Significance of the early fabric in the contact metamorphic aureole of the 590 Ma Ben Vuirich Granite, Perthshire, Scotland

Abstract

An early fabric is preserved in pseudomorphs of contact metamorphic cordierite and andalusite found in hornfelsed Dalradian rocks at the NW margin of the Ben Vuirich Granite. As previous work had suggested that this fabric may be of regional tectonic (D1) origin, and taken as evidence that a pre-590 Ma (Neoproterozoic) orogenic event had affected the Dalradian Supergroup, the textures found in the hornfelses and their porphryoblasts are illustrated and described in detail. Three types of hornfels are recorded: unspotted, cordierite-, and andalusite-bearing. Cordierite was altered to mica, garnet, and kyanite, and andalusite pseudomorphed by kyanite, during the subsequent D2 regional metamorphism.The early fabric is finely spaced and associated with a weak shape fabric, except in the higher grade hornfelses where it is more intensely developed and formed during the deformation of irregularly shaped, cross-cutting granitic veinlets. It is concluded that the hornfels fabric is probably bedding modified by intrusion-related deformation, rather than a D1 fabric parallel to bedding. The implication of this finding is that intrusion of the Ben Vuirich Granite occurred during the Vendian (c.650 Ma) rifting phase, at about the time that the Tayvallich lavas were being deposited, and before any significant deformation of the Dalradian rocks had taken place. There is no unambiguous evidence to show that the early fabric in the hornfels at Ben Vuirich formed during a Precambrian, regional-scale, deformation of the Dalradian block.