Geochronological constraints on Paleoproterozoic crustal evolution and regional correlations of the northern Outer Hebridean Lewisian complex, Scotland

Precambrian Research 105 (2001) 227– 245 www.elsevier.com/locate/precamres

Geochronological constraints on Paleoproterozoic crustal evolution and regional correlations of the northern Outer Hebridean Lewisian complex, Scotland Martin J. Whitehouse a,*, David Bridgwater b,1 a

Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden b Geological Museum, Copenhagen Uni6ersity, DK-1350 Copenhagen, 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

* Corresponding author. Fax: + 46-8-51954031. E-mail address: [email protected] (M.J. Whitehouse). 1 Deceased.

1. Introduction The Lewisian complex is exposed as the foreland to the Caledonian orogen in north-west

0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 0 ) 0 0 1 1 3 - 3

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mainland Britain, and throughout the ca. 200 km length of the Outer Hebrides archipelago. Broadly similar, late-Archean tonalite – trondhjemite – granodiorite (TTG) suite grey gneisses dominate these major outcrops, together with Paleoproterozoic mafic intrusions (Scourie dykes and related units), and substantial post-Scourie dyke structural reworking and granite intrusion. On a regional 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 (Nagsuggtoquidian and Svecokarelian and crust generation (Ketilidian and Svecofennian) events. The Outer Hebridean Lewisian has not attracted 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 Paleoproterozoic 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 isotopic 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 mainland complex and prefer to use the general term ‘early complex’) and post-dyke (‘Laxfordian’) periods.

2.2. The early complex The early complex is divided into the Eastern and Western Gneiss complexes by the Outer Isles Fault, a tectonic feature running the length of the archipelago, which may pre-date the dykes (Lailey et al., 1989), and was reactivated during Caledonian times (Kelley et al., 1994). The hanging wall (Eastern Gneiss) consists of highly tectonised gneissic units, including the mafic Corodale Gneisses, which have previously occupied an ambiguous position in Lewisian geological evolution (Coward, 1972) but have been dated at ca. 2.8 – 2.9 Ga (Whitehouse, 1993), and thus, clearly belong to the early complex. The Western Gneisses of the footwall, occupying most of the outcrop area of the Outer Hebrides, consist mainly of TTG gneisses with minor (but far more abundant than the mainland) occurrences of rocks of apparent supracrustal affinity (Coward et al., 1969). Compositionally, these gneisses resemble the TTG gneisses of the mainland (Fettes and Mendum, 1987), and similarly contain some rocks, which have experienced early (i.e. pre-dyke) granulite facies metamorphism. The boundary between early amphibolite facies and early granulite facies areas is not the sharp tectonic division (terrane boundary, Kinny and Friend, 1997) seen at Loch Laxford on the mainland and instead may represent an original prograde transition (Fettes and Mendum, 1987), with higher grade lithologies restricted to the southern parts of the Uists and Barra. Geochronological studies (summarised by

M.J. Whitehouse, D. Bridgwater / Precambrian Research 105 (2001) 227–245

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-microprobe 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 1.889 0.27 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 extensive Paleoproterozoic reworking and granite injection has strongly affected this region, obscuring the early history of the complex. Cliff et al. (1998) present three Sm–Nd tDM model ages from North Harris and South –West Lewis in the range 2.60 – 2.76 Ga, and tDM’s from 2.52 – 2.84 Ga have been reported from North Harris gneisses (Whitehouse, 1987). The large range of tDM ages 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 penetrative 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 Laxfordian involved granulite facies metamorphism, evidence for which is preserved in Scourie dykes in the low-strain areas of the southern Outer He-

