A Silurian age for the metasedimentary rocks of the Ekne Group, Trøndelag, Mid-Norwegian Caledonides: and inferences for a peri-Laurentian provenance

U–Pb analyses (LA–SF–ICP–MS) of detrital zircons from low-grade metasandstones, some tuffaceous, of the biostratigraphically-barren Ekne Group in the Støren Nappe of the Mid-Norwegian Caledonides have revealed that the sequence is almost entirely of Silurian age. The complete Ekne Group succession is about 2800 m thick and estimates of maximum depositional age based on the mean age of the youngest five zircon grains show an upward younging from 446.6 Ma at the base of the group to 432.9 – 427.2 Ma in the upper half. In the sample close to the very top of the succession, a small number of zircons yield latest Silurian to earliest Devonian ages. Considering the inferred staging, recycling and catastrophic release of sediments into the basin during deposition, it is possible that the younger ages reflect genuine Early Devonian sediment input. All samples are dominated by Early Palaeozoic and notably Late Ordovician (Sandbian–Katian) zircon ages, with some peaks suggesting derivation from Taconian plutons occurring in different parts of Mid Norway. Some 40% of zircons in the sample from the base of the Ekne Group range in age from Mesoproterozoic to Palaeoarchaean. Bearing in mind the palaeogeography at the time, the Archaean grains are considered to have been recycled from older sandstones, with their original provenance most likely being the c. 3.65 Ga gneiss complexes in SW Greenland. As the Iapetus Ocean had closed by latest Ordovician time, by comparing the Ekne Group and other Silurian successions in Trøndelag with similar volcanosedimentary sequences in the northern Appalachians of USA and Canada, it is highly likely that the Ekne Group was deposited in a basin peripheral to the eastern margin of Laurentia during the Salinic orogenic cycle.

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