Structural, mineralogical and petrophysical effects on deep crustal rocks of fluid-limited polymetamorphism, Western Gneiss Region, Norway

The Proterozoic banded granulite facies complex of Bardsholmen, Western Gne iss Region, Norway (T=815-845 degrees C) was locally transformed to eclogit e (T=455-510 degrees C, P>12 kbar) and amphibolite facies rocks (T=460 degr ees C) during the Caledonian continental collision. The granulite complex c onsists of mafic two-pyroxene granulite and leucocratic orthopyroxene+garne t-bearing layers alternating on a scale from 1 cm to 10 m. The granulite fa cies rocks change to eclogite facies rocks over centimetre-scale distances along well defined fluid-infiltration fronts. The mafic granulite was trans formed to omphacite+garnet-rich eclogites and the leucocratic rocks were co nverted to quartz+phengite-rich assemblages with minor garnet and local omp hacite. Melange-like lithologies consisting of mafic lenses of eclogite sur rounded by felsic material represent an advanced stage in the process of co nverting deep crust to eclogite facies. During amphibolitization this melan ge-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 Bardsholmen locality illustrates the profound control exerted b y fluids on the timing of metamorphism, the structural make up and petrophy sical properties such as density and rheology of crustal root zones. Fluid- induced metamorphism will therefore exert control on the attributes of orog enic belts such as topography and Moho depth and influence the dynamics of collision zones by controlling the time of orogenic collapse and the buoyan cy of the subducted crust. We suggest that orogens may develop differently depending on the fluid budget.