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  • Letter to the Editors


    SIRS—The geochronological studies of van Breemen et al. (1971) demonstrate the age of formation of the granitic sheets in the injection complex of South Harris to be c. 1750-1715 m.y. and generally relate this geological event to a ' Late Laxfordian episode of retrograde metamorphism and folding * (p. 140). However, the specific relationship between the phase of granitic injection and the structural evolution of the polyphase deformed gneisses which they intrude is not given, while the assignment to the * Laxfordian episode' is based on the sequence of events and correlation of Dearnley (1963). Examination of the relationships of (1) the discordant dyke of massive granite west of Clett Nisabost [NA 035960—roadside to 034961—shore] from which the dated zircon (sample 17) of van Breemen et al. (1971) was taken and (2) the granitic dykes near Loch Meurach [NG 062880], the locality indicated for sample 1, has shown that these granitic masses were emplaccd between the fifth and sixth phases of folding recognized to have affected the Lewisian rocks of south Harris.

    At these localities (Fig. 1), the granitic dykes and sheets cut the gneissose

    fie. 1. Relationship of granitic masses of injection complex to structures in polyphase deformed gneisses, South Harris; (a), (b)—west of Clett Nisabost, (c) near Loch Meurach.

    foliation (Sj), intrafblial folds (F,), close to tight folds (FJ which deform the folia- tion, asymmetrical folds (F,), close to open and generally symmetrical folds (Fj and open symmetrical folds (Fs) with sub-vertical axial planes and axes that trend E.-W. Some granitic and some pegmatitic veins are sited in the hinge zones of F, folds while the general NW.-NNW. trend and moderate to steep dip of the margins of many of the granitic sheets correspond with that of the axial planes of F4 folds. The hinge zones of Ff folds appear to exercise no major control on the emplacement of the granitic masses which, together with the adjacent gneisses, are affected by open warps (Fg) which plunge NNE.-NE., or SSW.-SW., parallel to mullions. In places an axial planar fracture cleavage (Sc) is weakly developed.

    The structural sequence determined at both these localities within the injection complex of South Harris corresponds, with some variation in expression of the

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    different structural elements, with that determined for the Lewisian rocks adjacent to the complex, both in the metasediments to the south [NG 025949] and in the gneisses to the east [NG 116913]. It also corresponds with that determined for the rocks of the north-western part of the injection complex, in south-western Lewis [NA 992233], where the dominant phase of injection of quartzo-feldspathic material was also between the fifth and sixth recognized fold phases (Hopgood and Bowes, in press (a)). This indicates that the age of determination of van Breemen et al. (1971) for granitic sheets in South Harris can be used to indicate both the age of the injection complex in other parts and also the lower and upper limits, respectively, of the age of the fifth and sixth phases of folding in the Lewisian of the region.

    The structural sequence determined in, and adjacent to the injection complex corresponds with that determined away from the injection complex in various parts of Lewis, both at individual localities and over large mapped areas. It also corresponds with that determined throughout the Lewisian of the Outer Hebrides to the south (Hopgood and Bowes, in press (a)). Hence it can be deduced, with reasonable certainty, that an approximate date of 1750-1715 m.y. can be given for the pegmatites emplaced between the Hecla Point and Skipisdale fold phases in Mingulay (Bowes and Hopgood, 1969, p. 331; cf. Hopgood, 1971a, table II) and for the fifth-sixth fold periods in Barra (Hopgood, 19716).

    The very marked correspondence of structural sequences, as well as igneous and metamorphic histories, of the Lewisian rocks of the Outer Hebrides with those of Rona and of the Loch Laxford-Durness region in the Lewisian of the North-West Highlands and Islands is used on a basis for correlating between the Outer Hebrides and the mainland (and associated islands), including the type area for the Laxfordian orogeny (Hopgood, 1971d, table V; Hopgood and Bowes, in press (b)). The pegmatitic and granitic veins emplaced on Rona between the fifth and sixth fold phases (Gillen et ah, 1969, p. 53) and the pegmatitic veins associated with the Leacach phase of deformation north of Loch Laxford (Dash, 1969, p. 365; Chowdhary and Bowes, in press) are correlated with the quartzo- feldspathic intrusions of the injection complex of Harris. This correlation of structural sequence indicates that the age of c. 1750-1715 m.y. determined by van Breemen et al. (1971) does, in fact, represent the age of a late phase of the Laxfordian orogeny. This, together with an age of c. 1900 m.y. (Lambert and Holland, 1971, p. 251—recalculated using the exposed Rb87 half-life of 5*0 x 1010

