The significance of lead isotope studies in ancient, high-grade metamorphic basement complexes, as exemplified by the Lewisian rocks of Northwest Scotland

EARTH

AND PLANETARY

SCIENCE LETTERS 6 (1969) 245-256. NORTH-HOLLAND PUBLISHING COMP., AMSTERDAM

THE SlGNIFICANCE OF LEAD ISOTOPE STUDIES IN ANCIENT, HIGH-GRADE METAMORPHIC BASEMENT COMPLEXES, AS EXEMPLIFIED BY THE LEWISIAN ROCKS OF NORTHWEST SCOTLAND S.MOORBATH,

H.WELKE * and N.H.GALE

Department of Geology and Mineralogy, University of Oxford, England

Received 9 June 1969

Lead isotope ratios, together with uranium and lead analyses, are reported for thirty-seven whole rock samples from the Precambrian Lewisian basement complex of Northwest Scotland. A fairly representative range of metamorphic and polymetamorphic gneisses, mainly in the amphibolite and pyroxene-granulite facies, has been analysed in this reconnaissance survey. The analysed rocks have a wide range of conventional radiometric dates in the range 2600 m.y. to 1600 m.y., depending on whether they are from the Scourian, lnverian or Laxfordian sectors of the Lewisian complex. The leads from these rocks cannot be interpreted by a single stage evolutionary model. The leads falJ below the primary growth curve and scatter rather closely about a straight line on a plot of 206Pb/204Pb versus 207Pb/204Pb, which cuts the primary growth curve (with 23sU/204Pb = n = 8.68) at t, = 2900 + 100 m.y. and t2 = 0 m.y. This shows that all the analysed rocks underwent varying degrees of uranium depletion at 2900 It 100 m.y. ago, presumably during the early pyroxene-granulite metamorphism. It also suggests that much of the Lewisian basement complex was already in existence about 2900 m.y. ago. Later metamorphisms of somewhat lower grade, such as the so-called Inverian and Laxfordian, have probably caused minor redistribution of uranium and/or lead leading to some scatter about the 206Pb/204Pb versus 207Pb/2”4Pb lead line, but, in general the uranium/lead ratios of these rocks have always been much lower than that indicated by the primary growth curve. It is suggested that lead isotope studies on metamorphic and polymetamorphic basement rocks can help to decide whether a given crustal segment represents essentially juvenile addition of material from the upper mantle, or reworked older crust.

1. INTRODUCTION In recent years, several studies have been published on the abundance of radioactive elements in crystalline shield rocks. In particular it has been noted that the uranium content (and to a lesser degree the thorium content) of metamorphic rocks decreases as the grade of metamorphism increases [l-5]. Possible mechanisms for,uranium depletion have been discussed in these papers. No studies of a similar kind are available for lead, but there is little geochemical reason to suppose that it would be depleted to any* Present address: Bernard Price Institute for Geophysical Research, Johannesburg,

South Africa.

thing like the same degree as uranium in high-grade silicic metamorphic rocks. It follows that high-grade metamorphic rocks with low uranium/lead ratios, that have remained closed, or nearly closed, systems with regard to uranium and lead since the time of metamorphism and concomitant uranium depletion, should contain lead whose isotopic composition bears a simple relationship to the time at which uranium depletion occurred. In some cases such lead model ages might even pre-date conventional radiometric (K-Ar, Rb-Sr, U-Pb) ages, particularly where any subsequent metamorphic events did not cause significant addition or removal of uranium and lead. By extracting and isotopically analysing lead from high-grade metamorphic rocks, it might

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S.MOORBATH, H.WELKE and N.H.GALE Table 1 Major events in the history of the Lewisian basement complex of the mainland of Northwest Scotland [8, 9, 10, 12, unpublished work by R.St.J.Lambert and colleagues, and S.Moorbath].

