presently enigmatic from a regional point of view. Recent
40
Ar –
39
Ar geochronology on hornblendes from a variety of lithologies throughout the
northern Outer Hebrides constrains cooling of this part of the complex through ca. 500°C by
1.7 – 1.6 Ga Cliff et al., 1998 and dates the termination of late Laxfordian events.
3. Sampling
The four samples studied here occur within, or close to, the major Paleoproterozoic belts of Ness,
northern Lewis, and South Harris Fig. 1. Diorite sample DB96-S117 was collected from accessible
cliff top outcrops south – east of the Butt of Lewis
2
lighthouse Great Britain national grid reference, NB 521 664 which expose a suite of
garnet-bearing dioritic rocks with marked catacla- sis, both in hand specimen and thin section. The
cataclasis, assumed to be related to ? Caledonian movements on the nearby Outer Isles fault, clearly
postdates
open ductile
folding ?Paleoprot-
erozoic, which is highlighted by a series of more leucocratic layers, from which sample DB97-S117
was collected. Tonalite sample DB96-S144 was collected at
the southern end of the coastal section through the Langavat meta-supracrustal belt, which forms
the northern margin of the South Harris igneous complex with the granite-gneiss terrane. At Ba`gh
Steinigidh NG 019939 marbles and semi-pelitic schists of the Langavat belt are exposed, together
with highly deformed diorite presumed part of the South Harris complex which has an ambigu-
ous relationship to the sediments. Immediately to the north of a fence running perpendicular to the
coastline, the tonalite is exposed truncating band- ing in the diorite, with which it shares a common
foliation; no earlier fabric is discernable in the diorite. The tonalite also contains discreet inclu-
sions of diorite, and it is possible that these rocks were co-magmatic.
Psammite MJW97-SH30 was collected from a thin B 20 cm band in the Leverburgh belt,
South Harris, the most extensive sequence of rocks of presumed supracrustal origin in the
Lewisian. At the sampling locality at the northern end of Tra`igh na Cleabhaig NF 979912, a se-
quence of psammites, pelites, semi-pelites, and calc-silicates are particularly well-exposed. Many
of the pelitic and psammitic lithologies, including the present sample, display pale-pink garnets, and
kyanite is present in the pelites as large laths.
Metasedimentary rocks displaying similar asso- ciations to the Langavat belt are exposed on the
islands of Pabbay, Killegray, and Berneray in the Sound of Harris, immediately to the south of the
South Harris complex Fettes et al., 1992; Fig. 1. Also exposed are meta-igneous rocks, including a
suite of diorites and tonalites visible around high- water mark in Loch a Bha`igh, Berneray. The
lithological association suggests possible correla- tion to the South Harris igneous complex, but
strong deformation in a NW – SE shear zone run-
Fig. 1. Simplified geological map of the northern Outer He- brides modified after Fettes et al., 1992 showing principal
geological features discussed in the text and sample localities. Abbreviations — WGC, Western Gneiss complex; ECG, East-
ern Gneiss complex.
2
In this paper, we use spelling of place names in the language, in which they appear on the most recent 1:50 000
topographical maps of the region Great Britain Ordnance Survey.
Fig. 2. Cathodoluminescence images of selected grains from Butt of Lewis diorite sample DB96-S117.
enhance secondary ion yields of
206
Pb to ca. 15 – 20 cps nA
− 1
ppm
− 1
Whitehouse et al., 1997a. PbU ratios were calibrated relative to the 1065
Ma Geostandards zircon 91500 Wiedenbeck et al., 1995 using a power law relationship between
PbU and UO
2
U ratios. Corrections for common Pb assume that this is largely derived from surface
contamination during polishing of grain mounts e.g. introduction into micro-cracks; this is moni-
tored using
204
Pb, with the correction assuming a present day terrestrial Pb-isotopic composition
Stacey and Kramers, 1975. During the course of this study, 69 analyses of the 91500 standard
zircon yielded a weighted average
207
Pb
206
Pb age of 1063 9 9 Ma MSWD = 3.2, no rejections and
1059 9 6 Ma MSWD = 0.5, n = 60. Analytical data and derived ages are presented in Table 1 all
ages are calculated with the decay constants of Steiger and Ja¨ger 1977. All weighted average
and regression ages have been calculated using IsoplotEx Ludwig, 1998, with errors reported at
the 95 ca. 2s confidence level.
5. Results and interpretation
