Some Aspects of the Geochemistry, Provenance and Palaeoclimatology of the Torridonian of NW Scotland
Journal of the Geological Society, London, Vol. 156, 1999, pp. 1097–1111. Printed in Great Britain.
Some aspects of the geochemistry, provenance and palaeoclimatology of the Torridonian of NW Scotland
GRANT M. YOUNG Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7 (e-mail: [email protected])
Abstract: Geochemical data, mostly from mudstones, are used in an attempt to investigate some aspects of the palaeoclimatology and provenance of the Torridonian succession in NW Scotland. The basal part of the Stoer Group, which has been interpreted as glaciogenic is thought to have formed in a warm arid setting. The presence of oscillation ripples and desiccation cracks in mudstones containing isolated clasts is thought to preclude their interpretation as ice-rafted debris. Some aspects of the major element geochemistry of the Stoer Group mudstones suggest deposition in an environment with little weathering, but chemical weathering is inhibited in both frigid and arid settings and sedimentological evidence favours the latter. The Stoer Group mudstones are enriched in Mg, Ca and Na relative to the Torridon Group samples, which are much closer in composition to an average shale estimate (PAAS). The Cr and Ni contents of the Stoer mudstones are significantly higher than those of the Torridon Group samples. There is, however, no evidence that enrichment in these elements was due to contamination with volcanic material like that of the Stac Fada Member. The Scourian basement appears to be an adequate source of the Ni and Cr in the Stoer Group sediments. Th/Sc ratios are higher in the Torridon than in the Stoer mudstones. This difference is interpreted as indicating that the Stoer mudstones were largely derived from local Archaean sources but that the Torridon Group mudstones incorporated materials from a much wider provenance, including younger and recycled materials. Evidence from rare earth elements also provides evidence for different source terrains for the Stoer and Torridon Groups. The Torridon Group mudstones are more weathered, have a higher REE content and display a more prominent Eu anomaly (lower value for Eu/Eu*) than do the Stoer samples. These results support evidence from Th/Sc ratios, that the Torridon Group was derived from a more varied, recycled and granite-rich terrain than the Stoer, much of which appears to have relatively local provenance. The geochemistry of mudstones can shed considerable light on problems of provenance and palaeoclimatology, because many useful trace elements tend to be concentrated in such fine grained rocks and some aspects of their major element chemistry may be related to climatic factors.
Keywords: Scotland, Torridonian, geochemistry, provenance, palaeoclimatology.
The Meso- to Neoproterozoic Torridonian succession crops study deals with the geochemistry of mudstones, which are out on the mainland of NW Scotland (Fig. 1) and in some particularly informative because the composition of the fine of the western islands such as Skye and Rum. It is a thick material indicates the degree of alteration that the source rocks (c. 11.5 km maximum thickness) sandstone-dominated have undergone (Nesbitt & Young 1996; Nesbitt et al. 1996). succession, but includes coarse breccias, conglomerates and Fine material also tends to contain a high concentration of shales, together with minor carbonates and volcanic rocks in trace elements, including rare earth elements (REE), which the basal Stoer Group. The main stratigraphic units of the can be useful provenance indicators. These geochemical mainland Torridonian in the northern part of the Scottish parameters, together with field observations, are used in this mainland are shown in Fig. 2, which also indicates units that study to investigate some aspects of the palaeoclimatology and were sampled for this investigation. The Torridonian strata provenance of the Torridonian succession. rest with profound unconformity on highly deformed crystal- line rocks of the Archaean to Palaeoproterozoic Lewisian complex. In the southern part of the outcrop area there is up to 600 m of relief beneath Torridon Group rocks but to the north Age and provenance of the Torridonian the contact is relatively planar (Williams 1969; Stewart 1991). The age of the Torridonian succession is poorly constrained. There has been considerable debate concerning the palaeo- There was probably a considerable time gap between climatic conditions prevailing at the time of deposition of the deposition of the Stoer and Torridon Groups (Moorbath basal units of the Stoer Group (Davison & Hambrey 1996, 1969). Recently a Pb–Pb age of 119970 Ma was obtained 1997; Stewart 1997). Peculiarities in the geochemistry of these from limestones of the Stoer Group and Rb/Sr isochrons from rocks have been attributed to deposition under closed-basin the Torridon Group yielded ages of 99448 Ma and ‘evaporitic’ conditions (Stewart 1990; van de Kamp & Leake 99739 Ma (Turnbull et al. 1996). On the basis of U–Pb 1997), whereas some of the compositional attributes have dating of individual zircon grains from the Applecross been related to inclusion of material from contemporaneous Formation, Rogers et al. (1990) concluded that ‘the volcanic activity (Stewart 1990). Most previous geochemical Torridonian must have had its major provenance dominated investigations have concentrated on sandstones, whereas this by Grenville aged crust and by 1650 Ma old crust’, with
1097 1098 G. M. YOUNG
Fig. 2. Outline of stratigraphic units of the Torridonan succesion in the mainland of NW Scotland. Stars show stratigaphic intervals from which samples were taken for this investigation.
and Allen (1991) suggested that many of these clasts represent a supracrustal assemblage of comparable age to the Laxfordian Complex, but derived from regions that suffered less severe metamorphism and erosion, possibly including Greenland and Labrador. Van de Kamp & Leake (1997) also suggested that the Torridon Group sediments could have origi- nated in Laurentia, perhaps shed from the Grenville orogen, but they proposed a local derivation and deposition in ‘closed basins’ for rocks of the Stoer Group and the topmost preserved unit of the Torridon Group, the Cailleach Head Formation. In a study of zircon geochronology of sedimentary rocks from the Palaeoproterozoic Loch Maree Group, which forms part of the Lewisian Complex in NW Scotland, Whitehouse et al. (1998) discovered numerous 2.0–2.2 Ga zircons. These zircons were attributed to contemporaneous magmatic activity and thought to be derived from a quartzo-feldspathic juvenile arc magmatic region that has subsequently been tectonically displaced. Williams (1968) documented the presence and nature of ‘exotic’ pebbles in the Torridon Group conglomer- Fig. 1. Sketch map to show the distribution of the Torridonian ates. He also argued that sediments of similar composition to succession in NW Scotland. the K-feldspar-rich sandstones of the Applecross Formation, could have formed as a result of weathering of a basement comparable to the underlying Lewisian rocks. He recognized contributions from late Archaean (c. 2.6–2.8 Ga) rocks. The the considerable compositional differences between the base- absence of very old zircons (>3.0 Ga) and evidence of ment to the Torridon Group and the sedimentary rocks, but rocks related to the Ketilidian orogeny (c. 1.8 Ga) led to the argued that weathering processes could have produced the suggestion (van de Kamp & Leake 1997) that a Greenland necessary transformations, especially the loss of plagioclase provenance was unlikely. More recently Rainbird et al. (1998) feldspar. His arguments (Williams 1968) were based in part on reported concordant Pb–Pb ages from four sandstone units of data collected from palaeosols that are locally preserved the Stoer Group and two formations of the Torridon Group. beneath the Torridon Group sediments in the northern part of They concluded that much of the sandstone detritus in the the Torridonian outcrop belt near Sheigra (Fig. 1). There sampled units of the Stoer Group was derived from local have been various opinions regarding the age and nature of basement of the Lewisian Complex but the Stoer Group the ‘palaeosols’ described by Williams (1968). Stewart units also contain evidence of a younger contribution (1995a) raised the possibility that the altered zone beneath the (1.91–1.75 Ga). The Torridon Group samples produced Torridon Group might be Tertiary in age but recent palaeo- ‘distinctive age modes at 1.80 Ga, 1.66 Ga and 1.10 Ga’, and magnetic work (Williams & Schmidt 1997) showed that the these results were interpreted to mean that the Torridon Group magnetic remanence direction in the weathered materials is sediments were derived in part from the eastern margin of similar to that obtained from red beds of the overlying Laurentia. Williams (1969) suggested that the source could Applecross Formation and it was concluded that the alteration have lain within the continental shelf regions presently situated took place penecontemporaneously with deposition of the off the east coast of Greenland and west coast of Scotland. sediments. Williams (1969) provided a detailed description of ‘exotic’ In a series of papers concerned with the geochemistry of pebbles in the Torridonian succession and Allen et al. (1974) the Torridonian sedimentary rocks, Stewart and colleagues DEPOSITIONAL ENVIRONMENT OF TORRIDONIAN 1099
(Stewart 1990, 1991, 1995a; Rodd & Stewart 1992; Stewart & (1996) suggested that the basal part of the Stoer Group may Donnellan 1992) investigated possible sources and palaeo- have been deposited under glacial conditions. This interpret- climatic settings. Stewart (1991) studied the composition of ation was later debated (Stewart 1997; Davison & Hambrey rocks of the Sleat and Torridon Groups of Skye and concluded 1997). In an effort to resolve these problems of interpretation that the lower part of the succession had a relatively local the exposures in question were restudied. origin but that the upper part may have been transported tens Of critical importance to the glacial interpretation of the of kilometres from the west. Stewart & Donnellan (1992) basal Stoer succession is the occurrence of isolated gneiss clasts suggested that the main Torridon Group source lay in in laminated to finely bedded sedimentary rocks (Fig. 3a) Scourian gneisses and Laxfordian granites of the Outer on the east side of a small ridge of basement gneiss on the Hebrides and beyond, with some contributions from sedimen- south shore of Enard Bay. These clasts possess many of the tary and volcanic rocks. Compositional differences within the classic features of dropstones such as deformed beds, which Applecross Formation were attributed by them to variations in were interpreted (Davison & Hambrey 1997) as ‘splash up’ tectonic relief. For example, the presence of plagioclase in the structures, indicating that they were vertically emplaced, as lower part of the formation was attributed to the existence of from floating ice. Some of these gneiss stones are over 2 m in high relief during deposition. diameter and presumably would have required a considerable In a study of the Stoer Group, Stewart (1990) noted the lack mass of ice to effect their transport. Such a hypothetical ice of Ca depletion and proposed that deposition took place in an mass could have varied greatly in its dimensions but, given the arid setting. Because of the presence, in the Stoer Group, of a constricted nature of the palaeotopography at the time of unit interpreted as a vitric tuff (the Stac Fada Member), deposition (see Stewart 1997, fig. 1) it is unlikely that a large Stewart (1990) raised the possibility that much of the Stoer slab of relatively thin ice could have carried the stones to their Group was deposited under an intermittent rain of fine ash, present position. It therefore seems likely that these large which he suggested might explain some of its chemical charac- ‘rafted’ clasts would have necessitated the existence of fairly teristics. Stewart also concluded that the sandstones forming deep water at the time of emplacement (see Davison & most of the Stoer Group could have formed by mixing of Hambrey, fig. 8). Careful examination of the strata laterally virtually unweathered material from the Lewisian basement equivalent to the ‘dropstone’-bearing unit reveals the presence (hornblende–biotite gneiss) with quartzites (found as pebbles of both oscillation ripples (Fig. 3b) and desiccation cracks in some of the sandstones). Mixing of these rock types in a (Fig. 3c), indicating the shallow-water to exposed nature of the ratio of c. 3:1 reproduced the major element composition of depositional environment (see discussion by Stewart 1997). the Stoer Group sandstones. Van de Kamp & Leake (1997) Although both sedimentary structures can form in glacial proposed that some of the Torridonian sediments were lakes, which may be subject to large depth fluctuations related, deposited in ‘closed basin’ conditions and, by analogy with for example, to ice-damming, their common occurrence makes deposits in Triassic–Jurassic rift basins of the northeastern this interpretation less likely than one involving constant United States, suggested that there was extensive contempor- shallow-water to emergent conditions. When the ‘dropstone’- aneous albitization of sandstones in the Stoer Group and bearing unit is followed laterally to the north it can be seen to the Diabaig Formation, which forms the basal part of the pass into a boulder-rich orthoconglomerate (Fig. 3d). Because Torridon Group. the ‘dropstone’-bearing unit passes to the south (distally) A variety of depositional settings has been proposed for the into desiccation cracked and ripple-marked sediments and Torridonian sedimentary assemblages. Stewart (1982, 1991) northwards (proximally) into boulder conglomerate, it is inter- proposed that they accumulated in a complex series of rifts and preted as the distal portion of a subaerial fan that debouched half grabens. Following erosion, peneplanation and deposition into an ephemeral lake or shallow marine setting. The of platformal Cambro-Ordovician sedimentary rocks, the asymmetrical nature of the deformation beneath the large clast region was variably affected by Silurian thrusting (Van shown in Fig. 3a also supports an interpretation involving Breemen et al. 1979). Many of the rift-bounding normal lateral rather than vertical emplacement. faults were thought to be reactivated as thrusts, including the Red to purple desiccation cracked and ripple-marked Moine Thrust (Fig. 1) that juxtaposes metasedimentary rocks mudstones were also observed at a similar stratigraphic of the Moine assemblage against the Torridonian and horizon on the west side of the more westerly of two elongate Cambro-Ordovician successions. This basic model has been basement ridges (see Stewart 1997, fig. 1). At this locality, they adopted by most subsequent workers. Soper & England (1995) are overlain by large (up to 1 m) gneiss boulders, which are proposed that the Torridonian rift deposits and the succeeding interpreted as the result of gravitational displacement from the Neoproterozoic Dalradian Supergroup were all products of an adjacent basement high. extended period of rift tectonics that preceded opening of the Davison & Hambrey (1996) also presented various