ca. 2600 m.y. (minimum) Scourian pyroxene-granulite or hornblende-granulite facies metamorphism, NE - SW trend. Plastic folding. K-Ar dating of retentive whole rocks and minerals in areas little affected by later events sets a minimum age. ca. 2310-2550 m.y. Intrusion of dyke-like potash feldspar pegmatites with NE-trend, cutting Scourian structures, but deformed locally by later folds. Dated by Rb-Sr method on coarse potash feldspar. ca. 2200 m.y. lnverian amphibolite facies metamorphism with NW-SE trend. Large scale plastic folding. Maximum ages from K-Ar dating of the least retentive minerals in neighbouring granulites, and from Rb-Sr dating of pegmatites. Minimum ages from K-Ar dating of amphibolised country rock and by K-At and Rb-Sr dating of post-lnverian dykes. ca. 2190(- 19007) m.y. Intrusion of NW- SE basic and ultrabasic dykes, ranging from unmetamorphosed where Laxfordian effects are absent, to amphibolised and foliated where Laxfordian effects are present. Unmetamorphosed dykes dated by K-Ar and Rb-Sr methods on whole rocks and separated minerals. ca. 1800- 1700 m.y. (minimum) Laxfordian amphibolite facies metamorphism with NW- SE trend. Dated by K-Ar and Rb-Sr methods on separated minerals of Laxfordian age and on non-resistant minerals and rocks of pre-Laxfordian age. ca. 1700- 1650 m.y. Intrusion of Laxfordian potash feldspar pegmatites and granites. Dated by Rb-Sr method on potash feldspar and micas. ca. 1450-- 1400 m.y. Termination of radiogenic argon and strontium diffusion episode in certain northern and southern areas, combined with minor recrystallisation of existing minerals and formation of muscovite. Dated by K-Ar and Rb-Sr methods on separated minerals,

therefore be possible to learn a good deal about the early history of certain basement complexes. Although there is no previously published lead isotope work on high-grade metamorphic rocks, somewhat analogous work on uranium-free lead ores (galenas) has been described by Kanasewich and Farquhar [6]. By means of a method of straight-line intercepts on a primary growth curve for the Cobalt-Noranda area of the Ontario-Quebec border, they showed that the Precambrian geology of North America extends back beyond about 3200 m.y. ago. In order to test the simple ideas outlined above, a reconnaissance lead isotope survey has been made of acid and basic gneisses of the Lewisian Complex of Northwest Scotland, together with analyses for total uranium and lead. The analysed rocks belong mainly to the amphibolite and pyroxene-granulite metamorphic facies. A great deal of further work particularly on pyroxene-granulite facies rocks from several areas is in progress, using extremely precise mass-spectrometric techniques being developed in this laboratory.

The Lewisian rocks of Northwest Scotland have been studied in more detail than those of almost any other ancient, metamorphic basement complex in the world. A voluminous literature extending over nearly one hundred years exists, the first phase of which culminates in the great Northwest Highlands Memoir published by the Geological Survey of Great Britain in 1907. Another landmark is the 1951 paper by Sutton and Watson [7] on the chronological classification of the Lewisian Complex. A detailed summary of Lewisian geology has recently been published by Watson [8]. A good deal of radiometric dating has been carried out on Lewisian rocks [9- 12], though much still remains to be published. The major outlines, however, are fairly clear and are briefly summarised in table 1. Sutton and Watson [7] used the metamorphic state of the widespread basic "Scourie" dykes to define and separate the Scourian and Laxfordian metamorphic episodes. Evans [ 12] first described the Inverian metamorphism in the Lochinver area, although it is now thought that it has a wider

ANCIENT HIGH-GRADE METAMORPHIC BASEMENT COMPLEXES

Fig. 1. Sketch map showing the principal Lewisian outcrops, as well as localities and sample numbers relevant to this work (table 2). Cross-hatched - complexes preserving pre-Laxfordian rocks or structures, dated as Scourian (or undated), including some reworked areas in the Outer Hebrides. Horizontal lines - Laxfordian complexes. Diagonal lines - undated complexes on foreland. L - Lewisian tectonic inliers in Caledonian orogenic belt. Inverian rocks are not separately shown, and have been described mainly from Lochinver area. (The basic diagram is reproduced by kind permission of Dr. Janet Watson, and Oliver and Boyd Limited). extent than previously realised (Evans and Lambert, in preparation). Fig. 1 shows the approximate distribution of Scourian and Laxfordian rocks, as well as specimen localities relevant to the present work. The local and regional effects of the Laxfordian metamorphism are very variable in intensity, the area between Scourie and Lochinver being by far the least affected. Here, some of the basic dykes are practically unmetamorphosed, and the true Scourian pyroxene-

