Discussion on garnet provenance studies, juxtaposition of Laurentian marginal terranes and timing of the Grampian Orogeny in Scotland

Abstract

P. Stone & J. A. Evans write: Hutchison & Oliver (1998) present a compelling interpretation for detrital garnet in late Ordovician turbidites from the Southern Uplands terrane as being of Barrovian type, derived by erosion of the Dalradian Supergroup. As the authors justifiably claim, the regional implications of their results are considerable, bearing on the timing of Caledonian terrane amalgamation across the Grampians, Midland Valley and Southern Uplands of Scotland. One of the lines of evidence cited by Hutchison & Oliver in support of their conclusion derives from Nd isotope investigations. They consider that the average Nd composition of the quartzo-feldspathic, basement-derived Kirkcolm Formation sandstone in the Southern Uplands is the same as the average value from Dalradian rocks. This was used to support a Dalradian provenance for the Kirkcolm Formation. It contradicts earlier assertions that the comparative Nd isotope compositions were sufficiently different to preclude derivation of the Southern Uplands sandstones from a mainly Dalradian provenance (O'Nions et al. 1983; Evans et al. 1991; Stone & Evans 1995). In view of that apparent contradiction, and the regional tectonic importance of the terrane relationships in question, we should like to explore further the implications of the Nd data. Two papers have published Nd isotopic data for Dalradian rocks from the Scottish outcrop and these provide the database against which comparisons are made. O'Nions et al. 1983 published seven data points and Frost & O'Nions (1985) published a further five (Table 1). They can be subdivided into three of the main