Abstract

The Proterozoic banded granulite facies complex of BÅrdsholmen, Western Gneiss Region, Norway (T=815–845°C) was locally transformed to eclogite (T=455–510°C, P>12 kbar) and amphibolite facies rocks (T=460°C) during the Caledonian continental collision. The granulite complex consists of mafic two‐pyroxene granulite and leucocratic orthopyroxene+garnet‐bearing layers alternating on a scale from 1 cm to 10 m. The granulite facies rocks change to eclogite facies rocks over centimetre‐scale distances along well defined fluid‐infiltration fronts. The mafic granulite was transformed to omphacite+garnet‐rich eclogites and the leucocratic rocks were converted to quartz+phengite‐rich assemblages with minor garnet and local omphacite. Melange‐like lithologies consisting of mafic lenses of eclogite surrounded by felsic material represent an advanced stage in the process of converting deep crust to eclogite facies. During amphibolitization this melange‐like lithology evolves to a rock where amphibolite lenses and layers are surrounded by granitoid gneiss, a lithology typical of the Western Gneiss Region. The BÅrdsholmen locality illustrates the profound control exerted by fluids on the timing of metamorphism, the structural make up and petrophysical properties such as density and rheology of crustal root zones. Fluid‐induced metamorphism will therefore exert control on the attributes of orogenic belts such as topography and Moho depth and influence the dynamics of collision zones by controlling the time of orogenic collapse and the buoyancy of the subducted crust. We suggest that orogens may develop differently depending on the fluid budget.