PETROTECTONIC EVOLUTION OF LINEATED ECLOGITE IN A SHEAR ZONE, WESTERN GNEISS REGION, NORWAY

The ultrahigh-pressure (UHP) Western Gneiss Region of Norway contains abundant localized (typically <100 m wide) sinistral shear zones in the basement gneiss; some of these contain eclogite pods and layers. We have conducted an integrated structural and petrologic study of the eclogite-bearing Salta shear zone (~50 m wide) in the southern UHP domain of the Western Gneiss Region. This sinistral shear zone is dominated by quartzofeldspathic gneiss that contains mafic bodies ranging from a 2 m thick layered, coesite-bearing eclogite lens to amphibolite (± garnet). Locally, a continuous transition from eclogite to amphibolite to hornblende-bearing gneiss is preserved. Foliation and lineation throughout the shear zone are consistent in all rock types, indicating that eclogite and gneiss were deformed together, likely in part under eclogite-facies conditions, as indicated by fabric-forming high-pressure minerals such as omphacite. Elongate omphacite defines the planar and linear fabrics, and EBSD analyses of omphacite are consistent with L-type constrictional fabrics. Quartz CPO shows both prism <a> and prism <c> slip consistent with deformation at high T. Garnet displays a random CPO, but preliminary 3D texture analysis based on X-ray computed tomography imaging shows that garnet is slightly elongate and has a larger angular distribution perpendicular to the lineation direction; this pattern is consistent with constriction. To determine conditions of recrystallization and fabric development, we analyzed Ti concentration in quartz in rutile-bearing eclogite. Deformed quartz in a garnet+quartz layer and quartz inclusions in garnet both preserve Ti concentration of ~6 ppm. Previous workers determined a peak T of ~800 C for this region; this T yields a TitaniQ P of ~2.6 GPa, consistent with the presence of coesite inclusions in garnet in the Salta eclogite. Structural and petrologic evidence shows that this shear zone was long-lived, from eclogite- to amphibolite-facies. The prevalence of sinistral shear zones in overall top-to-west sheared gneiss combined with constrictional strain at UHP conditions indicates that sinistral transtension is a likely model for the exhumation of the Western Gneiss Region.