The Loch Maree Group: Palaeoproterozoic Subduction–Accretion Complex in the Lewisian of NW Scotland

Precambrian Research 105 (2001) 205–226 www.elsevier.com/locate/precamres

The Loch Maree Group: Palaeoproterozoic subduction–accretion complex in the Lewisian of NW Scotland

R. Graham Park a,*, John Tarney b, James N. Connelly c

a Department of Geology, Keele Uni6ersity, Keele, Staffs ST55BG, UK b Department of Geology, Uni6ersity of Leicester, Leicester LE17RH, UK c Department of Geological Sciences, The Uni6ersity of Texas at Austin, Austin, TX 78712, USA

Received 15 February 1999; accepted 31 August 1999

Abstract

The Lewisian complex of northwest (NW) Scotland has long been correlated with intercontinental Palaeoprotero- zoic belts of the North Atlantic region but uncertainty about the age and origin of the supracrustal rocks of the Loch Maree Group (LMG) and the apparent lack of subduction-related intrusive rocks have precluded interpretations of a similar tectonic setting for the Lewisian. We present integrated field, geochemical and geochronological data that resolve both issues and are consistent with an intercontinental setting. The LMG is made up of two components, one oceanic (plateau basalts or primitive arcs, plus associated abyssal sediments, ferruginous hydrothermal deposits, and platform carbonates) and the other continental (deltaic flysch, greywacke shale). The metasediments have geochemical characteristics that imply a source outside the Archaean gneisses of the Lewisian, an interpretation that agrees with the detrital zircon populations (from the Flowerdale schists) that have a significant 2.2–2.0-Ga component. The Ard gneiss, formerly regarded by some as a tectonic sliver of basement, is a strongly foliated granodiorite that occurs in sheets intrusive into the LMG, and has given a UPb crystallisation age of 190393 Ma, consistent with its syntectonic relationship with the major D1/D2 phase of Proterozoic deformation. The gneiss has a rather primitive geochemistry, which implies that it was not generated by melting of the local metasediments but was derived by partial melting of a more mafic source. The most likely model is that the LMG evolved as an accretionary complex, modern parallels of which can be found in the Shimanto belt in Japan, Rhodope in north Greece and Colombia and the Caribbean. The various elements of the complex became tectonically intermixed and subject to extreme deformation during accretion to the overriding Lewisian continent. Eventual relaxation and exhumation of the accretionary complex may have resulted in the generation of the Ard gneiss (possibly by melting of the underplated oceanic plateau) followed by collision with the continental crust of the lower plate. The younger D3 phase of the Palaeoproterozoic deformation sequence was coincident with the emplacement of the Tollie pegmatites at 1.7 Ga, c 200 m. years after the main collisional event, and may be related to a younger accretionary event (Labradorian?). © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Gairloch; Geochemistry; Maﬁc volcanics; Palaeoproterozoic; Subduction-accretion; Tectonic models

* Corresponding author. Present address: Blackpark, Edderton, Tain, Ross-shire IV19 1LQ. E-mail address: [email protected] (R.G. Park).

1. Introduction There is a general consensus that the Nagssug- toqidian and Lapland–Kola belts, together with The Lewisian of northwest (NW) Scotland the Torngat and New Quebec orogens of Lauren- formed part of a widespread Palaeoproterozoic tia, are broadly coeval and have resulted from the system of orogenic belts throughout the North progressive accretion of a number of terranes Atlantic region. A recent palaeomagnetic recon- during the period 1.90–1.83 Ga (e.g. see refer- struction (Buchan et al., 2000; Fig. 1) of Lauren- ences in Park, 1995). While most of the orogenic tia and Baltica during the late Palaeoproterozoic belts show evidence of a continental margin set- to early Mesoproterozoic shows a close geometric ting, including subduction and sedimentation, and relationship between the Nagssugtoqidian belt of eventual continent–continent collision, a similar Greenland and the Lapland–Kola belt of Baltica history/setting has not been demonstrated for the within the combined continent. In this reconstruc- Lewisian. This mainly reﬂects an apparent lack of tion, the Nagssugtoqidian belt of East and West magmatic arc rocks and Palaeoproterozoic conti- Greenland appears to continue through the nental margin sediments and the similarity of the Lewisian complex of NW Scotland and has been Archaean basements on either side of the correlated with it by previous workers (e.g. Myers Palaeoproterozoic belt. 1987). Comparisons of the tectonic histories of the The pivotal central position of the Lewisian in Laurentia–Baltica reconstructions has warranted Nagssugtoqidian and Lapland–Kola belts have a re-evaluation of those lithotectonic units within been made by Bridgwater et al. (1990) and Kals- the Lewisian that might imply a continent–conti- beek et al. (1993), and Park (1994, 1995) has nent collisional setting, in spite of contrary histor- noted that the kinematics associated with the peak ical interpretations. This includes the Loch Maree temperatures in the two belts correspond best Group (LMG) that was formerly thought to rep- when the belts are approximately perpendicular as resent sediments and volcanics accumulated in an shown in Fig. 1. intra-continental extensional basin (Johnson et al., 1987). Utilising geochemistry and ﬁeld rela- tions, the LMG is re-interpreted here as a marine sedimentary sequence laid down on oceanic plateau basalts. The Ard gneisses, comprising granitoid orthogneisses interbanded with the LMG along its southwest margin, were originally thought to be part of the Archaean basement. However, a new UPb Palaeoproterozoic magmatic age is consistent with the existence of a previously unknown magmatic arc within the Lewisian sector of the Nagssugtoqidian–Lap- land–Kola collisional belt.

2. Regional setting of the Loch Maree Group

The Lewisian complex comprises mainly Fig. 1. Reconstruction of Laurentia and Baltica during the gneisses derived from igneous rocks and minor Palaeoproterozoic based on the palaeomagnetic ﬁt reported by metasediments of various ages and compositions. Buchan et al. (2000). Note that the Lewisian lies within a The bulk of the gneisses are of Archaean age, continuous Palaeoproterozoic belt extending from the Torngat of Laurentia through the Nagssugtoqidian to the Lapland– formed in the period 2900–2700 Ma (Hamilton et Kola belt of Baltica. The arrows show inferred movement al., 1979; Whitehouse, 1988, 1989a,b) and sub- directions of various crustal segments relative to Laurentia. jected to varying degrees of granulite-facies meta- R.G. Park et al. / Precambrian Research 105 (2001) 205–226 207

Table 1 Simpliﬁed Lewisian chronology. Source references: 1, Kinny & Friend, 1997; 2, Corfu et al., 1998; 3, Corfu et al., 1994; 4, Heaman & Tarney, 1989; 5, Whitehouse et al., 1997; 6, this paper; 7, Holland & Lambert, 1995; 8, Moorbath & Park 1971.

morphism and deformation during three separate torozoic supracrustal rocks in the mainland events: (1) Badcallian metamorphism that in- Lewisian. This region is subdivided by a line cludes pulses at :2730 and :2690 Ma (Corfu et extending southeast from Gruinard Bay: Ar- al., 1994, 1998; Kinny and Friend, 1997) (Table chaean structures and relict granulite-facies as- 1); (2) the Inverian tectono-thermal event at the semblages dominate the northeast part of the Archaean–Proterozoic boundary (2.5–2.4 Ga); region whereas NW–SE-trending Inverian and and (3) the Laxfordian tectonothermal events be- Laxfordian structures are common in the south- tween 1.9 and 1.5 Ga. The lithotectonic units of western part (Fig. 2). NW–SE-trending Scourie the mainland Lewisian resulting from this com- dykes occur throughout this region and help dif- plex history, which includes variable, polycyclic ferentiate Inverian and Laxfordian structures. deformation and metamorphism, can be divided The LMG occurs as two separate belts, one into three distinct domains, namely the northern, northeast of Loch Maree and the other at Gair- central and southern regions. loch (Fig. 2). In addition, a narrow zone of am- Whereas the early Badcallian affected the exist- phibolite with minor siliceous schist south of ing Archaean rocks across the entire Lewisian, the Gruinard Bay (Fig. 2) may also be of late Badcallian affected mainly the central region, Palaeoproterozoic age (Crane, 1978). which is relatively unaffected by subsequent Inve- The LMG outcrop at Gairloch, which is the rian and Laxfordian events. The deformation and subject of this paper, consists of a 3-km wide belt metamorphism of the Inverian and Laxfordian of metasedimentary and metavolcanic rocks (Figs. periods predominantly reworked the northern and 2 and 3) bounded on both sides by acid horn- southern regions and produced features that are blende- and biotite-gneisses cut by numerous am- only readily distinguished in the ﬁeld where the phibolitised basic dykes belonging to the Scourie Scourie maﬁc dykes intruded the central and dyke suite (see Park, 1991 for summary). The southern regions at 2.4 and 2.0 Ga. bounding gneisses are mostly Archaean in age but This paper focuses on the southern region as it have been extensively reworked during the Inve- contains the LMG, the only belt of Palaeopro- rian and Laxfordian (see Table 1). 208 R.G. Park et al. / Precambrian Research 105 (2001) 205–226

3. Lithology of the Loch Maree Group

The supracrustal rocks of the LMG consist of amphibolites of volcanic origin interbanded with metasediments. The metasedimentary rocks fall into two distinct categories: (1) semipelitic quartz–biotite schists that form the main part of the exposed LMG; and (2) narrow discontinuous bands of marble, banded iron-formation (BIF), graphite schist, chlorite-schist and other metasediments, which are spatially associated with the amphibolites, occurring either within or at their margins. The LMG exhibits amphibolite-facies metamorphic assemblages with stable oligoclase and aluminous hornblende in the basic rocks, oligoclase and garnet in the quartz–biotite schists,

