Ages of continental 'Old Red Sandstone' (ORS) deposits in Norway have been notoriously difficult to obtain due to the paucity of fossil-bearing horizons. In general, 40Ar/39Ar ages from detrital mica and K-feldspar from unmetamorphosed, first-cycle sedimentary units yield maximum ages for sediment deposition since the detrital grains retain the ages of their crystalline basement provenance. To determine the maximum age for a presumed Lower-Middle Devonian ORS sequence in west-central Norway – the Asenøya basin – we applied 40Ar/39Ar geochronology to detrital white micas in red sandstone, and to white mica, biotite and K-feldspar from three different whole-rock clasts in an overlying conglomerate. Single-grain laser fusion of white mica from the sandstone yielded ages ranging between 416 ± 4 and 386 ± 1 Ma, with the majority of grains between ca. 400 and 392 Ma. Furnace step-heating of white mica, biotite and K-feldspar from the three rock clasts yielded weighted-mean ages of 410.2 ± 3.5 Ma, 402.7 ± 3.5 Ma and 371.0 ± 3.2 Ma, respectively. Published white mica and biotite 40Ar/39Ar cooling ages from the Central Norway basement window (CNBW), to the east and north of Asenøya, range from ca. 398 to 388 Ma; white mica and biotite 40Ar/39Ar cooling ages from the Seve and Köli Nappes, above the CNBW, range from ca. 432 to 402 Ma. Both of these primary source regions—the CNBW and the nappes—are thus represented in the red sandstone and conglomerate detritus. K-feldspar 40Ar/39Ar ages from local, CNBW basement (published age of 371 ± 8 Ma) and from orthogneiss in an offshore, in situ basement horst (377.3 ± 3.4 Ma, this study) further confirm the presence of appropriately-aged, proximal basement sources of CNBW type for some of the ORS conglomerate clasts. The new provenance age data demonstrate that some of the west-central Norway ORS units are younger than ca. 371 Ma—or a minimum of Late Devonian to Early Carboniferous age if time for unroofing/erosion of the basement provenance is allowed. The Asenøya sedimentary deposits are, thus, the youngest yet documented in the Norway ORS and are the last remnants of the Paleozoic sedimentary deposits of the Norwegian Caledonides that subsequently became recycled in offshore Mesozoic sedimentary basins.
Modern techniques and Old Red problems – determining the age of continental sedimentary deposits with 40Ar/39Ar provenance analysis in west-central Norway