other Iapetus ocean. Thus the Torridonian rocks are considered to lines of evidence that they considered to favour a glacial have formed in a series of lakes or from large scale river interpretation. These included the presence of a clast fabric in systems (Williams 1969; Stewart 1991; Van de Kamp & Leake ‘diamictites’, the presence of ‘roche moutonne´e’-like forms, 1997) developed in an extensional regime. lee-side(?) accumulations of breccia and variations in clast shape in the ‘diamictites’. Most of these points were debated by Stewart (1997) and Davison & Hambrey (1997) but the ques- tions of the origin of the clast fabrics reported in conglomeratic Glacial deposits in the basal part of the Stoer Group? rocks by Davison & Hambrey (1996) and the use of the term Mainly on the basis of local development of erosional features ‘diamictite’ for some of these rocks are briefly discussed below. on the Lewisian basement rocks, the presence of conglomer- Clast fabrics in such rocks are not necessarily diagnostic of a ates and breccias considered to be diamictites and the occur- glacial origin. For example Selley (1965) described a similar rence of isolated gneiss boulders in finely bedded sedimentarty fabric from a conglomerate forming part of the Diabaig rocks, interpreted as ice-rafted clasts, Davison & Hambrey Formation on the island of Raasay. He interpreted these rocks 1100 G. M. YOUNG
Fig. 3. (a) Photograph on the south side of Enard Bay (Fig. 1) showing a large isolated gneiss clast (c. 2 m across) in finely bedded mudstones and granule-to-pebble breccias and conglomerates. Note that the mudstones are more strongly deformed on the left (south) side of the clast. These clasts are thought to have been emplaced by lateral movement, related to mass flow, rather than as dropstones from floating ice (Davison & Hambrey 1996). Photograph represents location A in (d). (b) Oscillation ripple marks on the surfaces of several mudstone beds that are laterally equivalent to the boulder shown in (a). Photograph taken at location B of (d). Lens cap is about 5 cm in diameter. (c) Ripple marks and desiccation cracks in mudstones at the same locality as (b). These beds are laterally equivalent to the beds with outsized clasts shown in (a) and (d) and suggest that the depositional environment was shallow-to-emergent. Coin is about 2 cm in diameter. (d) View on the south side of Enard Bay (see Fig. 1 for location), looking westwards at the basal beds of the Stoer Group, with large isolated gneiss clasts (A). Beds laterally equivalent to those with large isolated clasts contain ripple marks and desiccation cracks (location B). Note that the clast-bearing mudstones pass to the north (right on the photo) into a massive boulder conglomerate (C). Large rounded clast at middle left (A) is that shown in (a). as having formed as a subaerial mudflow and that the fabric casts some doubt on the interpretation as dropstones because it was imposed by a flood of water such as ‘typically follows a is unlikely that such large clasts could be rafted in shallow to mud-flow’ (Selley 1965, p. 375). Davison & Hambrey (1997) emergent conditions. The so-called ‘dropstones’ are interpreted argued that some of the rocks forming the basal part of the as boulders that became isolated by rolling or sliding ahead of Stoer Group are diamictites, citing papers by Eyles et al. (1983) a small subaerial fan into shallow ephemeral lake deposits, and Moncrieff (1989), which give definitions of diamicton/ where they depressed and deformed the existing bedded diamictite that include up to 80% clasts. Hambrey (1994), sediments (Fig. 3a). however, suggested that the term ‘diamictite’ should be restricted to sedimentary rocks with <50% of gravel and it is clear from the early literature (e.g. Pettijohn 1975, p. 170) that the term diamictite was intended for rocks that were not ‘the Previous geochemical investigations product of normal aqueous transport’ and that a ‘disrupted Most previous geochemical studies of the Torridonian rocks framework’ was implicit. According to this concept, which have been concerned with sandstones and with major element is surely more meaningful than the varied and somewhat geochemistry. Stewart (1990) pointed out some unusual arbitrary subdivisions proposed in the more recent literature, aspects of the chemical composition of sedimentary rocks of most of the large clast-bearing rocks in the basal part of the the Stoer Group. He suggested that these sediments were not Stoer Group should not be described as diamictites. solely derived from the Lewisian basement because they had The physical evidence at these localities can thus be inter- lower K/Rb ratios and a high K, Rb, and MgO content. preted without resorting to a glaciogenic origin. The presence Quoting Nd isotope data of O’Nions et al. (1983), Stewart of abundant ripple marks and desiccation cracks in sediments (1990) noted that two samples of red siltstone have crustal laterally equivalent to beds with large isolated gneiss boulders residence ages of 2.00 and 2.33 Ga. Because mantle-derived DEPOSITIONAL ENVIRONMENT OF TORRIDONIAN 1101 materials of this age were not known from northwest Scotland he interpreted these ages (together with geochemical argu- ments) to mean that the Stoer Group rocks were a mixture of Lewisian materials with a crustal residence age of c. 2.9 Ga (Hamilton et al. 1979) and volcanic material, contempor- aneous with deposition of the Stoer Group, as exemplified by the Stac Fada Member, the only known volcanic unit in the entire Torridonian. The Loch Maree Group, a Palaeo- proterozoic supracrustal succession in the Lewisian complex has, however, recently yielded zircons with U–Pb ages of about 2.0 Ga (Whitehouse et al. 1998), suggesting that a crystalline terrain of this age could also have contributed to the Torridonian sediments. Rainbird et al. (1998) have also demonstrated from Pb–Pb zircon dating that some of the sandstones in the Stoer Group also have a ‘young’ (1.91– 1.75 Ga) component. Stewart considered that some mudstones within the Stoer Group underwent expansion–contraction events due to the presence of abundant smectitic clays, related to the proposed contemporaneous fall-out of wind- borne volcanic materials. He considered the Stoer Group to have formed at low latitudes (10–20), as suggested by palaeomagnetic studies (Torsvik & Sturt 1987). Van de Kamp & Leake (1997) noted petrographic evidence of albitization in sandstones of the Stoer Group and suggested that the Na was concentrated in closed basins, then reacted with the minerals of the underlying sediments. They proposed a more mafic source for the lowest Stoer unit, the Clachtoll Formation and more felsic provenance for the overlying westerly-derived Bay of Stoer Formation and the easterly- sourced Meall Dearg Formation. Van de Kamp & Leake (1997) cited high K/Rb ratios in the Clachtoll sandstones as indicating a dominantly granulitic provenance. They consid- Fig. 4. Sketch map showing the geology of NW Scotland. Sampled localities are shown by stars. The number of samples from each ered the mafic nature of the Clachtoll Formation to be due to locality and unit is indicated by the numbers in parentheses. Note its provenance from the more mafic Central Region of the the division of the Lewisian terrain and its extensions beneath the Northwest Highlands (Bowes 1972), which underlies the Stoer Moine rocks into a central region (Scourian rocks, which generally Group rocks in the study area. They also noted high K/Ba exhibit granulite-facies metamorphism) and north and south regions ratios (relative to granulites) in the Torridonian sedimentary (regions affected by Laxfordian metamorphism, generally at rocks, including