Fig. 3. Simpliﬁed map of the LMG outcrop at Gairloch, consisting of amphibolites (maﬁc metavolcanics) and metasediments (semipelites, BIF, marble, graphitic schists) cut by granitic gneisses (Ard gneisses). The LMG forms a steeply-dipping, NW–SE-trending belt on the SW side of the F3 Tollie antiform, and bounded on its NE side by the Creag Bhan brittle-ductile shear zone.

tremolite or diopside in the calc-silicate rocks and garnet–grunerite in the BIF. Retrogression to greenschist facies occurs locally in the younger ductile shear zones, marked by the breakdown of Fig. 2. Simplified map of the southern area of the Lewisian garnet, feldspar and aluminous hornblende to outcrop on the Scottish mainland, representing the foreland to form biotite, epidote, albite, muscovite and the Caledonian orogenic belt. The Lewisian appears beneath actinolite. the Neoproterozoic Torridonian (970–790 Ma: Moorbath, 1969) arkosic sediments. Most of the Lewisian outcrop con- Both the compositional banding and the domi- sists of variably retrogressed and reworked Archaean gneisses nant foliation throughout the LMG outcrop are (protoliths: 2.85–2.75 Ga: Whitehouse et al., 1997; UPb steeply dipping and trend uniformly NW–SE. zircon 2.73 Ga: Corfu et al., 1998) cut by the Scourie Dyke The sequence of rock units from SW to NE swarm (2.42+1.99 Ga: Heaman and Tarney, 1989). The (structurally upwards) is shown in the cross-sec- Palaeoproterozoic LMG comprises highly deformed amphibolites and metasediments cut by granitic gneisses (Ard gneiss; tion (Table 2; Fig. 4). The depositional age of the 1.90 Ga) and later pegmatites (1.69 Ga) — see Figs. 3 and 4 LMG is presumed to be around 2.0 Ga, based on for details. aSmNd model age (O’Nions et al., 1983) and R.G. Park et al. / Precambrian Research 105 (2001) 205–226 209 detrital zircon dates (Whitehouse et al., 1997) (see (REE) by Floyd et al. (1989). The diversity of below). element abundances is consistent with original sedimentary compositions, as exhibited by the 3.1. Semipelites multi-element and REE diagrams (Fig. 5b,e). Al- though the compositions correspond closely to Semipelitic schists form several distinct belts that of greywacke, CaO values, ferromagnesian separated by metavolcanic amphibolites. The components and Na2O concentrations suggest Flowerdale schist represents the most prominent variable maturity. This is also reflected in the K2O member and occupies a broad belt about 700 m in values (0.45–3.38%), while K/Rb ratios cluster width. Narrower bands of semipelitic schist occur around the crustal average (:230–300). As the interbanded with amphibolites and granitoid or- local Archaean gneisses generally have much thogneisses (Ard gneisses) to the southwest of the lower K2O and higher K/Rb ratios, the sediments Flowerdale schist. Typical semipelitic schists are were apparently not of local derivation. fine-grained, intensely schistose, uniform-textured It is important to note the prominent negative rocks. Their main constituents are quartz, plagio- Eu, Sr and Ba concentrations of the metasedi- clase (oligoclase) and biotite. Those schists in ments (which are similar to the average post-Ar- contact with the Ard gneisses have appreciably chaean argillaceous sediments (PAAS) of Nance coarser grain size. Other varieties of semipelite and Taylor, 1976), in contrast to the surrounding include amphibole-bearing types that occur as Lewisian gneisses, which have either positive or narrow bands within the biotite schists. neutral concentrations (see Rollinson and Fowler, Twenty-five samples of semipelitic quartz–pla- 1987; Tarney and Jones, 1994). It is also notewor- gioclase–biotite schist representing the Flow- thy that the significant heavy rare earth element erdale, Charlestown and Kerrysdale units were (HREE) and Y depletions of the gneisses are not analysed for major, trace and rare earth elements apparent in the sedimentary patterns, which again

Table 2 Lithotectonic rock units of the Gairloch area (SW to NE) (see Fig. 4) (thicknesses are measured perpendicular to present orientation of S1/2)

Shieldaig gneiss Deformed basement gneisses with Scourie dykes, :650 m (transitional SW boundary). Cloiche marble Deformed Ard-type gneisses and amphibolite sheets with thin quartz-biotite-garnet schist and marble bands, belt 400 m Ard gneiss Deformed (gneissose) granodioritic sheet, enclosing several amphibolite sheets, up to 500 m Charlestown Quartz-biotite-plagioclase schists interpreted as metagreywackes with minor amphibole-bearing bands and schist thin amphibolites, 100–300 m Kerrysdale Amphibolite sheet containing bands (several m wide) of quartzite (metachert?), quartz-magnetite schist (BIF), basite biotite-garnet-grunerite schist and graphite-schist, and also several wider bands of highly siliceous quartz-biotite schist, the thickest of which is 150 m wide (all these bands lens out laterally), 350–500 m Flowerdale Quartz-chlorite schist with narrow (up to a few m wide) bands of BIF, graphite-schist and marble belt quartz-chlorite-phlogopite-marble, and containing a mylonite with a highly deformed tectonic melange, up to 100 m Flowerdale Quartz-biotite-plagioclase schists with narrow amphibole-bearing bands, 500–600 m, but wedging out to the schist NW Aundrary Amphibolite sheet, typically uniform in composition, but varying from highly schistose to unfoliated basite (epidiorite), enclosing a 30 m wide elongate lens of mylonitised siliceous schist, 500–750 m Am Feur-loch Intensely deformed narrow belt, co-extensive with the Creag Bhan brittle-ductile shear zone (q.v.); contains belt mylonitised to intensely deformed Buainichean gneisses (see below) enclosing a narrow (up to 20 m) band of quartz-biotite-garnet-schist and, locally, a 20 m-wide unit consisting of two thin marble bands with calc-silicate schist and pegmatite, and several thin schistose amphibolites, up to 150 m, but with a transitional boundary with: Buainichean Intensely deformed, ﬁnely foliated, granodioritic basement gneisses, enclosing numerous amphibolites (deformed Scourie dykes); transitional NE boundary.gneiss 210 R.G. Park et al. / Precambrian Research 105 (2001) 205–226

Fig. 5. Geochemistry of potential components of the Ard gneiss, Gairloch. Multi-element plots of (a) Archaean basement Gruinard gneiss and (b) LMG Flowerdale Schists vs rock/primordial mantle. REE plots of (c) the Ard gneiss and (d) Ard gneiss in comparison with Gruinard gneisses and (e) LMG metasediments vs rock/chondrite. Diagram (f) compares multi-element plots of the average Ard gneiss with averages of Gruinard tonalites and trondhjemites: the most signiﬁcant difference is that Ard gneiss is only mildly depleted in Y (and the HREE, see Fig. 5d). See text for sources of data. R.G. Park et al. / Precambrian Research 105 (2001) 205–226 211

Fig. 4. Simpliﬁed cross-section of the LMG outcrop at Gairloch (see Fig. 3) showing the main constituent units, which are described in Table 2.

are analogous to PAAS. This confirms the conclu- 3.2. Other metasediments sion that the sediments of the LMG were not locally derived. It is possible that highly weath- 3.2.1. Banded iron-formation ered basalts, enriched in HREE could have con- The BIF of the Gairloch district was the subject tributed to the high HREE patterns in the of detailed studies by Al-Ameen (1979) and sediments but they could not have caused the Williams (1986). Coarse layering (cm-scale), fine prominent negative Nb anomaly nor the highly lamination (mm-scale) and gradation observed in fractionated REE patterns (the amphibolites have the Gairloch BIF are considered by Williams to be original depositional features. Some of the flat patterns). Interestingly, this conclusion agrees textural relationships of the metamorphic phases with the ages of detrital zircon populations in the were thought to reflect the distributions of precur- LMG, as determined by Whitehouse et al. (1997). sor minerals in the protoliths; for example, the Palaeoproterozoic zircons with ages between 2.2 structures exhibited by the grunerite aggregates and 2.0 Ga are prominent in the sediments but suggest derivation from ooliths or granules as this range of zircon ages is not represented in any found in certain types of Fe-rich sediment. locally derived rocks (apart from the Scourie Williams (1986) concluded that the unmetamor- Dykes, which are unlikely to be the source of the phosed precursors of the BIF consisted of euhedral zircons). This further strengthens stratified mixtures of minerals derived from chem- the conclusion that the sources of the sedi- ical precipitates including silica (as chert), iron ments and their detrital zircons were much further oxides, chamosite–greenalite silicates and carbon- away. ates and probably small amounts of clastic grains 212 R.G. Park et al. / Precambrian Research 105 (2001) 205–226 that may have included additional silica as quartz. of the amphibolite sheets. They contain large, The massive garnet–grunerite–bearing rocks elongate garnet porphyroblasts in a highly schis- imply local concentrations of chamosite–green- tose matrix of quartz and biotite. As in the case of alite such as those observed within some the chlorite schists, the close association of these Archaean and Proterozoic BIF by various authors rocks with the metavolcanics suggests a genetic (cf. Williams, 1986). Many of these examples are relationship. The composition of these garnet associated with stratiform sulphide deposits, as schists is unlike that of the semipelitic schists in is the case at Gairloch. Oxide-bearing iron several respects and they are appreciably coarser formation in general has been considered indicat- grained. Six samples were analysed by Floyd et al. ive of a shallow-water origin (e.g. Goodwin, 1973; (1989) and despite the abundance of garnet, alu- Kimberley, 1978). This, combined with possible mina and alkalis are too low for them to be cross-lamination and primary oolitic structure in classified as pelitic. In comparison with the aver- the Gairloch rocks, was considered by Williams age metabasalt composition, however, they are

