Basement palaeogeography of late Neoproterozoic Scotland: constraints from exotic clasts within the lower Dalradian Supergroup
U–Pb geochronology of exotic igneous cobbles, together with detrital zircons from a nearby sedimentary heavy mineral band from the Glenshirra Group (basal Dalradian Supergroup), provide information on the age and nature of the adjacent crystalline basement that supplied detritus to the Dalradian Basin in the late Neoproterozoic. The igneous cobbles were sampled from a metaconglomerate interpreted as debris-flow deposits from a local igneous basement hinterland.
A granite cobble, and detrital zircons from a nearby psammite, gave c. 1800 Ma ages, indicating derivation from a presumed local Ketilidian–Makkovikian source akin to that exposed on Inishtrahull, Colonsay and the Rhinns of Islay. A syenite clast yielded crystallization ages of c. 1500 Ma (zircon cores), followed by a subsequent metamorphism at 1470 Ma (zircon rims). This corresponds to coeval Mesoproterozoic events that affected the Pinwarian terrane of SE Canada and Newfoundland. The Moine Supergroup and Glen Banchor Succession are unlikely sources of detritus for the Glenshirra Group, as palaeodrainage across the line of the future Great Glen Fault (in its pre-displacement disposition) would largely derive from local basement to the west of the Moine Supergroup.
Whilst the existence of nearby Ketilidian–Makkovikian basement has been noted previously, this study provides evidence that Pinwarian basement also lay adjacent to Scotland during the deposition of the early Dalradian basin fills. These data should be incorporated into palaeogeographical reconstructions at the time of the break-up of the Rodinian supercontinent in the late Neoproterozoic.