Journal of the Geological Society, London, Vol. 150, 1993, pp. 447-464, 10 figs. Printed in Northern Ireland

Volume 150 Celebration Paper

Unravelling dates through the ages: geochronology of the Scottish metamorphic complexes

G. ROGERS 1 & R.J. PANKHURST 2 l lsotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 OQU, UK 2British Antarctic Survey, c/o NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK

Abstract: The paper by Giletti et al. (1961) is seen as a major landmark in the evolution of dating techniques in polymetamorphic terrains. We consider certain critical issues from each of the main complexes of the Scottish Highlands studied by Giletti et al. to illustrate how subsequent develop- ments in geochronological methodology have influenced our understanding of metamorphic belts. Lewisian examples focus on the formation of Archaean crust, and the age of the main high-grade metamorphism and the Scourie dyke swarm. The antiquity of Moinian sedimentation, its relationship to the Torridonian sandstones, and the timing of Precambrian metamorphism have been controversial issues. The timing and nature of Caledonian orogenesis, most clearly expressed in the Dalradian complex, have been the focal points for the refinement of radiometric investigation. These complexes have been subject to successive developments in methodology, with ever-tighter constraints from Rb-Sr and K-Ar mineral dating, through Rb-Sr and Pb-Pb whole-rock studies, U-Pb dating of bulk zircon fractions, and Sm-Nd whole-rock and mineral investigation, up to the latest technologies of single-grain zircon and ion microprobe analysis. The rocks have released their secrets reluctantly, and many of the questions posed in 1961 have still not been definitively answered. However, the hope of unambiguous solution leads towards greater efforts, ever more reliable data, and a clearer evolution- ary picture.

The value of radioactive decay as a principle in determining Rb-Sr data and conclusions that, in general terms, have geological time was recognized almost as soon as proved correct. The initial impact on the geological radioactivity was discovered: a brief summary of this community may be judged by the six pages of written fascinating history is given by Faure (1986), who points out discussion following the paper. More importantly, they set that Rutherford, about 1905, was the first to make the goals and standards for future work. Only the most meaningful measurement of mineral ages (using the U-He enlightened of observers can have foreseen the growth of method). Naturally it took further discoveries and much the subject that has followed during the past 30 years, and work before theory and techniques (the work of Nier in the the degree to which modern geological theories now hinge development of mass-spectrometers was particularly critical on a reliable geochronological background (or, occasionally, (e.g. Nier 1940)) were developed to the point where its absence!). geochronology could be investigated in a practical way. This paper was so influential in both establishing the This occurred during the 1950s, when laboratories were set geochronological groundwork, and dictating the course of up world-wide and the first data were produced, mostly subsequent research, that it is all too easy to forget the using K-Ar and Rb-Sr mineral methods. Thus, by 1960, the relatively primitive technical facilities of the early days. This foundations of modern radiogenic geochronology had been is especially true of the mass-spectrometry where the laid, although theory was some way ahead of analytical subsequent advent of digital measurement (instead of the capability. use of chart-recorders for measuring isotope ratios) was The landmark represented by the publication of Giletti, accompanied by better methodology: Giletti et al. made Moorbath & Lambert (1961) was that of a regional empirical corrections for electron-multiplier discrimination, geochronological study, based upon well-constrained field but not for mass fractionation during ionization, and were relationships--the first in Britain and among the first restricted to Rb-enriched micas for dating, mostly without anywhere. The metamorphic rocks of the Scottish Highlands control of the initial 875r/86Sr ratio. They were content to had always been of great importance in the development of obtain errors of +5% on calculated ages, whereas today geological interpretation of both Archaean gneiss terrains errors of -t-0.5% are possible even on whole-rock isochrons and orogenic mobile belts, at times generating heated (with statistical tests for significance), and U-Pb zircon ages controversy. The lack of stratigraphical control, and the are often quoted to c. 2-3 Ma over the whole geological wide span of geological history represented in a relatively time-scale. Nevertheless, they demonstrated the need for small area, made this a prime target for testing methods that sound practices such as routine measurement of chemical could provide an absolute time frame. This was addressed blanks and inter-laboratory standards. Even after many of by Giletti et al. (1961) with a significant body of reliable these advances, the geological application of Rb-Sr 447

