The geology of the lewisian complex near Poolewe, Ross-shire

Abstract

The Lewisian rocks near Poolewe are part of an acid gneiss complex; the complex includes subordinate amounts of basic and ultra-basic gneiss (early basites) and also rocks of presumed metasedimentary origin (the Gleann Tulacha and Gruinard metasediments). Two hornblende schist bodies (the Beinn Airigh Charr and Gruinard basites) are intimately associated with the metasediments.
Members of a suite of basic igneous rocks (the Scourie dykes) have intruded the gneisses, early basites and also the Gruinard metasediments and basite; the chronological status of the Gleann Tulacha metasediments and Beinn Airigh Charr basite is in doubt since no member of the Scourie dyke suite intrudes them. All of the basic igneous rocks have been metamorphosed and converted into amphibolites.
Petrological variation within each of the main rock groupings is more a function of the metamorphic fabric domain in which a rock occurs than of pre-metamorphic lithological heterogeneities. Presentation of detailed petrological descriptions is therefore based upon the metamorphic fabric sequences recognised, attention being focussed upon shape rather than orientation fabrics. These fabric sequences are used for purposes of structural correlation.
Structural sequences are also established using the evidence of interfering fold sets and the structural relationships between members of the basic dyke suite and the country rocks. Dyke intrusion involved at least three phases of injection; these are however envisaged as stages in a single protracted event. The period of dyke intrusion is therefore used as a stratigraphical marker within the complex.
Structural correlations based upon fabric criteria are consistent with the hypothesis that dyke intrusion was a single protracted event. Development of the complex is therefore considered in terms of a pre-dyke (Scourian) complex and a post-dyke (Laxfordian) complex. Deformational events D2, D3, D4 and D5 have been recognised in the Scourian complex whilst D6, D7 and D8 events affected the Laxfordian complex. The original nature of the S1 surface is unknown.
Most of the rocks show evidence of metamorphism in amphibolite facies, although evidence is cited which implies that the M3 event was, in part, in granulite facies. This pre-D4 high-grade metamorphic complex is correlated with the Badcallian (Park 1970), whilst later Scourian events are considered to be of Inverian age.
A detailed geometrical analysis of the small-scale structures is presented. In addition, the areas mapped permit the study of parts of three large-scale Lewisian structures, namely the Carnmore antiform, the Letterewe synform and the Tollie antiform. The N.E. limbs of the Carnmore and Tollie antiforms are considered to be large-scale Inverian structures, whilst the S.W. limb of the Carnmore antiform and the adjacent Letterewe synform are of Laxfordian age.
The metasediments neither contain any evidence of the Badcallian fabrics nor show the effects of pre-Inverian migmatisation, apparent elsewhere in the complex. It is therefore concluded that the metasediments were originally deposited upon a Badcallian basement, but were involved in Inverian structural and metamorphic events. Both groups of metasediments occur at similar structural levels and are associated with large hornblende schist bodies. It is thus likely that the metasediments are stratigraphically equivalent and that the hornblende schist bodies probably represent post-Badcallian, pre Inverian meta-volcanic assemblages.
29 Samples of dyke material were analysed for major elements using "wet" techniques; 27 of these samples were analysed for Ni, Cr, Ba, Sc, V, Sr, Co, Zr and Cu by emission spectroscopy. The results of these geochemical analyses are presented. It is concluded that the chemical variation exhibited can be explained in terms of the differentiation of a magma having tholelitic affinities. Igneous structures in the dykes are discussed with respect to possible differentiation mechanisms. Consideration of the dyke fabrics and the structural relationships between dykes and country rock leads to the conclusion that the dykes were emplaced into a hot, but cooling, Inverian complex in which the Inverian structures exerted considerable control over dyke emplacement.