Precambrian Research 105 (2001) 227 – 245
Geochronological constraints on Paleoproterozoic crustal
evolution and regional correlations of the northern Outer
Hebridean Lewisian complex, Scotland
Martin J. Whitehouse
a,*, David Bridgwater
b,1aSwedish Museum of Natural History,Box50 007,SE-104 05Stockholm,Sweden bGeological Museum,Copenhagen Uni
6ersity,DK-1350Copenhagen,Denmark
Received 12 January 1999; accepted 23 June 1999
Abstract
Ion-microprobe U – Th – Pb geochronological data are presented for four samples from Paleoproterozoic belts in the Lewisian of the northern Outer Hebrides, north-west Scotland. Two of these samples, a tonalite sheet associated with the South Harris igneous complex, and a psammite from the Leverburgh metasupracrustal belt, South Harris, yield zircons with a dominant ca. 1.87 Ga age. These are interpreted as the igneous crystallisation age for the tonalite and the source rock for the psammite, and their age concordance suggests that the latter was developed in an arc basin sequence, derived largely from contemporaneous igneous rocks, and buried during collision, which resulted in documented \1.83 Ga high-grade metamorphism. A diorite from the Paleoproterozoic shear zone at the northern tip of Lewis has a probable 2.7 – 2.8 Ga protolith age, although its zircons have strongly been affected by Pb-loss during later events culminating in development of low Th/U overgrowths at ca. 1.86 Ga. Zircons from a tonalite from Berneray in the Sound of Harris yield an Archean crystallisation age of ca. 2.83 Ga, with no indication of later disturbance, thus providing a southern limit to the region affected by Paleoproterozoic tectonothermal events. The Paleoproterozoic arc in South Harris represents a major tectonic boundary (active margin) in the Lewisian of the Outer Hebrides, possibly correlated with the Laxford or Gairloch shear zones of the mainland Lewisian. Contrasts in the flanking region geology and geochronology, possibly reflecting lateral heterogeneities, may be introduced by major thrusts and/or extensional faults (e.g. the Outer Isles fault) developed between the shear zones. On a broader regional scale, evidence for a magmatic arc in the Lewisian is consistent with the tectonic style of other ca. 1.9 Ga Paleoproterozoic collisional orogens throughout Laurentia – Fennoscandia, suggesting a reappraisal of the formerly proposed intracratonic evolution of the Lewisian at this time. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:Lewisian; Paleoproterozoic; Absolute age (U – Pb zircon); Laurentia – Fennoscandia
www.elsevier.com/locate/precamres
* Corresponding author. Fax: +46-8-51954031.
E-mail address: martin.whitehouse@nrm.se (M.J. White-house).
1Deceased.
1. Introduction
The Lewisian complex is exposed as the fore-land to the Caledonian orogen in north-west
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 228
mainland Britain, and throughout the ca. 200 km length of the Outer Hebrides archipelago. Broadly similar, late-Archean tonalite – trondhjemite – gra-nodiorite (TTG) suite grey gneisses dominate these major outcrops, together with Paleoprotero-zoic mafic intrusions (Scourie dykes and related units), and substantial post-Scourie dyke struc-tural reworking and granite intrusion. On a re-gional basis, these same broad features suggest correlations to the Archean cratons of Laurentia (East Greenland and North Atlantic cratons) and Fennoscandia (or Baltica; Karelian craton), with their respective Paleoproterozoic reworking (Nag-suggtoquidian and Svecokarelian and crust gener-ation (Ketilidian and Svecofennian) events.
The Outer Hebridean Lewisian has not at-tracted the same intensive geochemical and geochronological investigations that have been carried out on the mainland over the past three decades, although there is a wealth of literature on structural aspects of the widespread Pale-oproterozoic events (see summaries by Fettes et al., 1992; Park et al., 1994). As a result, attempts to correlate the two main outcrops of the Lewisian remain speculative (e.g. Coward and Park, 1987). Recent geochronological and isotope geochemical studies of the mainland Lewisian (Whitehouse, 1989; Kinny and Friend, 1997; Whitehouse et al., 1997a) and the Outer Hebrides (Whitehouse, 1990a; Cliff et al., 1998) provide a better database for such correlations, both within the Lewisian and in a wider regional context. In this paper, we present new U – Pb zircon geochronology (ion-microprobe) for four rocks from key localities in the northern Outer Hebrides which, together with a synthesis of existing iso-topic and geochronological data, permit a better constrained evaluation of the Paleoproterozoic evolution and possible correlations.
2. The Outer Hebridean Lewisian complex
2.1. Early geological in6estigations — the basic
framework
The first modern accounts of the geology of the Outer Hebrides were presented in a series of
pa-pers by Jehu and Craig (1923, 1925, 1926, 1927, 1934). Further detailed mapping was carried out by Dearnley (1962) who used a suite of mafic dykes, tentatively correlated with the Scourie dykes of the mainland (the latter now known to consist of at least two suites intruded from ca. 2.4 – 2.0 Ga), to divide the Precambrian history of the complex into pre-dyke (‘Scourian’, by analogy with the pioneering mainland Lewisian study of Sutton and Watson (1951); in this paper, we note the increasing subdivision of the pre-dyke main-land complex and prefer to use the general term ‘early complex’) and post-dyke (‘Laxfordian’) periods.