247

granulite rocks are better preserved than anywhere else in the Lewisian. Of particular relevance to the present work is the fact that these rocks are highly depleted in rubidium and have very low present-day 87 Sr/86 Sr ratios mostly within the range 0.701 0.710 ( [ 12], R.St.J.Lambert, personal communication). The transitional zones (Claisfearn, Foindle, Badnabay) between Scourian pyroxene-granulites and Laxfordian amphibolite-facies gneisses in the ScourieLaxford Bridge area are of particular interest and have been described in detail by Sutton and Watson [7, 13] Another interpretation is proposed by Evans and Lambert (in preparation). The geology of the Outer Hebrides is in many ways analogous with that of the Mainland, although there are also some important differences. Since only two samples from this area have been analysed in the present work, no detailed geological discussion is given. Instead, reference is made to several relevant recent works [8, 14, 15]. One of the most fundamental problems in Lewisian geology - as, indeed, in many complexes of the same general type on other continents - is the extent to which later "orogenies" represent addition of new material to the continental crust, or the reworking of older crustal material. In view of the wide distribution of post-Scourian, pre-Laxfordian basic dykes in the Lewisian, exhibiting varying degrees of Laxfordian metamorphism, and for many other reasons, most workers would probably agree that a substantial part of the Lewisian basement consists of more ancient, reworked Scourian gneiss [8, 16]. The only major area in which basic dykes are largely absent or no longer recognisable is in the type Laxfordian between Laxford Bridge and the north coast. Here, there has been redistribution of much granitic and pegmatitic material in Laxfordian times, particularly in the Laxford Bridge-Rhiconich area. Discordant pods and lenses of amphibolite may possibly represent relict Scourie dykes. In contrast to the general views of the Lewisian outlined above, Bowes [17, 18] considers that the basic dykes in the Lewisian cannot be used as an essentially single important marker horizon to separate ologenic episodes (i.e. Scourian and Laxfordian). He postulates that intrusion of basic dykes occurred episodically from about 2200 m.y. ago to about

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ANCIENT HIGH-GRADEMETAMORPHICBASEMENTCOMPLEXES 1400 m.y. ago, i.e. right up to the end of the Laxfordian "orogeny". Most workers engaged in Lewisian research appear to disagree with these views including, it must be confessed, the present authors. Amongst other controversial points, Bowes attaches some importance to several published [11 ] whole-rock K-Ar age measurements on metamorphosed basic dykes from the Gairloch area, which gave apparent dates in the range 1530-1120 m.y. It must be pointed out that under no circumstances can these dates be interpreted as ages of intrusion. Indeed, subsequent (unpublished) K-At work by one of us (S.M.) on separated hornblendes from these rocks yields a minimum age of about 1700 m.y. for their metamorphism.

The scheme of geological classification adapted for convenience in the present work purely for the purpose of tabulating the 37 results (table 2) is therefore based on the more conventional views, and is as follows:

Group 1 Scourian gneisses, showing little or no later metamorphism, yielding true Scourian mineral dates of ca. 2600 m.y. and/or cut by relatively unmetamorphosed and undeformed basic "Scourie" dykes. Mostly (except for Torridon area) in pyroxene-granulite metamorphic facies (samples 1-8, table 2).

Group 2 Scourian gneisses affected by Inverian (pre-dyke) and/or Laxfordian (post-dyke) amphibolite facies metamorphism, and cut by variably metamorphosed and deformed basic "Scourie" dykes. Mineral dates are within the general range ca. 2 2 0 0 - 1 4 0 0 m.y. (samples 9-25, table 2).

Group 3 Scourian-Laxfordian transition zone gneisses, between Scourie and Laxford Bridge [7, 13]. Mineral and whole rock Rb-Sr isochron dates (Lambert and Holland, in preparation) are mainly 1600-1800 m.y., with a few higher values (samples 26-28, table 2).

Group 4 Presumed Scourian gneisses occurring as tectonic

251

inliers east of the Moine thrust, cut by basic "Scourie" dykes metamorphosed during the Laxfordian and Caledonian orogeny. This particular marginal area of the Caledonian mountain belt has been described by Ramsay [ 19]. Mineral dates range from 2200 m.y. to 420 m.y. (unpublished work by S.Moorbath, samples 29-32, table 2).

