Layered intrusive rocks in the Lewisian of the North-West Highlands of Scotland

Abstract

Layered ultrabasic and basic intrusive rocks occur in three structural environments in the Lewisian of the North-West Highlands of Scotland and 41 such layered masses, or groups of masses, in the area between Loch Laxford and Loch Torridon are recognized and described. Their igneous origin is indicated by the common mineralogical banding, the occurrence of sedimentation features, particularly current-and wedge-bedding, and by the association, in banded sequence, of ultrabasic, basic, and anorthositic rocks which are similar in chemical composition to corresponding rock-types in other layered igneous complexes. One group of metaperidotites and metagabbroic rocks has undergone coeval, or near-coeval, granulite-facies metamorphism and folding, probably after an earlier period of disruption. Subsequently, about 2400 m.y. ago, further disruption and amphibolitization took place. No evidence of the later events is shown by most of these masses, but they have moved tectonically, as units, by slip around their margins during subsequent episodes of deformation. The masses are usually in the form of massive metaperidotite lenses generally concordant with the banding of the adjacent gneisses or granulites, although a few serpentinite masses now have the form of tectonic diapirs. Low-grade retrogressive metamorphism in zones of steep reverse faulting affected some of the masses. Another group of metaperidotites has been deformed by isoclinal folding on axes trending nw-se with steeply dipping axial planes so that the mineralogical banding of the ultrabasic bodies is now vertical. Subsequent hydrous metamorphism, sometimes with the introduction of potassium, has caused marginal alteration of many of these masses to talc-trem-olite-carbonate-serpentine rocks, sometimes with biotite and muscovite. Complete minera-logical reconstitution to this low-temperature hydrous assemblage has taken place in some cases, but relict banding remains. After this episode, which was about 1500 m.y. ago, the rocks were deformed by a series of box-like folds, with steeply plunging axes, causing local deflexion of the banding from its nw-se trend. A small group of banded picrites and gabbros has intrusive relations to these gneisses and shows subsequent retrogression with the uralitization of the pyroxenes.