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geochronology was hampered by uncertainty over the K-feldspar and muscovite ages from a Laxfordian pegmatite half-life of 87Rb (not resolved until the work of Davis et al. were 1600 (mean) and 1500 Ma respectively, with mean 1977 and the subcommission report of Steiger & J~iger biotite ages from other pegmatites giving 1160-1510Ma. 1977), and poorly constrained calibration of the stratigraph- Giletti et al. concluded that the main Laxfordian ical time-scale (that of Holmes 1960 being the most recent metamorphism occurred between 1500 and 1600 Ma with available to Giletti et al.). All the progress made in these the spread to lower ages reflecting mild reheating later than aspects of analysis was stimulated by the demonstration, in 1100 Ma. The Scourian complex was considered to be older studies such as that of Giletti et al., that radiometric than 2460 Ma with the lower Scourian pegmatite ages being geochronology indeed had the potential to solve geological due to variable resetting during the Laxfordian. Giletti et al. problems. therefore established that there was c. 800 Ma between the Our objective in this paper is to use the progress that has two metamorphic events then recognized within the been made in our understanding of the Scottish Highlands Lewisian complex. since the pioneering work of Giletti et al. (1961) as a means The subsequent history of geochronology in the Lewisian of illustrating the evolution of methods of dating has been directed in the main at trying to establish the ages metamorphic events in general. This evolving research has of: (1) the protoliths to the Scourian and Laxfordian seen publications which themselves have been amongst the complexes, (2) the granulite and amphibolite facies first of their kind and represent landmark papers in their metamorphic events, (3) the intrusion of the Scourie dyke own right (e.g. Long 1964; Moorbath et al. 1969; Dewey & suite, (4) any regional variations in these events. The Pankhurst 1970; Pidgeon & Aftalion 1978; Hamilton et al. following sections will discuss certain aspects of these 1979). We do not intend to provide a comprehensive review problems in the light of evolving geochronological of all the geochronological data or the geology; thorough techniques and methodology. Detailed reviews of the and regularly up-dated reviews of Scottish geology have chronology of the Lewisian have been presented by Park been provided by Craig (1965, 1983, 1991)--the first of (1970), Bowes (1978) and Park & Tarney (1987). these already showing the impact of Giletti et al.'s work. Instead we highlight certain critical issues where the use of Protolith formation and Badcallian metamorphism new analytical techniques has helped to elucidate the complexities of polymetamorphic terrains and crustal Following Giletti et al.'s study, Evans conducted an development in orogenic belts (or, in some cases, to present extensive K-Ar investigation in the Lochinver area and, the challenge of a more confusing story!). All ages referred using hornblende data from ultrabasic gneisses, concluded to in this paper have been recalculated using the decay that the Scourian granulite facies metamorphism (termed constants recommended by Steiger & J~iger (1977); we have 'Badcallian' by Park 1970) occurred prior to 2600 Ma (Evans retained the error estimates quoted by the original authors 1965). He also identified an amphibolite facies event which in all cases. took place penecontemporaneously with the intrusion of the Scourie dykes, which he called the Inverian (see later). A major advance was made by Moorbath et al. (1969) who Lewisian showed that acid and basic gneisses from the Scourian and Following the basic chronological subdivision of the Laxfordian complexes gave a single Pb-Pb whole-rock Lewisian complex by Peach et al. (1907) the classic paper of isochron age of 2860-1-100Ma (Fig. 1), interpreted as the Sutton & Watson (1951) placed Lewisian evolution in an time of major U-depletion during the granulite and orogenic context, defining the following episodes. (1) amphibolite facies metamorphism. The data also showed Scourian: consisting of granulite facies gneisses and late that both the gneiss complexes had been in existence c. granitic pegmatites. (2) Intrusion of a suite of basic 2900Ma ago, and therefore that evolutionary models dykes--the Scourie dyke suite. (3) Laxfordian: reconstitu- proposing that the gneisses of the Laxfordian complex were tion of the Scourian gneisses and dykes, mainly to the north formed of completely reworked Scourian rocks (e.g. Sutton of Laxford Bridge (the northern region) and south of & Watson 1951; Bowes 1968; Holland & Lambert 1973) Gruinard Bay, under amphibolite facies conditions; granite were untenable. Moorbath et al. favoured an interpretation emplacement. Although a relative chronology had been in which the precursors to the gneisses had separated from erected there was no information regarding the absolute the mantle shortly before 2900Ma, and consequently that ages of the orogenic events, and hence it was impossible to crustal extraction and metamorphism were part of one assess the rates of geological processes or the genetic links 'superevent'. Such ideas led, through further work in between them. Greenland, to the 'Crustal Accretion Differentiation One of the major achievements of the paper by Giletti et Superevent' model of crustal growth (Moorbath 1977). al. was to place firm constraints on the timings of the main Evidence for a younger age of Badcallian metamorphism Scourian and Laxfordian events. They used Rb-Sr dating of was provided by a series of U-Pb zircon studies, and muscovites and K-feldspars from late Scourian pegmatites to subsequently by Rb-Sr and Pb-Pb whole-rock investigations. assess the age of the Scourian complex, as these Pidgeon & Bowes (1972) obtained zircon ages of 2663 + 20 geochronological systems were considered to be more robust and 2648 + 20 Ma for granulite facies gneisses from Kylesku than K-Ar ages or Rb-Sr biotite ages, observations later and Badcall, which they interpreted as dating the Badcallian encapsulated in the concept of blocking/closure tempera- granulite facies metamorphism. A zircon age of 2675 + tures (e.g. Macintyre et al. 1967; Dodson 1973). The oldest 20Ma from Rona (Lyon et al. 1973) indicated the K-feldspar age obtained was 2460Ma (mean of three widespread nature of this event, although in this case the determinations) with others ranging down to 2140Ma. metamorphic grade only reached amphibolite facies. A Biotite dates varied from 2090 to 1480 Ma, in each case zircon upper intercept age of 2702+ 10Ma from an giving a younger age than coexisting K-feldspar. Rb-Sr amphibolite gneiss from Harris (Pidgeon & Aftalion 1972)

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16.0 2o7pb

2o4pb 1000 500 0 M.Y. 15-5 Primary Growth Curve 150o for p=8-68 ~ 2000~.~~ . Fig. 1. Pb-Pb whole-rock isochron for granulite and amphibolite facies gneisses 15.0 from the Lewisian complex showing that 3000~ - both types of gneisses were in existence 14'5 2900 Ma ago. (Figure reproduced from 2o6pb Moorbath et al. 1969 with permission iO0 14'0 // 2o4pb from Elsevier Science Publishers BV and v,/ j l i i J i i x J I 1 I I S. Moorbath). 12-0 12-5 13.0 13.5 14-0 14.5 15-0 15-5 16.0 16.5 17-0 17-5 18.0 18"5 19-0

was considered to reflect the timing of Scourian granulite Lewisian), the Pb-Pb ages would have poor precision owing facies metamorphism, with the lower intercept age of to the reduction and homogenization of U/Pb ratios during 1625 -1- 60 Ma representing isotopic disturbance during the metamorphism. Laxfordian retrogressive metamorphism. The presence of Whereas the Rb-Sr, Pb-Pb and U-Pb work had given euhedral cores in some grains suggested an even older ages that were thought to date the Badcallian metamorph- zircon component, possibly related to igneous crystal- ism, the advent of Sm-Nd dating in the late 1970s, coupled lization, although Pidgeon & Aftalion (1972) argued that, if with the observation that Sm/Nd does not change during this were the case, the time interval between crystallization metamorphism (Green et al. 1969), led Hamilton et al. and the early metamorphism was short. However, as the (1979) to study the Lewisian complex using Sm-Nd on whole only granulite facies assemblages on Harris occur in the rocks in an attempt to determine the time of separation of South Harris igneous complex and are Laxfordian in age the gneissic precursors from the mantle. Using samples from (Cliff et al. 1983), it is possible that the 2700 Ma age may not both granulite and amphibolite facies areas they obtained an be dating a granulite facies event, but rather either igneous age of 2920 + 50 Ma (Fig. 2) which was interpreted as dating crystallization or an early amphibolite facies event. Lyon & the crustal generation of the complex. As this study used Bowes (1977) reported a zircon age of 2826 + 50 Ma from an such widely separated samples and different rock types, amphibolite facies gneiss from the Laxfordian complex near Whitehouse (1988) analysed tonalitic gneisses from the Durness, and, whereas the date again indicated an central region and obtained a Sm-Nd whole-rock age of Archaean age for the Laxfordian gneiss complex, the poorer 2600 + 155 Ma (MSWD = 5.7) with an endi of -- 2.4 + 1.9. precision hindered detailed interpretation. Rb-Sr whole- Sm-Nd whole-rock isochrons from ultramafic intrusions at rock ages of 2633 + 120 to 2731 + 210 Ma (Lyon et al. 1973, Drumbeg, Achiltibuie and Scourie gave ages of 2910 + 55, 1975; Moorbath et al. 1975), determined from various areas, 2810 + 95 and 2670 + 110 Ma (MSWD's = 0.2 - 5.0), which, and a Pb-Pb whole-rock age of 2665 + 26 Ma on samples coupled with their eN,~i values of between + 1 + 0.7 and from a small area near Scourie (Cohen et al. 1991) were +1.9-t-0.5 (Whitehouse 1989b), were interpreted as again interpreted as dating widespread Rb and U depletion indicating the dates of crystallization from mantle-derived during Badcallian metamorphism. A Pb-Pb whole-rock age of 2680 +60Ma was reported by Chapman & Moorbath (1977) on rocks considered on structural grounds to I I I / represent the oldest components of the Lewisian; the data 0.5135 o did not support a substantial crustal prehistory for these rocks. 0-5130 Recent work by Whitehouse has raised important o questions over the interpretation of Pb-Pb whole-rock ages from metamorphic terrains, and whether they have any Z 0.5125 geological significance. On the basis of assumed peak Y Scourian metamorphism at c. 2680 Ma a Pb-Pb whole-rock "--~0-5120 age of 2950 + 70 Ma from the amphibolite facies gneisses Z north of Laxford Bridge was interpreted as seeing through m 0,5115 the effects of the metamorphism to give the date of crustal accretion (Whitehouse & Moorbath 1986). In the light of his l 0.5110 J other Pb-Pb whole-rock ages (Whitehouse 1989a, 1990) he / argued that interpretation of Pb-Pb whole-rock ages from o 0'5105 amphibolite facies and lower grade suites must be treated /0" I I I with caution due to the effects of incomplete homogeniza- 0.10 0"15 0-20 0"25 tion of Pb isotopes during metamorphism. Consequently the 147Sm/144 Nd 'age' of 2950 Ma (Whitehouse & Moorbath 1986), although well-constrained by a low MSWD, was considered spurious. Fig. 2. Sm-Nd whole-rock isochron for Lewisian gneisses giving an Whitehouse further argued that the only 'reliable' data age of 2920 + 50 Ma, interpreted as dating Lewisian crustal could be generated from pyroxene-granulite facies as- accretion. (Figure reprinted with permission from Nature (Hamilton semblages. However, in granulite terrains where the et al. 1979): copyright (1979) Macmillan Magazines Limited, and reduction in U/Pb ratios has been severe (such as in the from P.J. Hamilton).