(1986) to reflect a shallow-water environment in much richer in SiO2 and K2O and poorer in MgO which chemically precipitated material was and CaO. The trace element values are equally reworked by current or wave action. anomalous. Most average values are similar to those of the semipelites (Ba, Nb, Pb, Rb and Zr) 3.2.2. Quartz–chlorite schist or are similar to both amphibolite and semipelite. Several narrow bands of quartz–chlorite schist There is a marked light rare earth element occur either within or at the margins of the (LREE) depletion relative to PAAS. metavolcanic amphibolites. Six analyses carried The geochemical data are best explained by out by Jones et al. (1987) showed wide chemical hydrothermal alteration of the basaltic protolith variations, notably in SiO2,Fe2O3 (total), MgO immediately adjacent to the schists, that produced and CaO, and low alkalis. Compared with the material enriched in Fe, Mn, Ti and V, but de- metavolcanic amphibolites with which they are pleted in Na and Ca. This was in turn mixed with associated, these rocks have high Si and low Ca the ‘normal’ sediment accumulating on the sea and Na contents. They occur stratigraphically floor. Comparisons can be made with the compo- above the metabasalts and are likely to represent sition of pelagic sediments influenced by hy- volcaniclastic sediments, mixed with additional drothermal activity in the Galapagos Mounds silica (from chert or possibly of clastic origin) area of the Eastern Pacific (Migdasov et al., from which Ca and Na have been partially 1983). Pelagic oozes formed at some distance leached out. from the hydrothermal mounds show a close One of the bands of chlorite schist includes a chemical similarity with the garnet schists except highly deformed fragmental rock consisting of for much lower Fe. This comparison is strength- flattened and elongate fragments of quartzite, up ened by the presence, at stratigraphically higher to several cm in width, together with minor quan- levels, of the hydrothermally derived stratiform tities of calcareous and chloritic schist, in a matrix ore horizon (see below) and the various ‘chemical’ of calcareous chlorite schist. Since the brecciation deposits. is considered to pre-date ductile shearing, the rock could be an olisthostrome or sedimentary slide 3.2.4. Stratiform sulphide deposits breccia, but we feel it is more likely to represent a Two laterally extensive sulphide horizons that tectonic breccia or melange along a decollement occur within one of the amphibolites have been surface that was subjected to very high strains described by Jones et al. (1987). One horizon, during early Laxfordian shearing. which has been traced intermittently for about 6 km parallel to the strike, consists mainly of iron 3.2.3. Garnet–biotite schist sulphides, the other, which is a copper-bearing Narrow bands of distinctive garnet-rich schist deposit about 4 m thick and 1 km long, is com- occur as bands within or at the margins of several posed of quartz–carbonate schist with pyrite, R.G. Park et al. / Precambrian Research 105 (2001) 205–226 213 pyrrhotite, chalcopyrite, sphalerite and native are minor constituents. Foliation is typically ex- gold. Both sulphide deposits are considered by pressed by the development of alternating thin Jones et al. (1987) to be of exhalative origin and bands or elongate lenses of hornblende- and the latter CuZn deposit to be of Besshi type. The feldspar-rich material, typically accompanied by a Mg-rich chlorite schists in the immediate footwall schistose fabric. The banding is attributed to of the ore deposit are thought by Jones et al. metamorphic segregation during intense deforma- (1987) to represent hydrothermal alteration of the tion rather than to primary compositional differ- basalt protolith. ences. Such banding, associated with marked grain coarsening, is much more pronounced in the 3.3. Amphibolites amphibolites in the southwest, which are interbanded with the Ard gneisses. The amphibolites occur in three main layers Johnson et al. (1987) report major- and trace- separated by the metasediments described above element XRF analyses of 154 amphibolite samples (Fig. 3) and also in several much thinner sheets. collected from seven separate sheets, comple- The amphibolites vary from well-foliated, fine- mented by 20 instrumental neutron activation grained hornblende–plagioclase schists to coarse- analyses of REE and other trace elements. Based grained, poorly-foliated or unfoliated varieties. on differences in SiO2,Al2O3,TiO2, Ba, Sr, Zr Plagioclase is either calcic oligoclase or andesine. and the REE, the amphibolites were divided into Quartz, epidote, biotite and occasionally garnet two groups: (1) Group A, represented by all the

Fig. 6. Geochemistry of the LMG amphibolites. (a) REE plot (vs rock/chondrite) showing the differences between the amphibolites of Group A (with flat patterns) and Group B (with LREE-enriched patterns); (b) Multi-element plots (vs rock/primitive mantle) of the average composition of Group A and B amphibolites — Group A has no Nb anomaly, Group B displays a prominent negative anomaly; also shown are the contrasting pattern for average N-type MORB (Sun & McDonough, 1989), but Group A amphibolites are closely similar to ocean plateau basalts (shaded field) from Ontong Java (Babbs, 1997; Mahoney et al., 1993); (c) REE patterns (vs rock/chondrite) of 2.0 Gyr Scourie dykes for comparison — these resemble the Group B amphibolites; (d) Multi-element plots (vs. rock/primitive mantle) of Scourie dykes, which show consistent negative Sr anomalies, unlike the Group B amphibolite average, which has high Sr. See text for sources of data. 214 R.G. Park et al. / Precambrian Research 105 (2001) 205–226 massive amphibolite sheets; and (2) Group B, values. In addition, they have much more frac- comprising a few thin discontinuous bodies in the tionated REE patterns and a significant NbTa southwestern part of the LMG outcrop. The dif- depletion (Johnson et al., 1987, Fig. 6b). The ferences between the two are most apparent in the closest modern analogues are primitive island-arc REE patterns (Fig. 6a), Group B having much systems such as the Marianas (W. Pacific). more fractionated REE patterns and much higher

Sr, Ba, Rb, SiO2,Fe2O3,K2O and V relative to 3.4. Granitoid intrusions associated with the Loch Zr. Maree Group There is little coherent variation in either major or trace elements within each body or between the 3.4.1. Ard gneisses mean compositions of each of the bodies in A narrow belt (up to 500 m wide) of homoge- Group A; rather there is a general scatter that is neous granodioritic and tonalitic gneisses occur- outside the analytical precision for most elements. ring on the southwest side of the Gairloch This is consistent with the typically homogeneous supracrustal belt have been termed the Ard macroscopic appearance of these bodies. Chemi- gneisses (Park, 1964). They typically exhibit a cal mobility within highly deformed zones may well-developed augen texture, and grade into have obliterated primary trends; however, it is quartz–biotite schists on each side. The gneisses encouraging to note that drilled basalts south of near the margins of the belt are highly mylonitised Iceland (Tarney et al., 1979) and on Cyprus (Tar- and difﬁcult to distinguish from the quartz–bi- ney and Marsh, 1991) retain coherent trends for otite schists. Although highly deformed during the

Zr vs Y, Zr vs TiO2, and Zr vs Nb despite severe Laxfordian, the Ard gneisses do not show any alteration. evidence of the pre-Laxfordian gneissose banding There is an appreciable iron-enrichment trend that is a common feature of the Archaean

