Abstract

B. J. Bluck, T. J. Dempster & G. Rogers write: In a recent Special paper, Soper (1994) contends that Dalradian deposition continued after the emplacement of the Ben Vuirich granite at 590 Ma (Rogers et al. 1989). The granite, however, contains metasedimentary xenoliths with an early (Dl) fabric (Bradbury et al. 1976; Rogers et al. 1989, fig. 2a) and produced a thermal aureole within the Dalradian country rocks (Barrow et al. 1905). The aureole overprints a metamorphic fabric (Dl) defined by 200-300 μm ilmenites (Tanner & Leslie 1994). We see this as clear evidence for a ductile, tectonothermal event which preceded the intrusion of the granite. With this in mind: (1) the model of Soper (1994) fails to address how significant deformation (possibly upright major D1 folds (Treagus 1987), associated greenschist facies metamorphism (Tanner & Leslie 1994) and granite emplacement (Rogers et al. 1989) are compatible with the Dalradian block being on a Laurentian passive margin at c. 590 Ma (cf. Bluck & Dempster 1991); and (2) it is also difficult to envisage deposition of the Dalradian continuing after the intrusion of the Ben Vuirich granite.

11 August 1994.

N. J. Soper replies: One purpose of my paper was to point out that the concept of end-Proterozoic orogeny in the Highlands, derived from structural and isotopic studies, is incompatible with upper Dalradian stratigraphy, which records the opening of an ocean. Placing the Dalradian elsewhere during deposition in the Vendian (Bluck & Dempster 1991) does not solve this problem. Bluck and

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