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Letter to the Editors
GRANITIC INJECTION COMPLEX OP HARRIS, OUTER HEBRIDES
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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290 LETTERS TO THE EDITORS
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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LETTERS TO THE E D I T O R S 291
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.
REFERENCES
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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