Group 5 Laxfordian acid gneisses in the Laxford BridgeRhiconich area. The gneisses are at amphibolite or epidote-amphibolite grade of metamorphism. The analysed samples were collected as far as possible away from pods, lenses, dykes etc. of pegmatite and granite which permeate many of the rocks in this area, and which is connected with the later stages of the Laxfordian orogeny. Mineral ages are in the range 1600-1800 m.y. (samples 33-37, table 2). The interpretation of the lead isotope abundances in this study is based on methods which are current amongst most lead isotope geologists. These methods have been reviewed in several books and papers [20, 21, 22].

2. ANALYTICAL METHODS The analytical methods used in the present work have been described previously [23] and are not repeated here in full. Lead was extracted from whole rock (and several mineral) powders by a simple volatilisation method, followed by ion-exchange purification and precipitation as PbS, ready for mass spectrometry. Lead concentration determinations were carried out by volatilisation, and isotope dilution with a 204pb spike. All isotope measurements were made on an A.E.I. MS-5 mass spectrometer with 90 °, 30 cm radius analyser tube, using a single tantalum filament source and electron multiplier collection. PbS was mounted on the filament with a 2% ammonium nitrate solution. Uranium concentrations were determined by the delayed neutron method, following the procedure of Gale [24] and using the Aldermaston (A.W.R.E.) Herald reactor.

252

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3. RESULTS

4. DISCUSSION OF RESULTS

The analytical results are shown in table 2. The average isotopic ratios for many repeated measurements of the Toronto Broken Hill galena standard T1003 during the course of this work, corrected for electron multiplier discrimination, were 206pb/204pb = 16.10 + 0.02, 207pb/204pb = 15.54 -+ 0.02, 208pb/204pb = 36.12 -+ 0.05. These figures compare closely with other recently published values [25, 26]. The average between-run error for these three ratios on individual standard and sample runs was approximately 0.3%, with individual within-run errors ranging from 0.06 to 0.15%. The average errors for the lead isotope dilution concentration determinations were calculated to be approximately 2% for the higher concentrations and about 5% for the lower concentrations. In the uranium determinations, the errors varied from about 3% to about 15% depending upon uranium concentration. It must be remembered that the actual concentrations in none of the analysed samples exceeds about 1.2 ppm, whilst in many of them it is much lower, extending right down to about 0.03 ppm (table 2). These, and many further samples, are being remeasured using improvements in the technique which should diminish the errors.

The observed isotope ratios obtained for the Lewisian rocks are plotted in fig. 2 on a graph of 206pb/204pb versus 2°Tpb]2°4pb. The most striking feature is that all the analysed Lewisian leads (except one) fall below the primary growth curve and scatter rather closely about a straight line which may be extrapolated at each end to cut the single-stage, primary growth curve at two points. The slope of this line was calculated by a least-squares method which takes into account correlated errors in the x andy coordinates (Cumming and Rollett, in press). A unique value of 238U/204pb (/~) = 8.68 --- 0.09 was computed for the primary growth curve with which the "anomalous" lead line intersects at 0 m.y. (because Lewisian leads have developed radiogenically until the present day). The lower intercept of the "anomalous" lead line cuts the normal growth curve at a point corresponding to an age of 2900 -+ iO0 m.y. (1 sigma error). The value o f ~ = 8.68 for the primary growth curve is in close agreement with that of the source regions of modern oceanic leads whose lead isotope compositions have been measured [23, 27, 28, 29]. The most plausible interpretation of the observed lead isotope pattern is that all the analysed Lewisian rocks, regardless of the geological group to which