2.2. The early complex
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 229
Whitehouse, 1990a) confirm a late-Archean age for these gneisses, but mostly with large errors and, to date, no modern U – Pb zircon ages (e.g. ion-mi-croprobe or small population conventional) have been reported. Sm – Nd tDM model ages of ca. 2.75 – 2.83 Ga for TTG gneisses from South Uist (Whitehouse, 1990a), have been interpreted as the age of the igneous protolith at ca. 2.8 Ga. These gneisses record extreme large-ion lithophile element (LILE) depletion resulting in an apparent (and clearly spurious) ca. 3.5 Ga Pb – Pb regression age, which has been interpreted as the result of extreme U-depletion at 1.8890.27 Ga (Whitehouse, 1990a). The remainder of the early gneiss complex, particularly in the northern islands of Lewis and Harris where it is poorly exposed, has received little geochronological attention, in part because exten-sive Paleoproterozoic reworking and granite injec-tion has strongly affected this region, obscuring the early history of the complex. Cliff et al. (1998) present three Sm – NdtDMmodel ages from North Harris and South – West Lewis in the range 2.60 – 2.76 Ga, andtDM’s from 2.52 – 2.84 Ga have been reported from North Harris gneisses (Whitehouse, 1987). The large range oftDMages probably reflects later disturbance, since most of these samples occur within the area affected by later granite injection, hence these ages are considered a less reliable indicator of TTG protolith age than those of South Uist.
2.3. Laxfordian reworking
Laxfordian structural modification of the early complex and its dykes is present throughout the Outer Hebrides in the from of regionally penetra-tive deformation, both on gently inclined and steep north-west trending axial planes, although the amount of strain is variable (Fettes and Mendum, 1987). A particularly interesting aspect of this reworking is the recognition of distinct early- and late-Laxfordian metamorphic events (Dearnley, 1962), in contrast to the mainland where only a single reworking episode is generally recognised. As proposed by Dearnley (1962, 1973), the early Lax-fordian involved granulite facies metamorphism, evidence for which is preserved in Scourie dykes in the low-strain areas of the southern Outer
He-brides, and in the South Harris complex (Dearnley, 1963, 1973), the latter dated at 1.8790.04 Ga by Cliff et al. (1983) and further constrained to \ 1.82790.016 Ga (Cliff et al., 1998; both ages from mineral Sm – Nd isochrons). The evidence for gran-ulite facies assemblages in the Scourie dykes has been disputed by Fettes et al. (1992) who suggest that it may, instead, represent an original crystalli-sation feature. Despite this, the 1.8890.27 Ga Pb – Pb model regression age for U-depletion of the southern Outer Hebridean gneisses (Whitehouse, 1990a), although insufficiently precise to be corre-lated directly with well-dated events in South Harris is, nonetheless, more likely to reflect early Laxfordian high-grade (?granulite facies) metamor-phism and (probable) associated LILE depletion than late-Laxfordian events, which are character-ised by granite injection and retrogression. A sim-ilar, again imprecise, 1.8690.24 Pb – Pb regression age from the early Proterozoic anorthosite at Ness, Lewis (Whitehouse, 1990b) is also more consistent with a high-grade metamorphic event capable of resetting U – Pb systematics. Development of the South Harris igneous complex also falls within the broad early Laxfordian framework, with ultra-mafic-anorthositic intrusion at ca. 2.2 – 2.0 Ga (Cliff et al., 1983), possibly overlapping some of the Scourie dyke suite intrusions, and later calc-alka-line diorites and tonalites (ca. 2.04 – 1.86 Ga, Cliff et al., 1983). The early Laxfordian is, therefore, recorded geochronologically by direct dates in South Harris and possibly by cryptic isotopic signatures throughout the Outer Hebrides.
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 230
presently enigmatic from a regional point of view. Recent 40Ar –39Ar 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 Lewis2 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 (B20 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.
2In this paper, we use spelling of place names in the
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 231
Fig. 2. Cathodoluminescence images of selected grains from Butt of Lewis diorite sample DB96-S117.
enhance secondary ion yields of 206Pb to ca. 15 – 20 cps nA−1 ppm−1 (Whitehouse et al., 1997a). Pb/U ratios were calibrated relative to the 1065 Ma Geostandards zircon 91500 (Wiedenbeck et al., 1995) using a power law relationship between Pb/U and UO2/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 204Pb, 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 106399 Ma (MSWD=3.2, no rejections) and 105996 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 Isoplot/Ex (Ludwig, 1998), with errors reported at the 95% (ca. 2s) confidence level.
