A diachronous record of metamorphism in metapelites of the Western Gneiss Region, Norway

Abstract

In this study, data from garnet‐kyanite metapelites in ultrahigh‐pressure (UHP) domains of the Western Gneiss Region (WGR), Norway, are presented. U–Pb geochronology and trace element compositions in zircon, monazite, apatite, rutile and garnet were acquired, and pressure–temperature (P–T) conditions were calculated using mineral equilibria forward modelling and Zr‐in‐rutile thermometry. Garnet‐kyanite gneiss from Ulsteinvik record a prograde evolution passing through ~690–710°C and ~9–11 kbar. Zircon and rutile age and thermometry data indicate these prograde conditions significantly pre‐date Silurian UHP subduction in the WGR and are interpreted to record early Caledonian subduction of continental‐derived allochthons. Minimum peak conditions in the Ulsteinvik metapelite occur at ~28 kbar, constrained by an inferred garnet+kyanite+omphacite+muscovite+rutile+coesite+H2O assemblage. The retrograde evolution passed through ~740°C and ~7 kbar, first recorded by the destruction of omphacite and followed by the partial replacement of kyanite and garnet by cordierite and spinel. Garnet‐kyanite metapelite from the diamond‐bearing Fjørtoft outcrop documents a polymetamorphic history, with garnet forming during the late Mesoproterozoic and limited preservation of high‐pressure Caledonian assemblages. Similar to the Ulsteinvik metapelite, zircon and rutile age data from the Fjørtoft metapelite also record pre‐Scandian Caledonian ages. Two potential tectonic scenarios are possible: (1) The samples were exhumed at different times during pre‐Scandian subduction of the Blåhø nappe, or (2) the samples do not share a history in the same nappe complex, instead the Ulsteinvik metapelite is a constituent of the Seve‐Blåhø Nappe, whilst the Fjørtoft metapelite shares its history within a separate nappe complex.

Isothermal compression of an eclogite from the Western Gneiss Region (Norway)

“…Few older ages, ranging between 430 and 500 Ma, are undistinguishable from the Scandian phase because of their large uncertainties (e.g., Griffin & Brueckner, 1985; Tamblyn et al, 2021). It appears that the pre‐Scandian phase of the Caledonian Orogeny is not recorded within the WGC basement (March et al, 2022).…”

H2O-rich rutile as an indicator for modern-style cold subduction

“… References: (1) Anczkiewicz et al ( 2007 ), (2) Barnicoat and Fry ( 1986 ), (3) Bissig and Hermann ( 1999 ), (4) Boston et al ( 2017 ), (5) Braga et al ( 2007 ), (6) Butler et al ( 2015 ), (7) Dale and Holland ( 2003 ), (8) Ewing et al ( 2013 ), (9) Ganade et al ( 2023 ), (10) Gauthiez-Putallaz et al ( 2016 ), (11) Groppo et al ( 2009 ), (12) Hacker et al ( 2015 ), (13) Hermann ( 2003 ), (14) Laurent et al ( 2018 ), (15) Liu et al ( 2014 ), (16) Liu et al ( 2020 ), (17) March et al ( 2022 ), (18) Mukhopadhyay and Basak ( 2009 ), (19) O’Brien et al ( 1997 ), (20) O’Brien and Rötzler ( 2003 ), (21) Piccoli et al ( 2021 ), (22) Reinecke ( 1998 ), (23) Schenk ( 1984 ), (24) Schwartz et al ( 2000 ), (25) Thöni et al ( 2008 ), (26) Trommsdorff et al ( 2000 ), (27) Vho et al ( 2020 ). Mineral abbreviations after Warr ( 2021 ) …”

Metamorphic evolution of sillimanite gneiss in the high-pressure terrane of the Western Gneiss Region (Norway)

“…The WGR is interpreted to represent the exhumed parts of the root zone of the Scandian orogen (e.g. Andersen et al, 1991;Roberts, 2003;Tucker et al, 2004;Hacker, 2007;Hacker et al, 2010;Walsh et al, 2013;DesOrmeau et al, 2015;March et al, 2022). P -T estimates reach 800 °C and 3.2 GPa, with the highest pressures and temperatures being reported from the northwestern part of the gneiss terrane (e.g.…”

“…5c-d) with microfractures perpendicular to the cleavage direction has been inherited after kyanite. Both kyanite and sillimanite gneisses are reported in the neighbouring coastal part of WGR (Tveten et al, 1998;March et al, 2022).…”