teristics of these magma types cf. Tarney and Jones, 1994, such as very high Sr and Ba, high
KRb ratios, very prominent Nb and Ti troughs and no significant Eu anomalies. On discrimina-
tion diagrams e.g. Rb vs NbY, they have a primitive arc signature that is confirmed by an
initial
87
Sr
86
Sr ratio of 0.7029 at 1.90 Ga. Bridg- water et al., 1997. This suggests a major juvenile
Proterozoic component added to the Gairloch sequence at : 1.90 Ga. These REE and multi-ele-
ment patterns differ so fundamentally from those of the metasediments Fig. 5e that there can be
negligible involvement of Gairloch schist in the petrogenesis of the Ard gneiss, a fact confirmed
by the more radiogenic Sr ratios of the metasedi- ment Bridgwater et al., 1997.
Calc-alkaline plutonic suites, ranging from diorites through tonalites to granodiorites, form a
volumetrically small but significant suite in the Palaeoproterozoic continent – continent collision
belts of the North Atlantic region, emplaced into both older sialic crust and Paleoproterozoic sedi-
ments. Emplacement ages range between 1.85 and 1.95 Ga e.g. Kalsbeek et al., 1987, 1993; Scott,
1995; Kalsbeek and Nutman, 1996.
3
.
4
.
2
. Tollie pegmatites Pegmatites that cut the Laxfordian D1D2 foli-
ation in the Archaean gneisses are widespread northeast of the supracrustal belt in the area of
the Tollie antiform. These bodies were emplaced after the main Laxfordian metamorphism and D2
deformation, since they are affected only by low- temperature D3 deformation. They, therefore,
play a critical role in the structural chronology. These pegmatites are rich in K and Rb, with
unusually low KRb ratios, relatively poor in Ba and Sr and have low levels of Li Holland and
Lambert, 1995. Like the pegmatites at Scourie in the northern zone, they have probably not been
derived from the local gneisses but from some underthrusted normal crustal material or sedi-
ment Tarney and Weaver, 1987.
Holland and Lambert also carried out RbSr isotopic analyses on a set of 13 samples of peg-
matite but the data do not form a single coherent isochron and are, therefore, difficult to interpret.
A selected nine-point isochron implies a crystalli- sation age of 1663 9 22 Ma with an initial
87
Sr
86
Sr ratio of 0.705, coincident with that of the Tollie gneisses. Some of the samples were taken
from the more deformed bodies on the southwest limb of the antiform but there is no correlation
between apparent age and amount of deforma- tion. Holland and Lambert considered that the
scatter of points might be the product of an open isotopic system involving variable loss of
87
Sr into the host gneisses. If this were the case and if there
were no comparable loss of Rb, the upper boundary of the data spread i.e. the nine-point
1663 Ma isochron would be expected to give the closest approximation to the age. However, even
if this isochron age can be relied upon, it probably indicates the date of the D3 deformation, rather
than the original crystallisation age of the peg- matite. A new precise zircon date of 1694 9 10
Ma is discussed below.
4. Palaeoproterozoic structure of the Gairloch area
Gneisses preserving pre-Laxfordian structures and metamorphic assemblages occur in the south-
west of the Gairloch area, around Loch Braigh Horrisdale forming part of the Ruadh Mheallan
block of Torridon and also to the northeast, in the Gruinard Bay area Fig. 2; Park et al., 1987.
The central part of the region consists of a belt of intense Palaeoproterozoic Laxfordian deforma-
tion that affects both the LMG and also a broad zone of basement gneisses and Scourie dykes on
each side of the LMG. This belt of deformation can be assumed to relate to the tectonic processes
that were responsible for the intercalation of the LMG slices within the basement complex.
Four phases of Laxfordian deformation D1 – D4 are recognised e.g. see Park et al., 1987 and
Table 1. The D1 and D2 structures represent ductile deformation that took place at mid-crustal
levels, whereas D3 and D4 are associated with retrogressive metamorphic events. D3 occurred at
low amphibolite- to greenschist-facies and af- fected much of the Gairloch area, in contrast to
D4 that occurred in more localised narrow belts at sub-greenschist facies.