(up to 18 wt.% Fe2O3) in the Group A analyses gneisses. They enclose thin marble bands and plotted in an AFM diagram and a similar Ti sheet-like bodies of LMG amphibolite that con- enrichment trend in a TiO2 vs Zr plot. By contain numerous feldspathic or quartzofeldspathic trast, samples of Group B exhibit a more calc-al- bands and occasional thin granodioritic sheets. kaline trend. Group A resembles Icelandic basalts They, therefore, post-date the LMG but do not (cf. Tarney et al., 1979) and basalts from other display such an intense D1/D2 Laxfordian fabric plateaus such as the Caribbean/Colombian exam- as the adjoining supracrustal rocks. They are, ple (Kerr et al., 1996, 1997a,b; White et al., 1999) therefore, considered to represent an intrusive and Ontong Java (Babbs, 1997; Mahoney et al., sheet or sheets emplaced into the metasediments, 1993). Interestingly, although most group A sam- probably during rather than before the Laxfor- ples plot within the mid-ocean ridge basalt dian D1/D2 deformation. This interpretation is (MORB) field in discrimination diagrams, they consistent with a new Palaeoproterozoic zircon are not LREE depleted like N-type MORB (see date of 1.90 Ga for the Ard gneiss presented Fig. 6b) but have rather flat patterns, without below (see Table 3). Nb/Ta depletion. This appears to be a characteris- While the geochemical data available on the tic of basalts from ocean plateaus (cf. Kerr et al., Ard gneisses are not comprehensive, the results of 1997a,b). The volumetrically subordinate Group Holland and Lambert (1973) and Park (unpubl.) B amphibolites differ in several respects. Superfi- clarify the petrological character and constrain cially they have some geochemical resemblances the petrogenesis quite well. Multi-element dia- to Scourie dykes (Fig. 6c,d) but apart from the grams and REE plots (Fig. 5) show significant fact that the Scourie dykes were probably em- HREE and Y depletion characteristic of TTG placed much earlier, they differ in having much (trondhjemite–tonalite–granodiorite) suites in the higher Sr than Scourie dykes. Compared with Archaean and adakitic magma compositions in Group A amphibolites, they have much higher the Phanerozoic (Defant and Drummond, 1990; Ba, Sr and P and much lower V at equivalent Zr Martin, 1993). Moreover they have other charac- R.G. Park et al. / Precambrian Research 105 (2001) 205–226 215 Pb Pb 1898 207 206 m); sub, m Pb U 207 235 1099 1434 Pb U 937 Ages [Ma] 206 238 Pb Pb 207 206 Pb U 207 235 2 Pb U 206 238 Pb Pb 0.0748 0.33041 88 5.2925 140 0.11617 14 1840 1868 0.0294 0.15653 52 1.9508 68 0.09039 22 Corrected atomic ratios 208 206 Pb Pb 474 206 204 [pg] T 54 4512 0.0760 0.33435 104 5.3562 158 0.11619 18 1859 1878 1898 61 62160 0.0257 0.22238 56 3.0285 90 0.09877 16 1294 1415 1601 CommonPb R 161.8 4 5.Zs clr sub prsms sm clr sub prsms 10 191.90.002 449560Z29 154.1Z1sm Concentration Measured [ppm] Pb UWeight 371 vsm cldy elong 300.20.002 1397Z13 2562 536.0 105 681 m). m .0Z2clr sub prsms[mg] 0.0210.003Z32 6912461156.0 19283 0.0659 0.33782 80 5.4192 36 0.11634 8 1876 1888 1901 50 B 1 All analyses are single and multi-grainRatios zircon corrected fractions. for fractionation, Abbreviations 1 and are: 25 cldy, pg laboratory cloudy; Pb clr, and U clear; blanks, elong, respectively and elongate; initial prsms, common Pb prisms; calculated sm, using Pb small isotopic (50–75 compositions of Stacey Pb data 1 2 Fraction subhedral; vsm, very small ( Ard gneiss Pegmatite Z3 4 vsm cldy prsms 0.003 1074 Z2 2 sm cldy sub prsms 0.02160.0020.21735 56 2.9417 76 0.09816 18 1268 1393 1589 Table 3 U and Kramers (1975). All fractions extensively abraded. Two-sigma uncertainties on isotopic ratios are reported after the ratios and refer to the final digits. 216 R.G. Park et al. / Precambrian Research 105 (2001) 205–226 teristics of these magma types (cf. Tarney and sation age of 1663922 Ma with an initial 87Sr/ Jones, 1994), such as very high Sr and Ba, high 86Sr ratio of 0.705, coincident with that of the K/Rb ratios, very prominent Nb and Ti troughs Tollie gneisses. Some of the samples were taken and no significant Eu anomalies. On discrimina- from the more deformed bodies on the southwest tion diagrams (e.g. Rb vs Nb/Y), they have a limb of the antiform but there is no correlation primitive arc signature that is confirmed by an between apparent age and amount of deforma- initial 87Sr/86Sr ratio of 0.7029 at 1.90 Ga. (Bridg- tion. Holland and Lambert considered that the water et al., 1997). This suggests a major juvenile scatter of points might be the product of an open Proterozoic component added to the Gairloch isotopic system involving variable loss of 87Sr into sequence at :1.90 Ga. These REE and multi-ele- the host gneisses. If this were the case and if there ment patterns differ so fundamentally from those were no comparable loss of Rb, the upper of the metasediments (Fig. 5e) that there can be boundary of the data spread (i.e. the nine-point negligible involvement of Gairloch schist in the 1663 Ma isochron) would be expected to give the petrogenesis of the Ard gneiss, a fact confirmed closest approximation to the age. However, even by the more radiogenic Sr ratios of the metasedi- if this isochron age can be relied upon, it probably ment (Bridgwater et al., 1997). indicates the date of the D3 deformation, rather Calc-alkaline plutonic suites, ranging from than the original crystallisation age of the peg- diorites through tonalites to granodiorites, form a matite. A new precise zircon date of 1694910 volumetrically small but significant suite in the Ma is discussed below. Palaeoproterozoic continent–continent collision belts of the North Atlantic region, emplaced into both older sialic crust and Paleoproterozoic sedi- 4. Palaeoproterozoic structure of the Gairloch ments. Emplacement ages range between 1.85 and area 1.95 Ga (e.g. Kalsbeek et al., 1987, 1993; Scott, 1995; Kalsbeek and Nutman, 1996). Gneisses preserving pre-Laxfordian structures and metamorphic assemblages occur in the south- 3.4.2. Tollie pegmatites west of the Gairloch area, around Loch Braigh Pegmatites that cut the Laxfordian D1/D2 foli- Horrisdale (forming part of the Ruadh Mheallan ation in the Archaean gneisses are widespread block of Torridon) and also to the northeast, in northeast of the supracrustal belt in the area of the Gruinard Bay area (Fig. 2; Park et al., 1987). the Tollie antiform. These bodies were emplaced The central part of the region consists of a belt of after the main Laxfordian metamorphism and D2 intense Palaeoproterozoic (Laxfordian) deforma- deformation, since they are affected only by low- tion that affects both the LMG and also a broad temperature D3 deformation. They, therefore, zone of basement gneisses and Scourie dykes on play a critical role in the structural chronology. each side of the LMG. This belt of deformation These pegmatites are rich in K and Rb, with can be assumed to relate to the tectonic processes unusually low K/Rb ratios, relatively poor in Ba that were responsible for the intercalation of the and Sr and have low levels of Li (Holland and LMG slices within the basement complex. Lambert, 1995). Like the pegmatites at Scourie in Four phases of Laxfordian deformation (D1– the northern zone, they have probably not been D4) are recognised (e.g. see Park et al., 1987 and derived from the local gneisses but from some Table 1). The D1 and D2 structures represent underthrusted normal crustal material or sedi- ductile deformation that took place at mid-crustal ment (Tarney and Weaver, 1987). levels, whereas D3 and D4 are associated with Holland and Lambert also carried out RbSr retrogressive metamorphic events. D3 occurred at isotopic analyses on a set of 13 samples of peg- low amphibolite- to greenschist-facies and af- matite but the data do not form a single coherent fected much of the Gairloch area, in contrast to isochron and are, therefore, difficult to interpret. D4 that occurred in more localised narrow belts A selected nine-point isochron implies a crystalli- at sub-greenschist facies. R.G. Park et al. / Precambrian Research 105 (2001) 205–226 217

Fig. 7. Simpliﬁed cross-section of the southern mainland Lewisian outcrop (see Fig. 2) showing the likely relationship between the outcrops of the LMG and the location of the principal major folds and sheared regions.

Major Laxfordian shear zones occur at various methods, including calcite–dolomite and Gairloch and Loch Torridon. The older Torridon garnet–biotite geothermometers and the garnet– shear zone (also known as the Diabaig shear plagioclase geobarometer. zone) is of D2 age, extends southwards to the southern margin of the Lewisian outcrop and is 4.1.1. Within the LMG refolded by the major NW–SE D3 folds (Fig. 7). The D1/D2 fabric in the metasediments and Its northeastern margin is inclined to the NE metabasites of the LMG is an intensely developed beneath the Ruadh Mheallan block, which foliation typically accompanied by a strong linear exhibits Archaean structures and is little affected fabric defined by elongated mineral aggregates by Laxfordian deformation. The younger, D3, (typically quartz) and aligned minerals (e.g. horn- Gairloch shear zone is about 5 km wide and blende), mica intersection and microfold axes. steeply dipping, extending southwest from the This pervasive, high-strain, LS fabric occurs Tollie antiform and including the whole of the throughout the supracrustal belt. Folds are not Gairloch supracrustal outcrop (Fig. 4). commonly associated with this fabric but are locally observed in the more siliceous lithologies 4.1. Laxfordian D1/D2 structure such as BIF. The foliation dips uniformly steeply, between 65° NE and vertical (average 70–75° NE) An S1 foliation is the first structure recognised and the lineation plunges typically at moderate in the LMG and the Scourie dykes. The D1 and angles (35–55°) SE in the main part of the D2 fabrics are distinguishable only where F2 folds supracrustal belt. Mylonitic banding that occurs are seen to affect earlier S1 foliation; however, in several narrow belts corresponds to the earliest such folds are uncommon in the supracrustal fabric in the surrounding schists and is at the rocks and the pervasive fabric produced in the same metamorphic grade. The sense of movement combined D1 and D2 deformations is considered in these mylonitic belts, parallel to the strong to be a composite D1/D2 fabric, probably related linear fabric, is sinistral, NE-up. This deformation to progressive deformation. The D1/D2 event is is interpreted to represent the earliest movement associated with an amphibolite-facies metamor- direction in the schist belt. phism that occurred probably close to 1.90 Ga as constrained by the Ard gneiss date. Droop et al., 4.1.2. SW of the LMG (1999) assign peak metamorphic P-T conditions As the LMG is approached from the south, the of 530–630°C and 6.5 kbar to this event using banding in the Archaean gneisses is gradually 218 R.G. Park et al. / Precambrian Research 105 (2001) 205–226 replaced by a more strongly developed planar D1/D2 movement was top to WNW on planes fabric (Laxfordian S1), with a NW–SE strike inclined gently NE. The dextral SW-up shear and steep dip, and a moderately SE-plunging sense, which relates to presently NE-dipping elongation lineation (Park et al., 1987). In a belt zones, and is therefore extensional in their pre-D3 :700 m wide, the dykes are rotated from a steep, orientation, may be due to extensional collapse of SW-dipping attitude to an average dip of 70–80° overthickened crust resulting from the D1/D2 NE. This fabric is still in amphibolite facies; dykes event. have recrystallised to a hornblende–calcic plagioclase assemblage and are penetratively 4.2. Laxfordian D3 and D4 structures deformed throughout. Along the southwest side of the LMG belt, the new fabric is associated with The later part of the Laxfordian deformation is a dextral, S-up sense of movement. divided into two phases, D3 and D4. The major Gairloch shear zone and associated Tollie an- 4.1.3. NE of the LMG tiform, together with the Letterewe synform and In the region of low Laxfordian strain immedi- Carnmore antiform to the north (Figs. 2 and 7) ately southeast of Loch Tollie (Fig. 3), steep have been ascribed to D3 (Odling, 1984; Park et NW–SE Inverian banding in the basement al., 1987). An upper limit for D3 is set at 1.69 Ga gneisses is folded on sub-horizontal axial planes from the post-D2, pre-D3 pegmatites. A cluster of and affected by a gently dipping Laxfordian LS KAr reset ages at around 1.7 Ga (Moorbath and fabric (S2), also at amphibolite facies. A strong Park, 1971) is also thought to reflect the D3 event, intersection/elongation lineation (L2) plunges SE. consistent within errors with the :1.69 Ga upper The dykes are divided into sectors that are alter- limit. nately discordant to and sub-parallel with the D4 includes a set of widely distributed steeply sub-horizontal fabric. The concordant sectors are plunging folds, which deform the S1/S2 and S3 more highly deformed and exhibit a well-devel- fabrics but lack any associated new fabric (Park, oped foliation, whereas the steeper discordant sec- 1964; Bhattacharjee, 1968; Park et al., 1987). The tors exhibit a mainly linear fabric. The sense of folds are typically small-scale (cm–m in ampli- shear implied by these more highly strained sec- tude), open and chevron in style and typically tors is top to the west. associated with narrow belts of cataclasis. A sec- ond cluster of KAr reset ages (Moorbath and 4.1.4. Interpretation Park, 1971) at around 1.5 Ga, ranging down to c The D1/D2 structure of the Gairloch area is 1.4 Ga, probably dates this more localised D4 interpreted by Park et al. (1987) as a major, deformation. originally low-angle, shear zone, including both the supracrustal schist belt and the neighbouring 4.2.1. D3 structure basement gneisses. The present steep attitude of The Tollie antiform (Figs. 2, 3 and 7) is the the schist belt is ascribed to D3 refolding (see most prominent D3 structure in the Gairloch area below). The high-grade D1/D2 fabrics are devel- (Peach et al., 1907; Park, 1969; Park et al., 1987). oped regionally and show a consistent NW–SE It has a moderately NE-dipping limb and steep to elongation lineation interpreted by Coward and vertical SW limb. The plunge varies from near Park (1987) as the movement direction on a wide- horizontal in the northwest to over 60° SE in the spread sub-horizontal shear zone network. southeast and the axial trace varies from NNW– Both dextral SW-up and sinistral NE-up shear SSE to NW–SE. L2 is oblique to the F3 axes sense are recorded in the Gairloch area, and also such that L2 lineations on the northeast limb of in Torridon to the south (Wheeler et al., 1987; the antiform plunge gently to the SE in contrast Niamatullah and Park, 1990). However, the to the southwest limb where they plunge NW, prevalence of a sinistral N-up sense of movement steepening to around 45° at the northeast margin on the early mylonites suggests that the main of the supracrustal belt. R.G. Park et al. / Precambrian Research 105 (2001) 205–226 219