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magmas. These results implied that the Nd isotopic metamorphism at 2660Ma to c. 600°C at 2490Ma. composition of the sub-Lewisian mantle was close to that of Although Cohen et al. (1988a, 1991) also stated that Sm-Nd depleted mantle (DePaolo 1981). The data for the Scourie and Pb-Pb mineral isochrons from mafic/ultramafic gneisses tonalitic gneisses which had low eNdi were therefore gave ages of c. 2420Ma (supporting data were not considered to be due to Nd isotopic homogenization of older presented) they interpreted the results as dating the final gneisses at 2660Ma during the Badcallian metamorphism. mineral crystallization of the granulite-facies assemblage Whitehouse (1989b) also dated hornblende-granulite gne- rather than simply the slow cooling of the complex, on the isses from Gruinard Bay and amphibolite-facies tonalitic basis of similar diffusion rates for Sm and Nd to the major gneisses from the northern region which gave Sm-Nd cations in garnet and clinopyroxene. Consequently, whereas

whole-rock ages of 2965 + 130Ma (MSWD=3.8; ~Ndi the Sm-Nd whole-rock data give the age of emplacement of +3.7 +2.2) and 2970 + 190Ma (MSWD =7.9; ~Nd = + these ultramafic bodies at c. 2700 Ma, Sm-Nd equilibration 4.7 + 2.6), but questioned the significance of these ages on at the mineral level was not achieved until about 250Ma the basis that their eNdi values were higher than DePaolo's later. Major regional U, Th and Rb depletion was (1981) depleted mantle ~ENd/ 2970 .~_. +1.8). He argued that as the accomplished during the peak of Badcallian metamorphism mafic bodies studied all appeared to have been derived from at 2660 - 2700Ma. This interpretation has a number of a mantle source similar to DePaolo's depleted mantle, and implications and associated problems. Most significantly, the that if the gneisses were derived from mafic/ultramafic 2420 Ma age of Cohen et al. is identical to the 2418 Ma U-Pb precursors, then, given that intracrustal melting and baddeleyite age of the oldest reliably dated Scourie dyke metamorphism have only limited affect on Sm/Nd ratios, (Heaman & Tarney 1989; see below). As the dykes cross-cut the TDMNd ages of the felsic gneisses should give the the Badcallian fabrics (see Park 1991) and were intruded approximate timing of crustal accretion of each region. penecontemporaneously with the Inverian amphibolite Thus, the Scourie region was deemed to have accreted at c. facies retrogression, it seems highly improbable that the 2930 Ma, the Gruinard Bay region at c. 2860 Ma, and the granulite facies fabrics could have been produced at northern region at c. 2780 Ma. However, if one chooses a 2420 Ma. different model for the evolution of the depleted mantle Burton & O'Nions (1992) have recently obtained Sm-Nd (e.g. Goldstein et al. 1984) then the eN,l_2,n. values for the and Pb-Pb mineral ages from an amphibolite facies agmatite tonalitic gneisses of Gruinard Bay and the northern region complex at Gruinard Bay. The host amphibolites give ages are broadly consistent with derivation from such a source, of c. 3300 Ma whereas the later trondhjemite gneisses are c. and both areas could have accreted at the same time. 2400 Ma. The date for the amphibolites is the oldest Sm-Nd Therefore, if one defines different boundary parameters-- age yet reported from the Lewisian complex. As the even within the same genre of model--significantly different amphibolite date is about 650 Ma older than the age of the conclusions may be drawn. Whereas Nd model ages may Badcallian metamorphism further north, Burton & O'Nions give some insight into the approximate average ages of (1992) concluded that information relating to the timing of components of Nd in rock units relative to a specific starting crustal differentiation or of prograde metamorphism would composition, great care must be taken in their inter- be unlikely to be preserved in rocks which had suffered pretation. (See Arndt & Goldstein 1987 for a review of Nd granulite facies metamorphism. Such a conclusion presup- model age applications). poses that the rocks further north in the central region were Cohen et al. (1991) have also recently addressed the formed significantly before the Badcallian metamorphism, problems of the physical extent and timing of depletion by and that Sm-Nd evidence for their original age has been studying a small area at Scourie. Sm-Nd whole-rock data for overprinted. mafic and ultramafic samples gave an age of 2702 + 52 Ma The geochronological history of Laxfordian activity does (eL,j = +1.8) which was interpreted as the time of Sm/Nd not uniquely serve to illustrate any points we wish to make, fractionation between these lithologies. This age is identical, and so is not considered in detail. However, for the sake of within error, to the best estimates of the time of Badcallian completeness the granite sheets along the Laxford Front metamorphism. Lower eNa27(•1 values for tonalites and have yielded Rb-Sr and Pb-Pb whole-rock ages of metasediments were taken to indicate that these had had a 1650 + 55 Ma to 1754 + 18 Ma (Taylor et al. 1984). Other previous crustal history, and that there had been minimal intrusive granites have given Rb-Sr whole-rock ages of exchange of Nd between the tonalites and more mafic 1429 + 350, 1645 + 170 and 1713 + 34 Ma (van Breemen et samples during Badcallian metamorphism at c. 2700Ma, al. 1971; Lyon et al. 1973; Lyon & Bowes 1977), and a U-Pb thus limiting Nd movement to no more than a metre scale. zircon age of 1678_1oMa+20 (van Breemen et al. 1971). Consequently, it was concluded that the 2702Ma age Lambert & Holland (1972) and Lyon & Bowes (1972) represented the time of igneous differentiation. Cohen et al. reported Rb-Sr whole-rock ages for quartzofeldspathic also argued that Whitehouse's apparently older ages of 2850 gneisses from the northern region of 1862-1-50 and and 2910Ma for the ultramafic bodies at Achiltibuie and 1713+ 135Ma, which they interpreted as the time of Drumbeg could be artefacts produced through mixing mafic Laxfordian metamorphism. Consequently, the timing of the magma emplaced at 2700 Ma with older tonalitic material: Laxfordian metamorphism and granite emplacement is this interpretation depends on these ultramafic intrusions thought to have occurred c. 1700 Ma. also being c. 2700 Ma old, for which there is no published Despite 30 years of effort there are still many major geochronological evidence. unresolved problems regarding the chronology of the Sm-Nd mineral data for basic gneisses around Scourie Lewisian gneiss complex. This discourse makes it apparent were presented by Humphries & Cliff (1982). They obtained that unravelling the geochronological evolution of such a uniform errorchron ages of 2490 Ma, largely controlled by polymetamorphic, high-grade terrain is a complex business. garnet, which, they argued, implied that the Scourian Not only can the structural geologists not always agree complex cooled slowly from the peak of Badcallian about which structures are being dated (e.g. Inverian or

