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Testing for the presence of a terrane boundary within Neoproterozoic (Dalradian) to Cambrian siliceous turbidites at Callander, Perthshire, Scotland

¹ Division of Earth Sciences, Gregory Building, University of Glasgow, Glasgow G12 8QQ, UK (e-mail: G.Tanner{at}earthsci.gla.ac.uk)
² Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 OQF, UK

The Southern Highland Group (Dalradian) and Keltie Water Grit Formation, which includes the Lower Cambrian Leny Limestone, form an inverted, 1.4 km thick, largely arenaceous, sequence at Callander. The grits have the same detrital mineralogy throughout, mainly quartz, plagioclase (An_1–3), muscovite, and biotite. Chlorite formed from detrital biotite during low-grade regional metamorphism (T<270°C). There are some vertical changes in major element (but not trace element) chemistry of the grits, and detrital muscovites have a wide, but comparable, range in composition throughout, apart from an influx of Na-rich micas in the Keltie Water Grits. ⁴⁰Ar/³⁹Ar laser fusion dating of detrital muscovites yields an age spectrum with a peak at 1600–1800 Ma in the Dalradian rocks; similar old ages occur in the Keltie Water Grits but are diluted by ages of 507–886 Ma. We interpret these new data as showing that the rocks were most likely deposited as a single sequence, possibly with a disconformity, in Neoproterozoic to Early Cambrian times, before the onset of Grampian orogenesis in the Early Palaeozoic. No major structural or stratigraphical breaks have been identified and there is no direct evidence for the presence of two separate terranes.

Keywords: Dalradian, Cambrian, Grampian orogeny, ⁴⁰Ar, ³⁹Ar, detritus.

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