The Tollie antiform forms the northeast margin matite is microcline rich with plagioclase and of the Gairloch shear zone (Odling, 1984; Park et muscovite and was collected from the top of the al., 1987) which is about 5 km wide and occupies hill immediately south of Loch Tollie (at the greater part of the belt of intense Laxfordian NG846778). The results are displayed with 2 deformation in the Gairloch area. Within this sigma errors in Table 3 and Fig. 8 and the shear zone, deformation is generally intense and methodology is described in Appendix A. the foliation is steeply dipping and trends NW– SE. The shear zone is considered to have steep- ened the originally gently NE-dipping D2 6. Results structure into a near-vertical attitude. Since the whole of the outcrop of this shear zone contains 6.1. Ard gneiss rocks already deformed in the earlier (D1/D2) deformations, it is often difﬁcult to distinguish D3 A 6-kg sample of Ard gneiss yielded subhedral effects from earlier fabrics. In the zone of highest to euhedral elongate (3:1), clear, colourless zir- D3 strain (in the SW limb of the Tollie antiform), cons that range in size from 50 to 150 mm. the elongation lineations plunge consistently NW Cathodoluminescence in prepared zircons from at moderate angles and this direction is taken to this population is typically weak but imaging be the shear direction of the shear zone (Odling, 1984). The D3 fabrics in the most highly strained gneisses indicate retrogression to greenschist facies. Feldspars are replaced by an assemblage of albite, microcline and epidote and myrmekitic texture is widespread; biotite is developed axial planar to small-scale F3 folds.