5. Results and interpretation
5.1. Butt of Lewis diorite, Lewis, DB96-S117
Zircons from the Butt of Lewis diorite sample range in size from ca. 150 – 500 mm and in mor-phology from anhedral, almost spherical, to euhe-dral bi-pyramidal grains. Some rounding of the crystal faces and vertices of all grains probably results from cataclasis and/or high-grade meta-morphism. Cathodoluminesence (CL) imaging of these grains (Fig. 2) reveals complex internal structures reflecting a number of growth and/or reworking phases. Most grains exhibit two or three growth phases, within which CL images identify CL dark, zoned ‘cores,’ which are par-tially resorbed/reworked (rounded margin and truncation of zoning) by a structureless CL bright phase, itself rounded (‘core overgrowth’). Three phase grains show both CL dark and CL bright pyramidal terminations (‘rims’), some of which show zoning. These CL bright terminations are developed on a thin (too small to analyse) CL dark phase.
ning through Berneray, precludes unambiguous assignment to Archean or Proterozoic crust gener-ation episodes. Sample MJW97-Ber43 (locality, NF 927819) is a tonalitic rock, which has been isoclinally folded together with flaggy diorites, but clearly cuts an earlier foliation in the diorite, and thus, represents a later phase (cf. Ba`gh Steinigidh ?co-magmatic rocks). This sample was collected in order to test whether Paleoproterozoic calc-alka-line magmatism, similar to that seen in the South Harris igneous complex, extends south into the Sound of Harris.
4. Analytical methods
M
U–Th–Pb analytical data and derived parametersa
207Pb/206Pb
0.193290.0015 -34 167695 113898 0.102890.0003
378
25b 81 0.005 0.49
0.109290.0031
29 11 0.014 3.28 0.325890.0035 1787952 1818917
15a*
7 3 0.013 0.111790.0027 0.312790.0029 -3 1827944 1754914
29e* 3.73
0.268490.0030 -17 1839957 1533915 0.112490.0035
0.015 5.97
29e rpt.* 43 15
0.112590.0006
345 125 0.006 0.32 0.324690.0035 1840910 1812917
31a
0.112890.0008
140 52 0.008 0.16 0.333590.0039 1845913 1855919
10b
0.328390.0030 184695 1830914
0.112990.0003
4a 1258 462 0.001 0.24
0.113290.0005
677 250 0.001 0.26 0.329690.0075 185197 1836937
22
0.329890.0041 1854914 1838920
0.113490.0009 134
31a rpt. 125 0.006 0.15
0.113790.0011
169 63 0.007 0.11 0.334590.0039 1860918 1860919
7b*
0.321990.0044 -1 1862915 1799921
7b rpt.* 70 25 0.006 0.02 0.113890.0010
0.326390.0032 -1 1864914 1820916 0.114090.0009
115
2 42 0.008 0.04
0.114590.0010
67 26 0.008 0.06 0.342490.0042 1873915 1898920
10b rpt.
0.06 0.114990.0004 0.340790.0032 187997 1890915
11a* 557 212 0.004
0.347990.0033 1 188397 1924916
0.115290.0005 0.002
53
19a* 137 0.09
0.117690.0053
9 4 0.011 5.50 0.325790.0025 -5 1920981 1818912
26c
0.43 0.130490.0017 0.366690.0037 -3 2103923 2013917
13 115 57 0.644
(ii)Cores,two and three phase grains
0.223390.0114 -34 2040936 1299960 0.125890.0026
0.121 1.63
32 270 73
0.138990.0005
921 397 0.301 0.06 0.350990.0039 -12 221396 1939918
31b
0.386390.00028 -6 2256911 2106913
17b* 21 11 0.664 0.50 0.142490.0009
0.348790.0045 -15 227598 1928922 0.143990.0006
398
31b rpt. 172 0.318 0.07
0.152790.0009
18 9 0.586 0.44 0.401590.0030 -9 2376910 2176914
28a
0.157190.0037
806 459 0.687 2.12 0.408290.0044 -9 2425940 2207920
10d
0.434490.0034 -5 245099 2326915
0.159590.0008 0.550 0.12
4b 25 14
0.162390.0010
261 160 0.680 2.03 0.439390.0055 -4 2479910 2347925
10d rpt.
