NEW 40Ar/39Ar MUSCOVITE CONSTRAINTS ON THE CALEDONIAN GRAMPIAN TERRANE AND THE OLD RED SANDSTONE IN SCOTLAND

Muscovite ⁴⁰Ar/³⁹Ar cooling ages from the Grampian Terrane (GT) in Scotland, combined with new depositional ages from the Midland Valley Old Red Sandstone (ORS), are at odds with the traditional view that the ORS of Scotland is entirely post-Caledonian. Single grain muscovite ⁴⁰Ar/³⁹Ar muscovite cooling (~400°C) ages from samples throughout the GT reveal distinct regions of similar age groupings ranging from 480-430 Ma, indicating tectonic components with slightly different histories within the Caledonian orogen. We augmented these results with four closely spaced sampling transects within the southernmost GT near the Highland Boundary fault (HBF). Step heating experiments yield ages that range from 482-447 Ma. Although published cooling ages younger than 460 Ma are scarce, and commonly deemed spurious, our new results suggest that they are the norm rather than an exception. Over one third of the GT ages measured for this study post-date 460 Ma. The age range of the ORS has been previously constrained as 413-393 Ma. The lower bound is provided by an ash bed U/Pb zircon age (413.7 ± 4.4 Ma) reported from near its purported base at Stonehaven (Suarez et al., 2017: PLoS ONE 12(6): e0179262). The youngest grains from detrital muscovite ⁴⁰Ar/³⁹Ar spectra are utilized here to track the maximum depositional age of the ORS, because contemporaneous muscovite-bearing sources throughout the ORS depositional environment are abundant. The maximum depositional age at Stonehaven is 444 Ma. Corresponding muscovite cooling ages in the adjacent GT range from 482-461 Ma. The maximum depositional age near Balmaha in the western part of the ORS outcrop is 450 Ma, with corresponding muscovite cooling ages in the nearby GT ranging from 490-444 Ma. An implication of these data is that the Dalradian continued to be exhumed from depths in excess of 10 km after ORS deposition began. This is surprising because the ORS was previously considered to be regionally unconformable on the GT. The mismatch between footwall and hanging wall rocks across the HBF suggests a combination of normal and strike slip substantially greater than generally assumed.