5. UPb geochronology

The age of the LMG is reasonably well constrained by the detrital zircon ages of Whitehouse et al. (1997) (see Table 1) but the Laxfordian events that affect the LMG have not previously been precisely dated. The Ard gneiss and Tollie pegmatites were chosen as targets because of the former’s syntectonic relationship with the main Laxfordian tectono-thermal event (D1/D2) and because the latter are clearly post-tectonic to D1/ D2 but affected by the D3 event. It was hoped that dating both bodies would constrain these events and enable the Lewisian to be correlated with the better-dated Palaeoproterozoic belts in Laurentia and Baltica. The Ard gneiss sample is a typical granodioritic augen gneiss containing quartz, plagioclase, K- feldspar, biotite, and epidote and was collected Fig. 8. U-Pb concordia plots of (a) Ard gneiss and (b) Tollie from fresh exposures behind the fish factory 150 pegmatite, with 2s error depicted for ellipses and intercept m west of Gairloch pier (NG805750). The peg- ages. 220 R.G. Park et al. / Precambrian Research 105 (2001) 205–226 shows zonation consistent with an igneous origin. indicate that these clastic sediments are derived Three fractions plot between 1.5 and 3.0% discor- from a continental upper-crustal source (Floyd et dant and define a single discordia line with inter- al., 1989). SmNd isotope data obtained by cepts at 1903+3/−2 and 2379190 Ma (44% O’Nions et al. (1983) gave two model crustal probability of fit) (Fig. 8a). Commensurate with an residence age estimates of 2.5 and 2.2 Ga for the igneous morphology and internal zonation, the sediment source material, obtained on semipelitic upper intercept is interpreted to represent the schist samples. These data are interpreted to reflect crystallisation age of the Ard gneiss (see Section 3.4 mixing of late Archaean (:2.5 Ga) source material above) and provides the best estimate to date of the with a component of juvenile crustal material with age of the Laxfordian D1/D2 event. The large an age closer to that of the deposition of the extrapolation and consequent large errors associ- sediments (:2.0 Ga). ated with the lower intercept preclude assigning any More detailed supporting evidence for this sedi- geological significance to this value. ment source model comes from the zircon study of Whitehouse et al. (1997). Zircons from a semipelitic 6.2. Tollie pegmatite schist sample (Flowerdale schist) yielded a range of ages that fall into two groups: an Archaean group A 3-kg sample of pegmatite yielded subhedral to with ages ranging from :2.5 to :3.1 Ga and an euhedral, elongate zircons (4:1) that range in size early Palaeoproterozoic group with ages clustering from 25 to 125 mm in length and vary in colour from around :2.0 and 2.2 Ga. The Archaean zircons cloudy and white to clear with a red coating. The are considered to have been derived from local least magnetic zircons correspond to the smallest Archaean basement sources with similar ages and grains (between 25 and 75 mm) such that only the thus confirm the continental source indicated from smallest, euhedral cloudy grains were analysed to the geochemical data. The early Palaeoproterozoic encourage greater concordancy. Three fractions zircons require a quartzofeldspathic source with weighing between 2 and 3 mg are variably and ages of :2.0 and 2.2 Ga and, although mafic highly discordant but define a line with intercepts magmatism occurred in the Lewisian at 2.0 Ga (the of 169495 and 436911 Ma with a 25% probabil- later Scourie dyke suite) and elsewhere in the North ity of fit (Fig. 8b). Consistent with the igneous Atlantic region at 2.2 Ga, no acid source rocks of morphology of the zircons analysed, the upper these ages are yet known either locally or region- intercept is interpreted to represent the crystallisa- ally. Whitehouse et al. (1997) conclude that the tion age of this pegmatite dyke (see Section 3.4 juvenile component of the sediment was probably above). It provides a lower limit to the age of the derived from a contemporaneous subduction-re- Laxfordian D1/D2 event and an upper limit to the lated volcanic arc that is now concealed or has been D3 event. The degree of discordance and the removed. maintenance of a single line over such a spread in concordia space would imply that the lower inter- 7.1.2. Other metasediments cept reflects an actual thermal event at 436911 The origin of those metasedimentary rocks Ma, probably related to movements on the Moine closely associated with the amphibolites must be thrust zone to the east (e.g. Freeman et al., 1998). considered separately from that of the semipelitic sediments. Current views on the origin of BIF favour a shallow-marine, biogenic origin, which 7. Discussion would also explain the presence of the associated graphitic schists. The marbles also may have had 7.1. Origin of the Loch Maree Group a biogenic origin. These largely chemical sediments must have formed on a substrate of submarine 7.1.1. Semipelites basalt, on which variable amounts of sediment The intermediate to high silica content and chemically indistinguishable from the larger units PAAS-like composition of the Gairloch semipelites of metagreywacke were also deposited. Important R.G. Park et al. / Precambrian Research 105 (2001) 205–226 221 components of this assemblage are the quartz– could have formed in close proximity, it is more chlorite schists and garnet schists. These are likely, given the evidence for an active margin thought to represent basaltic volcanic material setting, that they have been tectonically juxta- that has been hydrothermally altered via silica posed and subsequently invaded by the arc-like addition (possibly due to admixture of chert) and Ard gneiss. Ca redistribution. The stratiform sulphide deposits yield further supporting evidence for this 7.1.4. The subduction–accretion model model. The fact that the main ore zone has been With the depositional setting for each LMG affected by strong ductile deformation indicates lithotype better constrained, the question remains that the orebody is pretectonic in origin and its regarding the tectonic processes and history that close spatial association with the mafic igneous led to the intercalation of a plateau–arc sequence, rocks, together with the BIF and other chemical active continental margin sediments and the mag- sediments, suggests an exhalative origin. matic arc rocks of the Ard gneiss. To address this issue, we searched for modern examples with sim- 7.1.3. Amphibolites ilar lithotectonic assemblages and well-con- The chemistry of the group A amphibolites of strained tectonic histories. A possible high level the LMG (with their flat REE patterns) is consis- analogue lies at the continental margin of Colom- tent with an oceanic plateau origin, whereas the bia where an 87-Ma Caribbean oceanic plateau group B amphibolites (with more fractionated assemblage is intercalated with significant vol- REEs, high Sr and Ba, and negative NbTa umes of highly foliated and lineated shale- anomalies is consistent with some sort of primi- greywacke (flysch). Kerr et al. (1997a,b) suggest tive island arc of basaltic or basaltic andesite that the outboard plateau rocks were accreted composition. The oceanic plateau and arc tholei- during subduction to an arc-influenced, continen- ites may be independent in origin or they could be tal margin sedimentary sequence that was de- closely associated (Petterson et al., 1999; White et formed during the accretionary history. In this al., 1999). The Fe- and Ti-rich nature of the case, mainly the upper-level, altered basalts were Group A amphibolites could be due to the trap- accreted while the underlying gabbros and mantle ping of magma chambers within the overthick- components (Kerr et al., 1997a,b) were apparently ened (10–30 km) basaltic crust of a plateau (Kerr consumed in the subduction zone. et al., 1998), or because the mantle plume that A deep level, and thus, perhaps more appropri- forms the oceanic plateau is likely to contain a ate, analogue apparently lies in the crystalline large proportion of recycled basaltic crust that is massifs of N Greece (Barr et al., 1999). Although then available as basaltic partial melts (Cordery et similar to the Colombian example, there is a al., 1997; Tarney et al., 1997; Takahashi et al., much higher proportion of Tethyan limestones in 1998). In fact, Takahashi et al. (1998) have em- Greece that were converted to coarse marbles phasised that where thick lithosphere inhibits adi- (associated with blueschists and eclogites — abatic ascent of the plume, more silicic (basaltic many now retrogressed) during the accretionary andesite) magmas can result. Hence, most of the process. This requires that this component of the petrological and geochemical features in the Loch subduction complex was taken down to mantle Maree amphibolites (types A and B) are quite depths before its eventual exhumation 20–30 m. consistent with an oceanic plateau origin. years later, a process accompanied by widespread The LMG thus consists of two quite distinct migmatisation. Locally there are banded iron- assemblages: one dominated by metagreywackes stones and base metal mineralisation in N Greece, formed proximally to a continent and also influ- presumably originating from the ocean floor. enced by a contemporary magmatic arc; the other, The relative abundance of rock types and spe- consisting of the plateau-type metabasalts and cific structural attributes may differ between the their associated sediments, formed probably in an three areas but the first order similarities are oceanic setting. Although these two assemblages striking and appear to support a common tectonic 222 R.G. Park et al. / Precambrian Research 105 (2001) 205–226 setting and history. The differences are not sur- gneisses on both sides, and must partly relate to prising when considering the many variables of the collision process. However, it is not possible subduction–accretion and growth of accretionary to separate structures formed during the accretion wedges, many of which are only recently becom- stage from those formed during collision. ing better understood (von Huene and Scholl, 1991, 1993; Taira et al., 1997; Barr et al., 1999). 7.2. Summary One possible factor influencing the accretionary process is the existence of large rivers draining at Critical new evidence relating to the age and active margins (currently they mostly discharge at origin of the Ard gneisses and the nature, prove- passive margins because the present phase of plate nance and age of the LMG have led to a re-inter- tectonics has rifted the continents). The weight of pretation of the Palaeoproterozoic history of the sediment formed by such rivers loads and de- southern mainland Lewisian in terms of a colli- presses the downgoing plate and flattens it, with sional orogen. This model envisions the the result that the there is more contact between Palaeoproterozoic LMG representing a deformed the upper and lower plates, and sediment is off- accretionary complex caught up between two Ar- scraped against the continent and becomes chaean continental blocks to northeast and south- severely deformed. Moreover, the lithified mate- west. The Gairloch and Loch Maree outcrops rial from the ocean crust can be involved in this may represent separate thrust-bound slices or may process and off-scraped into the developing accre- have been isolated by D3 movements (Fig. 7). tionary prism (Kimura and Ludden, 1995). Formerly regarded as part of the Archaean Another factor is the nature of the subducting basement, the granodioritic to tonalitic Ard oceanic slab. Normal ocean floor is cold and gneisses, now precisely dated at 1903+3/−2Ma, dense, and provides most of the ‘slab-pull’ force, can be seen as evidence of a Palaeoproterozoic normally subducting completely. It is exotic com- magmatic arc indicating contemporary subduc- ponents of the oceans that tend to get scraped-off tion of oceanic material. This interpretation is during shallow subduction: namely island arcs, consistent with the revised view of the Loch Ma- seamounts, aseismic ridges and ocean plateaus, ree metavolcanics as oceanic plateau basalts and together with their carapaces of carbonate banks the associated sediments as a shallow marine as- and silicic chert. Oceanic plateaus can be rather semblage. The semipelites thus represent the im- large (e.g. the size of France) and were formed mature clastic sediments of an accretionary prism, over a very short time-span; they are normally too incorporating both juvenile and older basement warm and buoyant to subduct immediately source materials. The interleaving of the (Cloos, 1993) and have, therefore, a tendency to semipelites with the oceanic assemblage probably promote subduction-flip or back-step of the sub- reflects the accretion of separate tectonic slices. duction zone (Saunders et al., 1996). A possible sequence of events is represented in In summary, the Palaeoproterozoic history of cartoon form in Fig. 9. Stage 1 shows the original the Gairloch area is reinterpreted here to have accretionary complex and is followed by stage 2 been marked by lateral accretion of oceanic where the subduction complex becomes sand- plateaus and primitive island arcs. This involved wiched by thrusting between two Archaean conti- severe tectonic imbrication and mixing of oceanic, nental blocks. The D1/D2 deformation probably volcanic arc and continentally derived compo- reflects a continuous deformation from the accre- nents followed by continental collision. The major tionary through to the collision stage. Stage 3 D1/D2 deformation is interpreted as the result of represents an extensional phase caused by over- a major, low-angle shear zone with a westerly thickening of the crust during stage 2. The stage 3 directed sense of movement. The D1/D2 struc- geometry would have evolved into the present tures affect both the LMG and the adjoining geometry (see Fig. 8) by upright folding accompa- R.G. Park et al. / Precambrian Research 105 (2001) 205–226 223

Acknowledgements

The authors beneﬁted from discussions with David Bridgwater during the course of the project and acknowledge his help in contributing unpublished material on the Ard gneiss. They also thank Kathy Manser for her technical assistance in pro- ducing the UPb geochronology. NATO support is acknowledged by RGP and JNC.

