Ak. Engvik et al., Structural, mineralogical and petrophysical effects on deep crustal rocks of fluid-limited polymetamorphism, Western Gneiss Region, Norway, J GEOL SOC, 157, 2000, pp. 121-134
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.