Fabric development during exhumation from ultrahigh-pressure in an eclogite-bearing shear zone, Western Gneiss Region, Norway

Abstract

Petrofabrics and trace-element thermobarometry of deformed quartzofeldspathic gneiss and associated coesite-bearing eclogite in the Salt Mylonite Zone (Western Gneiss Region, Norway) document a pressure–temperature–deformation path from ultrahigh-pressure to amphibolite-facies conditions. The Salt mylonite zone is dominated by quartzofeldspathic gneiss with a strong foliation and lineation. Coesite-bearing eclogite within the shear zone contains a foliation and lineation (defined by elongate omphacite) consistent with that of the host gneiss, suggesting that gneiss and eclogite were deformed in the same kinematic framework. In eclogite, omphacite preserves LS- to L-type crystallographic preferred orientation, and quartz preserves prism <c> fabrics that developed in quartz near coesite–quartz transition conditions. The quartzofeldspathic gneiss in the mylonite zone records prism and rhomb <a> slip in quartz and reverse zoning in plagioclase (higher Ca rims) consistent with re-equilibration during decompression. The Ti concentration in quartz in gneiss is higher than that in quartz in eclogite, suggesting that quartz recrystallized at a lower pressure in the gneiss. Ti-in-quartz thermobarometry of rutile-bearing eclogite and titanite-bearing gneiss indicates equilibration at T > 750 °C and T < 650 °C, respectively. This mylonite zone preserves a discontinuous record of fabric development from incipient stages of exhumation of ultrahigh-pressure rocks to crustal conditions.