Fig. 7. Simplified cross-section of the southern mainland Lewisian outcrop see Fig. 2 showing the likely relationship between the outcrops of the LMG and the location of the principal major folds and sheared regions.
Major Laxfordian
shear zones
occur at
Gairloch and Loch Torridon. The older Torridon shear zone also known as the Diabaig shear
zone is of D2 age, extends southwards to the southern margin of the Lewisian outcrop and is
refolded by the major NW – SE D3 folds Fig. 7. Its northeastern margin is inclined to the NE
beneath
the Ruadh
Mheallan block,
which exhibits Archaean structures and is little affected
by Laxfordian deformation. The younger, D3, Gairloch shear zone is about 5 km wide and
steeply dipping, extending southwest from the Tollie antiform and including the whole of the
Gairloch supracrustal outcrop Fig. 4.
4
.
1
. Laxfordian D
1
D
2
structure An S1 foliation is the first structure recognised
in the LMG and the Scourie dykes. The D1 and D2 fabrics are distinguishable only where F2 folds
are seen to affect earlier S1 foliation; however, such folds are uncommon in the supracrustal
rocks and the pervasive fabric produced in the combined D1 and D2 deformations is considered
to be a composite D1D2 fabric, probably related to progressive deformation. The D1D2 event is
associated with an amphibolite-facies metamor- phism that occurred probably close to 1.90 Ga as
constrained by the Ard gneiss date. Droop et al., 1999 assign peak metamorphic P-T conditions
of 530 – 630°C and 6.5 kbar to this event using various methods, including calcite – dolomite and
garnet – biotite geothermometers and the garnet – plagioclase geobarometer.
4
.
1
.
1
. Within the LMG The D1D2 fabric in the metasediments and
metabasites of the LMG is an intensely developed foliation typically accompanied by a strong linear
fabric defined by elongated mineral aggregates typically quartz and aligned minerals e.g. horn-
blende, mica intersection and microfold axes. This pervasive, high-strain, LS fabric occurs
throughout the supracrustal belt. Folds are not commonly associated with this fabric but are lo-
cally observed in the more siliceous lithologies such as BIF. The foliation dips uniformly steeply,
between 65° NE and vertical average 70 – 75° NE and the lineation plunges typically at moderate
angles 35 – 55° SE in the main part of the supracrustal belt. Mylonitic banding that occurs
in several narrow belts corresponds to the earliest fabric in the surrounding schists and is at the
same metamorphic grade. The sense of movement in these mylonitic belts, parallel to the strong
linear fabric, is sinistral, NE-up. This deformation is interpreted to represent the earliest movement
direction in the schist belt.
4
.
1
.
2
. SW of the LMG As the LMG is approached from the south, the
banding in the Archaean gneisses is gradually
replaced by a more strongly developed planar fabric Laxfordian S1, with a NW – SE strike
and steep dip, and a moderately SE-plunging elongation lineation Park et al., 1987. In a belt
:
700 m wide, the dykes are rotated from a steep, SW-dipping attitude to an average dip of 70 – 80°
NE. This fabric is still in amphibolite facies; dykes have
recrystallised to
a hornblende – calcic
plagioclase assemblage and are penetratively deformed throughout. Along the southwest side
of the LMG belt, the new fabric is associated with a dextral, S-up sense of movement.
4
.
1
.
3
. NE of the LMG In the region of low Laxfordian strain immedi-
ately southeast of Loch Tollie Fig. 3, steep NW – SE Inverian banding in the basement
gneisses is folded on sub-horizontal axial planes and affected by a gently dipping Laxfordian LS
fabric S2, also at amphibolite facies. A strong intersectionelongation lineation L2 plunges SE.
The dykes are divided into sectors that are alter- nately discordant to and sub-parallel with the
sub-horizontal fabric. The concordant sectors are more highly deformed and exhibit a well-devel-
oped foliation, whereas the steeper discordant sec- tors exhibit a mainly linear fabric. The sense of
shear implied by these more highly strained sec- tors is top to the west.