0.163590.0006
26 16 0.219 0.06 0.490090.0041 2 249297 2571918
29b*
0.449990.0033 -4 250795 2395915
0.165090.0005
27a 85 47 0.154 0.89
0.06 0.165190.0008 0.471990.0109 250998 2492948
24* 237 156 0.740
0.455190.0042 -3 251798 2418919
0.166090.0008 201
14 121 0.589 0.05
0.167190.0008
526 301 0.749 0.08 0.410390.0029 -13 252898 2216913
30a rpt.*
0.167290.0014
1770 933 0.042 0.33 0.444690.0053 -5 2530915 2371924
30b rpt.*
0.487690.0112 253898 2560948
0.168090.0008
20 200 150 1.214 0.04
0.168990.0015
1033 548 0.026 0.12 0.449090.0031 -6 2547915 2391914
30b rpt.*
0.438790.0034 -8 254898 2345915
0.169190.0008 0.203
153
29b rpt.* 283 0.05
0.169590.0012
88 51 0.337 0.62 0.447290.0057 -6 2552912 2383926
15b rpt.*
0.17 0.171390.0015 0.493590.0055 2570915 2586924
M
Table 1 (Continued)
[U] ppm [Pb] ppm Th/U meas. 207Pb/206Pb 206Pb/238U Discordanced(%) 207Pb/206Pb 206Pb/238U
Sample/spotb f
206c(%)
(Ma9s) (Ma9s)
21b* 128 125 3.074 0.14 0.174790.0014 0.485990.0357 2603914 25539155
0.507090.0055 2608915 2644924
0.175290.0015 228
15b* 148 0.370 0.24
0.176390.0015
116 74 0.340 0.23 0.506190.0389 2618915 26409167
6b*
0.24 0.177290.0022 0.452690.0036 -9 2627920 2407916 82
12b 60 1.481
0.464890.0042 -6 263498 2461918
0.178090.0009 0.754 0.29
5 141 92
0.179990.0010
239 166 0.674 0.04 0.503490.0046 265299 2628920
7c rpt.*
0.182890.0022
27 16 0.260 0.28 0.484490.0037 -5 2678920 2546916
26b
0.505390.0055 2679914 2636924
0.182890.0015
8 232 166 0.821 0.07
0.03 0.183090.0005 0.496190.0062 -1 268095 2597927
11b rpt.* 179 145 1.660
0.493790.0036 -3 269299 2587915
0.184390.0010 21
11b* 18 1.796 0.07
0.187190.0012
860 641 0.950 0.04 0.515990.0058 2717910 2682925
7c*
19b* 68 49 0.451 0.80 0.199590.0026 0.540990.0069 2822922 2787929
0.558090.0069 2843914 2858929
0.202190.0017 20
27
19b rpt.* 0.460 0.76
(iii)Core o6ergrowths,two and three phase grains
0.367590.0054 -5 2162933
10c 87 42 0.614 0.37 0.134890.0026 2018925
0.368290.0063 -8 2226921 2021930 0.139990.0017
0.202 0.39
16 12 5
0.143990.0018
3 1 0.102 0.65 0.425590.0036 2275922 2286916
27b
0.147190.0025
54 26 0.146 0.42 0.396690.0043 -6 2312930 2153920
30c*
0.383590.0075 -8 2329927 2093935 0.148590.0024
30c rpt.* 20 9 0.137 0.32
0.86 0.154090.0050 0.377390.0055 -14 2391956 2064926 50
10c rpt. 26 0.569
0.447890.0047 2392919 2386921
0.154190.0018 0.056 0.25
26a 2 1
0.156290.0041
2 1 0.149 3.11 0.492290.0038 7 2415944 2580916
29c*
0.484090.0061 3 2426912 2545926
15c rpt.* 53 30 0.078 0.05 0.157290.0011
0.452990.0045 2434941 2408920
0.158090.0038 3
29a* 2 0.165 1.52
0.162290.0010
120 70 0.310 1.68 0.454790.0105 2479911 2416947
25a
0.164090.0046
7 3 0.030 0.29 0.420290.0084 -8 2497948 2261938
3
0.473990.0054 2530911 2500924
0.167290.0011 0.364 0.19
15c* 120 73
0.167590.0015
51 38 1.134 0.11 0.484690.0045 2533915 2547920
17a*
0.167890.0015
27 21 1.347 0.25 0.489090.0124 2536915 2566954
21a*
0.469090.0044 -4 2597913 2479919 0.174090.0013
18 146 96 0.711 0.02
0.28 0.175490.0026 0.442890.0040 -10 2609924 2363918 29
9 19 0.961
0.481090.0048 -3 262797 2531921
0.177290.0008 0.17
1 56 34 0.261
23* 67 48 0.954 0.12 0.181090.0015 0.489290.0113 2662914 2567949
Ba`gh Steinigie tonalite,South Harris,DB96-S144
0.02 0.111990.0012 0.342690.0064 0 1831919 1899931 227
16a 550 0.359
0.347590.0026 3 185098 1922913
0.113190.0005
5a 281 111 0.096 0.04
59 22 0.127 0.113790.0012 0.321890.0024 -2 1859919 1799912
11a 0.23
0.333190.0066 1863911 1853932
0.114090.0007 0.145
215
20a 551 0.56
0.114090.0004
1088 350 0.233 0.38 0.270190.0024 -18 186396 1541912
12a
0.01 0.114290.0004 0.334190.0061 186896 1858930
M
Table 1 (Continued)
[U] ppm [Pb] ppm Th/U meas. 207Pb/206Pb 206Pb/238U Discordanced(%) 207Pb/206Pb 206Pb/238U
Sample/spotb f
206c(%)
(Ma9s) (Ma9s)
0.333290.0062
14a 591 236 0.299 0.02 0.114390.0005 186997 1854930
0.343090.0026 0 186995 1901912
0.114390.0003 624
10a 264 0.410 0.03
0.114490.0006
653 272 0.378 0.04 0.342290.0064 187099 1897931
23a
0.01 0.114590.0004 0.345390.0061 187197 1912929 510
19a 212 0.312
0.343090.0031 187497 1901915
0.114690.0004 0.286 0.05
9a 443 182
0.114790.0005
506 203 0.355 0.04 0.331090.0061 187598 1843930
