Nature and origin of Lewisian basic rocks of Gairloch, Ross-shire

Abstract

Synopsis A series of basic sheets in the Lewisian complex of the Gairloch area was emplaced prior to the main tectonic phase (1400+m.y.) and metamorphosed in epidote-amphibolite facies. Several different modes of origin have previously been proposed for the various members of this suite, but most of the thinner bodies had been assumed to represent metamorphosed Scourie dolerite dykes although there was no evidence that they were of igneous origin. The suite is divided into eight types based mainly on macroscopic differences in texture and on the presence or absence of metasediment interlayers, but it is concluded that the marked textural variation is due largely to structural and metamorphic differences rather than to original heterogeneity. Petrographic data are given, and major and trace-element analyses, with modes and catanorms, presented for nineteen representative samples. It is suggested that in certain cases small changes in composition may have occurred through alkali metasomatism. A comparison of chemical variation trends with the igneous trends of the Karroo dolerites points to an igneous origin for the Gairloch basites which appear to represent tholeiites and olivine-tholeiites. The thicker bodies are either sills or lavas and the thinner bodies dykes or sills.