4
.
1
.
4
. Interpretation The D1D2 structure of the Gairloch area is
interpreted by Park et al. 1987 as a major, originally low-angle, shear zone, including both
the supracrustal schist belt and the neighbouring basement gneisses. The present steep attitude of
the schist belt is ascribed to D3 refolding see below. The high-grade D1D2 fabrics are devel-
oped regionally and show a consistent NW – SE elongation lineation interpreted by Coward and
Park 1987 as the movement direction on a wide- spread sub-horizontal shear zone network.
Both dextral SW-up and sinistral NE-up shear sense are recorded in the Gairloch area, and also
in Torridon to the south Wheeler et al., 1987; Niamatullah and Park, 1990. However, the
prevalence of a sinistral N-up sense of movement on the early mylonites suggests that the main
D1D2 movement was top to WNW on planes inclined gently NE. The dextral SW-up shear
sense, which relates to presently NE-dipping zones, and is therefore extensional in their pre-D3
orientation, may be due to extensional collapse of overthickened crust resulting from the D1D2
event.
4
.
2
. Laxfordian D
3
and D
4
structures The later part of the Laxfordian deformation is
divided into two phases, D3 and D4. The major Gairloch shear zone and associated Tollie an-
tiform, together with the Letterewe synform and Carnmore antiform to the north Figs. 2 and 7
have been ascribed to D3 Odling, 1984; Park et al., 1987. An upper limit for D3 is set at 1.69 Ga
from the post-D2, pre-D3 pegmatites. A cluster of KAr reset ages at around 1.7 Ga Moorbath and
Park, 1971 is also thought to reflect the D3 event, consistent within errors with the : 1.69 Ga upper
limit.
D4 includes a set of widely distributed steeply plunging folds, which deform the S1S2 and S3
fabrics but lack any associated new fabric Park, 1964; Bhattacharjee, 1968; Park et al., 1987. The
folds are typically small-scale cm – m in ampli- tude, open and chevron in style and typically
associated with narrow belts of cataclasis. A sec- ond cluster of KAr reset ages Moorbath and
Park, 1971 at around 1.5 Ga, ranging down to c 1.4 Ga, probably dates this more localised D4
deformation.
4
.
2
.
1
. D
3
structure The Tollie antiform Figs. 2, 3 and 7 is the
most prominent D3 structure in the Gairloch area Peach et al., 1907; Park, 1969; Park et al., 1987.
It has a moderately NE-dipping limb and steep to vertical SW limb. The plunge varies from near
horizontal in the northwest to over 60° SE in the southeast and the axial trace varies from NNW –
SSE to NW – SE. L2 is oblique to the F3 axes such that L2 lineations on the northeast limb of
the antiform plunge gently to the SE in contrast to the southwest limb where they plunge NW,
steepening to around 45° at the northeast margin of the supracrustal belt.
The Tollie antiform forms the northeast margin of the Gairloch shear zone Odling, 1984; Park et
al., 1987 which is about 5 km wide and occupies the greater part of the belt of intense Laxfordian
deformation in the Gairloch area. Within this shear zone, deformation is generally intense and
the foliation is steeply dipping and trends NW – SE. The shear zone is considered to have steep-
ened
the originally
gently NE-dipping
D2 structure into a near-vertical attitude. Since the
whole of the outcrop of this shear zone contains rocks already deformed in the earlier D1D2
deformations, it is often difficult to distinguish D3 effects from earlier fabrics. In the zone of highest
D3 strain in the SW limb of the Tollie antiform, the elongation lineations plunge consistently NW
at moderate angles and this direction is taken to be the shear direction of the shear zone Odling,
1984.
The D3 fabrics in the most highly strained gneisses indicate retrogression to greenschist fa-
cies. Feldspars are replaced by an assemblage of albite, microcline and epidote and myrmekitic
texture is widespread; biotite is developed axial planar to small-scale F3 folds.
5. UPb geochronology