13a
0.114790.0004
521 219 0.283 0.01 0.353290.0065 1 187697 1950931
21a
0.338590.0061 187696 1879929
0.114890.0004
18a 650 264 0.326 0.00
0.02 0.114990.0007 0.339590.0026 1879911 1884912 232
1a* 91 0.134
0.345390.0061 187995 1912929
0.115090.0003 976
17a 410 0.367 0.02
0.115090.0002
1661 685 0.335 0.01 0.341790.0065 188094 1895931
25a
0.115190.0005
685 265 0.314 0.04 0.320690.0057 -2 188298 1793928
24a
0.333090.0061 188397 1853929
0.115290.0004
15a 646 260 0.344 0.01
0.334690.0026 0 1887916 1861913
1b* 92 36 0.139 0.28 0.115590.0010
0.323790.0024 -4 189298 1808912
0.115890.0005 0.090 0.22
7a 315 117
0.116090.0005
305 119 0.095 0.11 0.341090.0025 189698 1892912
2a*
0.116390.0005
388 138 0.067 0.12 0.313290.0024 -7 189997 1757912
8a
0.348390.0026 191096 1926912
0.116990.0004
3b* 518 206 0.085 0.02
0.428290.0032 -8 2498910
6a 476 268 0.514 0.10 0.164190.0010 2297914
0.464890.0046 0 2505911 2461920
0.164890.0011
4a 267 159 0.422 0.05
3a* 877 508 0.215 0.17 0.216690.0020 0.454190.0034 -21 2955915 2414915
Tra`igh na Cleabhaig psammite,South Harris,MJW97-SH30
0.317490.0035 11 1586962
13a 286 106 0.175 2.45 0.098090.0032 1777917
0.323390.0037 1824910 1806918
0.111590.0006 363
28a 137 0.137 0.76
0.111990.0007
281 112 0.189 1.09 0.336390.0041 1831912 1869920
10a
0.56 0.112190.0005 0.352990.0042 5 183499 1949920 156
27a* 369 0.297
0.354890.0043 4 1854910 1958920
0.113490.0006 0.244 0.16
26a 398 167
0.113890.0004
528 188 0.070 0.10 0.313490.0232 186097 17579114
21a
0.114290.0003
426 174 0.225 0.06 0.347190.0039 1 186895 1921919
26b
0.335390.0075 187099 1864936
0.114490.0006
7a 332 131 0.207 0.22
292 118 0.188 0.114490.0005 0.347890.0045 1 187099 1924922
9a 0.04
0.360190.0046 4 187197 1983922
0.114490.0004 283
16a* 119 0.209 0.11
0.114590.0003
454 198 0.262 0.07 0.366790.0047 6 187294 2014922
12a
0.337790.0039 187596 1876919
5a* 308 121 0.174 0.22 0.114790.0004
0.351190.0040 2 1876910 1940919
0.114890.0007 711
1a* 280 0.014 0.41
0.115090.0002
1146 448 0.201 0.02 0.333490.0038 187993 1855918
3a
0.86 0.115090.0011 0.331990.0071 1880917 1848935
20a* 182 73 0.337
0.359190.0034 3 189295 1978916
0.115890.0003 0.256
418
27b* 972 0.32
0.116490.0006
767 317 0.052 0.60 0.360590.0037 3 190199 1984917
12b
0.12 0.118390.0003 0.346590.0040 193095 1918919
M
Table 1 (Continued)
[U] ppm [Pb] ppm Th/U meas. 207Pb/206Pb 206Pb/238U Discordanced(%) 207Pb/206Pb 206Pb/238U
Sample/spotb f
206c(%)
(Ma9s) (Ma9s)
6a 683 283 0.497 0.06 0.121290.0004 0.331990.0230 197496 18479111
0.364890.0079 197695 2005937
0.121390.0004 637
16b* 284 0.321 0.53
0.123190.0004
643 311 0.488 0.02 0.384790.0043 3 200295 2098920
19a
0.389990.0053 4
10b 337 160 0.338 0.04 0.123890.0003 201295 2122924
0.377290.00035 203894 2063917
0.125790.0003
11a 1588 850 1.050 0.13
0.383290.0044 207796 2091921
22a 248 123 0.597 0.02 0.128490.0004
0.360890.0085 -4 2131913 1986940 0.132590.0010
28b 218 106 0.655 0.32
0.03 0.143090.0015 0.419190.0089 2263918 2256940 1252
1b* 613 0.096
Loch a Bha`igh tonalite,Berneray,MJW97-Ber43
0.474590.0044 -9 274498
183 0.051 2503919
315
3a* 1.04 0.190290.0009
0.500390.0049 -4
6a* 283 171 0.053 0.16 0.191490.0008 275497 2615921
0.537690.0052 279794 2773922
0.196590.0005 0.13
8a* 335 218 0.052
0.497790.0046 -7 281696 2604920
1a* 308 235 1.102 0.66 0.198790.0007
0.496490.0047 -8 282595 2598920
0.199990.0006
9a 325 232 0.847 0.14
0.200190.0012
134 90 0.612 0.13 0.488590.0048 -10 282799 2564921
1b*
0.524990.0049 -3 282895 2720921
0.200290.0007 206
8b* 148 0.599 0.12
0.200490.0013
101 66 0.471 1.14 0.481590.0047 -11 2829911 2534921
2a*
0.09 0.201190.0012 0.523590.0049 -3 2835910 2714921
7b 102 71 0.452
0.473290.0045 -13 284498 2498920 0.202190.0009
0.643 0.31
7a 173 114
0.202390.0006
203 153 0.659 0.05 0.543090.0051 0 284595 2796921
4a
0.539890.0056 -1
5a 95 69 0.523 0.33 0.202590.0009 284797 2783923
0.502390.0047 -8 285999 2624920
0.204190.0012
103 0.491 0.10
10a 152
aAll errors quoted in this table are 1s. Data are presented in order of increasing207Pb/206Pb age within each sample group. bAsterix (*) indicates that CL image of grain is illustrated in Figs. 2 and 5 or Fig. 6.