229

brides, and in the South Harris complex (Dearnley, 1963, 1973), the latter dated at 1.879 0.04 Ga by Cliff et al. (1983) and further constrained to \ 1.8279 0.016 Ga (Cliff et al., 1998; both ages from mineral Sm–Nd isochrons). The evidence for granulite facies assemblages in the Scourie dykes has been disputed by Fettes et al. (1992) who suggest that it may, instead, represent an original crystallisation feature. Despite this, the 1.889 0.27 Ga Pb –Pb model regression age for U-depletion of the southern Outer Hebridean gneisses (Whitehouse, 1990a), although insufficiently precise to be correlated directly with well-dated events in South Harris is, nonetheless, more likely to reflect early Laxfordian high-grade (?granulite facies) metamorphism and (probable) associated LILE depletion than late-Laxfordian events, which are characterised by granite injection and retrogression. A similar, again imprecise, 1.869 0.24 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 ultramafic-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-alkaline 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. Late Laxfordian events are characterised by amphibolite facies retrogression, minor deformation and, locally, migmatisation and granite injection complexes, the latter particularly well developed in North Harris and the Uig Hills of southwest Lewis (Myers, 1971). An age for this granite magmatism of ca. 1.72 Ga has been reported by Breemen et al. (1971); conventional U–Pb zircon, corrected for modern decay constants), broadly correlating with granite magmatism on the mainland, particularly around Loch Laxford (Taylor et al., 1984). The significance of ca. 1.63 –1.65 Ga Sm –Nd mineral regression ages for granulite facies assemblages in two younger basic intrusions from the northern Outer Hebrides north of South Harris (Cliff et al., 1998) is

230

M.J. Whitehouse, D. Bridgwater / Precambrian Research 105 (2001) 227–245

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

Fig. 1. Simplified geological map of the northern Outer Hebrides (modified after Fettes et al., 1992) showing principal geological features discussed in the text and sample localities. Abbreviations — WGC, Western Gneiss complex; ECG, Eastern 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).

reference, NB 521 664) which expose a suite of garnet-bearing dioritic rocks with marked cataclasis, 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 (?Paleoproterozoic), 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 ambiguous relationship to the sediments. Immediately to the north of a fence running perpendicular to the coastline, the tonalite is exposed truncating banding in the diorite, with which it shares a common foliation; no earlier fabric is discernable in the diorite. The tonalite also contains discreet inclusions 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 sequence 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 associations 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 highwater mark in Loch a Bha`igh, Berneray. The lithological association suggests possible correlation to the South Harris igneous complex, but strong deformation in a NW –SE shear zone run-

M.J. Whitehouse, D. Bridgwater / Precambrian Research 105 (2001) 227–245

Fig. 2. Cathodoluminescence images of selected grains from Butt of Lewis diorite sample DB96-S117.

ning through Berneray, precludes unambiguous assignment to Archean or Proterozoic crust generation 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-alkaline magmatism, similar to that seen in the South Harris igneous complex, extends south into the Sound of Harris.

4. Analytical methods Ion microprobe U – Th – Pb analyses were performed using the Cameca IMS 1270 high massresolution instrument in Stockholm (Nordsim facility). Details of instrument parameters and basic analytical techniques and data reduction have been given by Whitehouse et al. (1997b). Analyses presented in this study were all obtained using an oxygen-bleed into the sample chamber to

231

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 monitored 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 207Pb/206Pb age of 10639 9 Ma (MSWD = 3.2, no rejections) and 10599 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 Isoplot/Ex (Ludwig, 1998), with errors reported at the 95% (ca. 2|) 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 morphology from anhedral, almost spherical, to euhedral bi-pyramidal grains. Some rounding of the crystal faces and vertices of all grains probably results from cataclasis and/or high-grade metamorphism. 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 partially 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.

232

Table 1 U–Th–Pb analytical data and derived parametersa Sample/spotb

[U] ppm

Th/U meas.

f206c(%)

207

0.005 0.014 0.013 0.015 0.006 0.008 0.001 0.001 0.006 0.007 0.006 0.008 0.008 0.004 0.002 0.011 0.644

0.49 3.28 3.73 5.97 0.32 0.16 0.24 0.26 0.15 0.11 0.02 0.04 0.06 0.06 0.09 5.50 0.43

0.121 0.301 0.664 0.318 0.586 0.687 0.550 0.680 0.219 0.154 0.740 0.589 0.749 0.042 1.214 0.026 0.203 0.337 0.857

1.63 0.06 0.50 0.07 0.44 2.12 0.12 2.03 0.06 0.89 0.06 0.05 0.08 0.33 0.04 0.12 0.05 0.62 0.17

Pb/206Pb

206

Pb/238U

Discordanced(%)

207

Pb/206Pb (Ma9 |)

206

Pb/238U (Ma9|)

0.1028 9 0.0003 0.1092 9 0.0031 0.1117 9 0.0027 0.1124 9 0.0035 0.1125 9 0.0006 0.1128 9 0.0008 0.1129 9 0.0003 0.1132 9 0.0005 0.1134 9 0.0009 0.1137 9 0.0011 0.1138 9 0.0010 0.1140 9 0.0009 0.1145 9 0.0010 0.1149 9 0.0004 0.1152 9 0.0005 0.1176 9 0.0053 0.1304 9 0.0017

