CONSTRAINTS ON THE PROVENANCE OF THE OLD RED SANDSTONE (DEVONIAN) OF SCOTLAND FROM 40AR/39AR MUSCOVITE GEOCHRONOLOGY

New muscovite ⁴⁰Ar/³⁹Ar ages from the Old Red Sandstone (ORS, Devonian) and Caledonide orogen of Scotland suggest that the sediment was derived mostly from local sources, but with contributions, particularly in the upper part, from as far afield as Scandinavia. The ORS has been subdivided in the Midland Valley basin into two grossly upward-fining conglomerate-sandstone units (fluvial and in part, alluvial fan). In the Orcadian basin of northeastern Scotland, an interval of cyclic sandstone and siltstone (lacustrine) constitutes a middle unit of the ORS. Preliminary data reveal the following overall pattern. Scottish Caledonide bedrock ages range from ~565 to ~418 Ma (Neoproterozoic to earliest Devonian), with a clustering of ages ~460 Ma (Middle Ordovician). Given additional exhumation since the Devonian, grains eroded from these same terranes at that time might be a little older today than grains we obtained at the present level of erosion. Population peaks for detrital ages range from ~475 to ~445 Ma in the Lower ORS and from ~455 Ma to ~415 Ma in the Upper ORS, broadly consistent with unroofing of the adjacent orogen. The ~415 Ma peak in the uppermost part of the ORS is of particular interest because we have yet to identify a source in Scotland. However, previous work by J.A. Templeton (Ph.D., 2015) in approximately correlative sandstones of the Hornelen basin of western Norway reveals a prominent peak at 414 Ma. A plausible interpretation therefore is that the Scottish sediments were derived in part from western Norway, and transported to their site of deposition by a through-going river system. This result is consistent with the interpretation of unusually preserved large bedforms (> 10 m high) that have been taken to imply the existence of large rivers (Bluck, 2000, GSL Spec. Publ. 180, 417-437).