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 236
Seventy-two spot analyses were performed on 32 grains from the Butt of Lewis diorite zircons; these include 15 replicate analyses made in order to improve the quality of data obtained from an analytical session with low sensitivity. Four analy-ses represent mixed phaanaly-ses and are not considered further in this discussion. Data are presented in Table 1 and Figs. 3 and 4a. The rim analyses with 207
Pb/206
Pb ages in the range ca. 1.8 – 1.9 Ga provide the most consistent group. A weighted average of 15 of these analyses yields an age of 1859910 Ma (MSWD=2.7, n=15; Fig. 6a). Together with an additional, highly discordant point, these analyses define a regression line with an upper intercept concordia age of 186197 Ma (MWSD=1.3; Figs. 3 and 4a). Although the lower intercept age of 427924 Ma is dependent largely upon a single discordant analysis, and so cannot be treated with the highest confidence, it is interesting to note that it overlaps the ca. 43096 Ma estimate for the latest reactivation of the
Fig. 4. U – Pb concordia diagram (207Pb/206Pb vs. 238U/206Pb)
plotting analyses from (a) Butt of Lewis diorite zircons (DB96-S117); (b) Ba`gh Steinigidh tonalite (DB96-S144); and (c) Tra`igh na Cleabhaig psammite (MJW97-SH30) in the same ca. 1.79 – 1.94 Ga concordia age range (a number of older ages for DB96-S144 and MJW97-SH30 are not plotted — refer to Table 1 for details). Error bars are plotted at 1s. Inset diagrams along right-hand axes are combined cumulative probability curves and histograms of the 207Pb/206Pb ratios.
Horizontal dashed lines and shaded area under the cumulative probability curves represent the weighted average 207Pb/206Pb
age (92s) for each sample — analyses included in this weighted average are shaded grey. Dash-dot-dash line in (a) is the same regression plotted in Fig. 3.
Fig. 3. U – Pb concordia diagram (207Pb/206Pb vs.238U/206Pb)
plotting analyses from Butt of Lewis diorite zircons (DB96-S117); error bars are plotted at 1s, in some cases obscured by the symbol. Classification of analyses follows that used in Table 1 and the text. Dashed outlined box shows the area expanded in Fig. 4a, and the dash-dot-dash line represents the 186197 Ma regression line through all rims with207Pb/206Pb
ageB2 Ga. The thick grey dashed line represents a hypothet-ical ancient Pb-loss trajectory from the oldest cores at ca. 2.83 Ga to the ca. 1.86 Ga age defined by the rim analyses. Present day Pb-loss on this concordia representation is a horizontal trajectory.
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 237
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 238
analyses are uniformly low (B0.02, Table 1), consistent with development of this phase of zir-con in a metamorphic environment (e.g. Williams and Claesson, 1987). One slightly older rim (analysis 13, 207Pb/206Pb age ca. 2.1 Ga dis-cordant) has a much higher Th/U ratio (ca. 0.64) and it is probable that this analysis is a mixed phase.
Analyses of zircon phases categorised as cores or core overgrowths occupy a broad range of the concordia diagram (Fig. 3), scattering about an ancient Pb-loss trajectory with its lower intercept at the ca. 1.86 Ga event defined by the rims. The oldest of the core analyses (grain 19) may define a minimum protolith age of ca. 2.83 Ga, al-though this is ca. 100 Ma older than the next youngest concordant ages and the possibility of an inherited older grain cannot be ruled out. An interesting feature of these data is that the ap-parent age range of the core overgrowths, and their upper age limit, is almost identical to that of the cores (Fig. 3). The pattern of analyses is difficult to interpret with a high degree of confi-dence, given the possibility of Pb-loss during tec-tonothermal events at ca. 1.86 Ga (rim overgrowths), ca. 2.2 Ga (Ness anorthosite em-placement, Whitehouse, 1990b), and ?earlier, as well as during the Caledonian (ca. 430 Ma) and at present day. Most of the analyses define a fan array from a ca. 2.7 – 2.8 Ga protolith towards both ca. 2.3 and 1.9 Ga. A single concordant analysis of a core overgrowth at ca. 2.3 Ga (analysis 27b) suggests that development of this phase might have occurred during events associ-ated with emplacement of the Ness anorthosite, with this particular analysis perhaps recording complete Pb-loss at this time, hence resetting to concordia. Development of core overgrowths at ca. 2.3 Ga without complete Pb-loss would result in an array of analyses (both cores and over-growths) between 2.3 and 2.7 – 2.8 Ga. Later par-tial Pb-loss, in particular at ca. 1.86 Ga, would then result in analyses plotting within the trian-gle defined by the protolith age, and events at ca. 2.3 Ga and 1.86 Ga, which is generally seen in Fig. 3. Given the broad spread of data and the polyphase Pb-loss history of these rocks, it is not possible to define the protolith age to better
than ca. 2.7 – 2.8 Ga, or to rule out the possibil-ity of early events (cf. the ca. 2.5 Ga Inverian event seen in the mainland central region, Kinny and Friend, 1997).
