A PETROLOGIC-MECHANICAL FEEDBACK IN DEFINING THE RHEOLOGY OF THE LOWER CRUST

Experimental studies predict that the strength of the lower continental crust will decrease with depth as increasing temperature facilitates the accommodation of strain by dislocation creep in framework silicates under amphibolite facies and higher metamorphic conditions. However, many rocks naturally deformed under these high-grade conditions exhibit dissolution features and overgrowth microstructures indicating the important role of solution creep. During solution creep the properties of the interstitial fluid film is paramount as it is the agent of dissolution and precipitation. In this contribution we recognize that the reactions producing garnet, staurolite and Al₂SiO₅ in paragneisses are commonly locally metasomatic, such that both an alkali silicate and H₂O are added to the interstitial fluid. For example, the common intergrowth of fibrolitic sillimanite with biotite may be described by the reaction: 6KFe₃AlSi₃O₁₀(OH)₂ + 14H⁺ = Al₂SiO₅(Sil) + 6Fe⁺⁺ + 5SiO₂(aq) + 2K⁺ + 9H₂O. Such reactions not only add H₂O to the grain boundary fluid, but also increase its pH and SiO₂ concentration. The generation of these aqueous reaction products has two consequences. 1) The change in solid volume in the forward reaction is negative; therefore, the differential stress that drives strain also drives the reaction. This stress sensitivity accounts for the generally strong preferred orientation of fibrolite intergrown with biotite. Moreover, the reaction leads to an increase in pH that facilitates further dissolution and lowers the differential stress needed to activate solution creep. It may lower the ambient differential stress to levels insufficient to activate dislocation creep. 2) The reaction produces H₂O and thus increases the fluid pressure. Where this fluid pressure exceeds the least compressive stress, hydro-fracture may occur. Evidence for this is the common increase in occurrence of quartz and quartz-feldspar veins in staurolite and higher-grade rocks. Thus, at the same time that metamorphic reactions expedite ductile solution creep, they also generate brittle deformation. These metamorphic processes illustrate the pivotal role of fluid composition and pressure in this chemo-mechanical feedback that helps to define the rheology of lower crustal rocks.