Late Ediacaran adakites from Middle Köli Nappe Complex in Northern Caledonides of Sweden

The Scandinavian Caledonides consist of a stack of thrust nappes emplaced during the Caledonian Orogeny. The Upper Allochthon of the Caledonides in Norway and Sweden is dominated by Iapetus derived rocks of the Köli Nappe Complex (KNC), which is traditionally separated into the Lower, Middle, and Upper KNC. In the Hammaren-Stáddátjåhkkå region, located to the North of the Sulitjelma ophiolite, the Middle KNC is composed of metasedimentary rocks of Cryogenian to early Ordovician age (Stephens et al. 1985), intruded by various igneous rocks including gabbros, trondhjemites and diabase dikes of unknown age.

Hereby we report new geochemical and geochronological results from three adakite samples, previously believed to be trondhjemites, from the region, and reveal unusually old magmatic ages within zircon grains. Collected samples were originally mapped as trondhjemite (Thelander 2009). However, bulk-rock geochemical data suggests that two of the samples are high-silica adakites related to a supra-subduction environment, which formed on an active continental margin or intra-oceanic arc, and the third is an adakite-like trachyandesite with the geochemical signature of a subduction-related environment. 

The absence of an Eu anomaly in zircon trace element patterns indicates that the source of melt was feldspar-free, while the low Ce anomaly suggests reducing conditions during melt formation. Such features also corroborate the thesis that the melt was derived from eclogitized oceanic crust in a subduction environment. In each sample, 14 zircons were analysed for 206Pb/U238 dating, and the calculated concordia ages are 549.3 ± 2.4 Ma (n=8), 551.9 ± 1.7 (n=13), and 559.8 ± 2.8 Ma (n=5), respectively. 

Both the geochemical signatures and the age of the adakites are quite rare in the Caledonides. Similar ages were only reported from the Seiland Igneous Province, however, they are believed to have formed in extensional settings. Regarding the age of the Northern branch of Iapetus opening (starting c. 590 Ma), it is highly improbable to develop a subduction zone in such a short time.  Thus, we claim the Middle KNC of the Hammaren-Stáddátjåhkkå area to be of exotic, possibly Timanian origin. However, the possibility that Iapetus was “infected” with early subduction, by a process similar to that described by Waldron et al. (2014), cannot be excluded.

This study underlines the importance of geochronological work on igneous and sedimentary rocks from the Hammaren area, which is emerging as a key locality to yield novel insights about the origin of the Iapetus terranes of the Northern Caledonides. 

 

Stephens, M.B., Furnes, H., Robins, B. and Sturt, B.A. 1985a. Igneous activity within the Scandinavian Caledonides. In: Gee, D. G. and Sturt, B. A. (eds) The Caledonide Orogen – Scandinavia and Related Areas, pp. 623–656.

Thelander, T., 2009: Berggrundskartan Kaledoniderna i norra Sverige, skala 1:250 000. Södra delen. Sveriges geologiska undersökning K 222:2.

Waldron J.W.F., Schofield D.I., Murphy J.B., Thomas C.W., 2014. How was the Iapetus Ocean infected with subduction? Geology  42 (12): 1095–1098.