Protolith ages and exhumation histories of (ultra)high-pressure rocks across the Western Gneiss Region, Norway

Abstract

The timing of protolith formation, ultrahigh-pressure (UHP) subduction, and subsequent exhumation for the ultrahigh-pressure to high-pressure units across the eastern part of the Western Gneiss Region, Norway, were assessed using U/Pb zircon, Th/Pb monazite, and 40 Ar/ 39 Ar white mica ages. U/Pb zircon ages from eclogites demonstrate that oceanic and continental allochthons were emplaced onto the Baltica basement before the entire mass was subducted to (ultra)high pressure. Eclogites within the allochthons across the entire Western Gneiss Region are Caledonian and show a degree of zircon (re)crystallization that increases with peak pressure, permitting the interpretation that the entire region underwent synchronous subduction. 40 Ar/ 39 Ar white mica ages of 399 Ma indicate that the eastern part of the Western Gneiss Region had been exhumed to shallow crustal levels while UHP metamorphism was ongoing farther west, indicating a westward dip to the slab. The 40 Ar/ 39 Ar white mica ages also show a clear east-to-west gradient across the entire Western Gneiss Region, indicating that the Western Gneiss Region rose diachronously to crustal levels from east to west between 399 and 390 Ma.

Monazite response to ultrahigh-pressure subduction from U–Pb dating by laser ablation split stream

“…Like most well-exposed and extensively investigated UHP terranes, the WGR is dominated by quartzofeldspathic gneiss; eclogite and (U)HP peridotite comprise only a few volume percent of the Region. The quartzofeldspathic gneiss and eclogite protoliths formed chiefly by magmatism at~1.68-1.64 Ga, 1.29-1.24 Ga, and~990-950 Ma [Corfu and Andersen, 2002;Walsh et al, 2007;Krogh et al, 2011;Corfu et al, 2013]; regional metamorphism [Roffeis and Corfu, 2014] associated with the~1.0-0.9 Ga magmatism reached temperatures of~775-825°C [Spencer et al, 2013]. During the Caledonian orogeny, subductionrelated magmatism related to closure of the Iapetus ocean ended at 430 Ma [Corfu et al, 2006], marginal basin ophiolites and island arcs were emplaced onto the Baltica craton in the Wenlock [Andersen et al, 1990], and the previously hyperextended continental margin [Andersen et al, 2012] began to contract.…”

“…Ar mica geochronology reveal at least two periods of metamorphism in most of the WGR: one Proterozoic and one Caledonian [Tucker et al, 1987;1990;Walsh et al, 2007;Kylander-Clark et al, 2008;Corfu et al, 2013]. In the eastern part of the WGC, metamorphic titanite and garnet are Proterozoic, whereas in the western part titanite and garnet at most locations are Caledonian [Johnston et al, 2007b;Peterman et al, 2009;Spencer et al, 2013] (Fig.…”

Strain within the ultrahigh-pressure Western Gneiss region of Norway recorded by quartz CPOs

“…2) (Griffin et al 1985;Kylander-Clark et al 2008). Subduction of the WGR was underway by 420 Ma and exhumation to mid-crustal levels was complete by 400-380 Ma (Kylander-Clark et al 2007;Walsh et al 2007).…”

Post-Caledonian brittle deformation in the Bergen area, West Norway: results from K–Ar illite fault gouge dating

“…1; Fossen & Hurich, 2005), still influence the landscape of western Norway today. While the high-temperature processes of the Caledonian Orogeny and early orogenic collapse can be dated by a variety of geochronological methods such as U-Pb, Lu-Hf, Sm-Nd, K-Ar and ArAr geochronology (e.g., Bingen et al, 2001;Fossen & Dunlap, 2006;Walsh et al, 2007;Bingen & Solli, 2009;Smit et al, 2010), the multiphase Late Palaeozoic-Mesozoic rift history of West Norway is more difficult to constrain. The main rift structures are located offshore, but the tectonic evolution of onshore and offshore areas is strongly linked (e.g., Fossen et al, 2016) and needs to be considered as a cohesive system in order to understand western Norway's evolution from a high-standing Caledonian mountain range to the elevated rifted margin of today.…”