5.2. Ba`gh Steinigidh tonalite, South Harris, DB96-S144
Zircons from the Ba`gh Steinigidh tonalite, DB96-S144, show a range of morphologies, from anhedral, rounded grains B200 mm to euhedral prisms B500 mm. Prismatic grains are mostly clear or pale-brown in colour, while anhedral grains are mostly pale to dark-brown. CL imag-ing (Fig. 5a) shows that the (near-) prismatic grains display finely growth-banded internal structure, occasionally with a rounded core and some minor recrystallisation of the external part of the grain. Some of the more anhedral grains are uniformly CL-dark and no internal structure can be discerned, while a few others show CL-bright, structureless reworking of older cores. The range of zircon morphologies present is con-sidered to be consistent with a possible mixed magma origin suggested by field relationships.
Analyses of zircons from this sample are mostly concordant, yielding 207
Pb/206
Pb ages in the range 1.91 – 1.83 Ga, with a few older ages (Table 1, Fig. 4b). A weighted average age of 187695 Ma (MSWD=2.2, n=20) obtained from these analyses is interpreted as the proba-ble igneous crystallisation age of the tonalite. This age agrees with the lower limit for magma-tism in the South Harris complex derived from the 2.06 – 1.87 Ga range of Sm – Nd tDM model ages for the main diorite body (Cliff et al., 1983), and an unpublished ca. 1.88 Ga zircon age (R.T. Pidgeon and M. Aftalion, cited in Cliff et al., 1983). Three older ages have been ob-tained, two as cores in grains with ca. 1.9 Ga rims. These yield 207
Pb/206
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 239
5.3. Tra`igh na Cleabhaig psammite, South Harris, MJW97-SH30
Zircons from the Tra`igh na Cleabhaig psam-mite sample, MJW97-SH30, range in size from ca. 100 – 200 mm, with the characteristic rounded/ an-hedral morphology of detrital grains. Some grains show elongation up to 2.5:1, also with rounded grain surfaces, although rare facets may be pre-served. Irregular pitting of some of the grain surfaces may be seen in transmitted light. CL imaging of the grains shows a range of internal structure, with several polyphase grains in which the cores are rounded (previous early sedimentary cycle or resorption in a magma?) and overgrown by zoned, sometimes finely growth-banded zircon. A sedimentary cycle, and hence detrital origin, is indicated by rounding of this latest phase, with truncation of banding against grain margins and off-centre cores (e.g. grains 25 and 27, Fig. 5b). Many grains display a thin (B5 mm) CL bright rim, which probably results from the post-deposi-tion high-grade metamorphism of these rocks; this final growth phase cannot be dated.
Near-concordant to concordant analyses of zir-cons from the Tra`igh na Cleabhaig psammite yield 207
Pb/206
Pb ages ranging from ca. 2.3 – 1.8 Ga (Table 1, Fig. 4c). A small degree of reverse discordance is observed in these analyses, of un-known cause, but probably arising from an error in the Pb/U calibration for this particular analyti-cal session which will not affect207Pb/206Pb ratios and derived ages. On a combined cumulative probability/histogram diagram, there is a pro-nounced peak at ca. 1.87 Ga made up by 12 analyses, which yield a weighted average of 187395 Ma (MSWD=1.3, n=11, one age rejected).
Three analyses yield 207 Pb/206
Pb ages slightly younger than ca. 1.87 Ga, and one considerably younger but strongly reverse discordant (13a). These four analyses indicate a relatively high level of apparent common Pb (as determined by 204Pb counts; f206ca. 0.6 – 2.5%, Table 1) and the accu-racy of the corrected 207Pb
/206Pb ages is, thus, dependent upon the validity of assumptions that (1) counts at mass 204 represent only 204Pb, and (2) the exact composition of this common Pb can
be predicted. Since neither of these assumptions can be given a particularly high degree of confi-dence, we prefer to interpret these younger ages as analytical artefacts, rather than assigning any geochronological significance to them.
Ten analyses yield207Pb/206Pb ages greater than ca. 1.87 Ga, ranging from ca. 1.9 – 2.3 Ga (Table 1). Six of these, spanning the complete range of ages, are analyses of distinct rounded cores in grains whose outer regions yield ca. 1.87 Ga ages. Interpretation of these ages is complicated by the lack of any grouping and the possibility of modifi-cation by Pb-loss during the ca. 1.87 Ga event recorded by the outer regions of many of these grains. Pb-loss along a short chord close to con-cordia would be undetectable given the analytical errors associated with these data (in this case, exacerbated by reverse discordance). An early Proterozoic source, perhaps related to ca. 2.3 Ga components within the South Harris igneous com-plex is considered most likely, but inheritance of late-Archean zircon which experienced later Pb-loss cannot be ruled out.
