Basic volcanism contemporaneous with the Sturtian glacial episode in NE Scotland David M. Chew 1 , Nicola Fallon 1 , Christine Kennelly 1 , Quentin Crowley 1,2 and Michael Pointon 1 1 Department of Geology, Trinity College Dublin, Ireland 2 NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, UK Email: chewd@tcd.ie, fallonna@tcd.ie, kennellc@tcd.ie, crowleyq@tcd.ie, pointonm@tcd.ie ABSTRACT: The Dalradian Supergroup contains three distinct glacigenic units, formerly termed ‘Boulder Beds’, which are correlated with widespread Neoproterozoic glaciations. The oldest and thickest unit, the Port Askaig Formation, marks the Appin–Argyll group boundary of the Dalradian Supergroup and has been correlated with the Middle Cryogenian (Sturtian) glaciation. The Auchnahyle Formation, a diamictite-bearing sequence near Tomintoul in NE Scotland, exhibits strong lithological similarities to the Port Askaig Formation. Both these glacigenic ‘Boulder Bed’ units contain abundant dolomite clasts in their lower parts and more granitic material at higher levels. Both metadiamictite units are overlain by thick shallow-marine quartzite units. C isotope data from Appin Group carbonate strata below the Auchnahyle Formation support this correlation. U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) detrital zircon data from the Auchnahyle Formation metadiamictite differ slightly from the Port Askaig Forma- tion, but are similar to detrital zircon spectra obtained from the Macduff Formation, a diamictite unit in the younger Southern Highland Group of the Dalradian Supergroup; both apparently reflect derivation from local basement rocks. No detritus younger than 0·9 Ga is observed, so the data do not constrain significantly the depositional age of the glacial strata. A thin tholeiitic pillow basalt unit in the lower part of the Auchnahyle Formation is geochemically distinct from pre-tectonic metadolerite sills and from basic metavolcanic rocks up-section. A Sturtian (c. 720–700 Ma) age for the Auchnahyle Formation metadiamictite would imply that this basaltic volcanism represents the oldest recorded volcanic activity in the Dalradian Supergroup and is inferred to represent an early, local phase of proto-Iapetan rifting within the Rodinian supercontinent. KEY WORDS: C isotopes, Cryogenian, Dalradian, geochronology, U–Pb, zircon The ‘Boulder Beds’ of the Dalradian Supergroup of Scotland and NW Ireland are schistose dominantly metasiltstone units containing angular to rounded granitic and carbonate clasts. They were recognised as distinctive lithologies in the early nineteenth century (e.g. Jameson 1800; MacCulloch 1819) and a glacigenic origin was suggested in the latter part of the nineteenth century (e.g. Thomson 1871, 1877). Kilburn et al. (1965) compiled all known exposures of the ‘Boulder Bed’ localities in Scotland and NW Ireland, and demonstrated that they formed part of a uniform succession for over 600 km along strike. The best exposed sections of ‘Boulder Bed’, the Port Askaig Formation, crop out on the Garvellach Islands and Islay in western Scotland (Fig. 1), where a combined thickness of over 750 m of metadiamictite and related meta- sandstone is exposed. These sections have been described in detail in the classic memoir of Spencer (1971). With the exception of the Early Cambrian trilobites and inarticulate brachiopods of the Leny Limestone (Pringle 1939; Fletcher & Rushton 2008), the Dalradian Supergroup is un- fortunately almost entirely devoid of fossils. Lithostratigraphi- cal correlation of this diverse series of Laurentian margin metasedimentary and mafic volcanic rocks is also hampered by complex polyphase deformation and locally by rapid lateral facies changes. Despite these difficulties, a coherent litho- stratigraphy from western Ireland to the Shetland Islands has been established (e.g. Harris et al. 1978, 1994). It comprises four Groups – the Grampian, Appin, Argyll and Southern Highland – with the Port Askaig Formation and its strati- graphical equivalent units in the lowermost Argyll Group forming a key marker horizon in the Dalradian succession (Fig. 2). McCay et al. (2006) and Brasier & Shields (2000) suggest that this formation is best correlated with the global Sturtian glacial episode. Other studies (e.g. Condon & Prave 2000; McCay et al. 2006) demonstrate the presence of two further possible glacigenic units within the Dalradian Super- group (Fig. 2). The Stralinchy ‘Boulder Bed’ occurs within the Easdale Subgroup in Donegal (Hutton & Alsop 2004) and has been interpreted as a glacigenic diamictite and correlated with the global Marinoan glaciation (McCay et al. 2006). The Stralinchy ‘Boulder Bed’ is overlain by a 6 m-thick dolostone that exhibits a pronounced negative C isotopic trend (McCay et al. 2006), characteristic of Neoproterozoic cap carbonates (e.g. Hoffman & Schrag 2002). The Loch na Cille and Inishowen ‘Boulder Beds’ occur within the uppermost Argyll Group and basal Southern Highland Group strata respectively, and are correlated with the more restricted Gaskiers glacial event (Condon & Prave 2000; Bowring et al. 2003). A glacial affinity for the Stralinchy ‘Boulder Bed’ is based on the presence of ice- rafted debris, although this is harder to prove for the Loch na Cille ‘Boulder Bed’. The precise stratigraphical position of other glacial units within the Dalradian, such as those within the Macduff Formation on the Banffshire coast, remains uncertain. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 100, 399–415, 2010 (for 2009)  2010 The Royal Society of Edinburgh. doi:10.1017/S1755691009009037