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

Abstract

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.

The Skuggliberga unit of the Oppdal area, central Scandinavian Caledonides: calc-alkaline pyroclastic volcanism in a fluvial to shallow-marine basin following a mid-Ordovician orogenic event

“…Based on the age data provided in Stokke et al (2018), Roberts et al (2019) suggested that the Skuggliberga unit may correlate with the Ekne Group northeast of Trondheim (Fig. 1A).…”

“…1A). Four detrital zircon samples from the Ekne Group are dominated by Palaeozoic detrital zircons which show main peaks at 460-450 Ma, interpreted by Roberts et al (2019) to indicate deposition in the Late Ordovician to Silurian. However, the Ekne Group is lithologically different from the Skuggliberga unit, as it is dominated by turbiditic sandstones and siltstones and does not contain volcanic rocks.…”

Accretion, Soft and Hard Collision: Similarities, Differences and an Application from the Newfoundland Appalachian Orogen

“…The Scandian Caledonides in Greenland and Baltica are a good example of a hard collision that resulted in a bivergent orogenic wedge. The pro-wedge preserves Taconic deformed rocks and upper plate arc rocks in the highest east-directed nappes in western Nor-way (Roberts et al 2007(Roberts et al , 2019, whereas west-directed nappes of the retro-wedge occur in eastern Greenland (Higgins et al 2004). The nappe stack in the Swiss-Italian Alps comprises penetratively deformed and metamorphosed rocks of both the lower European as well as the upper Adrian plates, elegantly illustrated and described in the kinematic-geometric evolutionary reconstructions made by Escher and Beaumont (1997).…”