Appendix A

Rock samples were crushed to mineral size under clean conditions using a jaw crusher and disc pulverizer and minerals were separated using Fig. 9. Cartoon profiles illustrating the possible structural a Wilfley table, disposable sieves, heavy liquids evolution of the LMG. A) Stage 1(:1.90Ga): thick sequence of plateau basalts, accompanying oceanic sediments and is- and a Frantz magnetic separator at The Univer- land-arc material is scraped off the lower plate, tectonically sity of Texas at Austin. Zircons were character- mixed with turbiditic clastic sediments derived from the upper ised using a binocular reflected-light microscope, plate, and underplated beneath the continental crust of the transmitted light petrographic microscope (with : upper plate. B) Stage 2 ( 1.87Ga??): lower-plate continental condenser lens inserted to minimize edge refrac- crust has underthrust the upper plate and imbricate slices of lower-plate crust have been emplaced beneath the previously tion) and a scanning cathodoluminescence (CL) underplated oceanic material. C) Stage 3 (:1.86Ga??): exten- imaging system on a JEOL 730 scanning electron sional collapse of the overthickened stage 2 crust. NB. Dates microscope. of B and C are very tentative; there is no direct evidence. Multiple or single grains of each population were selected for analysis on the basis of optical nied by the development of the steep Gairloch and magnetic properties to ensure that only the shear zone. The rotation of the Gairloch highest quality grains were analysed. All mineral supracrustal belt into its present near-vertical atti- fractions analysed were strongly abraded (Krogh, tude was accomplished by SW-down movements, 1982), subsequently re-evaluated optically and with the development of the highly sheared SW then washed successively in distilled 4 N nitric limb of the Tollie antiform, and the accompany- acid, water and acetone. They were loaded dry ing tight D3 folds with steep axial planes. into TEFLON capsules with a mixed 205Pb/235U The direction of movement undergoes a marked isotopic tracer solution and dissolved with HF change from D2 to D3, since the earlier direction and HNO3. Chemical separation of U and Pb is around NW–SE with a sinistral strike-slip com- from zircon using 0.055-ml columns (after Krogh, ponent and the later around N–S with a dextral 1973) resulted in a total Pb procedural blank of strike-slip component. This could be attributed to :1 pg over the period of analyses. The U proce- a regional kinematic change from :WNW–ESE dural blank is estimated to be 0.25 pg. Pb and U to :N–S convergence (Park, 1995). The earlier were loaded together with silica gel and phospho- (D2) convergence direction is consistent with ric acid onto an outgassed filament of zone- other kinematic indicators in the Lapland–Kola, refined rhenium ribbon and analyzed on a Nagssugtoqidian and Torngat collisional belts multi-collector MAT 261 thermal ionization mass (Fig. 1) which suggests that a common move- spectrometer, either operating in static mode ment pattern prevailed during the Palaeo- (with 204Pb measured in the axial secondary elec- proterozoic amalgamation of the North Atlantic tron multiplier (SEM)-ion counting system) or supercontinent. dynamic mode with all masses measured sequen- 224 R.G. Park et al. / Precambrian Research 105 (2001) 205–226 tially by the SEM-ion counting system. Ages were Corfu, F., Heaman, L.M., Rogers, G., 1994. Polymetamorphic calculated using decay constants of Jaffey et al. evolution of the Lewisian Complex, NW Scotland, as (1971). Errors on isotopic ratios were calculated recorded by U Pb isotopic compositions of zircon, titanite and rutile. Contrib. Miner. Petrol. 117, 215–228. by propagating uncertainties in measurement of Corfu, F., Crane, A., Moser, D., Rogers, G., 1998. UPb isotopic ratios, fractionation and amount of zircon systematics at Gruinard Bay, northwest Scotland: blank. Results are reported in Table 1 with 2s implications for the early orogenic evolution of the errors. Linear regressions were performed using Lewisian complex. Contrib. Miner. Petrol. 133, 329–345. the procedure of Davis (1982). The goodness of fit Coward, M.P., Park, R.G., 1987. The role of mid-crustal shear of a regressed line is represented as a probability zones in the early Proterozoic evolution of the Lewisian. In: Park, R.G., Tarney, J. (Eds.), Evolution of the of fit, where 10% or better is considered accept- Lewisian and Comparable Precambrian High Grade Ter- able and corresponds to a Mean Square of rains. Geol. Soc. Lond. Spec. Publ. 27, pp. 127–138. Weighted Deviates (MSWD) of 2 or less. Crane, A., 1978. Correlation of metamorphic fabrics and the age of Lewisian metasediments near Loch Maree. Scott. J. Geol. 14, 225–246. References Davis, D.W., 1982. Optimum linear regression and error esti- mation applied to UPb data. Can. J. Earth Sci. 23, 2141–2149. Al-Ameen, S.I., 1979. Mineralogy, petrology and geochemistry Defant, M.J., Drummond, M.S., 1990. Derivation of some of the Banded Iron Formation, Gairloch, NW Scotland. Unpublished M.Sc. Thesis. Keele University, Keele. modern arc magmas by melting of young subducted litho- Babbs, T.L., 1997. Geochemical and petrological investiga- sphere. Nature 347, 662–665. tions of the deeper portions of the Ontong Java Plateau: Droop, G.T.R., Fernandes, L.A.D., Shaw, S., 1999. Laxfor- Malaita, Solomon Islands. Unpublished Ph.D. Thesis. Uni- dian metamorphic conditions of the Palaeoproterozoic versity of Leicester, Leicester. Loch Maree Group, Lewisian Complex, NW Scotland. Barr, S.R., Temperley, S., Tarney, J., 1999. Lateral growth of Scott. J. Geol. 35, 31–50. the continental crust through deep level subduction–accre- Floyd, P.A., Winchester, J.A., Park, R.G., 1989. Geochemistry tion: a re-evaluation of Central Greek Rhodope. Lithos 46, and tectonic setting of Lewisian clastic metasediments from 69–94. the Early Proterozoic Loch Maree Group of Gairloch NW Bhattacharjee, C.C., 1968. The structural history of the Scotland. Precambrian Res. 45, 203–214. Lewisian rock northwest of Loch Tollie, Ross-shire, Scot- Freeman, S.R., Butler, R.W.H., Cliff, R.A., Rex, D.C., 1998. land. Scott. J. Geol. 4, 235–264. Direct dating of mylonite evolution: a multi-disciplinary Bridgwater, D., Mengel, F.C., Park, R.G., Tarney, J., 1997. geochronological study from the Moine Thrust Zone, NW Paleoproterozoic calc-alkaline magmatism in the Lewisian Scotland. J. Geol. Soc. Lond. 155, 745–758. — a link between NW Scotland and contemporaneous Goodwin, A., 1973. Archaean iron formation and tectonic collisional orogens in Labrador, Greenland and the Baltic basins of the Canadian Shield. Econ. Geol. 68, 915–933. Shield. (Abstract, EUG-8, Strasbourg). Terra Nova 00, Hamilton, P.J., Evensen, N.M., O’Nions, R.K., Tarney, J., 00–00. 1979. SmNd systematics of Lewisian gneisses: implica- Bridgwater, D., Austrheim, H., Hansen, B.T., Mengel, F., tions for the origin of granulites. Nature 277, 25–28. Pedersen, S., Winter, J., 1990. The Proterozoic Nagssug- Heaman, L.M., Tarney, J., 1989. UPb baddeleyite ages for toqidian mobile belt of southeast Greenland: a link be- the Scourie dyke swarm, Scotland: evidence for two dis- tween the Eastern Canadian and Baltic Shields. Geosci. tinct intrusion events. Nature 340, 705–708. Canada 17, 305–310. Holland, J.G., Lambert, R.St.J., 1973. Comparative major Buchan, K.L., Mertanen, S., Park, R.G., Pesonen, L.J., Elm- element geochemistry of the Lewisian of the mainland of ing, S-A., Abrahamsen, N., Bylund, G., 2000, Pesonen, Scotland. In: Park, R.G., Tarney, J. (Eds.), The Early L.J., Park, R.G., Leino, M.A.H., Bylund, G., Abraham- sen, N., 2000. Comparing the drift of Laurentia and Precambrian of Scotland and Related Rocks of Greenland. Baltica in the Proterozoic: the importance of key palaeo- Keele University, Keele, pp. 51–62. magnetic poles. Techonophysics 319, 167–198. Holland, J.G., Lambert, R.St.J., 1995. The geochemistry and Cloos, M., 1993. Lithospheric buoyancy and collisional oroge- geochronology of the gneisses and pegmatites of the Tollie nesis: subduction of oceanic plateaus, continental margins, antiform in the Lewisian complex of northwestern Scot- island arcs, spreading ridges, and seamounts. Geol. Soc. land. Can. J. Earth Sci. 32, 496–507. Am. Bull. 105, 715–737. Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C., Cordery, M.J., Davies, G.F., Campbell, I.H., 1997. Genesis of Essling, A.M., 1971. Precision Measurements of Halflives flood basalts from eclogite-bearing mantle plumes. J. and Specific activities of 238U and 235U: Physical Reviews Geophys. Res. 102, 20179–20197. C, pp. 1889–1906. R.G. Park et al. / Precambrian Research 105 (2001) 205–226 225

Johnson, Y.A., Park, R.G., Winchester, J.A., 1987. Geochem- Krogh, T.E., 1982. Improved accuracy of UPb zircon ages by istry, petrogenesis and tectonic significance of the early the creation of more concordant systems using an abrasion Proterozoic Loch Maree Group amphibolites of the technique. Geochim. Cosmochim. Acta 46, 637–649. Lewisian Complex, NW Scotland. In: Pharaoh, T.C., Beck- Mahoney, J.J., Storey, M., Duncan, R.A., Spencer, K.J., insale, R.D., Rickard, D. (Eds.), Geochemistry and Miner- Pringle, M., 1993. Geochemistry and age of the Ontong Java alization of Proterozoic Volcanic Suites. Geol. Soc. Lond. Plateau. In: Pringle, M., Sager, W., Sliter, W., Stern, S. Spec. Publ. 33, pp. 255–269. (Eds.), The Mesozoic Pacific: Geology, Tectonics and Vol- Jones, E.M., Rice, C.M., Tweedie, J.R., 1987. Lower Protero- canism. American Geophysical Union Monograph 77, pp. zoic stratiform sulphide deposits in Loch Maree Group, 233–261. Gairloch, northwest Scotland. Trans. Instn. Min. Metal. 96, Martin, H., 1993. The mechanisms of petrogenesis of the B128–140. Archaean continental crust — comparison with modern Kalsbeek, F., Pidgeon, R.T., Taylor, P.N., 1987. Nagssugtoqid- processes. Lithos 30, 373–388. ian mobile belt of West Greenland: cryptic 1850 Ma suture Migdasov, A.A, Gradusov, B.P, Bredanova, N.V, Bezrogova, between two Archaean continents — chemical and isotopic E.V, Savaliev, B.V., Smirnova, O.N., 1983. Major and minor evidence. Earth Planet. Sci. Lett. 85, 365–385. elements in hydrothermal and pelagic sediments of the Kalsbeek, F., Austrheim, H., Bridgwater, D., Hansen, B.T., Galapagos Mounds area, Leg 70, Deep Sea Drilling Project, Pedersen, S., Taylor, P.N., 1993. Geochronology of Ar- Init. Repts. DSDP 70, Washington, 277–288. chaean and Proterozoic events in the Ammassalik area, Moorbath, S., 1969. Evidence for the age of deposition of the South-East Greenland, and comparisons with the Lewisian Torridonian sediments of north-west Scotland. Scott. J. of Scotland and the Nagssugtoqidian of West Greenland. Geol. 5, 154–170. Precambrian Res. 62, 239–270. Moorbath, S., Park, R.G., 1971. The Lewisian chronology of Kalsbeek, F., Nutman, A.P., 1996. Anatomy of the Early the southern region of the Scottish mainland. Scott. J. Geol. Proterozoic Nagssugtoqidian orogen, West Greenland, ex- 8, 51–74. plored by reconaissance SHRIMP UPb zircon dating. Myers, J.D., 1987. The East Greenland Nagssugtoqidian Geology 24, 515–518. mobile belt compared with the Lewisian complex. In: Park, Kerr, A.C., Marriner, G.F., Arndt, N.T., Tarney, J., Nivia, A., R.G., Tarney J. (Eds.) Evolution of the Lewisian and Saunders, A.D., Duncan, R.A., 1996. The petrogenesis of Comparable High Grade Terrains. Geol. Soc. Lond. Spec. Gorgona komatiites, picrites and basalts: new field, petro- Publ. 27, 235–246. graphic and geochemical constraints. Lithos 37, 245–260. Nance, W.B., Taylor, S.R., 1976. Rare earth patterns and crustal Kerr, A.C., Marriner, G.F., Tarney, J., Nivia, A., Saunders, evolution, I: Australian post-Archaean sedimentary rocks. A.D., Thirlwall, M.F., Sinton, C.W., 1997a. Cretaceous Geochim. Cosmochim. Acta 40, 1539–1551. basaltic terranes in Western Colombia: Elemental chrono- Niamatullah, M., Park, R.G., 1990. Laxfordian structure, strain logical and SrNd isotopic constraints on petrogenesis. J. distribution and kinematic interpretations of the Kenmore Petrol. 38, 677–702. Inlier, Loch Torridon: anatomy of a major Lewisian shear Kerr, A.C., Tarney, J., Marriner, G.F., Nivia, A., Saunders, A.D., 1997b. The Caribbean–Colombian Cretaceous Ig- zone. Trans. R. Soc. Edin. Earth Sci. 81, 195–207. neous Province: the internal anatomy of an oceanic plateau. Odling, N.E., 1984. Strain analysis and strain path modelling In: Mahoney, J., Coffin, M. (Eds.), Large Igneous Provinces: in the Loch Tollie gneisses, Gairloch, N.W. Scotland. J. Continental, Oceanic and Planetary Flood Volcanism. Struct. Geol. 6, 543–562. American Geophysical Union, Geophysical Monograph O’Nions, R.K., Hamilton, P.J., Hooker, P.J., 1983. A Nd 100, pp. 123–144. isotope investigation of sediments related to crustal develop- Kerr, A.C., Tarney, J., Nivia, A., Marriner, G.F., Saunders, ment in the British Isles. Earth Planet. Sci. Lett. 63, 229–240. A.D., 1998. The structure of an oceanic plateau: evidence Park, R.G., 1964. The structural history of the Lewisian rocks from obducted Cretaceous terranes in western Colombia. of Gairloch, Wester Ross. Quart. J. Geol. Soc. London 120, Tectonophysics 292, 173–188. 397–434. Kimberley, M.M., 1978. Palaeoenvironmental classification of Park, R.G., 1969. The structural evolution of the Tollie antiform iron formations. Econ. Geol. 73, 215–229. — a geometrically complex fold in the Lewisian north-east Kimura, G., Ludden, J., 1995. Peeling oceanic crust in subduc- of Gairloch, Ross-shire. Quart. J. Geol. Soc. Lond. 125, tion zones. Geology 23, 217–220. 319–349. Kinny, P.D., Friend, C.L.R., 1997. UPb isotopic evidence for Park, R.G., 1991. The Lewisian complex. In: Craig, G.Y. (Ed.), the accretion of different crustal blocks to form the Lewisian The Geology of Scotland, 3rd Edition. The Geological complex of northwest Scotland. Contr. Miner. Petrol. 129, Society, London, pp. 25–64. 326–340. Park, R.G., Crane, A., Niamatullah, M., 1987. Early Protero- Krogh, T.E., 1973. A low-contamination method for hydrother- zoic structure and kinematic evolution of the southern mal decomposition of zircon and extraction of U and Pb for mainland Lewisian. In: Park, R.G., Tarney, J. (Eds.), isotopic age determination. Geochim. Cosmochim. Acta 37, Evolution of the Lewisian and Comparable High Grade 485–494. Terrains. Geol. Soc. Lond. Spec. Publ. 27, pp. 139–151. 226 R.G. Park et al. / Precambrian Research 105 (2001) 205–226

