The Outer Hebridean Lewisian complex

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 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 Caledo- nian 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 am- biguous position in Lewisian geological evolution Coward, 1972 but have been dated at ca. 2.8 – 2.9 Ga Whitehouse, 1993, and thus, clearly be- long 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 appar- ent 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 repre- sent an original prograde transition Fettes and Mendum, 1987, with higher grade lithologies re- stricted to the southern parts of the Uists and Barra. Geochronological studies summarised by 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 t DM 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 1.88 9 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 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 – Nd t DM model ages from North Harris and South – West Lewis in the range 2.60 – 2.76 Ga, and t DM ’s from 2.52 – 2.84 Ga have been reported from North Harris gneisses Whitehouse, 1987. The large range of t DM 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 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 1.87 9 0.04 Ga by Cliff et al. 1983 and further constrained to \ 1.827 9 0.016 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 1.88 9 0.27 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, 1.86 9 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 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. Late Laxfordian events are characterised by amphibolite facies retrogression, minor deforma- tion and, locally, migmatisation and granite in- jection 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 re- ported by Breemen et al. 1971; conventional U – Pb zircon, corrected for modern decay con- stants, broadly correlating with granite magma- tism 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 He- brides north of South Harris Cliff et al., 1998 is presently enigmatic from a regional point of view. Recent 40 Ar – 39 Ar geochronology on hornblendes from a variety of lithologies throughout the northern Outer Hebrides constrains cooling of this part of the complex through ca. 500°C by 1.7 – 1.6 Ga Cliff et al., 1998 and dates the termination of late Laxfordian events.

3. Sampling