the Stoer and attributed these either to amphibolite grade), after Weaver & Tarney (1981). For explanation derivation from a more granite-dominated provenance, or to of patterns see Fig. 1. concentration of K-feldspars in the sediments, as plagioclase was lost during weathering. Stewart (1990) and van de Kamp & Leake (1997) proposed Sampling and analytical methods that the sediments of the Stoer Group formed in closed basins Rock samples were collected from coastal regions of NW Scotland, in a warm, evaporitic low-latitude setting, contrary to the extending from Sheigra in the north (Fig. 1) to an area north of suggestion by Davison & Hambrey (1997), that the basal part Aultbea in the south. All of the samples from the Stoer Group were of the Stoer was deposited under glacial conditions. taken from the ‘central region’, which is underlain by rocks of the Scourian part of the Lewisian Complex. Samples from the Diabaig Formation of the Torridon Group were collected from near Achiltibuie where the Torridonian beds overlie rocks of the central Present investigation Lewisian region and from the Loch Torridon area in the southern region (Fig. 4). Samples of the Applecross Formation (Torridon Previous geochemical investigations of the Torridonian Group) were collected at Sheigra (Fig. 4), which is in the northern rocks have concentrated mainly on major (and some trace) region of the Lewisian Complex, and others were taken from the Loch elements and have been concerned largely with sandstones (e.g Torridon area in the southern region. Stewart 1990, 1991, 1995a; Stewart & Donnellan 1992; van de In preparation for chemical analysis, care was taken to remove any Kamp & Leake 1997). In this study, attention is focused on surfaces that showed evidence of ‘recent’ chemical weathering. The mudstones, which are considered to be more sensitive indica- samples were washed in distilled water to remove any possible tors of weathering conditions (Nesbitt & Young 1982, 1984) salt-water contaminants. Some of the samples were analysed in and are therefore potentially useful in palaeoclimatic investi- laboratories at the University of Western Ontario, using standard gations. This study also concentrates on trace elements, many X-ray fluorescence (XRF) techniques. Some trace elements and rare earth elements were analysed by Instrument Neutron Activation of which tend to be more abundant in fine-grained sediments Analysis (INAA). Other samples were analysed by Activation than in sandstones. Some trace elements, because of their low Laboratories Ltd, in Ancaster, Ontario where major and trace solubilities, tend to be relatively immobile during diagenesis elements were determined by fusion ICP/MS analysis. In both and metamorphism and thus may provide useful provenance laboratories standard and duplicate analyses were carried out in order indicators. to check that precision and accuracy were within acceptable limits. 1102 G. M. YOUNG
Normalizing factors used for REE are from Masuda et al. (1973) and of post-Archaean Australian shale (PAAS of Taylor & Taylor & McLennan (1985). McLennan 1985). The relatively low CIA values for the lower Stoer Group appear to support the suggestion by Davison & Hambrey (1996) that these rocks formed in a glacial environ- Major elements ment, where chemical weathering would have been inhibited New major and trace element analyses were obtained from a (Nesbitt & Young 1982). A suite of Recent sediment samples total of 36 rock samples. Of these analyses, 23 are from derived from a granite in the Mojave Desert of California mudstones of the Torridonian succession, two represent lapilli (Fedo et al. 1997a), however, displays similar low CIA values, tuff from the Stac Fada member and 11 represent matrix so that this aspect of the major element geochemistry cannot material (i.e. excluding large clasts) of the partly volcanic Stac be used to differentiate between mudstones from desert Fada Member. Average values for the various lithologies (shaded area on Fig. 5a) and glacial environments. In both studied are listed in Table 1. regimes chemical weathering appears to be inhibited. As a technique for investigating and quantifying the degree When analysed samples of mudstones from the overlying of weathering to which sediments and sedimentary rocks have Torridon Group are plotted in A–CN–K space (Fig. 5b), they been subjected, Nesbitt & Young (1982, 1984) introduced a show a slightly greater range of CIA values, with some samples chemical index of alteration (CIA), which is calculated as from the Applecross Formation reaching values comparable to follows: the average Australian post-Archaean shale. Samples from the Diabaig Formation, have lower CIA values, possibly reflecting CIA=[Al2O3/(Al2O3+CaO*+Na2O+K2O)] 100. their local derivation (van de Kamp & Leake 1997), with incorporation of a lower percentage of recycled material than Molar proportions of the oxides are used in the calculation of samples from the overlying formations. the CIA. CaO* is CaO incorporated in silicates as opposed to Spider diagrams (Fig. 6) representing the ratio of the carbonates or phosphates. Details of the calculations and average analysed Applecross Formation mudstones (5) to the corrections may be found in Fedo et al. (1995). The CIA may average Diabaig mudstones (4) show that the Torridonian be expressed as an absolute number, usually ranging from samples are relatively depleted in Ca and Na and contain more about 50 for unweathered rocks to 100 for extremely altered K. These differences may be explained by incorporation of materials that are composed entirely of secondary aluminous more weathered material into the Torridon Group samples. clay minerals such as gibbsite and kaolinite. The CIA may also However, strong similarities in the trace elements, especially in be shown in triangular diagrams (Fig. 5), with Al2O3, REE suggest that the Diabaig and Applecross samples had CaO*+Na2O and K2O (molar) at the apices. This triangle, a similar provenance. For this reason the Torridon and known as the A–CN–K triangle, permits the identification of Diabaig samples have been combined to calculate an average ‘anomalous’ samples and trends that deviate (because of Torridon value in Table 1 but the two formations have been metasomatic alteration) from the expected weathering trends differentiated in most diagrams. as established both by theoretical considerations and empirical The strong deviation of the trend representing the Torridon observations on modern weathering profiles (Nesbitt & Young Group samples (dotted arrow in Fig. 5b) from the expected 1984; Fedo et al. 1995). The CIA values for the analysed Stoer weathering trend (arrow parallel to the A–CN join, after Group samples range from about 38 to 66, with an average of Nesbitt & Young 1984, 1989), suggests that they have under- about 53 for the lower Stoer (all of these samples are from gone K-metasomatism, as can be illustrated for various ancient below the Stac Fada Member and close to the basement, so weathering profiles (Rainbird et al., 1990), including those that they are considered equivalent to the breccia sandstone developed beneath the Applecross Formation at Sheigra facies of Stewart, 1990) and about 50 for the upper part of the (Young 1999). If lines drawn through the points representing group. individual mudstone samples and the K-apex are projected The mudstone samples from the Stoer (black dots and open onto the ‘expected’ weathering trend, an estimate of the circles) are plotted in Fig. 5a, together with an average pre-metasomatism CIA value may be obtained (Fedo et al. estimate for the composition of the Lewisian basement, based 1995). Such a procedure results in much higher CIA values on a 9:1 