FLUIDS AND SHEAR ZONES IN THE DEEP CRUST

The continental deep crust is normally nearly fluid-absent. During and after active tectonism (extension, thrusting or shearing), fluids may be abundant in the deep crust and may be associated with such diverse phenomena as granulite-facies metamorphism, shear zone alkaline granites, carbonatite-lamprophyre-syenite complexes, and various kinds of metasomatism, including region carbonatization, fenitization, granitization, and, probably, extreme large-ion lithophile element depletion. The fluids are H2O-CO2 mixtures with variable halogens, alkalis and dissolved silicate constituents. CO2-rich fluids are inferred from dense carbonic inclusions in some deep-crustal granulites, both volcanic ejecta and uplifted deep-crustal terrains. CO2-H2O-dominated fluids must be fairly oxidizing, and may account for upward transport of Au, Sb, As and S from the lower crust and upper mantle, with enrichment of these elements at shallow levels. Crustal sources of abundant CO2 are not likely, either from meteoric sources or from buried carbonates because of their refractory nature. The most plausible source is asthenospheric carbonatite, kimberlite, and alkali basalt liquids, and volatile-enriched lithosphere. Exsolution of fluids of varying CO2/H2O ratios is possible upon freezing of the magmas, depending on depth of magma generation, velocity of ascent and degree of volatile presaturation of the lithosphere. The enriched lithosphere in areas of deep-seated magmatism is an intermediate storage site for CO2 and H2O. Melts and fluids are liberated from the mantle under tensional conditions, which can include lithospheric strike-slip faulting (transtension) and even localized tension generated in convergence. The fluids can transport heat upward, which augments their ability to metasomatize and melt the lower crust. Melting of the crust can occur under high-CO2 fluids at granulite facies conditions to produce syenitic magmas, and this could conceivably be accomplished by action of mantle-derived fluids without the need for large-scale basaltic underplating of the crust. The possibility of crust-mantle interactions is much greater if mantle-derived fluids permeate the lower crust; effects on isotopes and trace elements may be substantial in zones of enhanced permeability like the Nordre Stromfjord shear of Greenland and comparable megashears. © 1990.