0.1932 9 0.0015 0.3258 9 0.0035 0.3127 9 0.0029 0.2684 9 0.0030 0.3246 9 0.0035 0.3335 9 0.0039 0.3283 9 0.0030 0.3296 9 0.0075 0.3298 9 0.0041 0.3345 9 0.0039 0.3219 9 0.0044 0.3263 9 0.0032 0.3424 9 0.0042 0.3407 9 0.0032 0.3479 9 0.0033 0.3257 9 0.0025 0.3666 9 0.0037

-34

1676 9 5 1787 9 52 1827 9 44 1839 9 57 1840 9 10 1845 9 13 1846 9 5 1851 9 7 1854 9 14 1860 9 18 1862 9 15 1864 9 14 1873 9 15 1879 9 7 1883 9 7 1920 9 81 2103 9 23

113898 1818 917 1754 914 1533 915 1812 917 1855 919 1830914 1836937 1838 920 1860 919 1799 921 1820 916 1898 920 1890915 1924916 1818 912 2013 917

0.1258 9 0.0026 0.1389 9 0.0005 0.1424 9 0.0009 0.1439 9 0.0006 0.1527 9 0.0009 0.1571 9 0.0037 0.1595 9 0.0008 0.1623 9 0.0010 0.1635 9 0.0006 0.1650 9 0.0005 0.1651 9 0.0008 0.1660 9 0.0008 0.1671 9 0.0008 0.1672 9 0.0014 0.1680 9 0.0008 0.1689 9 0.0015 0.1691 9 0.0008 0.1695 9 0.0012 0.1713 9 0.0015

0.2233 9 0.0114 0.3509 9 0.0039 0.3863 9 0.00028 0.3487 9 0.0045 0.4015 9 0.0030 0.4082 9 0.0044 0.4344 9 0.0034 0.4393 9 0.0055 0.4900 9 0.0041 0.4499 9 0.0033 0.4719 9 0.0109 0.4551 9 0.0042 0.4103 9 0.0029 0.4446 9 0.0053 0.4876 9 0.0112 0.4490 9 0.0031 0.4387 9 0.0034 0.4472 9 0.0057 0.4935 9 0.0055

-34 -12 -6 -15 -9 -9 -5 -4 2 -4

2040 9 36 2213 9 6 2256 9 11 2275 9 8 2376 9 10 2425 9 40 2450 9 9 2479 9 10 2492 9 7 2507 9 5 2509 9 8 2517 9 8 2528 9 8 2530 9 15 2538 9 8 2547 9 15 2548 9 8 2552 9 12 2570 9 15

1299 960 1939918 2106 913 1928922 2176 914 2207 920 2326915 2347 925 2571918 2395915 2492948 2418919 2216913 2371 924 2560948 2391 914 2345915 2383 926 2586 924

DB96 -S117 81 11 3 15 125 52 462 250 125 63 25 42 26 212 53 4 57

(ii ) Cores, two and three phase grains 32 270 73 31b 921 397 17b* 21 11 31b rpt. 398 172 28a 18 9 10d 806 459 4b 25 14 10d rpt. 261 160 29b* 26 16 27a 85 47 24* 237 156 14 201 121 30a rpt.* 526 301 30b rpt.* 1770 933 20 200 150 30b rpt.* 1033 548 29b rpt.* 283 153 15b rpt.* 88 51 30a* 1124 784

-3 -17

-1 -1

1 -5 -3

-3 -13 -5 -6 -8 -6

M.J. Whitehouse, D. Bridgwater / Precambrian Research 105 (2001) 227–245

Butt of Lewis diorite, Lewis, (i) Rims 25b 378 15a* 29 29e* 7 29e rpt.* 43 31a 345 10b 140 4a 1258 22 677 31a rpt. 134 7b* 169 7b rpt.* 70 2 115 10b rpt. 67 11a* 557 19a* 137 26c 9 13 115

[Pb] ppm

Table 1 (Continued) Sample/spotb

f206c(%)

