U-Pb Zircon Dating of Migmatitic Paragneisses and Garnet Amphibolite from the High Pressure Seve Nappe Complex in Kittelfjäll, Swedish Caledonides

Abstract

The Seve Nappe Complex exposed in the Kittelfjäll area of the northern Scandinavian Caledonides comprises a volcano-sedimentary succession representing the Baltica passive margin, which was metamorphosed during the Iapetus Ocean closure. Garnet amphibolites, together with their host migmatitic paragneisses, record a potential (U)HP event followed by decompression-driven migmatization. The garnet amphibolites were originally thought to represent retrogressively altered granulites. The petrological and geochemical features of a studied garnet amphibolite allow for speculation about a peridotitic origin. Zirconium (Zr) content in rutile inclusions hosted in garnet in paragneisses points to near-peak temperatures between 738 °C and 780 °C, which is in agreement with the c. 774 °C obtained from the matrix rutile in the garnet amphibolite. The matrix rutile in multiple paragneiss samples records temperatures below 655 °C and 726 °C. Whereas the LA-ICP-MS U-Pb dating of zircon cores revealed the age spectrum from Paleoproterozoic to early Paleozoic, suggesting a detrital origin of zircon cores in paragneisses, the metamorphic zircon rims show an Early Ordovician cluster c. 475–469 Ma. Additionally, zircon cores and rims from the garnet amphibolite yielded an age of c. 473 Ma. The REE patterns of the Caledonian zircon rims from the paragneisses show overall low LREE concentrations, different from declining to rising trends in HREE (LuN/GdN = 0.49–38.76). Despite the textural differences, the cores and rims in zircon from the garnet amphibolite show similar REE patterns of low LREE and flat to rising HREE (LuN/GdN = 3.96–65.13). All zircon rims in both lithologies display a negative Eu anomaly. Hence, we interpret the reported ages as the growth of metamorphic zircon during migmatization, under granulite facies conditions related to exhumation from (U)HP conditions.

Diachronous collision in the Seve Nappe Complex: Evidence from Lu–Hf geochronology of eclogites (Norrbotten, North Sweden)

“…One possible explanation would be that the Seve Nappe in Norrbotten represented a promontory in the passive continental marging, leading to a younging of the collision process towards the south (e.g. Barnes et al., 2018; Bukała, Majka, et al., 2020; Petrik et al., 2019; Figure 12).…”

“…Soper, 1994). Caledonian HP to UHP rocks are found in the Seve Nappe Complex in Jämtland (Janák et al., 2013; Klonowska et al., 2016, 2017; Majka et al., 2014), Västerbotten (Bukała, Majka, et al., 2020; Petrík et al., 2019), and Norrbotten (Bukała et al., 2018). Ages of this UHP metamorphism are between 445 and 500 Ma (e.g.…”

“…While the age of 472 Ma has been interpreted as related to monazite growth under near‐peak UHP conditions, Bukała, Majka, et al. (2020) report a zircon U–Pb age of c . 473 Ma, interpreted to date the exhumation‐related migmatization of garnet amphibolites and host paragneisses from Kittelfjäll (central Västerbotten).…”

Zircon and monazite reveal late Cambrian/early Ordovician partial melting of the Central Seve Nappe Complex, Scandinavian Caledonides

“…1B; Petrík et al 2019). In contrast, the paragneisses and amphibolites near Kittelfjäll, ~ 30 km northeast of Saxnäs, demonstrate that the Marsfjället Gneiss underwent partial melting at 474.2 ± 9.6 Ma, yielded by zircon U-Pb geochronology (Bukała et al 2020b). Orogenic spinel peridotites are exposed close to the Kittelfjäll paragneisses and preserve metamorphic conditions of 650-830 °C and 1-2 GPa during subduction, followed by exhumation of the peridotite to 550-600 °C and 0.45-0.60 GPa (Clos et al 2014).…”

“…1A). Two were obtained from the Sippmikk creek exposure of the Avardo Gneiss (samples SM19-02, and SM19-03) and one was obtained from a roadcut exposure of the Marsfjället Gneiss northwest of the town of Kittelfjäll (sample MJ18-05A), which was previously studied by Bukała et al (2020b). All paragneisses were investigated using transmitted light microscopy to characterize mineral assemblages, textural relationships, and microstructures.…”