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Laxfordian), but many of the ages so far produced are by age of c. 2200 Ma for the intrusion of the dykes as a whole, and large of dubious chronological validity. There are often although one fresh tholeiite gave an age of 1950Ma, major difficulties encountered in the interpretation of suggesting that dyke emplacement may have been a whole-rock ages, and in certain instances the ages may even protracted or pulsed affair. The oldest ages (>2200Ma) be meaningless. K-Ar ages may be largely a function of were attributed to the presence of excess Ar, whereas thermal resetting or excess Ar; Rb-Sr mineral ages may younger ages (with the exception of the sample cited above) reflect post-crystallization effects and are subject, in some were considered to be due to variable Ar loss during cases, to the choice of initial 875r/86Sr ratio leading to Laxfordian metamorphism. potentially spurious ages; earlier published U-Pb ages based In contrast to Rb-Sr mineral and K-Ar studies, which upon highly discordant data may be inaccurate; and Sm-Nd may be reset by later thermal events, Rb-Sr whole-rock mineral data require a detailed knowledge of the textural dating may in certain circumstances see through these later relations in order to interpret correctly the precise 'ages' events to yield information regarding the igneous crystal- that may be obtained. It is likely that only through detailed lization of protoliths. Using this philosophy, Chapman mineral geochronology, coupled with perhaps whole-rock (1979) analysed three dolerite dykes--two from Scourie and data to obtain information on process length scales, will the one from Kylesku--using Rb-Sr whole-rock techniques. temporal complexities of the processes involved be further Owing to the low Rb/Sr ratios and to the lack of unravelled. fractionation of Rb/Sr within each individual dyke the errors on the ages were high, although it was stated that the ages and the initial 87Sr/S6Sr ratios for the individual dykes The Scourie dykes were not significantly different. However, each dyke had a The emplacement of the Scourie dyke swarm presents one distinct (if restricted) range of Rb/Sr, and so, by combining of the most critical and controversial events in the the data for the three dykes, a greater spread of Rb/Sr was development of the Lewisian complex. Their generally achieved and hence a reduction in the calculated error. The fine-grained texture and basic composition and the effects of combined age of 2390 + 20 Ma was thought to represent the post-intrusive tectono-thermal events all militate against emplacement of the Scourie dykes; this date also placed a straightforward radiometric dating. The history of study of minimum age on the Inverian metamorphism. No evidence the Scourie dykes thus provides an interesting example of was presented to substantiate the c. 1950 Ma age of Evans & how evolving analytical methodology may be progressively Tarney (1964). used to tackle more difficult geochronological problems with In order to obtain more precise ages from individual increased precision and accuracy, and hence may be able to dykes Cohen et al. (1988c) undertook a Sm-Nd mineral address regional geological evolution. study of Scourie dykes in which an apparently primary Sutton & Watson (1951) considered structures that were igneous mineralogy was still preserved: the detailed results cut by the dykes to be correlated with the Scourian orogeny of this study were given in Waters et al. (1990). Olivine and those that affected the dykes to be Laxfordian. Such a gabbro and quartz dolerite dykes gave Sm-Nd mineral- model assumed that there was only one phase of dyke whole-rock ages of 2015 + 42 to 2102 + 77 Ma and 1758 + 7 injection, and therefore that it represented a unique time and 1982 + 44 Ma respectively (Table 1). In the case of the marker. This interpretation was challenged by Bowes and Graveyard dyke a feldspar datum lay below the regression co-workers (e.g. Bowes & Khoury 1965; Bowes 1968; Dash line and was therefore excluded from the calculation. A et al. 1974) who argued on the basis of structural evidence Rb-Sr mineral-whole-rock age of 1978 + 13 Ma was also that dyke injection must have occurred over a considerable obtained from the Rhegreanoch dyke, within error of the timespan. In contrast to the view of Sutton & Watson (1951) Sm-Nd age. Rb-Sr mineral-whole-rock ages for the other that the dykes were anorogenic and emplaced into a cold dykes generally gave younger ages than the Sm-Nd data. An gneiss complex, O'Hara (1961) and Tarney (1963) proposed exception was the Graveyard dyke which gave a Rb-Sr that the dykes were intruded at depth into hot country amphibole-feldspar age of 2027-1-11 Ma compared to the rocks. It was also recognized that the Inverian deformation Sm-Nd age of 1758 + 7 Ma. This discrepancy was explained and amphibolite facies metamorphism occurred after the by invoking recrystallization and growth of garnet and Badcallian granulite facies event, but prior to and ilmenite during Laxfordian metamorphism; amphibole and penecontemporaneous with the intrusion of at least some of feldspar were deemed to have formed shortly after dyke the dykes (Tarney 1963; Evans 1965). It was argued that the emplacement, and to have remained closed systems foliated, more deformed, Scourie dykes represented the throughout the later event. Clearly a primary igneous earliest members of the suite intruded at higher mineralogy had not remained undisturbed in this case. No temperatures, whereas the unfoliated variants were evidence was found for dyke emplacement at 2400Ma, emplaced later into cooler crust (Tarney 1963, 1973; Park which led Cohen et al. (1988c) to state that the Rb-Sr age of 1970). The objections highlighted by Bowes were attributed Chapman (1979) was 'erroneous', and probably reflected by Park (1970) to both Inverian and Laxfordian structures contamination with continental crust. Interestingly, the having similar orientations coupled with ambiguities in the Graveyard dyke at Scourie was also one of those used by interpretation of early geochronological data. Chapman in the Rb-Sr study. Although Giletti et al. (1961) did not date any of the Whereas the studies considered so far have either been Scourie dykes, their importance in the chronological prone to post-emplacement thermal disturbance of isotope framework of the Lewisian was clearly recognized. Shortly systematics, or have had to make assumptions about the afterwards Evans & Tarney (1964) presented the first K-Ar geochronological significance of isotopic equilibration whole-rock data for the dykes covering a range of between minerals and/or whole-rocks, U-Pb geochronology compositions, which gave ages between 1390-3860 Ma. Due on phases such as zircon, baddeleyite and titanite, to an overall cluster of ages Evans & Tarney preferred an potentially represent self-contained systems with high