The similarity of the ca. 1.87 Ga weighted average ages obtained from this metasupracrustal rock and the Ba`gh Steinigidh tonalite suggests an origin during the same tectonothermal event. The detrital zircons were probably derived from a 1.87 Ga calc-alkaline igneous rock, probably the high-level volcanic equivalent of the presently exposed South Harris plutonic complex, of which the Ba`gh Steinigidh tonalite is a part. In this scenario, the zircons would be eroded from a magmatic (?volcanic) edifice, deposited as a clastic sedimen-tary precursor to the psammite, and raised to their present granulite-facies metamorphic grade within a few tens of millions of years, given the minimum age estimate of 1.82790.016 Ga for this event (Cliff et al., 1998). This available timescale is more than adequate given typical rates of burial of ca. 10 – 30 mm a−1
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 240
Fig. 6. Cathodoluminescence images of selected grains from Loch a Bha`igh tonalite sample MJW97-Ber43.
ca. 200 – 500mm in size. CL imaging reveals a very simple population of zircons dominated by fine-scale oscillatory zoning typical of igneous zircons. Obvious cores are absent, or very small, and there are no overgrowths, although some of the grains exhibit embayment by CL-dark zircon, which is probably related to a post-igneous crystallisation metamorphic reworking (e.g. grain 3, Fig. 6).
All zircons from the Loch a Bha`igh tonalite are late-Archean, with 207Pb
/206Pb ages ranging from 2.74 – 2.86 Ga (Table 1, Fig. 7). Three analyses, corresponding to reworked parts of the grains (3a, 6a, 8a) have the youngest 207Pb/206Pb ages, ac-companied by very low Th/U ratios (ca. 0.05). These three grains define a Pb-loss trajectory to ca. 1 Ga, with the concordant analysis 8a indicat-ing a probable age for the reworkindicat-ing event of ca. 2.80 Ga. The possibility of a ca. 1 Ga Pb-loss event may be supported by evidence for a Grenville age thermal event in the Outer Hebrides (Cliff and Rex, 1989). Although these documented ages all occur to the north of the Langavat belt, it remains possible that a regional Greenville event might have caused Pb-loss in damaged/metamict older zircons in rocks, which did not experience
5.4. Loch a Bha`igh tonalite, Berneray, MJW97-Ber43
Zircons from this sample are mostly euhedral,
Fig. 7. U – Pb concordia diagram (207Pb/206Pb vs.238U/206Pb) plotting analyses from the Loch a Bha`igh tonalite (MJW97-Ber43).
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 241
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 207Pb/206Pb age of 283499 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-Archean/early-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.
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 242
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;tprepresents protolith age,tmlrepresents 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 (190393 Ma, J.N. Con-nolly, personal communication; Bridgwater et al., 1997), together with contemporaneous (B2.0 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
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 243
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 and/or 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
The geochronological data presented in this paper allow us to make the following conclusions. 1. Paleoproterozoic tectonothermal events are recorded by zircon crystallisation at ca. 1.87 Ga in South Harris (igneous crystallisation of zircons) and northern Lewis (metamorphic overgrowths), strengthening evidence for the widespread nature of this early Laxfordian event.
2. Contemporaneous magmatic activity in the South Harris igneous complex and the source region of detrital zircons in a Leverburgh belt metasediment is consistent with a magmatic arc environment with subsequent collision and high-grade metamorphism.
3. Paleoproterozoic events in South Harris are not recorded in Archean zircons from Bern-eray, a few kilometres to the south, suggesting that the strongest effects of these events are limited to the narrow zones represented by the Paleoproterozoic shear zones. Previously pub-lished ca. 1.9 Ga ages from the southern Outer Hebridean grey gneisses indicate some perva-sive reworking in the flanking gneiss terranes. 4. Major tectonic boundaries within the Lewisian (South Harris, Laxford, and Gairloch shear zones) provide a structural framework for cor-relating gneiss terranes of the mainland and Outer Hebridean Lewisian, although detailed geology of these terranes is not directly com-parable and their geochronology remains poorly constrained. Such correlations must, in any case, consider the possibility of lateral heterogeneities in the gneiss terranes along boundaries perpendicular to the Paleoprotero-zoic shear zones (e.g. the Outer Isles fault). 5. The Paleoproterozoic evolution of the
M.J.Whitehouse,D.Bridgwater/Precambrian Research105 (2001) 227 – 245 244
Acknowledgements
The authors wish to express their thanks to Jessica Vestin and Torbjo¨rn Sunde (Stockholm), Lis Pedersen, Birgitte Lassen and Peter Venslev (Copenhagen) for analytical assistance. The Nordic geological ion-microprobe facility (Nord-sim) is jointly funded by Denmark, Finland, Nor-way and Sweden. Fieldwork was supported by grants from Swedish NFR (to MJW) and Danish NSF (to DB). Robert Frei, Flemming Mengel and Bunessan Gnomes provided stimulating discus-sion in the field. Constructive reviews by Bob Cliff and Hugh Rollinson are acknowledged gratefully. Nordsim contribution number 12.
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