ratio (after Stewart 1990) of the average of 254 (c. 80) for many of the Applecross Formation samples. Stewart granulite facies gneisses from the Drumbeg area (Weaver & (1990) suggested that features such as the high K content of Tarney 1981) and 54 diabase dykes from the Assynt area some of the Torridon Group rocks could be explained as a (Tarney 1973). Two estimates of the average composition of result of derivation from older sedimentary or metasedimen- the Laxfordian crust from the northern area (Fig. 4) are also tary rocks but the fact that there has been significant in situ shown by the open squares on Fig. 5b. The trend shown by the K-addition to the sub-Torridon palaeosols shows that meta- lower Stoer samples passes close to the Scourian basement somatism has occurred. As noted by Nesbitt & Young (1989) estimate, suggesting that these sediments were ‘locally’ derived. and Fedo et al. (1997c), K-metasomatism is common in The three samples with the lowest CIA values are from sedimentary successions such as the Torridonian, that formed mudstone that fills fractures in the basement near Clachtoll in continental settings. just south of Stoer (Fig. 1), and from mudstone and granule Stratigraphic variation in CIA values is shown in Fig. 7, conglomerate just above the basement on the south side of which illustrates the relatively low values for many of the Stoer Enard Bay (Fig. 4). The trend for the upper Stoer mudstone Group mudstones and the higher values of the Torridon samples falls close to an upper crustal estimate, which is also Group samples. The low CIA values for some of the Stoer close to the Laxfordian average estimates on this plot (see samples can be shown to to be related to unusually high CaO
Fig. 5b), perhaps supporting the observation (Stewart 1990) and Na2O values. Values for these two oxides (normalized to that the associated sandstones include ‘exotic’ materials. TiO2) are plotted in Fig. 8a. The Stac Fada samples are rich in The Stoer mudstones (dots and open circles) fall much lower Na and several of the Stoer samples show both high CaO and on the triangle than an estimate of the average composition Na2O. Three of these samples (labeled B on Fig. 8a) were DEPOSITIONAL ENVIRONMENT OF TORRIDONIAN 1103
Table 1. Average chemical composition of Torridonian samples and average estimates of Laxfordian (LAX) and Scourian basement (SC), post-Archaean Australian shale (PAAS) and Upper continental crust (UCC)
SF (11)1 LT (2) SFM (4) STM (10) TM (9) LAX SC PAAS UCC
Major elements
SiO2 65.39 68.60 60.21 60.03 56.96 66.70 60.06 62.80 66.00 TiO2 0.52 0.54 0.68 0.67 1.08 0.34 0.66 1.00 0.50 Al2O3 13.69 13.66 15.78 14.17 17.82 16.00 15.25 18.90 15.20 t Fe2O3 5.58 5.23 7.22 6.47 8.39 3.60 6.87 7.23 5.00 MnO 0.09 0.06 0.08 0.09 0.12 0.04 0.09 0.11 — MgO 4.12 2.57 4.23 4.01 3.11 1.40 3.66 2.20 2.20 CaO 1.12 0.92 0.76 3.52 1.03 3.20 6.00 1.30 4.20
Na2O 3.86 5.39 2.52 3.09 1.69 4.90 4.20 1.20 3.90 K2O 2.27 1.07 4.13 2.71 5.10 2.10 0.97 3.70 3.40 P2O5 0.18 0.18 0.15 0.18 0.32 0.14 0.18 0.16 — LOI 3.07 1.73 3.94 5.46 4.37 ———— Total 99.92 99.96 99.69 100.41 99.99 98.42 97.94 98.60 100.40 CIA 56.77 54.86 62.09 51.23 65.30 50.28 45.29 70.36 46.15 Trace elements Ba 803.7 273.5 829.8 610.9 758.1 713.0 275.2 650.0 550.0 Sr 201.2 183.5 137.8 159.1 147.5 580.0 532.6 200.0 350.0 Y 13.4 13.0 20.8 21.7 38.4 7.0 10.6 27.0 22.0 Sc 9.0 9.5 12.5 13.5 18.7 ——16.0 11.0 Zr 157.8 157.0 191.8 185.9 235.1 193.0 194.5 210.0 190.0 Be 1.1 1.0 2.5 2.1 3.7 ———3.0 V 74.5 64.5 127.2 91.8 112.9 ——150.0 60.0 Cr 214.0 155.5 170.8 122.3 74.3 32.0 94.0 110.0 35.0 Co 31.5 24.0 27.2 23.5 24.1 ——23.0 10.0 Ni 219.2 199.5 93.0 79.0 55.9 20.0 62.6 55.0 20.0 Cu 18.7 56.0 23.7 28.7 49.9 ——50.0 25.0 Zn 60.7 112.0 50.2 72.9 94.6 ——85.0 71.0 Ga 15.2 15.5 21.5 19.1 26.6 ——20.0 17.0 Ge 0.9 1.0 1.1 1.2 1.4 ———1.6 As 11.5 5.0 7.0 8.2 6.5 ———1.5 Rb 66.8 29.8 128.5 85.0 168.7 74.0 11.2 160.0 112.0 Nb 5.5 5.9 7.5 8.7 18.6 6.0 5.0 19.0 25.0 Mo 0.7 0.6 0.3 0.8 0.8 ——1.0 1.5 Sn 0.8 1.0 1.6 2.3 2.8 ——4.0 5.5 Sb 0.3 0.3 0.5 0.6 0.5 ———0.2 Cs 0.9 0.4 3.5 2.6 5.9 ——5.0 3.7 Ba 802.2 265.0 803.2 613.4 664.9 — 710.8 650.0 550.0 Hf 3.8 4.2 5.3 5.0 7.0 3.8 3.6 5.0 5.8 Ta 0.3 0.3 0.5 0.6 1.2 0.5 0.6 5.0 2.2 W 0.5 0.2 2.0 0.5 1.0 — 0.6 2.7 2.0 Tl 0.4 0.2 0.5 0.5 0.8 ——2.7 0.75 Pb 15.4 26.0 11.0 24.0 22.0 22.0 12.6 20.0 20.0 Bi 0.1 0.1 0.1 0.3 0.3 — 12.6 20.0 0.13 Th 4.9 4.9 8.4 8.2 14.1 8.4 — 14.6 10.7 U 1.0 1.3 2.1 3.2 4.7 ——3.1 2.8 Rare earth elements La 33.9 45.6 32.3 32.3 57.9 36.0 22.0 38.0 30.0 Ce 66.4 79.0 64.1 65.3 144.3 69.0 43.2 80.0 64.0 Pr 7.3 8.4 8.6 7.4 14.1 — 7.5 8.9 7.1 Nd 28.5 34.1 32.0 29.0 64.4 30.0 18.5 32.0 26.0 Sm 4.7 5.4 5.1 5.3 16.1 4.4 3.3 5.6 4.5 Eu 1.2 1.2 1.2 1.2 3.5 1.1 1.2 1.1 0.9 Gd 3.9 4.3 4.9 4.6 7.0 ——4.7 3.8 Tb 0.5 0.5 0.6 0.7 1.5 0.4 0.5 0.8 0.6 Dy 2.5 2.7 3.5 4.0 8.3 — 2.9 4.4 3.5 Ho 0.5 0.5 0.7 0.8 0.7 — 0.5 1.0 0.8 Er 1.4 1.4 2.3 2.4 4.3 — 1.3 2.9 2.3 Tm 0.2 0.2 0.3 0.4 0.6 0.1 0.2 0.4 0.3 Yb 1.1 1.2 2.0 2.2 4.6 0.8 1.3 2.8 2.2 Lu 0.2 0.2 0.3 0.3 0.6 — 0.2 0.4 0.3
SF, Stac Fada Member; LT, Lapilli tuff samples from the Stac Fada member; SFM, mudstones within the Stac Fada Member; STM, Stoer Group Mudstones; TM, Torridonian mudstones. Numbers in parentheses are the number of samples used to calculate the averages. LAX, Laxfordian basement estimate from Weaver & Tarney (1981, table 1, column 4). SC, Scourian Crust estimate from Weaver & Tarney (1981) and Tarney (1973), with average estimates for gneiss and amphibolite in a ratio of 9:1 (following Stewart, 1990). Values for post Archaean Australian shale (PAAS) and upper continental crust (UCC) from Taylor & McLennan (1985). Dashes indicate that no data are available. 1104 G. M. YOUNG
Fig. 5. Al2O3–CaO*+Na2O–K2O(A–CN–K) plots (Nesbitt & Young 1984, 1989) for mudstones of the Stoer (a) and Torridon (b) Groups. Samples from the Stoer Group (a) plot quite low on the triangle, close to the feldspar join (50–50 line), suggesting that they have not undergone significant weathering. Some of the lower Stoer samples (black dots) may have undergone K-metasomatism because they depart from the ‘expected’ weathering trend, parallel to the Al–CN boundary. The shaded area shows the field representing a suite of Recent sediments from an arid area in the Mojave desert of California. From (b) it can be seen that the material in the Torridon Group mudstones has undergone more severe weathering. The general trend shown by the Torridon mudstone samples (dotted arrow) indicates that these rocks have undergone significant K-metasomatism. The arrow parallel to the A–CN join shows the expected weathering trend, if the rocks were derived from average upper continental crust. After ‘correction’ to remove excess K from the samples, many of the values are considerably higher. taken from beds lying directly on, or very close to the Scourian Scourian basement. By contrast the samples from the Torridon basement and may simply reflect the high value for the average Group contain much lower amounts of both MgO and Ni, basement estimate (x). Three other samples with high CaO comparable to an average shale estimate (PAAS). content (but less elevated Na2O values) are from the upper A comparison between an estimate of the major element part of the Stoer Group (Poll a’ Mhuilt Member of Stewart composition of the Scourian basement and the average com- 1990). These rocks (labeled C on Fig. 8a) include carbonate position of the Torridonian mudstones is shown in Fig. 9a. beds and have been interpreted as a series of lake deposits, The mudstones of the Stoer Group show the strongest overall possibly in a region of internal drainage. The high CaO similarity to the basement rocks, supporting earlier suggestions content of some of the Stoer Group rocks (those near the that they were mainly locally derived (Stewart 1990; van de basement) is thus thought to reflect the composition of the Kamp & Leake 1997). The MgO content of the analysed local Scourian rocks whereas those from the upper part of mudstones from the Stoer is not significantly different from the Stoer Group appear to owe their unusually low CIA values that of the basement estimate but Mg is normally removed to the presence of carbonate minerals. during weathering (compare the data for the average Torridon A high MgO content (together with high K, Rb, Y and Th) Group muds—crosses in Fig. 9a). The Stoer mudstones show was reported from the sediments of the Stoer Group by evidence of some Ca loss and K addition (relative to the Stewart (1990), who suggested that these characteristics might basement composition), as is usually the case during weather- be due to the presence of fine-grained volcanic material, ing and subsequent diagenesis. The major element composition incorporated by airfall into the sediments. He considered that of the Stac Fada Member is similar to that of the Stoer Scourian rocks could not provide the relatively high MgO mudstones except for the much lower Ca content of the Stac content of the Stoer sediments, without supplying a much Fada. The similarities are thought to be due to the abundant higher Ni content than that observed. In order to look for sedimentary component in the matrix of the Stac Fada which evidence of incorporation of volcanic materials like those of was probably derived from the Stoer Group (Lawson 1972; the Stac Fada Member into the Stoer mudstones, Ni was Sanders & Johnson 1989; Stewart 1990). The Torridon plotted against MgO after normalizing to TiO2. The Stac Fada mudstones have relatively high values for ‘resistate’ elements Member and the Stoer mudstones fall into distinct fields such as Ti, Fe and Mn. Compared to the Stoer samples without overlap (Fig. 8b). The estimated average MgO and Ni the Torridon mudstones show strong depletion in ‘mobile’ contents (normalized) of the Scourian basement are similar to elements such as Mg, Na and especially Ca. These character- those of the Stoer mudstones. Stoer mudstones with slightly istics suggest that the materials in the Torridon muds are much higher MgO content are from very close to the basement, more chemically mature than those of the Stoer. The Torridon suggesting that they reflect local variability within the Scourian samples also exhibit strong K-enrichment, which is typical of basement, rather than incorporation of volcanic materials more highly weathered materials as a result of metasomatic from the Stac Fada. A few samples that fall out of the addition to kaolinite-rich muds. Comparison of the average designated fields are lapilli tuff (black triangles) or mudstones major element composition of the Torridon Group mudstones from within the Stac Fada Member (open circles), both of to an estimate of the composition of the Laxfordian basement which would be expected to show mixing. The high MgO in part of the northern Lewisian region (after Weaver & content of the Stoer mudstones may be explained as a result of Tarney 1981, table 1, column 4) is also shown in Fig. 9a. This their provenance (without significant weathering) from the plot shows that the Torridon Group muds deviate more from DEPOSITIONAL ENVIRONMENT OF TORRIDONIAN 1105
Fig. 7. Stratigraphic variations in CIA values. Dots represent individual analyses. The dark line joins the average values for each unit. The shaded area shows one standard deviation on each side of the mean. Note that all of the Torridonian mudstones have values lower than average shale estimates but the values obtained from most of the samples are probably lowered due to K-metasomatism (see Fig. 5).
and Na, elements that are susceptible to removal by dissolu- tion during weathering. This plot confirms the more mature nature of the Torridon Group materials.
Trace elements There are few reports dealing with trace elements in the Torridonian succession. Certain trace elements are considered to be conservative during weathering, erosion, transport, deposition, diagenesis and even metamorphism (Taylor & Fig. 6. (a) Spider diagrams showing the average major element McLennan 1985). Although the rare earth elements (REE) values (wt%) for mudstones (five samples) of the Applecross have been shown to be somewhat mobile in certain Formation, divided by those of mudstones (four samples) from the environments (Nesbitt 1979; Duddy 1980; Nesbitt & Diabaig Formation. The compositions are fairly similar (close to Marcovics 1997), mixing of materials derived from large unity) except for relative depletion of the Applecross samples in the drainage basins is thought to result in REE distributions that mobile elements Ca and Na and relative enrichment in K. These reflect the composition of the provenance terrain. differences are compatible with greater weathering of the Applecross samples. (b) Trace element ratios for the same samples as those used in constructing (a). (c) Chondrite-normalized rare earth element ratios for the Applecross and Diabaig samples. Note the close Incorporation of volcaniclastic material into the similarities between the two sample suites in both (b) and (c). Stoer Group? Figure 10a shows a comparison between the average trace the Laxfordian estimate than they do from the Scourian element compositions of the analysed Stoer and Torridon basement. Perhaps more significantly, comparison between the mudstone samples. The most obvious difference is the higher average REE composition of Laxfordian gneisses and the Cr and Ni content of the Stoer samples. In Fig. 10b the average Torridon Group mudstones reveals significant differences (see compositions of the Stoer and Torridon mudstones are com- following section on rare earth elements). pared with an average estimate for post-Archaean Australian Figure 9b shows a comparison between the major element shale (PAAS of Taylor & McLennan 1985). The Torridon composition of the average post-Archaean shale (PAAS of samples show a fairly close correspondance with PAAS Taylor & McLennan 1985) and the Stoer and Torridon (crosses in Fig. 10b) whereas the Stoer Group samples differ mudstones. The Torridon mudstones are not significantly considerably in some trace elements. Ni and Cr are particularly different from PAAS, except for slight enrichment in Na and abundant and there is considerable depletion in some elements K. By contrast the Stoer mudstones are enriched in Mg, Ca such as Ba, Sr and Rb. 1106 G. M. YOUNG
Fig. 8. Scatter diagrams showing the distribution of CaO v. Na2O and Ni v. MgO (all Ti-normalized) in the analysed Torridonian rocks. The sodic nature of the Stac Fada Member samples (triangles) can be seen in (a). The Torridon Group mudstones and most of the Stoer samples contain much less Na. Some samples collected very close to the Lewisian basement (indicated by the letter B) have a high Na content, probably reflecting that of the basement (x). Three samples (labelled C) from the upper part of the Stoer Group have an unusually high CaO content, which is probably due to the presence of carbonate. The high Ni and Mg content of the Stac Fada Member can be seen in (b). The Stoer Group mudstones have a lower content of both elements and plot around the average Scourian estimate. The Torridon mudstones contain lower amounts of both elements and fall closer to an estimate for the upper continental crust and an average post-Archaean shale (PAAS).
Fig. 9. Spider diagrams to compare the average major element compositions of Torridonian rocks with the estimated composition of the Scourian and Laxfordian basement (a) and an average shale estimate (PAAS) (b).
