Monazite U-Th-total Pb dating of migmatites from the Krutfjellet Nappe, Upper Köli Nappes, Swedish Caledonides

The Köli Nappe Complex (KNC) in the Scandinavian Caledonides of Sweden originated as terranes within the Iapetus Ocean derived from subduction-related magmatic and basin systems. The Krutfjellet Nappe is part of the Upper Kӧli Nappes in Västerbotten, Sweden. Siliclastic, carbonate and  volcanic protoliths^[3] underwent amphibolite facies metamorphism involving extensive migmatisation, which was of a distinctly higher grade than the other Koli Nappes. No modern P-T-t studies have been made in this nappe. Foliations and early folds in the metasediments (D1 and D2) are cut by latest Ordovician to earliest Silurian metagabbros and metagranites. Regional metamorphism and intrusion were syn-to-post D2. All these predate Scandian thrusting over the middle and lower KNC^[3]. A trondhjemitic pebble in a metaconglomerate was dated to c. 489 Ma^[4] so the main fabric-forming event is constrained to some time in the Ordovician. The mafic intrusions were partially converted to amphibolite and greenschist^[2] and the main greenschist-amphibolite metamorphism in the subjacent KNC was early Silurian, followed by early Devonian thrusting^[1], so a Scandian metamorphic imprint in the Krutfjellet Nappe is implied.

Four sillimanite and/or kyanite-bearing pelitic migmatite samples from the Norra Storfjället lens of the Krutfjellet Nappe were selected for U-Th-total Pb electron microprobe dating of monazite. Monazites from a variety of fabric elements including matrix, leucosome and inclusions within garnet yielded ages spanning the range 484-390 Ma. The monazites often have complex zoning patterns in Th and Y. However, discrimination of monazite populations based on trace element measurements was not resolvable so zoning appears to be decoupled from ages. There is also no discernable relationship between ages and location of the monazite within fabric elements. Weighted mean specimen ages were found to be 427 ±3.8 Ma, 442.5 ±4.0 Ma, 433.3 ±3.0 Ma and 438.3 ±2.7 Ma.

The large span of ages obtained suggests that more than one metamorphic event is recorded, however, some mixing and/or partial resetting of ages has occurred. The oldest ages (474-484 Ma), often outliers, are close to the early Ordovician conglomerate clast age^[4] and may have either been inherited from detrital monazite or formed during an early metamorphic event close to the clast age. The youngest ages (c. 430-400 Ma) are likely to be related to final thrusting of the Scandian nappe assemblage. The predominant age population falling around 445-435 Ma is similar to the ages of nearby early Silurian intrusions^[3], so monazite may have been generated or reset by the early Silurian intrusions, or by regionally-enhanced thermal regime associated with this magmatism.

 

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

[1] Bender, H., Glodny, J. and Ring, U. 2019. Lithos, 344–345, 339–359.

[2] Senior, A. and Otten, M.T. 1985. In: Gee, D.G. and Sturt, B.A., 953–978.

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

[4] Stephens, M.B., Kullerud, K. and Claesson, S. 1993. GSL, 150, 51–56.