Provenance of Neoproterozoic sediments in the Särv nappes (Middle Allochthon) of the Scandinavian Caledonides: LA-ICP-MS and SIMS U-Pb dating of detrital zircons

Abstract

We present U-Pb age data for detrital zircons from dike-intruded Neoproterozoic sedimentary rocks of the Caledonian Middle Allochthon in central Sweden and Norway. Detrital zircons from 11 samples from the Särv, Sætra and upper Leksdal nappes (informally referred to as the Särv nappes) are clustered within ca. 0.9-1.75. Ga, but display a bimodal distribution with major ca. 1.45-1.75. Ga and ca. 0.9-1.2. Ga components. An apparent increase of younger (0.9-1.2. Ga) components to the northwest reflects varying source terranes. Detrital zircons from an additional sample from the lower part of the Leksdal Nappe, of uncertain affiliation to the Särv has a prominent 1.75-1.85. Ga component supporting previous suggestions that this part of the nappe belonged to a more proximal basin. Comparison of the Särv age probability patterns with data from basement windows and basement slices within the Middle Allochthon in central Sweden and Norway supports the derivation of the sediments from the attenuated Baltican continental crust on which they were presumably deposited. Similar comparisons suggest that derivation from the southern segment of the Fennoscandian Shield or from eastern segments of Laurentia is less likely, mostly because they include also older components.We infer that the ca. 200. km wide belt of attenuated Baltican continental crust included northern extensions of Mesoproterozoic to early Neoproterozoic terranes exposed in the southern part of the Fennoscandian Shield and the easternmost part of Laurentia, which at ca. 900. Ma were still adjacent. Pre-1.75. Ga terranes of the Fennoscandian Shield were probably isolated from the Särv distal basin(s) by intra-cratonic basins and uplifted margins associated with early development of this extended continental crust. The significantly older ages in the lower part of the Leksdal Nappe and its inferred more proximal position support this model. The proposed northern extension of Mesoproterozoic-early Neoproterozoic terranes can explain in a simpler way the occurrence of such detritus in many Caledonide-Appalachian allochthons exposed at the margins of the North Atlantic, with no need to infer large displacement along the axis of the Caledonide Orogen or to postulate selective transport of Grenville-age material from the south over large distances. One of our Särv samples located at the Norwegian coast revealed Caledonian reworking at ca. 395. Ma. This age agrees with ages of late-tectonic amphibolite-facies metamorphism and pegmatite intrusion recorded in this part of the Caledonides.