    years) for an early phase (Gualin phase—cf. Dash, 1969) of the Laxfordian orogeny in the type area, means that an approximate time range for the Laxfordian orogeny has been established. As the Gualin phase is the second deformational phase recognized in the region north of Loch Laxford (Chowdhary and Bowes, in press), the Laxfordian orogeny in and adjacent to its type area thus embraces a number of phases of folding, metamorphism and igneous activity, and its effects are widespread (Bowes, 1971). This is also the case in the Outer Hebrides where

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    at least the second to the fifth of the recognized phases of deformation appear likely to represent the effects of this orogeny. The sixth phase, which affects the c. 1750-1715 m.y. granitic sheets, is also likely to be a related but very late phase, not much younger than the granites. The age of the earliest recognized set of folds (Fx) and the associated foliation (Sx) has yet to be convincingly demonstrated either in the Outer Hebrides or in the North-West Highlands and Islands. This and other geochronological aspects of the Lewisian are under active investigation in conjunction with the Isotope Geology Unit at the Scottish Research Reactor Centre.

    Dr. O. van Breemen, Dr. M. Aftalion and Dr. R. T. Pidgeon kindly made available their manuscript and maps to assist in the integration of structural history and age determinations.


    BOWES, D. R. 1971. Lewisian chronology. Scott. J. Geol. 7, 180-3. and H O P G O O D , A. M. 1969. The Lewisian gneiss complex of Mingulay, Outer Hebrides

    Scotland. In Larsen, L. H. (ed.), Igneous and Metamorphic Geology. Mem. Geol. Soc. Amer. 115, 317-60.

    CHOWDHARY, P. K. and BOWES, D . R. (in press). Structure of Lewisian rocks between Loch Inchard and Loch Laxford, Sutherland, Scotland. Krystalinikum, 9.

    DASH, B. 1969. Structure of the Lewisian rocks between Strath Dionard and Rhiconich, Sutherland, Scotland. Scott. J. Geol. 5, 347-74.

    DEARNLEY, R. 1963. The Lewisian complex of South Harris. Q.Jl. geol. Soc. Lond. 119, 243-312. GILLEN, C., BOWES, D. R. and H O P G O O D , A. M. 1969. Glasgow University Exploration

    Society Expedition to Rona, 1968 and 1969. Outcrop, Univ. of Glasgow, 2 , 49-56. H O P G O O D , A. M. 197la. Correlation by tectonic sequence in Precambrian gneiss terrains. Spec.

    Publ. Geol. Soc. Aust. 3 (in press). 1971b. Structure and tectonic history of Lewisian Gneiss, Isle of Barra, Scotland. Krystalinikum

    7,27-60. —— and BOWES, D . R. (in press (a)). Application of structural sequence to the correlation of

    Precambrian gneisses, Outer Hebrides, Scotland. Bull. geol. Soc. Am. (in press (&)). Correlation by structural sequence in Precambrian gneisses of north-western

    British Isles. Int. geol. Congr. Montreal, 1972. LAMBERT, R. ST. J. and HOLLAND, J. G. 1971. A geochronological study of the Lewisian of

    the Laxford area, north-west Scotland. Proc. geol. Soc. Lond. 1664, 251. van BREEMEN, O., AFTALION, M. and PIDGEON, R. T. 1971. The age of the granitic injection

    complex of Harris, Outer Hebrides. Scott. J. Geol. 7, 139-152.

    D. R. BOWES A. M. HOPGOOD Department of Geology, MARTHA B . TAFT The University, Department of Geology, Glasgow, W2 University of St Andrews,

    St Andrews, Fife

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