The Precambrian gneiss complex of Ness, Lewis, in relation to the effects of Laxfordian regeneration

Abstract

The Lewisian gneisses of Ness, at the northern tip of the Outer Hebrides, contain abundant metasedimentary relics including calc-gneisses, quartzites, pelites and semi-pelites. These rocks were partially migmatised during the earliest recognisable metamorphic episode and subsequently developed a characteristic flaggy structure and fine-grained texture during two later episodes of deformation associated with incomplete recrystallisation. An irregular belt of anorthosite, at least five kilometres in length, was probably emplaced before migmatisation. It is inferred, from the age-relationships of Late Laxfordian pegmatites and of metadolerites comparable with the post-Scourian basic suite of the Outer Hebrides, that the early migmatisation was Scourian and that the two later tectonic episodes were Laxfordian. The style of Laxfordian regeneration contrasts with that of the ‘grey gneisses’ in adjacent parts of Lewis and it is suggested that this difference depended partly on the operation of temperatures and pressures rather lower than those of most other regions and partly on the inherited effects of lithological and structural contrasts established in Scourian times.