K/Rb ratios and provenance basement. Analytical data from the Stoer and Torridon group Rb is much more abundant in all the Torridonian mudstones mudstones, are shown in Fig. 11. Estimated average values for than in the Scourian or Laxfordian basement rocks (Table 1 the Scourian and Laxfordian gneisses are also shown, together and Fig. 11). The high Rb content appears to be related to the with an estimate of the composition of the present upper crust high degree of K-metasomatism that these rocks have under- and PAAS. The K/Rb ratios shown by all of the sedimentary gone (see Fig. 5). Although K and Rb enrichment are most samples fall within a relatively restricted range, illustrating the marked in the Torridon mudstones, both elements are also strong correlation between the two elements. Perhaps the most significantly enriched in the analysed Stoer mudstones, relative significant difference between the Torridon and Stoer samples to the Scourian basement, which was their likely source is that the former have much higher contents of both Rb and (Table 1). K/Rb ratios have been proposed as a provenance K. The K:Rb ratio of the average Laxfordian estimate is indicator, mainly for sandstones, in previous studies of the comparable to those of both the Stoer and Torridon Group Torridonian (Stewart 1990; van de Kamp & Leake 1997). mudstones but both elements are much more abundant in the Differences in K/Rb values between the Scourian basement sedimentary rocks. That such differences can be brought about and the Torridonian sedimentary rocks were taken as evidence by weathering (followed by K-metasomatism) is shown by the that their provenance was not entirely from the Scourian trends from evenly spaced samples representing a c. 3 m thick DEPOSITIONAL ENVIRONMENT OF TORRIDONIAN 1107
Fig. 10. (a) Spider diagrams comparing average trace element compositions of mudstones from the Stoer and Torridon Groups and (b) comparison of the average Torridonian mudstone compositions with an average shale estimate (PAAS). palaeosol (Williams 1968; Williams & Schmidt 1997) of the K (and Rb) in the palaeosols is probably metasomatic in developed on the Laxfordian gneissic basement at Sheigra origin (Nesbitt & Young 1984; Fedo et al. 1995) the K:Rb (thick arrow in Fig. 11). Both the Rb and K contents of rocks ratios in the sedimentary rocks most likely reflect the com- may therefore be changed by weathering processes, involving position of metasomatic fluids rather than that of the formation of clay minerals such as kaolinite that act as provenance area. By analogy with the palaeosol, the relatively receptors for subsequent addition of K to form sericite/illite high K and Rb contents of the Torridon Group samples during diagenetic and later metasomatic processes. Since most suggest that these materials underwent more weathering than the Stoer Group sediments. This interpretation is also in keeping with the CIA data from these rocks (Fig. 5). The fact that most of the Stoer Group values are lower than that of average present-day upper crust suggests that they were not derived from crustal material of this composition because the weathering processes involved in production of fine grained siliciclastic rocks would normally lead to elevated K and Rb values. Thus K/Rb ratios in siliciclastic sedimentary rocks are probably controlled to a large degree by the composition of metasomatic fluids rather than the composition of the provenance area.
Th/Sc ratios and provenance Th tends to be an incompatible element during igneous crys- tallization processes, whereas Sc usually accumulates in early formed minerals in mafic magmas. Thus the Th/Sc ratio has been used (Taylor & McLennan 1985; Fedo et al. 1997b; Panahi & Young 1987) as an indicator of crustal differentia- tion, and may have decreased from the Archaean to the Proterozoic. The Stoer samples have Th/Sc ratio values com- parable to that suggested by Taylor & McLennnan for the Fig. 11. Plot of K v. Rb (both expressed as ppm) for the analysed >3.0–2.5 Ga crust (Fig. 12). Estimated ratios for younger mudstone samples from the Stoer Group (shaded area) and (Proterozoic) crust are higher, in keeping with incorporation of Torridon Group (area enclosed by line). Note that the K/Rb ratio is a higher percentage of differentiated crustal materials. PAAS similar for all samples but that the Torridon Group samples are has a ratio similar to that of post-Archaean upper crustal richer in both elements. The average Scourian basement estimate estimates. The Torridon Group mudstones fall between the show a much higher K/Rb ratio but the amounts are significantly lower than those found in the sedimentary rocks. The Laxfordian younger upper crustal estimates and Archaean values but have gneiss estimate (open squares) Most of the K (and Rb) was higher Th:Sc ratios than the Stoer rocks, again supporting the probably metasomatically added so that the K/Rb ratio in the rocks idea that these sediments had a more varied provenance and more closely reflects the nature of the metasomatic fluids than the were derived, at least in part, from crustal materials that were composition of the provanance area.. The heavy black arrow shows younger than the Scourian basement (Rogers et al. 1990; the observed changes in Rb and K content in evenly spaced samples Rainbird et al. 1998). Weaver & Tarney (1981) reported a from a c. 3 m-thick paleosol developed on the Laxfordian basement higher Th value (8.4 ppm) from Laxfordian crust than from near Sheigra (see Fig. 1 for location). Symbols are as in Fig. 8. the Scourian (0.42). The Laxfordian values are, however, 1108 G. M. YOUNG
Fig. 12. PLot of Th v. Sc for the analysed Torridonian rocks. The Stoer Group mudstones generally have lower Th/Sc ratios than the Torridon Group samples, in keeping with their derivation from older crust. The crustal estimates for different ages are from Taylor & McLennan (1985). Symbols for Torridon Group are as in Fig. 8. considerably lower than those of the Torridon Group samples so that a Laxfordian source does not appear to be appropriate for these sediments.
Rare earth elements There are few published analyses of rare earth elements from the Torridonian succession. For this study data on REE were obtained from mudstones of the Stoer and Torridon groups and the mixed volcanic and sedimentary materials of the Stac Fada Member. The REE are commonly used as provenance indicators in the study of sedimentary terrains (e.g. McLennan et al. 1979; Morey & Setterholm 1979). Chondrite-normalized REE patterns for the Torridonian averages were plotted together with an average shale estimate (PAAS), an average upper crustal estimate and a partial REE analysis of the average Scourian (Fig. 13a). All of the patterns indicate much greater REE abundances (from 10 times to over 100 times) than in average chondrites. Among the samples represented in Fig. 13a the Torridon Group mudstones have the greatest concentration of REE. All of the analyses represented in Fig. 13a show enrichment in LREE. The Stac Fig. 13. (a) Chondrite-normalized REE patterns (average values) Fada Member is relatively depleted in HREE. for the analysed Torridonian mudstones and samples of the Stac Average chondrite-normalized REE patterns for both the Fada Member, together with estimates for the upper continental crust and PAAS (after Taylor & McLennan 1985) and an average Stoer and Torridon Group mudstones are generally similar Scourian estimate based on analyses reported by Weaver & Tarney (Fig. 13b). The Stoer Group, however, has a somewhat lower (1981) and Tarney (1973). (b) Comparison between average REE concentration of REE and in particular of LREE. The Stoer composition of Torridon Group and Stoer Group mudstones and an samples also have a less pronounced Eu anomaly. The average estimate of the composition of the Laxfordian gneisses (after Weaver REE composition of Laxfordian gneisses from an area in the & Tarney 1981). Note that the Laxfordian estimate shows much northern sector of the Lewisian complex (Weaver & Tarney greater depletion in heavy rare earth elements than either the 1981) is also shown in Fig. 13b. The LREE content of these samples from the Stoer or Torridon Group mudstones. gneisses is similar to that of the Stoer mudstones, but they (c) Comparison between chondrite-normalized Stoer mudstones and show significant depletion in HREE, so that they are unlikely samples of the Stac Fada Member. Note that the Stac Fada samples to be the sole source of either the Stoer or Torridon Group are depleted in HREE. DEPOSITIONAL ENVIRONMENT OF TORRIDONIAN 1109 mudstones. The Stoer mudstones are compared with the average from samples of the Stac Fada Member in Fig. 13c. The distribution of LREE is quite similar but the Stac Fada is considerably more depleted in HREE, confirming the con- clusion, based on other elements (e.g. Fig. 8b), that significant amounts of Stac Fada volcanic material were not incorporated into the Stoer Group during deposition (Stewart 1990). Incor- poration of ash fall materials would be greatest in mudstones, which presumably accumulated more slowly than associated sandstones. The patterns for the Stoer mudstones and the Scourian basement are basically similar, except that most of the REE are more abundant in the Stoer mudstones and a weak negative Eu anomaly appears in the sedimentary rocks. Thus the REE data are not incompatible with derivation of the Stoer mudstones, at least in part, from the local basement and do not require significant input of Stac Fada volcanic material. Apart from greater abundance of REE and development of a stronger negative Eu anomaly, the REE patterns for the Stoer and Torridon Groups are, however, quite similar.
