62nd Annual Report on Research 2017

Considerable progress has been made within the last year to characterize the Silurian-Carboniferous Old Red Sandstone (ORS) muscovite detrital and potential source terrane signatures within the Caledonide orogen of Scotland, Norway, and Greenland. Our primary goal remains to identify the ultimate source of the ORS, which some have considered to be analogous to the Indus and Ganges draining the Himalaya today. Of the 116 sites sampled in summer, 2016, 27 potential source terrane and 17 ORS localities have been analyzed with muscovite Ar⁴⁰/Ar³⁹ geochronology – a total of 81 source terrane and 442 detrital ORS single grain analyses. When combined with U/Pb detrital zircon ages for two ORS sites, these new data provide the geochronological framework needed to resolve subtle provenance signatures within the ORS.

Source terrane analysis reveals ages ranging from 1,636 to 413 Ma, with the majority falling between 530 and 424 Ma. These ages define the range of the latest unroofing events in the Scottish terranes, and hence the youngest possible ages that could be representative of Scottish sourcing in the ORS detrital signature. Therefore, any ages younger than those exposed today at the surface must have been derived from a more distant source. Detrital muscovite ages from the ORS reveal an age range from 540 to 330 Ma, with the majority lying between 480 and 370 Ma. Considering that the youngest portion of the age spectrum for Scottish source terranes is ~420 Ma, this suggests that the ORS is at least in part derived from outside the area studied. Many localities appear to be for the most part locally sourced. However, many sites reveal minor components younger than what is currently observed in Scotland.

Two localities southwest of Glasgow record a suite of local grains and a robust Scandian sourced suite. These two localities correspond with the uppermost ORS (Lower Carboniferous), and the upper part of the younger of two upward fining sequences that characterize the Lower and Upper ORS. It has been suggested that the flux of distant sediment was largely controlled by local deformation, wherein there were two periods of tectonism and widespread clastic sheets were deposited following tectonic quiescence. The two localities near Glasgow appear to support this hypothesis, in that the uppermost portion of the upward fining ORS reveals the most distinct Scandian signal within our dataset. Our data, at present, do not definitively support this same conclusion for the lower upward-fining sequence. However, scarce Scandian ages in samples from other localities suggest that this is a possibility.

Further work is now needed to firm up subtle geochronological peaks within the ORS succession. In many cases, indication of a younger source is suggested by no more than one to several grains. More analyses are needed to make these interpretations statistically robust. We plan to continue analyzing ORS components in an effort to reinforce potential Scandian peaks. The work will be presented by Ph.D. student Michael DeLuca at the Annual Meeting of the Geological Society of America in Seattle (22-25 October, 2017).

The research is contributing to DeLuca’s dissertation. He has become an integral member of the Argon Geochronology in Earth Sciences lab., where he is acquiring the skills needed to become an academic scientist in geochronology.