High salinity fluids: The result of retrograde metamorphism in thrust zones

Abstract

Highly saline fluid inclusions in vein quartz from Caledonian Thrust Zones of northern Norway are associated with late orogenic, retrograde metamorphic reactions. Several retrograde reactions have been observed in pelitic lithologies and all involve the rehydration of earlier higher grade metamorphic assemblages. The most common products of these reactions are chlorite and sericite, which have formed at the expense of garnet, kyanite, staurolite, and hornblende. Chlorite geothermometry suggests temperatures between 330 and 370°C for the formation of the retrograde chlorite. A range of total fluid salinities has been determined by microthermometry between 27 and 50 wt%. These high fluid salinities result from relatively rapid rehydration reactions during limited infiltration of the thrust zones by aqueous fluids. Crush leach analyses show the fluids to be multi-component fluids whose electrolyte contents are Na and Cl dominated but with appreciable contents of Ca, K, Fe, and Sr. Geochemical modelling of the fluid compositions suggests that the concentrations of the major elements in solution were buffered by the retrograde rehydration reactions. The source of the infiltrating fluids which caused the retrogression is not identifiable from their major element chemistry. Surface derived (meteoric, sea, or sedimentary formation waters) fluids may have utilised pre-existing thrust zones as fluid pathways and are considered to have been important in the retrogression in the structurally higher parts of the Caledonian allochthon. At deeper levels, however, metamorphic fluids produced by devolatilisation reactions in underthrust units may have migrated upwards into the thrust zones and resulted in the retrogression of the hangingwalls of overlying, higher grade nappes.