207

206

125 148 74 60 92 166 16 166 145 18 641 49 20

3.074 0.370 0.340 1.481 0.754 0.674 0.260 0.821 1.660 1.796 0.950 0.451 0.460

0.14 0.24 0.23 0.24 0.29 0.04 0.28 0.07 0.03 0.07 0.04 0.80 0.76

0.1747 90.0014 0.1752 9 0.0015 0.1763 9 0.0015 0.1772 9 0.0022 0.1780 9 0.0009 0.1799 9 0.0010 0.1828 9 0.0022 0.1828 9 0.0015 0.1830 90.0005 0.1843 9 0.0010 0.1871 9 0.0012 0.1995 90.0026 0.2021 9 0.0017

0.4859 9 0.0357 0.5070 90.0055 0.5061 90.0389 0.4526 90.0036 0.4648 90.0042 0.5034 90.0046 0.4844 90.0037 0.5053 9 0.0055 0.4961 90.0062 0.4937 90.0036 0.5159 90.0058 0.5409 9 0.0069 0.5580 90.0069

(iii ) Core o6ergrowths, two and three phase grains 10c 87 42 0.614 16 12 5 0.202 27b 3 1 0.102 30c* 54 26 0.146 30c rpt.* 20 9 0.137 10c rpt. 50 26 0.569 26a 2 1 0.056 29c* 2 1 0.149 15c rpt.* 53 30 0.078 29a* 3 2 0.165 25a 120 70 0.310 3 7 3 0.030 15c* 120 73 0.364 17a* 51 38 1.134 21a* 27 21 1.347 18 146 96 0.711 9 29 19 0.961 1 56 34 0.261 23* 67 48 0.954

0.37 0.39 0.65 0.42 0.32 0.86 0.25 3.11 0.05 1.52 1.68 0.29 0.19 0.11 0.25 0.02 0.28 0.17 0.12

0.1348 90.0026 0.1399 9 0.0017 0.1439 9 0.0018 0.1471 9 0.0025 0.1485 9 0.0024 0.1540 90.0050 0.1541 9 0.0018 0.1562 9 0.0041 0.1572 90.0011 0.1580 9 0.0038 0.1622 9 0.0010 0.1640 9 0.0046 0.1672 9 0.0011 0.1675 9 0.0015 0.1678 9 0.0015 0.1740 9 0.0013 0.1754 90.0026 0.1772 9 0.0008 0.1810 90.0015

0.3675 9 0.0054 0.3682 90.0063 0.4255 90.0036 0.3966 90.0043 0.3835 90.0075 0.3773 9 0.0055 0.4478 90.0047 0.4922 90.0038 0.4840 90.0061 0.4529 9 0.0045 0.4547 90.0105 0.4202 90.0084 0.4739 9 0.0054 0.4846 90.0045 0.4890 90.0124 0.4690 9 0.0044 0.4428 90.0040 0.4810 90.0048 0.4892 90.0113

-5 -8

Ba`gh Steinigie tonalite, South Harris, DB96 -S144 16a 550 227 0.359 5a 281 111 0.096 11a 59 22 0.127 20a 551 215 0.145 12a 1088 350 0.233 22a 1644 666 0.355

0.02 0.04 0.23 0.56 0.38 0.01

0.1119 9 0.0012 0.1131 9 0.0005 0.1137 90.0012 0.1140 9 0.0007 0.1140 9 0.0004 0.1142 90.0004

0.3426 90.0064 0.3475 9 0.0026 0.3218 9 0.0024 0.3331 9 0.0066 0.2701 90.0024 0.3341 9 0.0061

0 3 -2

21b* 15b* 6b* 12b 5 7c rpt.* 26b 8 11b rpt.* 11b* 7c* 19b* 19b rpt.*

128 228 116 82 141 239 27 232 179 21 860 68 27

Pb/206Pb

Pb/238U

Discordanced(%)

-9 -6 -5 -1 -3

-6 -8 -14 7 3

-8

-4 -10 -3

-18

207

Pb/206Pb (Ma9 |)

206

Pb/238U (Ma9 |)

26039 14 2608 9 15 2618 9 15 2627 9 20 2634 98 2652 99 2678 9 20 2679 9 14 2680 9 5 2692 9 9 2717 9 10 28229 22 2843 9 14

2553 9 155 2644 9 24 2640 9 167 2407 9 16 24619 18 2628920 2546 9 16 2636 9 24 2597927 25879 15 2682 9 25 2787 9 29 2858 9 29

2162 9 33 2226 9 21 2275 9 22 2312 930 2329 9 27 2391 956 2392 919 2415 944 2426 9 12 2434 941 2479 911 2497 9 48 2530 9 11 2533 9 15 2536 9 15 2597 913 2609 9 24 2627 9 7 26629 14

20189 25 2021 9 30 2286 9 16 2153 920 2093 935 2064 9 26 2386 9 21 2580 916 2545 926 2408 9 20 2416 947 2261 9 38 2500 9 24 2547 9 20 2566 9 54 2479 9 19 2363 9 18 25319 21 2567 9 49

1831 919 1850 9 8 1859 9 19 1863 9 11 1863 96 1868 96

1899 9 31 19229 13 1799 9 12 1853 9 32 15419 12 18589 30

233

Th/U meas.

