cl-rich scapolite and amphibole formed during ductile shear deformation associated with the infiltration of an externally derived Cl-bearing fluid in a gabbroanorthosite of the Flakstadøy Basic Complex, Lofoten, Norway. Amphibole and scapolite formed along the contacts between incompletely altered igneous mafic minerals (orthopyroxene, clinopyroxene, biotite, and ilmenite) and plagioclase and internally in the grains of primary igneous plagioclase. The secondary minerals show large compositional variations on thin-section scale. The Cl content of scapolite varies between 0.3 and 0.95 apfu (atoms per formula unit), whereas amphibole shows Cl concentrations from 0 to 1.5 apfu. The primary igneous plagioclase (An 50-An 60) underwent extensive recrystallization and alteration during the fluid–rock interactions. Secondary plagioclase shows compositions in the range from An 20 to An 55. In general, plagioclase did not equilibrate with the fluid phase during alteration, due to the sluggishness of the cation exchange reactions between plagioclase and fluid. Occasionally, however, equilibrium among plagioclase, scapolite, and the fluid phase was attained. The compositional variations of amphibole, scapolite, and plagioclase that equilibrated with the fluid are principally related to variations in the fluid activity ratios a Cl − /a (CO 3) 2− and a Cl − /a OH − . The large compositional variations of the minerals on thin-section scale thus indicate steep gradients in the fluid activity ratios a Cl − /a (CO 3) 2− and a Cl − /a OH − . The activity gradients of the fluid phase developed as a result of the preferential extraction of water from the grain-boundary fluid during the formation of hydrous silicates. Scapolite and the most Cl-enriched amphibole formed in equilibrium with an evolved fluid phase, enriched in Cl and CO 2.