In situ laser ablation Lu–Hf geochronology of garnet across the Western Gneiss Region: campaign-style dating of metamorphism

Abstract

The development of in-situ laser ablation Lu–Hf geochronology of apatite, xenotime and garnet has opened avenues to quickly and directly date geological processes. We demonstrate the first use of campaign-style in-situ Lu–Hf geochronology of garnet across the high- to ultrahigh-pressure Western Gneiss Region in Norway. Mafic eclogites from this region have been the focus of much work, and were clearly formed during continental subduction during the Caledonian Orogen. However, abundant quartzofeldspathic and pelitic lithologies record a more complex history, with some preserving polymetamorphic age data, and most containing no indication of high-pressure mineral assemblages formed during subduction. Twenty metapelitic and felsic samples spanning 160 lateral kilometers across the Western Gneiss Region have been analysed using garnet Lu–Hf geochronology. The results reveal Caledonian ages for the majority of the garnets, suggesting some quartzofeldspathic and metapelitic lithologies were reactive and grew garnet during high- to ultrahigh-pressure metamorphism. However, two ultrahigh-pressure eclogite locations, Verpeneset and Fjørtoft, preserve both Caledonian and Neoproterozoic-aged garnets. Despite significant uncertainties on some of the Lu–Hf geochronologic ages, laser ablation Lu–Hf efficiently identifies the polymetamorphic history of parts of the Western Gneiss Region, illustrating the effectiveness of this novel analytical method for rapid mapping of metamorphic ages.Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycleSupplementary material:https://doi.org/10.6084/m9.figshare.c.5715453

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

“…Recently obtained Lu-Hf age data from the sample used in this study indicates the garnets in the Fjørtoft sample, WGC2019J-31A, formed during the Mesoproterozoic (Simpson et al, 2021;Tamblyn et al, 2021). This is consistent with the presence of Late Mesoproterozoic-aged monazite inclusions inside garnet (Figure 8a; Terry et al, 2000b;Holder et al, 2015).…”

“…Similarly, Tual et al (2020) obtained a Lu–Hf garnet age of 422 ± 2 Ma from the Fjørtoft metapelite. In contrast, the Ulsteinvik metapelite from this study only records two Mesoproterozoic‐aged zircons and garnet Lu–Hf dating gives Caledonian ages (Tamblyn et al, 2021). This regional and local heterogeneity in the recorded timing of garnet formation highlights complexity in the metamorphic record, possibly arising as the result of either high‐grade Caledonian‐aged overprinting of local previously retrogressed Mesoproterozoic‐aged assemblages, the resistance of low a H2O assemblages to Caledonian metamorphic change, or the incorporation of previously unmetamorphosed rocks into the Caledonian tectonic system.…”

“…Despite c . 423 Ma monazites occurring within the Mesoproterozoic‐aged garnet (Simpson et al, 2021; Tamblyn et al, 2021), these grains are connected to the matrix via fractures. This connection with the matrix may have facilitated monazite recrystallization in response to fracture‐hosted fluid migration (e.g.…”

“…Recent advances in Lu-Hf dating allowed for garnets from the same sample as the Fjørtoft metapelite used in this study (WGC2019J-31A) to be dated in situ. Mesoproterozoic garnet ages (Simpson et al, 2021;Tamblyn et al, 2021) indicate the Fjørtoft metapelite sample used in this study had a Mesoproterozoic-aged metamorphic protolith. These findings are corroborated by U-Pb results from monazite and zircon, which record unambiguous evidence of a Mesoproterozoic event (e.g.…”

“…Relicts of this event within the WGC are rare but tend to be more prevalent within allochthons that were thrust over the Baltican basement during the early stages of the Caledonian Orogeny (Jolivet et al, 2005). Garnet Sm–Nd and Lu–Hf dating has shown that garnet in some high and ultrahigh‐pressure locations formed during the late Mesoproterozoic and early Neoproterozoic, ascribing a polymetamorphic character to some high‐P assemblages (Root et al, 2005; Simpson et al, 2021; Tamblyn et al, 2021). Low H 2 O activity, coarse grain size, and limited deformation have been attributed to the sporadic preservation of these granulite facies assemblages during the subsequent Caledonian Orogeny (Austrheim, 1987; Krabbendam et al, 2000).…”