M.J. Whitehouse, D. Bridgwater / Precambrian Research 105 (2001) 227–245

[Pb] ppm

[U] ppm

234

Table 1 (Continued) [U] ppm

[Pb] ppm

Th/U meas.

f206c(%)

207

Pb/206Pb

206

Pb/238U

14a 10a 23a 19a 9a 13a 21a 18a 1a* 17a 25a 24a 15a 1b* 7a 2a* 8a 3b* 6a 4a 3a*

591 624 653 510 443 506 521 650 232 976 1661 685 646 92 315 305 388 518 476 267 877

236 264 272 212 182 203 219 264 91 410 685 265 260 36 117 119 138 206 268 159 508

0.299 0.410 0.378 0.312 0.286 0.355 0.283 0.326 0.134 0.367 0.335 0.314 0.344 0.139 0.090 0.095 0.067 0.085 0.514 0.422 0.215

0.02 0.03 0.04 0.01 0.05 0.04 0.01 0.00 0.02 0.02 0.01 0.04 0.01 0.28 0.22 0.11 0.12 0.02 0.10 0.05 0.17

0.1143 90.0005 0.1143 9 0.0003 0.1144 9 0.0006 0.1145 90.0004 0.1146 9 0.0004 0.1147 9 0.0005 0.1147 9 0.0004 0.1148 9 0.0004 0.1149 90.0007 0.1150 9 0.0003 0.1150 9 0.0002 0.1151 9 0.0005 0.1152 9 0.0004 0.1155 90.0010 0.1158 9 0.0005 0.1160 9 0.0005 0.1163 9 0.0005 0.1169 9 0.0004 0.1641 90.0010 0.1648 9 0.0011 0.2166 90.0020

0.3332 90.0062 0.3430 9 0.0026 0.3422 90.0064 0.3453 90.0061 0.3430 90.0031 0.3310 90.0061 0.3532 90.0065 0.3385 90.0061 0.3395 9 0.0026 0.3453 90.0061 0.3417 90.0065 0.3206 90.0057 0.3330 9 0.0061 0.3346 90.0026 0.3237 9 0.0024 0.3410 90.0025 0.3132 90.0024 0.3483 9 0.0026 0.4282 90.0032 0.4648 9 0.0046 0.4541 90.0034

Tra`igh na Cleabhaig psammite, South Harris, MJW 97 -SH30 13a 286 106 0.175 2.45 28a 363 137 0.137 0.76 10a 281 112 0.189 1.09 27a* 369 156 0.297 0.56 26a 398 167 0.244 0.16 21a 528 188 0.070 0.10 26b 426 174 0.225 0.06 7a 332 131 0.207 0.22 9a 292 118 0.188 0.04 16a* 283 119 0.209 0.11 12a 454 198 0.262 0.07 5a* 308 121 0.174 0.22 1a* 711 280 0.014 0.41 3a 1146 448 0.201 0.02 20a* 182 73 0.337 0.86 27b* 972 418 0.256 0.32 12b 767 317 0.052 0.60 25a* 877 347 0.067 0.12

0.0980 9 0.0032 0.1115 90.0006 0.1119 90.0007 0.1121 90.0005 0.1134 90.0006 0.1138 90.0004 0.1142 90.0003 0.1144 90.0006 0.1144 9 0.0005 0.1144 90.0004 0.1145 90.0003 0.1147 90.0004 0.1148 90.0007 0.1150 90.0002 0.1150 90.0011 0.1158 90.0003 0.1164 90.0006 0.1183 90.0003

0.3174 90.0035 0.3233 90.0037 0.3363 90.0041 0.3529 9 0.0042 0.3548 90.0043 0.3134 90.0232 0.3471 90.0039 0.3353 90.0075 0.3478 90.0045 0.3601 90.0046 0.3667 90.0047 0.3377 90.0039 0.3511 90.0040 0.3334 90.0038 0.3319 9 0.0071 0.3591 90.0034 0.3605 90.0037 0.3465 9 0.0040

