P-T conditions of high- and ultrahigh-pressure metamorphism along the Western Gneiss Region, Norway

The Western Gneiss Region (WGR) is a well-known UHP metamorphic terrane in SW Norway, where eclogite bodies crop out among gneisses. These rocks experienced high- and ultrahigh-pressure (HP-UHP) metamorphism during the Caledonian Orogeny and display pressure-temperature (P-T) gradient increasing from the south to the north (e.g., Cuthbert et al., 2000). Eclogites from nine localities along the entire length of WGR, namely Drøsdal, Vårdalsneset, Verpeneset, Halnes, Saltaneset, Grytting, Ulsteinvik, Solholm, and Juvika, have been chosen for P-T condition estimates. Here we present a reevaluation based on the new geothermobarometric techniques in the WGR regional study.

Eclogite varies in the amount of amphibole, kyanite, phengite, zoisite, and inclusions of quartz and rutile in garnet. Phengite occurs mainly in the eclogites in the south, whereas those from the north contain orthopyroxene. Garnet in the south is almandine-rich with the average composition of Alm_0.41–0.57Grs_0.15–0.31Prp_0.10–0.37Sps_0.01–0.07, with increasing Mg in the rims and often showing complex compositional zoning. Garnet from the other localities is homogenized and dominated by pyrope with the composition of Prp_0.41–0.56Alm_0.33–0.46Grs_0.09–0.13Sps_0.01–0.04. Clinopyroxene in the northern part is poorer in jadeite component with X_Na=0.20-0.29 and X_Fe=0.12-0.17 than clinopyroxene in the south with X_Na=0.40-0.52 and X_Fe=0.12-0.19. Silicon in phengite is up to 3.30 atoms per formula unit (apfu) in most localities, with a maximum of 3.48 apfu in the Verpeneset locality.

Coesite and polycrystalline quartz inclusions in garnet, evidence of UHP metamorphism, are typical of Saltaneset and Verpeneset localities, which is also validated by thermobarometric calculations. We estimated the peak P-T equilibration condition by Grt-Cpx-Phe thermobarometry, Ti-in-Quartz and Zr-in-Rutile thermometry, along with Quartz-in-Garnet elastic geobarometry. Preliminary estimates give slightly higher P values than previously reported in the two southernmost localities Drøsdal and Vårdalsneset [720-830°C and 1.9-2.1 GPa (Foreman et al., 2005), 635°C and 2.3 GPa (Engvik et al., 2007), accordingly] yielding the peak conditions of 680-740°C and 2.7-2.85 GPa. The obtained results from other localities give P conditions close to the boundary of quartz and coesite stability fields and T in the range of 650-750°C, with the highest P of 3.28 GPa in the Verpeneset locality. The highest T of 875°C has been obtained in Ulsteinvik locality. Those results may help refine previous studies and understand the history of the WGR.

This work was funded by the National Science Centre of Poland project no. 2021/43/D/ST10/02305.

 

References:

Cuthbert, S.J., Carswell, D.A., Krogh-Ravna, E.J., Wain, A. (2000). Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides. Lithos, 52 (1–4), 165-195.

Foreman, R., Andersen, T.B., Wheeler, J. (2005). Eclogite-facies polyphase deformation of the Drøsdal eclogite, Western Gneiss Complex, Norway, and implications for exhumation. Tectonophysics, 398, 1-32.

Engvik, A.K., Andersen, T.B., Wachmann, M. (2007). Inhomogeneous deformation in deeply buried continental crust, an example from the eclogite facies province of the Western Gneiss Region, Norway. Norwegian Journal of Geology, 87, 373-389.