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ANCIENT HIGH-GRADEMETAMORPHICBASEMENT COMPLEXES they belong, underwent variable degrees of uranium depletion 2900 + 100 m.y. ago, and that all the rocks already existed at that time, though not necessarily in the chemical and metamorphic condition in which we see them today. From 2900 + 100 m.y. to the present day all'the rocks developed radiogenically along their own characteristic growth curves (not shown in fig. 2), but with much lower/a values than that of the primary (/a = 8.68) source region. In the ideal, but unlikely, case that each rock sample remained a completely closed system as regards uranium and lead throughout its entire history, the observed lead isotope ratio data points would be expected to lie on the straight lead line (within analytical error) at distances from the lower intercept directly proportional to the 238U/204pb (i.e. U/Pb) ratios of the individual samples. Considerable deviations from ideal closed system behaviour are evidenced by the scatter of points about the "anomalous" lead line, which exceeds analytical error in some cases by a factor of two or three. Later metamorphisms (i.e. lnverian, Laxfordian) undoubtedly induced some redistribution of uranium and lead in the rocks, although it appears that in all cases the U/Pb ratios remained much lower than in the source regions of normal leads as represented by the primary growth curve (p = 8.68) of fig. 2. The possibility also exists that not all the samples commenced their uraniumdepleted development at the same point in time, or on exactly the same primary growth curve, so that there could have been some variation in initial 2°6pb/204pb and 207pb/204pb ratios. Despite all these possible complications the fact remains that the lead isotope data points show a statistical scatter about a straight line and that this line yields a highly plausible primary age and/l value. Despite the limited amount of data (table 2) it appears that some of the most uranium-depleted rocks are the Scourian pyroxene-granulites of Group 1, with uranium contents down to about 0.03 ppm. Only a few of the other rock-types have a uranium content greater than 1 ppm, and none of them approach the crustal average of 2.7 ppm given by Taylor [30], which was based mainly on data from igneous rocks. The average uranium value for all the analysed Lewisian samples is 0.24 ppm, although from the preliminary data in table 2 it may well be that the average for true Scourian pyroxene-granulites

253

is between five and ten times lower. Despite the overall uranium depletion, many of these rocks, other than those of Group 1, contain accessory amounts of up to about 1% of such minerals as apatite, sphene and zircon. It is quite possible that such minerals in high-grade metamorphic rocks are highly impoverished in uranium as compared with similar minerals in igneous rocks. On the other hand, there is undoubtedly a close, direct relationship between the content of radioactive accessory minerals (particularly sphene and zircon) seen in thin section, and the measured 208pb/2°4pbratios, which vary in the range 33.7 to 64.9 (table 2). Unfortunately, thorium analyses are not yet available for the Lewisian rocks. It should be noted that there is no obvious or consistent relationship between the observed 206pb/204pb and 2°Spb/ 204pb ratios. Thus, uranium appears to be much more strongly fractionated than thorium during high-grade regional metamorphism. This was also found by Fahrig et al. [3] in the Canadian Shield. The average lead content of all the analysed rocks is 7.9 ppm, with individual values ranging from 0.30 ppm to 17.5 ppm. The crustal average given by Taylor is 12.5 ppm [30]. There is no obvious correlation as yet with metamorphic grade. Rocks with the highest proportion of basic minerals such as hornblende, pyroxene, garnet and the lowest proportion of feldspar (e.g. No. 22, table 2) have the lowest lead contents. In view of the considerable variability in lead contents, and to a lesser extent of the uranium contents, it follows that the 238U/204pb values also exhibit quite a wide scatter, namely from 0.10 to 6.3. As pointed out previously, the observed values are considerably lower than that of the source region of the primary Lewisian lead (/a = 8.68, fig. 2). Examination of the data given in table 2 shows a very broad, general proportionality between the observed 238U/204pb and 2°6pb/204pb ratios. The scatter is, however, too great to obtain any meaningful 238U-2°6pb isochron ages, so that a plot of these parameters is not given in this paper. The reasons for this scatter are, of course, in part identical to those responsible for the scatter about the "anomalous" lead line of fig. 2, namely i) lack of completely closed system behaviour for uranium and lead during later history of the rocks, ii) comparatively high analytical error in the uranium analysis for samples with the lowest uranium contents, iii) variation in initial lead

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S.MOORBATH, H.WELKEand N.H.GALE

isotope ratios at time of uranium depletion. (This latter factor can cause analogous uncertainties in the interpretation of Rb-Sr isochrons.) Clearly the data as a whole can only be described to a first approximation as a two-stage model, and a three-stage model may fit at least some of the data more closely. Increased precision in lead isotope and U/Pb measurements should eventually help to decide between these alternatives. The above uncertainties will have a much greater effect on a plot of 238U/204pb versus 206pb/204pb than on a plot of 206pb/204pb versus 207pb/204pb (fig. 2). Whilst, in one sense, it is not strictly necessary to measure uranium and lead contents at all for a plot of the latter type, these are of the greatest geochemical interest, and also of importance for checking the consistency of the isotope data with the assumed model.