Discordanced(%)

0

1

-2 0 -4 -7 -8 0 -21 11

5 4 1 1 4 6 2

3 3

207

Pb/206Pb (Ma9 |)

206

Pb/238U (Ma9 |)

186997 1869 95 1870 99 1871 9 7 1874 9 7 1875 9 8 1876 9 7 1876 9 6 1879 9 11 1879 9 5 1880 9 4 1882 9 8 1883 97 1887 9 16 1892 9 8 1896 9 8 1899 9 7 1910 96 2498 9 10 2505 911 2955 9 15

18549 30 19019 12 1897931 1912929 1901915 18439 30 19509 31 18799 29 1884 9 12 19129 29 18959 31 17939 28 18539 29 1861 9 13 18089 12 18929 12 17579 12 19269 12 22979 14 2461 9 20 2414 9 15

1586 9 62 1824 910 1831 9 12 1834 9 9 1854 9 10 1860 9 7 1868 95 1870 99 1870 9 9 1871 9 7 1872 94 1875 96 1876 9 10 1879 93 1880 9 17 1892 9 5 1901 99 1930 9 5

17779 17 1806 918 1869 9 20 19499 20 1958 9 20 17579 114 19219 19 18649 36 19249 22 19839 22 20149 22 18769 19 1940 9 19 18559 18 1848 9 35 19789 16 19849 17 19189 19

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Sample/spotb

Table 1 (Continued) [U] ppm

[Pb] ppm

Th/U meas.

f206c(%)

207

206

6a 16b* 19a 10b 11a 22a 28b 1b*

683 637 643 337 1588 248 218 1252

283 284 311 160 850 123 106 613

0.497 0.321 0.488 0.338 1.050 0.597 0.655 0.096

0.06 0.53 0.02 0.04 0.13 0.02 0.32 0.03

0.1212 90.0004 0.1213 9 0.0004 0.1231 9 0.0004 0.1238 90.0003 0.1257 9 0.0003 0.1284 90.0004 0.1325 9 0.0010 0.1430 90.0015

0.3319 9 0.0230 0.3648 90.0079 0.3847 90.0043 0.3899 9 0.0053 0.3772 90.00035 0.3832 90.0044 0.3608 90.0085 0.4191 9 0.0089

1.04 0.16 0.13 0.66 0.14 0.13 0.12 1.14 0.09 0.31 0.05 0.33 0.10

0.1902 90.0009 0.1914 9 0.0008 0.1965 90.0005 0.1987 9 0.0007 0.1999 90.0006 0.2001 9 0.0012 0.2002 90.0007 0.2004 90.0013 0.2011 90.0012 0.2021 90.0009 0.2023 90.0006 0.2025 9 0.0009 0.2041 90.0012

0.4745 90.0044 0.5003 9 0.0049 0.5376 90.0052 0.4977 90.0046 0.4964 90.0047 0.4885 9 0.0048 0.5249 90.0049 0.4815 90.0047 0.5235 9 0.0049 0.4732 90.0045 0.5430 90.0051 0.5398 90.0056 0.5023 90.0047

Loch a Bha`igh tonalite, Berneray, MJW 97 -Ber43 3a* 315 183 0.051 6a* 283 171 0.053 8a* 335 218 0.052 1a* 308 235 1.102 9a 325 232 0.847 1b* 134 90 0.612 8b* 206 148 0.599 2a* 101 66 0.471 7b 102 71 0.452 7a 173 114 0.643 4a 203 153 0.659 5a 95 69 0.523 10a 152 103 0.491 a

Pb/206Pb

Pb/238U

Discordanced(%)

3 4

-4

-9 -4 -7 -8 -10 -3 -11 -3 -13 0 -1 -8

207

Pb/206Pb (Ma9 |)

206

Pb/238U (Ma9 |)

19749 6 1976 9 5 2002 9 5 20129 5 2038 9 4 2077 9 6 2131 9 13 2263 918

18479 111 20059 37 20989 20 21229 24 20639 17 20919 21 1986 940 2256 9 40

2744 9 8 27549 7 2797 9 4 2816 9 6 2825 9 5 2827 9 9 2828 9 5 2829 9 11 2835 9 10 2844 9 8 2845 9 5 28479 7 2859 9 9

25039 19 26159 21 27739 22 26049 20 25989 20 25649 21 27209 21 2534 9 21 2714 9 21 24989 20 27969 21 27839 23 26249 20

All errors quoted in this table are 1|. Data are presented in order of increasing 207Pb/206Pb age within each sample group. Asterix (*) indicates that CL image of grain is illustrated in Figs. 2 and 5 or Fig. 6. c Percentage of 206Pb contributed by common Pb, assuming Stacey and Kramers (1975) approximation of present day terrestrial Pb isotopic composition. d Discordance of data (if\2| error on analysis).