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Table 1. Geochronological data for Scourie dykes (Waters et al. 1990)

Dyke Rock type Phases Isotope System Age (Ma)

Badnaban Olivine gabbro Fsp-cpx-amph-WR Sm-Nd 2031 + 62 Bi-WR Rb-Sr 1714 + 8 Rhegreanoch Olivine gabbro Fsp-cpx-amph-WR Sm-Nd 2015 + 42 Fsp-cpx-amph-WR Rb-Sr 1978 + 13 Loch Torr Olivine gabbro Fsp-WR Sm-Nd 2102 + 77 an Lochain Graveyard Quartz dolerite Amph-gt-ilm-WR Sm-Nd 1758-1- 7 Amph-fsp Rb-Sr 2027 4- 11 Ilm-WR Rb-Sr 1738 + 11 Poll Eorna Quartz dolerite Fsp-cpx-ilm-WR Sm-Nd 1982 + 44 IIm-WR Rb-Sr 1733 + 7

Fsp, feldspar; Cpx, clinopyroxene; Amph, amphibole; Bi, biotite; Gt, garnet; llm, ilmenite; WR, whole-rock

closure temperatures to parent-daughter migration. Altho- data of Waters et al. (1990) discussed above. The c. 2200 Ma ugh material for analysis has to involve the selection of K-Ar whole-rock ages of Evans & Tarney (1964) may high-integrity grains in order to avoid the effects of therefore represent prolonged cooling following dyke low-temperature Pb loss, and there is always the possibility intrusion at deep crustal levels, whereas the younger K-Ar of analysing grains which have experienced multiple whole-rock age of 1950 Ma may indicate more rapid cooling episodes of growth (e.g. Pidgeon & Aftalion 1978), the of later dykes intruded at higher crustal levels. presence of two internal U-Pb radiometric clocks enables In conclusion, if the Scourie dykes do, in fact, represent departure from simple closed-system behaviour to be a unique structural time marker, then the time interval over generally readily identified. The development of improved which there has to be structural quiescence is about 400 Ma. techniques for the production of accurate and precise U-Pb The final recognition of such a long interval led Park (1991) data on small samples (e.g. Krogh 1982b; Parrish & Krogh to state that 'in view of the geochronology, it is likely that 1987), and the discovery that mafic dykes may contain trace such tectonic activity did take place [during the 400 Ma] and amounts of zircon and/or baddeleyite (Heaman et al. 1986; may ultimately be proved by more adequate dating.' The Krogh et al. 1987) enabled Heaman & Tarney (1989) to studies of Cohen et al. (1988c), Waters et al. (1990) and obtain ages on three individual Scourie dykes. A bronzite Chapman (1979) indicate some of the problems of picrite from Beannach and an olivine gabbro from Strathan geochronological techniques which require an assumption of gave baddeleyite ages of 2418~ 7Ma and 1992+_~Ma isotopic equilibration between coexisting phases or whole- respectively (Fig. 3). The latter was interpreted as the time rocks, and highlight the great care required in determining of dyke emplacement whereas the former, owing to the which phases are primary and which may be either slight discordancy of the data, was thought to be the secondary or have recrystallized. If incorrect textural minimum age of intrusion with the true age being a little observations are made then isolated results may yield older. Zircons from a norite from Badcall Bay yielded erroneous interpretations. It is clear, however, that if discordant data but their 2°Tpb/2(~pb ages of 2166-2179 Ma suitable material can be found and careful petrographic were considered to represent minimum estimates for the age analysis undertaken, then both detailed Sm-Nd and U-Pb of the dyke. The U-Pb data provide clear evidence for two mineral studies may yield meaningful emplacement ages for phases of dyke emplacement, the first at c. 2418 Ma and the mafic dyke suites. The significance of such ages in the second at 1992 Ma, this latter date being consistent with the structural evolution of the Lewisian complex requires a

2,4oo~j~ I b 0-,5 2,36o~~J / 2,000

2,360 ~ 0"36 Fig. 3. U-Pb concordia diagrams for two Scourie dykes, confirming the 0.43 / ~/ episodic emplacement of the suite. 418+7 M 1,992+3,,_2M,rl (a) Beannach dyke. (b) Strathan dyke. Error ellipses are drawn at the 20 level. (Figures reprinted with 71Myr . 86Mvr permission from Nature (Heaman & 0-41 J I J L ~ l Ii 0'34 l a Tarney 1989): copyright 8"6 8"8 9'0 9"2 9'4 9"6 9"8 58Y 5"9 6"0 6!1 6'2 (1989) Macmillan Magazines Lim- 207pb / 235 U 207pb / 235 ited, and from L.M. Heaman).

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more reliable knowledge of the timing of the igneous and / A CARN CHUINNEAG / / metamorphic events within the region, without which the erSr GRANITE / ~ - structural debate may continue in sterile argument.

