Rapid fluid-driven transformation of lower continental crust associated with thrust-induced shear heating

Abstract

The rate and progress of regional-scale prograde metamorphism is commonly assumed to be controlled by conductive heat supply. In contrast, the rate of retrograde metamorphism is in most cases controlled by the rate of fluid supply. Crustal-scale heat conduction is a slow process, typically causing temperature changes at rates on the order of 10 °C/million years, but convective fluid fluxes can be substantial on much shorter time scales. Therefore, it is often assumed that prograde metamorphism usually takes place near thermodynamic equilibrium, whereas retrograde metamorphism may operate far from equilibrium. In many settings, including convergent plate margins, fluid-driven retrograde metamorphism is probably strongly coupled to fluid-producing prograde metamorphism. Here, we present new geochemical and geochronological results from the Lindås Nappe in the Bergen Arcs of Western Norway, which show that fluid-driven (retrograde) eclogite- and amphibolite-facies metamorphism of lower crustal granulites was synchronous with the emplacement of numerous pegmatites and dykes. These have geochemical characteristics suggesting an origin by fluid-present melting of the granulites themselves along with underlying schists. The morphology and chemical zoning of garnets from the exposed part of these schists suggest growth during rapid prograde metamorphism associated with emplacement of the Lindås Nappe. We present a simple model that indicates that rapid prograde metamorphism of the schists may have been driven by shear heating near the base of the Lindås Nappe, and that temperatures may have reached wet-melting conditions for both schists and granulites. The Lindås Nappe thus represents an example of how strong and dry lower crust may experience rapid retrograde metamorphic and structural reworking in direct response to shear-heating induced prograde metamorphism taking place near its base.