Park, R.G., 1994. Early Proterozoic tectonic overview of the Tarney, J., Saunders, A.D., Weaver, S.D., Donnellan, N.C.B., northern British Isles and neighbouring terrains in Lauren- Hendry, G.L., 1979. Minor element geochemistry of tia and Baltica. Precambrian Res. 68, 65–79. basalts from Leg 49, North Atlantic Ocean. Init. Repts. Park, R.G., 1995. Palaeozoic Laurentia/Baltica relationships: a DSDP 49, 657–691. U.S. Government Printing Office, view from the Lewisian. In: Coward, M.P., Ries, A.C. Washington. (Eds.), Early Precambrian processes. Geol. Soc. Lond. Tarney, J., White, R.V., Barr, S.R., Kerr, A.C., Saunders, Spec. Publ. 105, pp. 299–310. A.D., 1997. Crustal growth by vertical and by lateral Peach, B.N., Horne, J., Gunn, W., Clough, C.T., Hinxman, accretion: the mantle plume connexion. In: GSA (Ab- L.W., Teall, J.J.H., 1907. The geological structure of the stracts with Programmes, Fall Meeting, Salt Lake City, north-west Highlands of Scotland. In: Mem. Geol. Surv. Oct. 1997), pp. 244–245. Great Britain, p. 668. Tarney, J., Weaver, B.L., 1987. Mineralogy, petrology and Petterson, M.G., Babbs, T.L., Neal, C.R., Mahoney, J.J. geochemistry of the Scourie dykes: petrogenesis and crys- Saunders, A.D., Duncan, R.A., Tolia, D., Magu, R., Qo- tallisation processes in dykes intruded at depth. In: Park, poto, C., Mahoa, H., Natogga, D., 1999. Geological–tec- R.G., Tarney, J. (Eds.), Evolution of the Lewisian and tonic framework of the Solomon Islands, SW Pacific: Comparable Precambrian High-grade Terrains. Geol. Soc. crustal accretion and growth within an intra-oceanic set- Lond. Spec. Publ. 27, pp. 217–233. ting. Tectonophys 301, 35–60. von Huene, R., Scholl, D.W., 1991. Observations concerning Rollinson, H.R., Fowler, M.B., 1987. The magmatic evolution sediment subduction and subduction erosion, and the of the Scourian complex at Gruinard Bay. In: Park, R.G., growth of the continental crust. Rev. Geophys. 94/B2, Tarney, J. (Eds.), Evolution of the Lewisian and Compara- 1703–1714. ble High Grade Terrains. Geol. Soc. Lond. Spec. Publ. 27, von Huene, R., Scholl, D.W., 1993. The return of sialic pp. 57–71. material to the mantle indicated by terrigenous material Saunders, A.D., Tarney, J., Kerr, A.C., Kent, R.W., 1996. The subducted at convergent margins. Tectonophysics 219, formation and fate of large oceanic igneous provinces. 163–175. Lithos 37, 81–95. Wheeler, J., Windley, B.F., Davies, F.B., 1987. Internal evolu- Scott, D.J., 1995. UPb geochronology of a Palaeoproterozoic tion of the major Precambrian shear belt at Torridon, NW continental magmatic arc on the western margin of the Scotland. In: Park, R.G., Tarney, J. (Eds.), Evolution of Archaean Nain craton, northern Labrador, Canada. Can. the Lewisian and Comparable High Grade Terrains. Geol. J. Earth Sci. 32, 1870–1882. Soc. Lond. Spec. Publ. 27, pp. 153–164. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial White, R.V., Tarney, J., Kerr, A.C., Saunders, A.D., Kemp- lead isotopic evolution by a two-stage model. Earth Planet. ton, P.D., Pringle, M.S., Klaver, G.T., 1999. Modification Sci. Lett. 26, 207–221. of an ocean plateau, Aruba, Dutch Caribbean: Implica- Sun, S-S., McDonough, W.F., 1989. Chemical and isotopic tions for the generation of continental crust. Lithos 46, systematics of oceanic basalts: implications for mantle 43–68. compositions and processes. In: Saunders, A.D., Norry, Whitehouse, M.J., 1988. Granulite facies Nd-isotopic ho- M.J. (Eds.), Magmatism in the ocean basins. Geol. Soc. mogenisation in the Lewisian complex of N.W. Scotland. Lond. Spec. Publ. 42, pp. 313–345. Nature 331, 705–707. Taira, A., Kiyokawa, S., Aoike, K., Saito, S., 1997. Accretion Whitehouse, M.J., 1989a. Pb-isotopic evidence for UThPb tectonics of the Japanese islands and evolution of the behaviour in a prograde amphibolite to granulite facies continental crust. C.R. Acad. Sci. Paris (Earth Planet. Sci.) transition from the Lewisian complex of north-west Scot- 325, 467–478. land: implications for PbPb dating. Geochim. Cos- Takahashi, E., Nakajima, K., Wright, T.L., 1998. Origin of mochim. Acta 53, 717–724. the Columbia River basalts: melting model of a heteroge- Whitehouse, M.J., 1989b. SmNd evidence for diachronous neous plume head. Earth Planet. Sci. Lett. 162, 63–80. crustal accretion in the Lewisian complex of N.W. Scot- Tarney, J., Jones, C.E., 1994. Trace element geochemistry of land. Tectonophysics 161, 245–256. orogenic igneous rocks and crustal growth models. J. Geol. Whitehouse, M.J., Bridgwater, D., Park, R.G., 1997. Detrital Soc. Lond. 151, 855–868. zircon ages from the Loch Maree Group, Lewisian Com- Tarney, J., Marsh, N.G., 1991. Major and trace element plex, NW Scotland: confirmation of a Palaeoproterozoic geochemistry of Holes CY-1 and CY-4: Implications for Laurentia — Fennoscandia connection. Terra Nova 9, petrogenetic models. In: Gibson, I.L., Malpas, J., 260–263. Robinson, P.T., Xenophontos, C. (Eds.), Cyprus Crustal Williams, P.J., 1986. Petrology and origin of iron-rich silicate– Study Project: Initial Report, Holes CY-1 and 1A. Geol. magnetite–quartz rocks from Flowerdale, near Gairloch, Surv. Canada Paper 90–20, pp. 133–176. Wester Ross. Scott. J. Geol. 22, 1–12.