: /~ ~ MineralIsochron The Moinian Supergroup 1"0 J'-/ ...... ~ \ ///° M 403 • 5 Ma The age, metamorphic history and wider affinities of the Moinian Supergroup have long been contentious issues. / F -WR Following the great debates of the last century (see Oldroyd 0.4 ! 1990 for a review) two main hypotheses emerged regarding the evolution of the area: (1) that the Moinian and Torridonian sediments were of the same age and that 0-8-1J / w R ~b='-" 's°chr°n 1, L~ 548 + 10 Ma I orogenesis was entirely Caledonian (Peach in Peach & 548+ lOMa 87Rb~ Horne 1930); (2) that the metamorphism was, at least in part, pre-Cambrian in age (Horne in Peach & Horne 1930). O' 7 W R . . , i 8-8-~-SrIP With the advent of radiometric dating techniques Giletti et 10 20 30 750 1000 al. (1961) set out to address this problem through Rb-Sr and Fig. 4. Rb-Sr isochron diagram for whole-rocks and minerals from K-Ar dating of micas from Moinian metasediments and the Carn Chuinneag granite (Long 1964) illustrating isotopic pegmatites. Biotites from schists covering much of the strike homogenization of the minerals during regional metamorphism length of the Moine gave an average age of 420-1-15 Ma while the whole-rocks remained closed systems. which Giletti et al. interpreted as dating widespread metamorphism of the region. The age also placed a two metamorphic episodes (Shepherd 1973; Wilson & maximum age on the movement of the post-metamorphic Shepherd 1979), gave a Rb-Sr whole-rock age of Moine thrust. More interesting, however, were Rb-Sr 548+10Ma, but that the minerals from one of these muscovite ages of 690-750Ma from pegmatites from whole-rocks yielded an age of 403 + 5 Ma (Fig. 4). These Knoydart and Sgurr Breac. Giletti et al. put forward several results were taken to indicate that the granite was intruded hypotheses in which the dates might be a function of later at 548 Ma, but that during the later metamorphism, whereas isotopic disturbance, yet all reasonable solutions required the whole-rocks remained closed systems, the minerals the pegmatites to be Precambrian in age. This led Giletti et within the whole-rocks isotopically equilibrated to give the al. to conclude 'that the Moine sediments, at least in the age of the reheating event. The whole-rock data clearly Knoydart-Morar area are older than 740 m.y.' and that the indicated that the earlier metamorphism had to be pegmatites were formed 'possibly at the time of the first, or pre-550Ma. Pidgeon & Johnson (1974) performed bulk an early, metamorphism of the Moine sediments', thus fraction U-Pb zircon analyses on three facies of the pluton. supporting Horne's view of a Precambrian metamorphism. Data from the Inchbae and Lochan a' Chairn phases defined The Moinian Supergroup has been shown to have a reverse discordia with lower intercept ages of 563 + 10 and experienced polyphase deformation and two main metamor- 502 Ma respectively, whereas the riebeckite gneiss gave a phic events (e.g. Ramsay 1963; Powell 1974). The timing of simple discordia line but also with an upper intercept of the metamorphism and associated deformational episodes 522 + 20 Ma. All these phases were considered to have been are largely constrained by dating intrusive bodies which both emplaced at c. 560Ma, broadly consistent with the Rb-Sr pre- and post-date the events. The initial results of Giletti et date. A Rb-Sr whole-rock age of 416 + 15 Ma from the al. stimulated further research into the geographical extent riebeckite gneiss showed that although the Inchbae and of the Precambrian event and the timing of the early and Lochan a' Chairn facies had remained closed to Rb-Sr late metamorphisms. The results of this work, inextricably migration on the scale of the whole-rocks during the linked with the highly contentious issue of the structural Caledonian, the riebeckite gneiss had been open. Conse- interpretation of the Moine (see Harris & Johnson (1991) quently, whereas the studies at Carn Chuinneag indicated and references therein), have prompted much heated debate the potential benefits of combined mineral and whole-rock and several major issues are still unresolved. studies, they also sounded a severe note of caution Various studies have shown that the late event in the regarding whole-rock methods in polymetamorphic terrains. Moine is dated by: (1) a bulk fraction U-Pb zircon age of Further work on the older pegmatites of Giletti et al. has 456 + 5 Ma for the Glen Dessarry syenite (van Breemen et produced Rb-Sr muscovite ages from 647 + 20 to 776 + al. 1979) which was deformed during this event (Roberts et 15 Ma (Long & Lambert 1963; van Breemen et al. 1974, al. 1984); (2) a concordant U-Pb monazite age of 1978; Powell et al. 1983; Piasecki & van Breemen 1983). In 450 + 10 Ma and Rb-Sr muscovite ages of 438-450 Ma for a detailed study of the Carn Gorm locality van Breemen et late pegmatites (van Breemen et al. 1974); (3) concordant al. (1974) found that there was probably only minor U-Pb monazite ages of 455 + 3 Ma from the Glenfinnan area disturbance of the Rb-Sr muscovite systematics during the (Aftalion & van Breemen 1980); (4) Rb-Sr muscovite ages Caledonian metamorphism. In contrast K-Ar muscovite from semi-pelitic units of 427+ 8 to 462 + 10Ma (van dates from the Ardnish pegmatite (Powell et al. 1983) yield Breemen et al. 1978). The high closure temperature of the younger ages of 498-410 Ma, indicating partial or total Ar U-Pb monazite system (c. 725 °C; Parrish 1990) suggests loss during the Caledonian, in accord with other K-Ar that the peak of Caledonian metamorphism was c. 455 Ma, determinations from the NW Highlands (Giletti et al. 1961; with pegmatite injection and cooling below 500 °C following Miller & Brown 1965; Brown et al. 1965a, 1965b). In an shortly thereafter. attempt to address the possibility that the Rb-Sr muscovite In a classic piece of work Long (1964) showed that the ages might reflect partial Caledonian overprinting of Carn Chuinneag granite, which was intruded between the Grenvillian pegmatites van Breemen et al. (1974, 1978)

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performed U-Pb analyses on monazite and zircon - which analysed bulk fraction zircons from an MP1 pegmatite lit have higher closure temperatures - from two localities. from the gneiss which gave a lower intercept age of Monazite analyses were concordant at 780 + 10 Ma whereas 556 + 8 Ma. Yet again there was no hint of a Grenvillian age discordant zircon data gave ages of 740+30 and in the data (Fig. 5). Nonetheless, Aftalion & van Breemen, 815 +30Ma. Given that these pegmatites were emplaced largely on the basis of the 1028Ma age, constructed into rocks of low metamorphic grade, van Breemen et al. elaborate models of multi-stage Pb loss to account for the (1978) concluded that the ages must represent the time of observed zircon discordance in terms of a Grenvillian pegmatite intrusion rather than reflecting slow cooling. crystallization age for the gneiss. Furthermore, the field evidence of pegmatite concordance Sm-Nd mineral dating has also thrown a spanner into the with the main foliation in the host lithologies suggested that works regarding the presence of a Grenvillian event in the this was also the time of peak metamorphism (termed the Moine. Sanders et al. (1984) obtained Sm-Nd garnet- 'Knoydartian' by Bowes (1968) and the 'Morarian' by clinopyroxene-whole-rock ages of 1082 +24 and 1010 + Lambert (1969)). In this interpretation they agreed with the 13Ma for eclogites from the Glenelg inlier which are views of Giletti et al. (1961), Long & Lambert (1963), considered to date the eclogite facies metamorphism. As the Bowes (1968) and Lambert et al. (1979) in ascribing the Morar group sits unconformably on the Glenelg inlier pegmatites to orogenic activity. Evidence against this (Ciough in Peach et al. 1910; Ramsay 1958) and is only at interpretation was provided by Powell et al. (1983) who low metamorphic grade (e.g. Fettes et al. 1985) it follows showed that the Ardnish pegmatite, which they had dated that the Morar group must have been deposited after c. using Rb-Sr on muscovites at 776 + 15 Ma, post-dated the 1000Ma. Given that there is stratigraphic continuity early folding and metamorphism of the Moinian metasedi- throughout the Moinian succession, and that the early ments and was deformed during a later event. The timing of Moinian metamorphism is considered to have occurred at the early metamorphism, however, remained unclear, pressures of about 6.5 kbar (Fettes et al. 1985), there is although a Grenvillian age was plausible in the light of other considerable difficulty in reconciling the Sm-Nd data with data (see below). the Rb-Sr data from the Ardgour gneiss (1028 + 46 Ma). Perhaps the most controversial aspect of Moinian In summary, whereas the ages of the Caledonian geochronology has centred on the age of the Ardgour metamorphism and the Knoydartian pegmatite emplace- gneiss. This was originally held to have been produced by in ment are now fairly well constrained, the significance of the situ metasomatism during the peak of metamorphism (e.g. Knoydartian event and the timing of the Precambrian Dalziel 1966), but has since been shown to have been metamorphism are still unclear. intruded during the early metamorphism (Barr et al. 1985). Brook et al. (1976), using large samples, produced a Rb-Sr whole-rock age of 1028 + 46 Ma which they interpreted as Torridonian indicating that the early metamorphism was Grenvillian in The dating of unfossiliferous sedimentary successions which age. Pidgeon & Aftalion (1978) investigated the gneiss using do not contain volcanic horizons or igneous intrusions bulk fraction U-Pb zircon techniques. The data defined a presents a considerable geochronological challenge as such reverse discordia giving a lower intercept age of 574 + 30 Ma combinations militate against dating strategies such as have and an upper intercept age of 1556 ~7~,Ma, which they had been used in the Moine and Dalradian. The Torridonian great difficulty in reconciling with the 1028 Ma Rb-Sr age of sandstones are one such succession. Brook et al. (1976). Aftalion & van Breemen (1980) also Although samples from the Torridonian sandstones

