WHOLESALE TRANSFORMATION OF CONTINENTAL CRUST DURING UHP TECTONISM? GARNET GEOCHRONOLOGY OF THE WESTERN GNEISS REGION, NORWAY

It is now widely accepted that continental crust has been and continues to be subducted to depths >100 km and that a single orogenic belt can undergo multiple stages of (ultra)high-pressure (UHP) metamorphism. The Western Gneiss Region (WGR) of Norway is one of two giant UHP terranes on Earth, and as such constitutes an important natural laboratory for investigating processes attending the subduction of continental margins. Central to understanding these processes—and yet poorly constrained—is the extent to which the WGR was deformed and transformed to UHP minerals during subduction.

Garnet constitutes perhaps the single-most useful vector for answering these outstanding questions about the deformation and transformation of continental margins at UHP conditions because of its presence throughout the WGR, and its ability to record i) PT changes in its composition, ii) deformation in its textures, and iii) history in its age(s). This research uses Sm → Nd decay to date garnets in eight quartzofeldspathic gneisses from both within and outside the UHP/HP boundaries. The garnet ages are expected to be i) c. 950 Ma grains that predate and were unaffected by UHP tectonism, ii) c. 425–390 Ma UHP phases, or iii) <390 Ma neoblasts grown during the supra-Barrovian to Buchan metamorphism overprint that occurred during exhumation of the UHP terrane to crustal depths. Accompanied by PT data and detailed petrography, the ages of these eight samples will establish whether continental crust in the WGR transformed to UHP mineralogy and the degree of deformation to the terrane attributed to UHP tectonism.