5. GENERAL SIGNIFICANCE AND IMPLICATIONS OF THE RESULTS From the foregoing discussion it appears that 2900 _+ 100 m.y. ago, the greater part of the Lewisian Complex was already in existence. At that time, the rocks underwent substantial but variable depletion in uranium, and have retained their low U/Pb ratios until the present time. The uranium which was lost from the deeper levels of the Lewisian Complex presumably migrated into the upper crust, where it has always been concentrated. Much of this uranium undoubtedly ended up in late Precambrian (Moine, Torridonian, Dalradian) and Phanerozoic sediments, as well as in some of the late Laxfordian granites and pegmatites. Individual rock samples may not have remained completely closed systems for uranium and/or lead during reheating or recrystallisation caused by later (e.g. lnverian and Laxfordian) metamorphisms. However, whilst strictly limited gain or loss of uranium and/or lead may have increased the statistical scatter of the points about the "anomalous" lead line of fig. 2, it does not appear to have been sufficient in the present case to destroy the essential regularity. The event that led to uranium depletion (and most probably to the depletion of other "incompatible" trace elements such as rubidium) is considered to be the pyroxene-granulite metamorphism itself. The somewhat younger maximum radiometric ages of ca.

2 6 0 0 - 2 7 0 0 m.y. found in those parts of the Scourian Complex which are hardly, if at all, affected by later metamorphisms, may well refer to later uplift and cooling events in the history of the Scourian belt, or to overprinting by later thermal events. As mentioned earlier, the value ofta = 8.68 for the primary growth curve (fig. 2) is essentially the same as the values found by other workers for the source regions of modern oceanic lead, where the majority of values are closely grouped around 8.6 to 8.8 [23, 27, 28, 29]. This places certain restrictions on the pre-2900 m.y. history of the Lewisian Complex. There are two principal possibilities: i) The pre-Scourian Complex separated from the upper mantle source region at any time prior to ca. 2900 m.y. ago, but retained the characteristic # value of the source region; i.e. the lead isotope composition changed along the primary growth curve from the time of separation to 2900 m.y. ago, when uranium depletion occurred. Whether a crustal se~aent can "differentiate" from upper mantle source regions without a significant change in # is speculative. The observed tz values in modern basic and acid oceanic igneous rocks are generally highly variable, and often much higher than any plausible/a values (ca. 8.7) for the primary source region. However, conditions may well have been different during the early history of the 6arth. ii) The pre-Scourian Complex separated from the upper mantle source region only a very short time prior to ca. 2900 m.y. ago, possibly with an enhanced/a value. Shortly afterwards, and perhaps as part of the same general episode, high-grade metamorphism and uranium depletion occurred, so that the initial lead isotope composition did not have time to change significantly through radiogenic additions between the two events. This essentially relegates both events into one "superevent" at essentially 2900 m.y. ago, and dispenses with the necessity for a pre-2900 m.y. crust. It is not easy to use the present-day rocks to decide between the two alternatives, because the highgrade metamorphism has been sufficient to obliterate the original nature of the rocks. Many of the more basic gneisses appear to represent metamorphosed or migmatised igneous rocks, whilst locally there are some recognisable metasediments showing the same degree of metamorphism as the gneisses with which

ANCIENT HIGH-GRADEMETAMORPHICBASEMENTCOMPLEXES they are associated. Watson [8, pp. 67-68] writes that these, and many other features "... suggest that the parts of the Scourian Complex preserved in northwest Scotland were formed in the interior of a much larger area of plutonic activity and at a considerable depth in the crust. There is no conclusive evidence to indicate that the period of formation was connected with an orogenic cycle, but the incorporation of some sedimentary rocks shows that normal geological processes were already in action ...". Both of the principal possibilities mentioned above raise a number of geological and geochemical problems, though the present authors favour the second as regards the bulk of the gneisses. Further, much more detailed work on the pyroxene-granulites may help to elucidate the preScourian history of the rocks. In this work, choice of samples has been restricted to rather typical Lewisian gneisses. It is unlikely that certain other types of Lewisian rocks such as the i) various recognisable metasedimentary sequences, ii) basic "Scourie" dykes (ca. 2200 m.y.), iii) Laxfordian granites and pegrnatites, would fit the relatively simple pattern demonstrated for the gneisses in fig. 2. These later intrusive rocks, in particular, might be expected to have the lower intercept on the primary growth curve corresponding closely with their radiometric age, provided that no isotopic exchange of the type described by Moorbath and Welke [31 ] has occurred between intrusive and country rocks. Such rocks might also exhibit uranium enrichment, and higher # values than the primary source region, which would affect the isotopic pattern correspondingly. Lead isotope studies on metamorphic rocks could have considerable significance for studying the early history of the continents, and for deciding between the rival hypotheses of crustal accretion (i.e. addition of differentiated material from the upper mantle) and crustal rejuvenation (i.e. reworking of older crustal material) within a given orogenic belt. In some cases this can be tackled by means of discordant K-Ar, Rb-Sr and U-Pb mineral dates, as well as Rb-Sr whole rock dates. In the past 10 years it has been clearly demonstrated that many mountain belts (including those in northwest Scotland) are at least partly underlain by, or represent reworked, older crust. Some of the most recent Rb-Sr whole rock work of this type has shown that some parts of the Grenville rocks of