r a : 206pb / 238U

GRANITIC GNEISS Rb/Sr AGE 2ooou,~ 84+ 53~M 1° Calculated I~/~~f Primary Age ..~1~ ~ +106uNM ,<106+84~NM ..... 1.... ~f ~uNM ~f fs~ GRANITE- 1400Ma /~~/ 0 2 EMPLACEMENT 1200M Fig. 5. U-Pb concordia diagram for

lOOOMa00 Ma~/~-~ samples from Glenfinnan. Inset shows RC909 r 8 ~ ~/.-- RCTO5÷l16~NMRC705 116pNM Zircon size fractions: a. RC1524 modelled zircon evolution assuming Pb a¢CM~ Q RC1524 Paragneiss Glenfinnan loss during Caledonian and Grenvillian Ol 600M~5pNM [] RC705 Granitic Gneiss T2~//~ENITE ~ RC909 Lit in Granitic Gneiss times. See Aftalion & van Breemen ~MO;AaZnI:EAIGsE2OF (1980) for detailed discussion. (Figure 200Ma reproduced from Aftalion & van Bre- o ,o =!o 3!0 4o rs s!o a'o 20 30 4'0 emen 1980 with permission from r 5 = 207pb / 235U Springer-Verlag and M. Aftalion).

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which unconformably overlie the Lewisian complex were which gave ages from 802-2926 Ma. Taking the results at not analysed by Giletti et al. (1961), the data they obtained face value and using known palaeocurrent indicators they regarding the age of the Laxfordian led them to conclude concluded that the data were consistent with a source in the that the Torridonian must be younger than c. 1600 Ma, or Preketilidian and Ketilidian belts of Greenland and the possibly ll60Ma based on an Rb-Sr biotite age from a Grenville of Labrador. However, it is now recognized that gneiss from Loch Torridon whose interpretation was tourmalines are prone to excess Ar and give anomalously problematical. If the Moine and Torridonian sediments were old ages (Damon & Kulp 1958); consequently no reliable lateral equivalents (e.g. Sutton 1963) then the latter must be information can be gleaned from the data. older than the c. 740 Ma pegmatites in the Moine. However, Rb-Sr dating of biotites has provided evidence of uplift given that the Torridonian is unconformably overlain by and cooling of the Lewisian complex of northern Harris and Cambrian sediments, Giletti et al.'s data permitted the Lewis at c. 1100 Ma (Cliff & Rex 1989). By comparing these Torridonian to be younger than the Moine and to have been data with Rb-Sr and K-Ar ages for detrital muscovites from derived from it. the Torridonian and a K-Ar biotite age from a gneiss In order to assess the age and provenance of the boulder in a Lower Torridonian tilloid (Moorbath et al. Torridonian, Moorbath et al. (1967) analysed both detrital 1967), Cliff & Rex suggested that the northern part of the grains and individual pebbles. In the Applecross Formation Outer Hebrides could have acted as a source for some of the muscovites from schistose pebbles gave K-Ar ages of Torridonian sediment. 1659-1802Ma whereas Rb-Sr K-feldspar dates from As zircon has a high closure temperature for U-Pb microcline and quartz porphyry pebbles ranged from compared with either K-At or Rb-Sr in micas, U-Pb dating 1320-1637 Ma. Detrital muscovites from the Diabaig of individual zircon grains can potentially yield information Formation gave a restricted range of Rb-Sr and K-Ar ages regarding the ages of formation of the sources of from l160-1190Ma. From these results Moorbath et al. sedimentary piles rather than later uplift and/or metamoi- concluded that the schistose pebbles were not derived from phic ages. Moreover, because they are generally mechani- the Moine metasediments as these had been metamor- cally and chemically resistant they are most likely to survive phosed at least 740 Ma ago, and that some of the pebbles sedimentary processes and so provide a full coverage of the had a provenance in rocks which had been last provenance spectrum, provided, of course, that all the metamorphosed c. 1700Ma, which they equated with the source rocks contain zircon. Rogers et al. (1990) analysed Laxfordian complex. The ages from the detrital micas single detrital zircon grains from the Applecross Formation. indicated that the Torridonian must be younger than about The analyses were generally less than 1% discordant and fell 1190 Ma. into three groups: 1088-1193Ma; 1625-1662Ma; 2628- In an attempt to define the age of the Torridonian more 2857 Ma. These ages provide a maximum age for the the closely Moorbath (1969) used Rb-Sr whole-rock analyses of Applecross Formation of 1088 Ma consistent with both the shales to construct isochrons for the Stoer and Applecross palaeomagnetic and earlier geochronological results. Rogers Formations, which gave ages of 968 + 24 and 788 + 17 Ma et al. concluded that the data were more consistent with a respectively (Fig. 6). Moorbath argued that these reflected provenance in Labrador than E Greenland. Given the lack isotopic homogenization during diagenesis which would of published evidence for zircons of c. 1150 and c. 1650 Ma have closely followed deposition. This interpretation was from the Outer Hebrides, a source in such an area also questioned by Smith et al. (1983) who suggested, on the seems unlikely. basis of palaeomagnetic results which indicated that the Thirty years on, and despite the increasing sophistication Stoer and Applecross Formations were c. 1100 and 1040 Ma of geochronological approaches, the problems of the age respectively, that diagenesis could have occurred sig- and affinities of the Torridonian are still unresolved. nificantly later than deposition. Stratigraphical correlation with the Moine is still equivocal Allen et al. (1974) presented 4°Ar-39Ar data on exotic and would appear to require more detailed knowledge about quartz tourmaline pebbles from the Applecross Formation the age of the Torridonian sediments and the tectono-

a 0-85 0"85 _ b • 2 A 1A Sr 87 26-7 Sr 87 Sr 86 Sr~ _ ~//~ 6_~4 26-3 0.80 0-80 26-6 2A

1 /4°_. 12~12h Fig. 6. Rb-Sr whole-rock isochrons for 0"75 0.75 12f~12g 6-~oy 26-~2 968±24Na ~/ 12b 788+_17Ma siltstones and shales from (a) the Stoer ~) 26-11 f3A and (b) the Applecross Formations of 26-9 Rb87 the Torridonian sandstone. (Figure ~4A Rb87 adapted from Moorbath 1969 with Sr ~1~ Sr~-6 I~ 0.7086+0.0016 "~ 0 -7215+0.0014 permission from Scottish Journal of 0.70 I 07C I 1 Geology). 0 10 0 5 10

