THE RECORD OF GIANT UHP TERRANE EXHUMATION PRESERVED IN LITHOLOGICALLY DIVERSE, METERS-SCALE SHEAR ZONES: AN INTEGRATED TRACE ELEMENT THERMOBAROMETRY AND MICROSTRUCTURAL STUDY OF ECLOGITE AND GNEISS FROM THE WESTERN GNEISS REGION, NORWAY

The presence of ultrahigh-pressure (UHP) minerals in crustal rocks is a record of the subduction and subsequent return of continental crust from mantle depths. These dynamic processes are fundamental to collisional orogenesis and are best understood through a detailed P-T-t-d record. Norway’s Western Gneiss Region (WGR) is a large (>25,000 km²) UHP terrane formed by subduction of Baltica crust beneath Laurentia. In the case of this UHP terrane, much of the WGR does not preserve mineral assemblages or structures related to Caledonian UHP metamorphism. Instead, inherited pre-UHP assemblages and structures dominate, especially in quartzofeldspathic gneiss. The best outcrop evidence of terrane exhumation is found in discrete, meters-scale shear zones that are kinematically compatible with terrane-bounding extensional and strike-slip structures. These shear zones contain eclogite bodies mantled by (garnet) amphibolite and hornblende gneiss within quartzofeldspathic host gneiss. Outcrop mapping shows that the amphibolites and mafic gneisses are spatially associated with eclogite and therefore likely represent retrogressed eclogite. The shear zones thus preserve evidence for the discrete segments of the eclogite retrogression path followed during UHP terrane ascent.

We collected a suite of samples representing the transition of eclogite to gneiss from one such shear zone on the island of Finnøya in the highest P-T, northern (Nordøyane) UHP domain to investigate the extent to which shear zones can preserve the P-T-t-d record of terrane exhumation. We present Ti-in-quartz thermobarometry and crystallographic preferred orientation data of major (quartz, feldspar) and accessory (titanite, rutile) phases. Results of these analyses, when integrated with outcrop-scale relationships of eclogite, amphibolite, and gneiss, track eclogite retrogression and shear zone fabric development over multiple stages of decompression during UHP terrane ascent. Additionally, microstructural orientation mapping of titanite and rutile serves as a key way to evaluate deformation mechanisms that influence the distribution of U-Pb isotopic signatures and trace element abundances. These data are essential for accurate interpretation of how the recorded ages and temperatures/pressures relate to terrane exhumation.