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Sample/spotb

b

235

236

M.J. Whitehouse, D. Bridgwater / Precambrian Research 105 (2001) 227–245

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 analyses represent mixed phases 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/206Pb 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. 4309 6 Ma estimate for the latest reactivation of the

Fig. 3. U – Pb concordia diagram (207Pb/206Pb vs. 238U/206Pb) plotting analyses from Butt of Lewis diorite zircons (DB96S117); error bars are plotted at 1|, 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 18619 7 Ma regression line through all rims with 207Pb/206Pb age B 2 Ga. The thick grey dashed line represents a hypothetical 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.

Fig. 4. U – Pb concordia diagram (207Pb/206Pb vs. 238U/206Pb) plotting analyses from (a) Butt of Lewis diorite zircons (DB96S117); (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 1|. 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 ( 9 2|) 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.

Outer Isles fault (Kelley et al., 1994), probably responsible for development of the cataclastic fabric in these rocks. Th/U ratios from these rim

M.J. Whitehouse, D. Bridgwater / Precambrian Research 105 (2001) 227–245

237

Fig. 5. Cathodoluminescence images of selected grains from (a) Ba`gh Steinigidh tonalite sample DB96-S144; (b) Tra`igh na Cleabhaig psammite sample MJW97-SH30.

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analyses are uniformly low (B0.02, Table 1), consistent with development of this phase of zircon in a metamorphic environment (e.g. Williams and Claesson, 1987). One slightly older rim (analysis 13, 207Pb/206Pb age ca. 2.1 Ga discordant) 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, although 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 apparent 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 confidence, given the possibility of Pb-loss during tectonothermal events at ca. 1.86 Ga (rim overgrowths), ca. 2.2 Ga (Ness anorthosite emplacement, 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 associated 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 overgrowths) between 2.3 and 2.7 – 2.8 Ga. Later partial Pb-loss, in particular at ca. 1.86 Ga, would then result in analyses plotting within the triangle 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 possibility 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 B 200 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 imaging (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 CLbright, structureless reworking of older cores. The range of zircon morphologies present is considered to be consistent with a possible mixed magma origin suggested by field relationships. Analyses of zircons from this sample are mostly concordant, yielding 207Pb/206Pb 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 probable igneous crystallisation age of the tonalite. This age agrees with the lower limit for magmatism 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 obtained, two as cores in grains with ca. 1.9 Ga rims. These yield 207Pb/206Pb ages of ca. 2.5 Ga (one concordant, one discordant) and 2.95 Ga (discordant), indicating a relatively minor input of older material into the tonalite, consistent with previous reports of Sm –Nd tDM model ages of ca. 2.1 –2.5 Ga from calc-alkaline intrusives of the South Harris igneous complex (Cliff et al., 1983, 1998).

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5.3. Tra`igh na Cleabhaig psammite, South Harris, MJW 97 -SH30 Zircons from the Tra`igh na Cleabhaig psammite sample, MJW97-SH30, range in size from ca. 100 – 200 mm, with the characteristic rounded/anhedral morphology of detrital grains. Some grains show elongation up to 2.5:1, also with rounded grain surfaces, although rare facets may be preserved. 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-deposition high-grade metamorphism of these rocks; this final growth phase cannot be dated. Near-concordant to concordant analyses of zircons from the Tra`igh na Cleabhaig psammite yield 207Pb/206Pb 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 unknown cause, but probably arising from an error in the Pb/U calibration for this particular analytical session which will not affect 207Pb/206Pb ratios and derived ages. On a combined cumulative probability/histogram diagram, there is a pronounced 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 207Pb/206Pb 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; f206 ca. 0.6 – 2.5%, Table 1) and the accuracy 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

239

be predicted. Since neither of these assumptions can be given a particularly high degree of confidence, we prefer to interpret these younger ages as analytical artefacts, rather than assigning any geochronological significance to them. Ten analyses yield 207Pb/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 modification 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 concordia 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 complex is considered most likely, but inheritance of late-Archean zircon which experienced later Pbloss 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 highlevel 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 sedimentary 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 1.8279 0.016 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 during collisional tectonics (e.g. Alps and Himalayas), but requires a major heat source to attain necessary temperatures. This heat could perhaps be provided by continued magmatism documented by the granulite-facies fabric-cutting net-veined gabbro, which provides the lower age constraint on this metamorphism (Cliff et al., 1998).

