Enigmatic migmatites from the North Scandinavian Caledonides: Unpicking the metamorphic history of the Upper Kӧli Nappes using in-situ (Th)-U-Pb dating of monazite and zircon

The Köli Nappe Complex in the Scandinavian Caledonides of Sweden originated as terranes within the Iapetus Ocean, derived from subduction-related magmatic and basin systems. The Krutfjellet Nappe in Västerbotten, Sweden and the Gasak Nappe in Nordland, Norway are both part of the Upper Kӧli Nappes. Siliclastic, carbonate and volcanic protoliths underwent metamorphism up to amphibolite facies, in places involving extensive migmatisation which is not found in other Kӧli Nappe units. Owing to a lack of previous studies, the exact age and origins of these migmatites is currently unknown. Foliations and early folds in the metasediments are cut by c. 445-434 Ma intrusions [1], followed by a regional metamorphic overprint which is assumed to be Scandian. Monazites and zircons from pelitic migmatites in the Norra Storfjället lens of the Krutfjellet Nappe, and monazites from pelitic schists and gneisses in the Sulitjelma area of the Gasak Nappe were dated in-situ using LA-ICP-MS.

Monazites yield Th-U-Pb concordia ages of between 428-424 Ma. Individual concordant analyses span between 443-416 Ma. The monazites yield a large proportion of discordant analyses which fall on discordia lines at high angles to the concordia curve, interpreted to be due to the presence of initial Pb in the monazites. Lower intercept ages range between 428-416 Ma. These isotopic monazite ages are younger than the majority of EPMA Th-U-total Pb ages obtained for the Krutfjellet Nappe in the same grains [2]. The monazites often have complex zoning patterns in Y and REEs, however this zoning appears to be decoupled from the Th-U-Pb isotopic systems. We suggest that the monazites from the Krutfjellet and Gasak Nappes underwent metamorphism and/or fluid-related alteration at between 428-416 Ma, associated with assembly of the Caledonian orogenic wedge. This may have involved pervasive resetting of older monazites through dissolution-reprecipitation, leading to contamination with initial Pb and decoupling of the Th-U-Pb system from Y and REE zoning. The actual age of migmatisation still remains uncertain.

Zircons yield a spectrum of U-Pb dates which appear to be predominantly detrital. The majority are Mesoproterozoic to earliest Neoproterozoic (1600-900 Ma) and a small number around 600-550 Ma. A significant proportion of dates are discordant. These discordant dates often occur in grains which have a spongey, mottled appearance, indicating that the zircons have also experienced some alteration (metamictisation?). Lower discordia intercepts are poorly constrained, but appear to be Ordovician to Silurian. A zircon included in kyanite produced a single concordant date of 440 Ma, which matches the age for nearby earliest-Silurian intrusions [1]. On the basis of this so-far limited zircon dataset we tentatively propose that these rocks underwent migmatisation at c. 440 Ma, perhaps associated with enhanced heat flow from rift-related magmatism, but the monazite age record has been severely overprinted during subsequent Scandian orogenic wedge assembly and translation and its associated lower-grade metamorphism.

Funded by the National Science Centre (Poland) grants no. 2021/41/N/ST10/04298 and 2021/41/B/ST10/03679.

[1] Stephens, M.B. 2020. GSL Memoirs, 50, 549–575.

[2] Carter, I. S. M., Cuthbert, S. & Walczak, K. 2023. EGU Gen. Assem.