Discussion on a Pre-D2 age for the Ben Vuirich Granite

Abstract

P. A. R. Nell & J. E. Treagus write: In their paper concerned with the structural context of the 590 Ma Ben Vuirich granite, Tanner & Leslie (1994) demonstrate that the contact metamorphism post-dates the first (S1) slaty cleavage. It is also pleasing to see Maclachlan & Treagus's (1991) assertion confirmed, that it is the regional schistosity (S2) which foliated the granite; this schistosity has been well-documented to the southwest of the granite ( Nell 1986 ; Treagus 1987 ) as well as to its northeast (Upton 1986). The question will be asked, however, particularly by workers unfamiliar with the Dalradian, how could the crucial structural context of this granite be misconstrued by previous workers ( Bradbury et al. 1976 ; Rogers et al. 1989 ) as post-D2 and pre-D3. These interpretations, which had placed the important D2 nappe-forming event of the Grampian fold-belt before intrusion of the dated granite, have had far-reaching effects upon tectonic models of the late Precambrian and early Palaeozoic evolution of Britain. The answer lies partly in the apparent mis-interpretation of local detail, relating to the granite margins and the fabric in xenoliths, matters which Tanner & Leslie (1994) have addressed. We would like to address here the more important question of why local fabrics were miscorrelated with those elsewhere in the fold-belt. All locations mentioned below are located on Fig. 1. The continuity of the ubiquitous second deformation (D2) event in the Dalradian can be seen from the observations of many workers: from the Southwest Highlands, where it is first