FLUID-TRIGGERED, RHEOLOGICALLY BUFFERED ECLOGITE METAMORPHISM, BERGEN ARCS, WESTERN NORWAY

The transformation of subducted and tectonically buried crustal rocks to denser eclogite plays a fundamental role in the dynamics of mountain building and crustal recycling. But a complex of partially eclogitized granulites on the island of Holsnøy in the western Norwegian Caledonides reveals that such densification is not thermodynamically inevitable and may be suppressed entirely in the absence of an aqueous fluid phase. In these rocks the conversion to eclogite was apparently delayed until lower crustal earthquakes allowed fluids to enter the dry and anomalously strong granulite. Pseudotachylyte veins containing microlites of hydrous eclogite-facies minerals are the first structures to cut the older granulite facies fabric and indicate that brittle, seismic faulting at depths of 60 km or more may have opened the previously anhydrous system to external fluids.

Initially, macroscropic fractures would have allowed for long-distance fluid transport and accounted for most of the rocks’ bulk permeability. Then, as the water-rich fluid reacted with the metastable granulite assemblages, microfracture networks would have developed as a result of volume reduction upon formation of eclogite minerals. For a time, conversion to eclogite would have been a positive feedback process driven by self-propagating microfracture fronts that created their own fluid pathways to new mineral surfaces on which reactions could continue. Ubiquitous mylonitic fabrics in the eclogitized areas indicate that hydration and recrystallization caused profound weakening of the rocks. Ultimately, therefore, the process of eclogitization would have been self-limiting, arrested by the rheological change from brittle granulite to ductile eclogite, which would have inhibited further faulting and cut off the influx of external fluids.

A reservoir-flux systems model is used to explore the interplay among the hydrologic, metamorphic and deformational processes recorded in these rocks. The model suggests that the metamorphic ‘event’ may have been remarkably brief (<<1 My) and governed by subtle interactions among phenomena over a wide range of scales.