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the Canadian Shield, which yield mineral dates in the range 900 +- 100 m.y., really t~onsist of much older rocks whose essential characteristics were established during an earlier metamorphism about 1 5 0 0 - 1 8 0 0 m.y. ago [32]. However, it is widely recognised nowadays that migration of radiogenic strontium during the subsequent thermal history of a given rock unit can, in some cases, cause considerable blur and scatter of Rb-Sr whole-rock isochrons. Lead isotope measurements are potentially capable of delving farther back into the history of a polymetamorphic basement complex, provided that uranium depletion was associated with the first (and most intense) metamorphism, and that at least to a first approximation the rocks have remained closed systems to uranium and lead ever since. If a straight line can be fitted to the analytical data, as in fig. 2, giving a single age intercept on a primary growth curve with a plausible/.t value, it may be surmised that all the analysed samples form part of a single, primary basement complex, which may, or may not, have been partially or wholly reworked in the course of geological time. In those cases, however, where different age intercepts are found for different rock units or rock types within a basement complex on a similar primary growth curve, it would be valid to conclude that these different rock units and rock types came into existence (by some sort of upper mantle differentiation process indistinguishable by present methods from the unimodal case above) at different times. The first case obviously corresponds to crustal rejuvenation and the second to crustal accretion (as defined earlier).

ACKNOWLEDGEMENTS The authors are indebted to Professor G.L.Cumming, Dr. J.Watson, Dr. R.St.J.Lambert and Mr. G. Muecke for much helpful discussion and constructive criticism. Skilled technical assistance was provided by Miss J.Tompkins.

REFERENCES [ 1] K.S.Heier and J.A.S.Adams, Concentrations of radioactive elements in deep crustal material, Geochim. Cosmochim. Acta 29 (1965) 53.

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[ 2] W.F.Fahrig, K.E.Eade and J.A.S.Adams, Abundance of radioactive elements in crystalline shield rocks, Nature (London) 214 (1967) 1002. [31 W.F.Fahrig and K.E.Eade, The chemical evolution of the Canadian Shield, Can. J. Earth Sci. 5 (1968) 1247. [4] I.B.Lambert and K.S.Heier, Estimates of the crustal abundances of thorium, uranium and potassium, Chem. Geol. 3 (1968) 233. [5] I.B.Lambert and K.S.Heier, Geochemical investigations of deep-seated rocks in the Australian Shield, Lithos 1 (1968) 30. [6] E.R.Kanasewich and R.M.Farquhar, Lead isotope ratios from the Cobalt-Noranda area, Canada, Can. J. Earth Sci. 2 (1965) 361. [7] J.Sutton and J.Watson, The pre-Torridonian metamorphic history of the Loch Torridon and Scourie areas in the North-West Highlands, and its bearing on the chronological classification of the Lewisian, Quart. J. Geol. Soc. (London) 106 (1951) 241. [ 8] J.Watson, The Lewisian, in: The Geology of Scotland, ed. G.Y.Craig (Oliver and Boyd, Edinburgh, 1965) 50. [9] B.J.Giletti, S.Moorbath and R.St.J.Lambert, A geochronological study of the metamorphic complexes of the Scottish Highlands, Quart. J. Geol. Soc. (London) 117 (1961) 233. [ 10] C.R.Evans and J.Tarney, Isotopic ages of Assynt dykes, Nature (London) 204 (1964) 638. [ 111 C.R.Evans and R.G.Park, Potassium-argon age determinations from the Lewisian of Gairloch, Ross-shire, Scotland, Nature (London) 205 (1965) 350. [ 121 C.R.Evans, Geochronology of the Lewisian basement near Lochinver, Sutherland, Nature (London) 207 (1965) 54. [13] J.Sutton and J.Watson, Further observations on the margin of the Laxfordian complex of the Lewisian near Loch Laxford, Sutherland, Trans. Roy. Soc. (Edinburgh) 65 (1962) 89. [ 14] R.Dearnley, An outline of the Lewisian Complex of the Outer Hebrides in relation to that of the Scottish Mainland, Quart. J. Geol. Soc. (London) 118 (1962) 143. [ 15] R.Dearnley and F.W.Dunning, Metamorphosed and deformed pegmatites and basic dykes in the Lewisian Complex of the Ou ter Hebrides and their geological significance, Quart. J. Geol. Soc. (London) 123 (1968) 335. [16] J.Watson, Evidence of mobility in reactivated basement complexes, Proc. Geol. Assoc. 78 (1967) 211. [ 17] D.R.Bowes, An orogenic interpretation of the Lewisian of Scotland, Proc. 23rd Internat. Geol. Congr. 4 (1968) 225. [ 18] D.R.Bowes, The absolute time scale and the subdivision of Precambrian rocks in Scotland, Geol. Fi~r. Stockh. F6rhand. 90 (1968) 175.