The negative relationship between Eu/Eu* and CIA
The Eu anomaly is a measure of fractionation of Eu relative to Fig. 14. Plot to show the weak negative relationship between CIA Sm and Gd or Tb, using chondrite-normalized values. The and Eu/Eu* (after Gao & Wedepohl 1995) for the analysed development of Eu anomalies in crustal rocks is thought to Torridonian samples. Most of the more highly weathered samples result from intracrustal melting and fractionation, which sep- (high CIA, low value for Eu/Eu*) are from the Torridon Group and arates granitic melts (negative Eu anomalies) from plagioclase- are probably derived from a wide provenance that includes ++ rich residues that are rich in Eu (positive Eu anomalies). abundant recycled material (contributing to the high CIA value) and The presence of a negative Eu anomaly has been considered a high percentage of granitic material (low value for Eu/Eu*). The significant from the point of view of crustal development Stoer Group mudstones are thought to be mostly derived from local (McLennan et al. 1979; Taylor & McLennan 1985; McLennan materials (with a high Eu/Eu*) that have not been recycled or highly & Taylor 1991) although the relatively significant change weathered and thus have relatively low CIA values. proposed by these authors at the Archaean–Proterozoic boundary (c. 2.5 Ga) has been challenged by others (Gibbs sediments (CIA values were probably even higher, prior to et al. 1986; Condie & Wronkiewicz 1990). K-metasomatism, Fig. 5) are preserved over a much greater Gao & Wedepohl (1995) pointed out a negative relationship region than the Stoer Group (Fig. 1) and are commonly between the degree of weathering, calculated as CIA (Nesbitt considered to have a wider provenance (Stewart 1991; & Young 1982) or CIW (Harnois 1988) and the value of the Eu Williams 1969; van de Kamp & Leake 1997). This interpret- anomaly (Eu/Eu*). This relationship was found both for ation is also supported by U–Pb geochronological studies of Archaean pelites and for young deep sea sediments. They zircons from the Torridonian succession (Rogers et al. 1990; presented evidence that the presence of a strong negative Eu Rainbird et al. 1998), which indicate a generally local source anomaly in more highly weathered materials is not due to (Scourian) for the Stoer Group sediments and a much more weathering, a conclusion that is supported by the absence of varied source (including c. 1.0 Ga rocks from the Grenville any correlation between CIA values and Eu/Eu* for samples Province and other post-Archaean Laurentian(?) terrains) for from a number of weathering profiles and palaeosols. Gao & the Torrridon Group. Wedepohl (1995) suggested that the observed negative corre- Previous studies (Moorbath et al. 1967; Williams 1969; Allen lation between Eu/Eu* and CIA values in the pelites and muds et al. 1974; Allen 1991) have documented the presence of that they examined was due rather to a provenance factor. The supracrustal rock fragments in the Torridon Group rocks and relatively unweathered (low CIA, CIW) samples were mainly geochronological studies have indicated the presence of rela- derived from active arc settings and were composed mainly of tively young (c. 1700–1150 Ma) K–Ar and Rb–Sr ages. Some juvenile materials that lacked a strong negative Eu anomaly. of these ages could represent Laxfordian and later events that The more weathered materials were considered to have been appear to have affected some parts of the Lewisian complex, derived from a larger source area that incorporated significant particularly in the Outer Hebrides (e.g. Cliff et al. 1983; amounts of granitic material, resulting in muds with a signifi- Whitehouse 1990). However, the presence of significant cant negative Eu anomaly (low Eu/Eu* values). When the amounts of Grenville age and older Proterozoic zircons analysed Torridonian samples are plotted on a Eu/Eu* v. CIA (Rogers et al. 1990; Rainbird et al. 1998) in Torridon Group diagram, however, they also show a negative correlation sandstones and the presence of REE patterns in the Torridon (Fig. 14). Almost all of the analysed Torridon Group samples Group mudstones that differ markedly from those of the have relatively high CIA values and prominent negative Eu Laxfordian gneisses in the northern region of the Lewisian anomalies, whereas most of the Stoer Group samples have (Weaver & Tarney 1981) support the suggestion by Rogers lower CIA values and weaker negative Eu anomalies. These et al. (1990) and Rainbird et al. (1998) that much of the findings are consistent with the interpretation of Gao & Torridon Group may have been derived from distant sources, Wedepohl (1997). The more highly weathered Torridon Group including parts of the Laurentian Grenville province. Thus the 1110 G. M. YOUNG weak negative Eu anomalies and the flat HREE pattern in the contribute to the solution of problems involving both palaeo- Stoer Group mudstones may reflect the REE characteristics of climatology and provenance. the basement rocks (Weaver & Tarney 1983; Fig. 13a), whereas the Torridon Group rocks more closely resemble upper crustal or average shale (PAAS) values in having relatively strong This study was made possible by financial support from the National Scientific and Engineering Research Council of Canada. I negative Eu anomalies (Fig. 13a). am grateful for helpful discussions of Torridonian problems in the field with A. D. Stewart, S. Davison and A. Panahi. I am par- ticularly grateful to H. W. Nesbitt, for providing valuable assistance Conclusions in many aspects of the geochemistry of sedimentary rocks. The manuscript benefitted especially from insightful comments by A. D. Field evidence such as the presence of desiccation cracks and Stewart and by journal reviewer R. C. Selley, and an anonymous oscillation ripples, suggests shallow to emergent conditions, so reviewer. M. J. Hambrey also provided valuable comments. C. Wu that it is unlikely that large ‘isolated’ clasts in laminated to supervised the geochemical analyses carried out at the University of finely bedded sediments near the base of the Stoer Group in Western Ontario. the vicinity of South Enard Bay were dropped from floating ice. A lateral transition into boulder conglomerates suggests, rather, that the large gneiss boulders were emplaced from References subaerial fans into shallow water bodies, possibly ephemeral lakes. Mudstones in the lower part of the Stoer Group A, P. 1991. Provenance research: Torridonian and Wealden. In:M, A.C., T S.P. & H, P.D.W. (eds) Developments in Sedimentary typically exhibit low values for a Chemical Index of Alteration, Provenance Studies. Geological Society, London, Special Publications, 51, suggesting that their constituent materials were not strongly 13–21. weathered. Such conditions are typical of frigid environments ——,S,J.&W, J.V. 1974. Torridonian tourmaline-quartz pebbles but are equally typical of hot arid settings, which are consid- and the Precambrian crust northwest of Britain. Journal of the Geological ered much more likely during deposition of the Stoer Group. Society, London, 130, 85–91. Mudstones of the Torrridon Group have higher CIA values B , D.R. 1972. Geochemistry of Precambrian crystalline basement rocks, North-West Highlands of Scotland. In: Twenty-fourth International than those of the Stoer and are significantly enriched in K. The Geological Congress, Montreal, Part 1, 97–103. Stoer samples are enriched in Mg relative to the Torridon C, R.A., G,C.M.&H, H. 1983. A Sm–Nd isotopic study of the Group samples and they are enriched in Ca, Mg and Na, South Harris Igneous Complex, the Outer Hebrides. 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Received 21 September 1998; revised typescript accepted 1 March 1999. Scientific editing by Mike Hambrey.