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Fig. 6. Cathodoluminescence images of selected grains from Loch a Bha`igh tonalite sample MJW97-Ber43.

5.4. Loch a Bha`igh tonalite, Berneray, MJW 97 -Ber43 Zircons from this sample are mostly euhedral,

Fig. 7. U – Pb concordia diagram (207Pb/206Pb vs. Error bars are plotted at 1|.

238

ca. 200 –500 mm in size. CL imaging reveals a very simple population of zircons dominated by finescale 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, accompanied 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 indicating a probable age for the reworking 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

U/206Pb) plotting analyses from the Loch a Bha`igh tonalite (MJW97-Ber43).

M.J. Whitehouse, D. Bridgwater / Precambrian Research 105 (2001) 227–245

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 28349 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. Significantly, there is no record either of Paleoproterozic (South Harris equivalent) age events, or lateArchean/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, together 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 tectonothermal 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 recrystallisation or disturbance (Pb-loss) at this time. Data from a Leverburgh belt psammite suggest derivation of clastic material largely from a ca. 1.87 Ga magmatic precursor, in agreement with the observation of Cliff et al. (1998; Sm – Nd model age data) that this belt must contain a ‘significant post-Archean component’. Given geochemical evidence 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 accretionary wedge (Baba, 1997), a possible tectonic scenario emerges, in which the rocks of South Harris represent a magmatic arc, complete with contemporaneously derived clastic sediments, developed in a collisional orogen, which culminated in granulite 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-

241

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 Paleoproterozoic shear zones (Fig. 8) but, as pointed out by Coward and Park (1987), the most obvious correlation of the South Harris (SHSZ) and Gairloch shear zones (GSZ, Fig. 8a) is not supported by the geology of the flanking regions, and the alternative 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 generation 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 Paleoproterozoic collisional orogen must now be considered 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 history (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

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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; tp represents protolith age, tml 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 central 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 recognised 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 terrane. Unlike the LSZ, the GSZ contains evidence for juvenile Paleoproterozoic arc-magmatic activity in the Ard Gneisses (19039 3 Ma, J.N. Connolly, 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 developed 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, indicate a ca. 2.8 Ga protolith age, with no apparent early (ca. 2.5 Ga Inverian) granulite facies metamorphism 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 alternative correlation of the SHSZ with the GSZ is more favourable in terms of evidence for a Paleoproterozoic 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 apparent metamorphic history. Correlation of mainland

M.J. Whitehouse, D. Bridgwater / Precambrian Research 105 (2001) 227–245

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 motion could also account for differences observed in the flanking gneiss regions (whether the SHSZ correlates with the LSZ or the GSZ) by the development of thrusts and/or extensional detachments parallel to the transport direction. Such structures could readily introduce contrasting terrane features 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) Corodale 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 number of major tectonic breaks, which must be considered 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 comparison of tectonic data from Paleoproterozoic belts throughout Laurentia and Fennoscandia, in which the Lewisian, together with the Ammassalik 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 magmatic arcs in collisional orogens brings their evolution into line with other Paleoproterozoic belts

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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 Berneray, 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 published ca. 1.9 Ga ages from the southern Outer Hebridean grey gneisses indicate some pervasive 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 correlating gneiss terranes of the mainland and Outer Hebridean Lewisian, although detailed geology of these terranes is not directly comparable 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 Paleoproterozoic shear zones (e.g. the Outer Isles fault). 5. The Paleoproterozoic evolution of the Lewisian is not simply an intracontinental rift, but involves magmatic arcs and collisional orogeny at ca. 1.9 Ga, similar to other regions of Laurentia –Fennoscandia at this time.

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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 (Nordsim) is jointly funded by Denmark, Finland, Norway 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 discussion in the field. Constructive reviews by Bob Cliff and Hugh Rollinson are acknowledged gratefully. Nordsim contribution number 12.

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