conditions that would reset Ar – Ar systematics in biotite. Analyses of the main, finely banded zircon
phase define a present-day Pb-loss trajectory, with a weighted average
207
Pb
206
Pb age of 2834 9 9 Ma MSWD = 3.5, n = 10. This age is interpreted
as the igneous protolith age for the tonalite, clearly placing it in the early complex. Signifi-
cantly, there is no record either of Paleoproterozic South Harris equivalent age events, or late-
Archeanearly-Proterozoic events corresponding to the ca. 2.5 Ga Inverian event of the mainland
central region.
6. Discussion
6
.
1
. Paleoproterozoic e6olution of the Outer Hebrides
The geochronological data presented here, to- gether with previously published data from the
South Harris igneous complex Cliff et al., 1983, 1998 indicate a ca. 1.87 – 1.83 Ga high-grade tec-
tonothermal event in the Paleoproterozoic belts of the northern Outer Hebrides Lewis and Harris.
This event is not recorded south of the Sound of Harris, where zircons from an Archean tonalite
on Berneray show no evidence for recrystallisa- tion or disturbance Pb-loss at this time. Data
from a Leverburgh belt psammite suggest deriva- tion of clastic material largely from a ca. 1.87 Ga
magmatic precursor, in agreement with the obser- vation of Cliff et al. 1998; Sm – Nd model age
data that this belt must contain a ‘significant post-Archean component’. Given geochemical evi-
dence for an andesitic arc character to the South Harris igneous complex e.g. Fettes et al., 1992;
Bridgwater et al., 1997, and lithological evidence that the Leverburgh belt represents an accretion-
ary wedge Baba, 1997, a possible tectonic sce- nario emerges, in which the rocks of South Harris
represent a magmatic arc, complete with contem- poraneously derived clastic sediments, developed
in a collisional orogen, which culminated in gran- ulite facies metamorphism. This metamorphism is
recorded in contemporaneous shear zones at Ness, and possibly in many of the Archean
gneisses throughout the Outer Hedrides as cryp- tic isotopic signatures, see discussion in Section
2.3. In this case, South Harris should be regarded as a major Paleoproterozoic active margin and
tectonic boundary within the Lewisian.
6
.
2
. Correlations within the Lewisian Several studies have attempted to correlate the
Lewisian of the Outer Hebrides with that of the mainland see discussion in Coward and Park,
1987 and their figures 2 and 9. These are based primarily upon matching the major Paleoprotero-
zoic shear zones Fig. 8 but, as pointed out by Coward and Park 1987, the most obvious corre-
lation of the South Harris SHSZ and Gairloch shear zones GSZ, Fig. 8a is not supported by
the geology of the flanking regions, and the alter- native correlation of the SHSZ with the Loch
Laxford shear zone LSZ, Fig. 8b requires a large strike-slip displacement along the Permo-Triassic
Minch fault. These authors prefer a model, in which the Outer Hebridean Lewisian represents a
large-scale shear zone flat acting as a detachment zone, against which the major structures die out.
In this model, there would be no a priori reason for any of the mainland structures to correlate
with those of the Outer Hebrides and hence, no requirement for Minch strike-slip, although gen-
eration of structures in both blocks in the same stress field would produce similar orientations.
The status of the SHSZ and related rocks as a major tectonic boundary representing a Pale-
oproterozoic collisional orogen must now be con- sidered in Lewisian correlations. In the mainland
Lewisian outcrop, there are two major tectonic boundaries, 1 the LSZ separates the northern
and central blocks; and 2 the GSZ separates the central and southern blocks. The profound nature
of the tectonic break across the LSZ is highlighted by Sm – Nd isotopic Whitehouse, 1989 and U –
Pb geochronological Kinny and Friend, 1997 data, which indicate a contrast both in protolith
age ca. 2.8 – 2.84 northern region; ca. 2.95 – 3 Ga central region and subsequent metamorphic his-
tory ca. 2.5 Ga granulite facies in central region; absent ? in northern region. These two terranes
were apparently juxtaposed after late Laxfordian ca. 1.75 Ga granite emplacement in the northern
Fig. 8. Possible reconstructions of the mainland and Outer Hebridean Lewisian complex across the Minch fault, showing presently available geochronological constraints on the major crustal blocks adapted from Coward and Park, 1987; see their 9 for structural
and geological detail. The Outer Hebridean Lewisian is separated into a northern NOH and southern SOH block by the South Harris shear zone SHSZ; the mainland Lewisian is divided into northern NR, central CR and southern SR regions by the
Laxford shear zone LSZ and Gairloch Shear zone GSZ. Ages are presented in Ga; t
p
represents protolith age, t
ml
represents the time of metamorphism in the early i.e. pre-dyke complex. Abbreviations in parentheses after ages indicate method used — n,
Sm – Nd model age; iz, ion-microprobe zircon; cz, conventional zircon. Refer to text for detailed discussion.
