A pre-D2 age for the 590 Ma Ben Vuirich Granite in the Dalradian of Scotland

Abstract

Regional mapping demonstrates that S2 is the main fabric in the Central Highland Dalradian and that it is this fabric, not S3 as previously thought, which overprints the 590 Ma Ben Vuirich granite. The granite appears to have been emplaced following a tectonic event (D1) in the Dalradian rocks, for a finely-spaced penetrative fabric seen in newly-discovered hornfelses from the granite aureole is overgrown by contact metamorphic cordierite and andalusite porphyroblasts (now both altered) which predate the D2 metamorphism. The Ben Vuirich granite cuts the Neoproterozoic Dalradian sequence of the Central Highlands of Scotland (Fig.1, inset), and was first recognized as an older foliated granite by Barrow et al. 1905. Later workers ( Bradbury et al. 1976 ; Bradbury 1978 ) concluded that the granite was intruded between D2 and D3 in the Dalradian rocks ( Harris et al . 1976 ) and for many years an U-Pb zircon age of 514 ±7 Ma from the intrusion (Pankhurst & Pidgeon 1976) provided a valuable datum point in the evolution of the Scottish Caledonides. However, Dalradian chronology has recently undergone a radical revision following the determination of a precise U-Pb age of 590 ± 2 Ma for the emplacement of the Ben Vuirich granite ( Rogers et al. 1989 ). In accepting the interpretation of Bradbury et al. 1976 that the regional penetrative fabric which affects the country rocks (and is continuous with that in the granite) is of D3 age, Rogers et al. were also guided by the conclusions of Harte et al. 1984 and Dempster & Harte