[ 19] J.G.Ramsay, Moine-Lewisian relations at Glenelg, Inverness-shire, Quart. J. Geol. Soc. (London) 113 (1958) 487. [20] R.D.Russell and R.M.Farquhar, Lead Isotopes in Geology (Interscience, New York, 1960). [21] W.F.Slawson and R.D.Russell, Common lead isotope abundances, in: Geochemistry of Hydrothermal Ore Deposits, ed. H.L.Barnes (Holt, Rinehart and Winston, New York, 1967) 77. [22] E.R.Kanasewich, The interpretation of lead isotopes and their geological significance, in: Radiometric Dating for Geologists, eds. E.I.Hamilton and R.M.Farquhar (Interscience, London, 1968) p. 147. [23] H.Welke, S.Moorbath, G.L.Cumming and H.Sigurdsson, Lead isotope studies on igneous rocks from Iceland, Earth Planet. Sci. Letters 4 (1968) 221. [24] N.H.Gale, Development of the delayed neutron techniques as a rapid and precise method for determination of uranium and thorium at trace levels in rocks and minerals, with applications to isotope geochronology, in: Radioactive Dating and Methods for Low-Level Counting (International Atomic Energy Agency, Vienna, 1967) p. 431. [25] R.G.Ostic, R.D.Russell and R.L.Stanton, Additional measurements of the isotopic composition of lead from stratiform deposits, Canad. J. Earth Sci. 4 (1967) 245. [26] J.A.Cooper and J.R.Richards, Solid-source lead isotope measurements and isotopic fractionation, Earth Planet. Sci. Letters 1 (1966) 58. [27] M.Tatsumoto, Isotopic composition of lead in volcanic rocks from Hawaii, Iwo Jima and Japan, J. Geophys. Res. 71 (1966) 1721. [ 28] M.Tatsumoto, Genetic relations of oceanic basalts as indicated by lead isotopes, Science 153 (1966) 1094. [ 29] R.D.Russell, W.F.Slawson, T.J.Ulrych and P.H.Reynolds, Further applications of Concordia plots to rock lead isotope abundances, Earth Planet. Sci. Letters 3 (1968) 284. [30] S.R.Taylor, Abundance of chemical elements in the continental crust: a new table, Geochim. Cosmochim. Acta 28 (1964) 1273. [ 311 S.Moorbath and H.Welke, Lead isotope studies on igneous rocks from the Isle of Skye, Northwest Scotland, Earth Planet. Sci. Letters 5 (1968) 217. [32] T.E.Krogh, G.L.Davis, L.T.Aldrich, S.R.Hart and A. Stueber, Geological History of the Grenville Province, Carnegie Institution Geophys. Lab. Ann. Rep. (19661967) 528. [33] R.G.Park, The structural history of the Lewisian rocks of Gairloch, Wester Ross, Scotland, Quart. J. Geol. Soc. (London) 120 (1964) 397.