region equivalent granites are absent in the cen- tral region, but prior to a common ca. 1.73 Ga
event recorded in titanites in both regions Kinny and Friend, 1997. No juvenile Paleoproterozoic
rocks of arc affinity have, however, been recog- nised in the LSZ. A similar contrast is apparent
across the GSZ, with a protolith age of ca. 2.8 Ga for the southern region gneisses Chamberlain et
al., 1986; conventional U – Pb and whole-rock Sm – Nd suggesting another distinct gneiss ter-
rane. Unlike the LSZ, the GSZ contains evidence for juvenile Paleoproterozoic arc-magmatic activ-
ity in the Ard Gneisses 1903 9 3 Ma, J.N. Con- nolly, personal communication; Bridgwater et al.,
1997, together with contemporaneous B 2.0 Ga metasediments Whitehouse et al., 1997a, and
would appear to represent a magmatic arc devel- oped at an active margin.
There is insufficient data currently available from the gneiss terranes of the Outer Hebrides to
constrain correlations unequivocally. Correlation of the SHSZ with the LSZ would imply that rocks
equivalent to the mainland central region, with protolith ages of ca. 2.9 – 3.0 Ga, should be found
immediately to the south. Zircon geochronology from the Loch a Bha`igh tonalite this study, and
Sm – Nd data from the southern Outer Hebrides South Uist, Whitehouse, 1990a, however, indi-
cate a ca. 2.8 Ga protolith age, with no apparent early ca. 2.5 Ga Inverian granulite facies meta-
morphism as recorded in the central region. To the north of South Harris, the few available Sm –
Nd model ages Whitehouse, 1987; Cliff et al., 1998 indicate a similar, albeit poorly constrained,
protolith age to that of the northern region ca. 2.8 Ga, and the relationship of extensive granite
magmatism occurring north of the tectonic boundary only is similar to the LSZ. The alterna-
tive correlation of the SHSZ with the GSZ is more favourable in terms of evidence for a Pale-
oproterozoic magmatic arc of similar age ca. 1.9 Ga, but this would make the gneisses of Lewis
and Harris the equivalents of the mainland central region, which is not strongly supported either by
to date, limited protolith age studies, or appar- ent metamorphic history. Correlation of mainland
southern region and the southern Outer Hebrides implied by this fit is, however, more favourable.
Correlation of tectonic boundaries and gneiss terranes in the Lewisian must also account for the
possibility of extensive strike-slip motion on the major sutures or transforms. This was suggested
by Whitehouse et al. 1997a as a mechanism for removing the inferred magmatic arc rocks from
the
present location
of the
Loch Maree
supracrustals and the GSZ. Such strike-slip mo- tion could also account for differences observed in
the flanking gneiss regions whether the SHSZ correlates with the LSZ or the GSZ by the devel-
opment of thrusts andor extensional detachments parallel to the transport direction. Such structures
could readily introduce contrasting terrane fea- tures along boundaries perpendicular to the major
transforms. An example of this might be the juxtaposition
across the
Outer Isles
fault ?reactivated in the Laxfordian, Lailey et al.,
1989 of the high-grade granulite-facies Coro- dale Gneisses of South Uist with the lower grade
Western Gneisses.
Correlation of gneiss terranes between the mainland and Outer Hebridean Lewisian outcrops
thus remains problematical. The observations from this study supporting a collisional orogen in
South Harris, together with similar observations from Gairloch Whitehouse et al., 1997a, and the
work of Kinny and Friend 1997, require a num- ber of major tectonic breaks, which must be con-
sidered in correlation models as further data from the flanking gneiss terranes becomes available.
6
.
3
. Regional implications Park 1994, in his table 1, presents a compari-
son of tectonic data from Paleoproterozoic belts throughout
Laurentia and
Fennoscandia, in
which the Lewisian, together with the Ammassa- lik belt of East Greenland is interpreted as an
intra-continental rift at ca. 1.87 Ga, while all other belts show evidence for development of
magmatic arcs and collisional orogens at this time. Identification of South Harris this study
and Gairloch Whitehouse et al., 1997a as mag- matic arcs in collisional orogens brings their evo-
lution into line with other Paleoproterozoic belts of this region and requires a reappraisal of the
tectonic setting of the Lewisian at this time.
7. Concluding remarks
