Paper No. 2
Presentation Time: 9:00 AM-6:30 PM

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


RENEDO, Roxanne N.1, TEYSSIER, Christian2, NACHLAS, William O.1 and WHITNEY, Donna L.1, (1)Earth Sciences, University of Minnesota, Minneapolis, MN 55455, (2)Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, rened001@umn.edu

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.