Reading the record of ultrahigh-pressure terrane exhumation preserved in meters-scale shear zones of the Western Gneiss Region, Norway

Abstract

The occurrence of large (>10,000 km2) (ultra)high-pressure (UHP) metamorphic terranes in eroded mountain belts indicates that significant tracts of continental crust can be subducted to and returned from mantle depths. Despite the recognition of UHP terranes in many orogens, the relationship between the pressure-temperature-time (P-T-t) exhumation path primarily from eclogites and deformation (d) within large UHP terranes is incompletely understood. Compared with outcrop-scale structures in UHP terranes, terrane-bounding structures inform exhumation models, receive more research focus, and tend to be extensively overprinted by the last increment of finite strain. This produces a disconnect between proposed exhumation mechanisms, the P-T-t history derived from eclogites, and the response of the terrane interior to subduction and exhumation. This thesis therefore focuses on eclogite-bearing high-strain zones within the Western Gneiss Region (WGR) of Norway, a large and exceptionally well-exposed UHP terrane. Accessory phase (titanite and rutile) petrochronology, trace-element thermobarometry, and (micro)structural characterization of WGR shear zones deepens our understanding of titanite as a deformation petrochonometer, yields detailed P-T-t-d histories from single outcrops, and highlights the variability of exhumation mechanisms recorded across the terrane. Titanite is a promising accessory phase for dating and describing the metamorphic environment of ductile deformation if the relationships between (re)crystallization, Pb loss, and trace element concentrations can be characterized. To that end, titanite grains affected by a strain gradient preserved in a mylonitized, syn-kinematic pegmatite dike were analyzed to compare their U-Pb dates with the timing of dike crystallization and deformation (U-Pb zircon age). Zr-in-titanite temperatures indicate crystallization below Tc estimates for Pb volume diffusion, and Ti-in-quartz thermometry indicates mylonitization at T <600℃. Titanite yielded a lower intercept date of 387.2 ± 7.7 Ma (MSWD = 2.2) compared to zircon crystallization at 396.0 ± 7.9 Ma (MSWD = 1.0), and titanite kernel average misorientation maps show a correlation between younger single-grain dates and greater lattice distortion. Pb in titanite is impacted by mylonitization at these temperatures (<600℃), but the resultant U-Pb dates do not correspond to the timing of deformation; rather, deformation-induced fast pathways generated during deformation subsequently lower Tc for Pb. Despite the difficulty in using titanite to directly date crystal-plastic deformation, trace and rare-earth element concentrations in titanite provide reliable information on temperature, co-(re)crystallizing phases, and sources of common Pb at the time of (re)crystallization. Samples yielding nondispersed lower-intercept dates were investigated using REE pattern shape coefficients and Zr concentrations to identify two generations of titanite formed along the WGR exhumation path: one during eclogite partial melting and another, ~10 myr younger, during the onset of constrictional deformation. This approach explains the locally variable U-Pb titanite dates reported in the WGR and highlights the power of trace and rare earth elements to identify geologically significant (re)crystallization events in seemingly uncomplicated U-Pb datasets. Titanite and rutile petrochronology applied to samples of shear zone-hosted retrogressed eclogite across the WGR offers insight into the conditions under which eclogite was retrogressed during exhumation-related deformation. In Nordfjord and Sørøyane UHP domains, rutile and quartz thermobarometry preserve a lower-temperature/higher-pressure history than in the Nordøyane UHP domain and in HP rocks from Roan to the north. Titanite in the latter two areas records higher temperatures than the Nordfjord and Sørøyane domains, pointing to a period of high-temperature reequilibration not experienced in the two southernmost UHP domains. Exhumation histories have the potential to vary systematically across large UHP terranes.