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thermal history of the Moine. Nonetheless, significant following a brief early Ordovician climactic episode of progress has been made regarding the provenance of the metamorphism. They drew age contours ('chrontours' or sediments which points to areas of fruitful research in the 'thermochrons'), taken to represent lines of synchronous future. cooling through the respective closure temperatures, and proposed early uplift of the low-grade rocks against the bounding faults and later uplift of the hotter, high-grade The Dairadian Supergroup rocks in the central areas (Fig. 7). Their model also related Nowadays we look to geochronology to determine both the the cycle of deposition, deformation, metamorphism, uplift time of metamorphic recrystallization within a complex belt and erosion to the still new ideas for subduction and closure and its post-orogenic uplift history. These two aspects were of the Iapetus Ocean to the south, and they further already inherent in the treatment of the Dalradian suggested that the post-orogenic Newer Granites (which had metamorphic complex presented by Giletti et al. mostly given ages of 420-370Ma) were triggered by pressure-release during the uplift phase. Mineral ages: cooling histories Seven Rb-Sr mica model ages from the Grampian Highlands and three from the Connemara schists of western Ireland # were reported by Giletti et al. By subsequent standards this is a small body of data, but they were enough to reveal an unexpected problem. Whereas their data for the Moinian i I/I Supergroup (see above) had suggested a Late Silurian/Early K-Ar Age m.y. Devonian metamorphism at about 420 Ma, consistent with >500 490-499 ~) the stratigraphically-controlled age of Caledonian deforma- 480-489 © 470-479 • tion in the Southern Uplands, England and Wales, most of , gx'// 460-469 [] 450-459 0 the Dalradian and Connemara schist ages were significantly ,,J 440-449 4, older at about 475Ma. Although this supported a ',i/ 430-439 A 420-429 ! 410-419 • correlation of the Dalradian and Connemara schists, it also 400-409 v >400 v indicated that both had experienced an Early-Mid- 7-."-t" / / ~. ~',ll t Ordovician metamorphism. Additionally, three of the Dalradian biotite ages were younger and within error of the ¢~ T -/<' 't~'l !i~ "~ Moine schist results, suggesting that the later Moinian event 0 0 !ic J .i ...... ;,,j! !lJ t had also affected the Dalradian rocks; consequently, 475 Ma i~°~

could be regarded only as a minimum age for the early l metamorphism of the Dalradian. The idea of mineral phases having different 'blocking AA "''v temperatures', with muscovite being more resistant to 4, . ./,5 overprinting than biotite, was already emerging at the time of Giletti et al.'s paper, and in their Discussion reply to R. Rutland, the authors cautioned that the younger age of 420Ma might not represent the real time of prograde metamorphism but merely that of post-metamorphic cooling. In this they admitted a fundamental property of mineral geochronology, but also predicted how this might be turned to advantage in the interpretation of the thermal history of metamorphic belts. ,G The following years saw the acquisition of a great deal more mineral data in an attempt to resolve this issue (K-Ar: Miller & Brown 1965; Brown et al. 1965a; Harper 1967: Rb-Sr: Bell 1968). These showed a wide spread of ages, /~1 40 miles i from 500 to less than 400Ma in both the Moine and Dalradian. This was still generally taken to represent the effects of overprinting of a late metamorphism on an early Fig. 7. K-Ar muscovite age-contours ('chrontours' or 'thermo- one, although authors such as Brown et al. (1965a) carefully chrons') for the Scottish Highlands, as presented by Dewey & Pankhurst (1970). These were interpreted as representing lines of discussed the alternative possibility of slow cooling. synchronous uplift and cooling through the blocking temperature (c. Particularly influential was the idea of Harper (1967) that 350 °C) following a relatively brief climactic episode of deformation low-grade rocks that had only reached their maximum and metamorphism, 480-500 Ma ago. Early uplift occurred along temperature during a short period prior to uplift, should the bounding faults in the marginal parts of the orogen (Highland give a better estimate for the metamorphic age than Boundary fault and Moine thrust), whereas the central high-grade high-grade rocks; he obtained K-Ar whole-rock ages of c. areas that had been most deeply buried did not finally cool to 350 °C 490-520 Ma from the southern margin of the belt, close to until 80-100 Ma later. Subsequent work has suggested a more the Highland Boundary fault. The case for the cooling complex pattern of local uplift events, and attainment of peak hypothesis was advanced by Dewey & Pankhurst (1970), metamorphism as late as 455 Ma in the NW Highlands (Figure who interpreted the entire body of data for both the Moine adaptation reproduced by permission of the Royal Society of and Dalradian rocks as representing uplift and cooling Edinburgh from Dewey & Pankhurst 1970).

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This was a very 'broad-brush' approach to the problem. have recently argued for a closure temperature of c. 600 °C Proper control of time-temperature trajectories for meta- in metamorphic garnets <5 cm in diameter. This implies morphism requires a great deal of high-quality data. The that for rocks above middle amphibolite facies the Sm-Nd principles usually applied have been developed from the ages obtained using garnet will generally record cooling pioneering work in the Swiss Alps (e.g. J~iger 1979). K-Ar rather than prograde mineral growth. hornblende and Rb-Sr muscovite are generally thought to The most detailed studies that have been made on the have relatively high closure temperatures of about 550 and Dalradian metamorphic complex have suggested that the 500°C respectively, K-Ar muscovite about 350°C and cooling history was more complicated than that considered biotite (Rb-Sr and K-Ar) about 300°C. The highest by Dewey & Pankhurst (1970). Dempster (1985) presented temperatures of metamorphism require independent con- Rb-Sr and K-Ar data for two transects from the Highland trol, possibly using Rb-Sr whole-rock or U-Pb zircon Border into the Central Highlands, which he interpreted in geochronology, whereas fission track data are necessary to terms of thermal modelling and tectonics. He found the date cooling down to 100°C. These methods must be expected decrease in ages northwards with increasing grade, applied to a volume of rock small enough to have had a but with a reversal in Rb-Sr muscovite ages in the area of uniform cooling path, and with sufficient precision to the Tarfside Culmination, which he explained by early distinguish the separate stages: even then, no direct uptilting. From the comparison of the various isotope time-temperature information can be obtained for the systems he deduced large fluctuations in cooling rates prograde heating path prior to the maximum temperature. It (1-25°C/Ma) in different sectors, and at different times is rare that the full set of such measurements is available; within individual sectors (Fig. 9). Dempster also favoured the only case in Scotland is that of the Glen Dessarry syenite the idea that the metamorphic climax might have been intruded into the Moinian Supergroup (see above). The data reached at progressively later times at greater depths, which for Glen Dessarry, collated and interpreted by Cliff (1985), could be seen as a reconciliation of the two hypotheses (one are consistent with a fairly simple pattern, showing an initial metamorphism or two?) first propo.~ed by Giletti et al. cooling rate of 30°C/Ma, falling to 10°C/Ma over the (1961). first 40 Ma, and followed by dramatically slower cooling Consequently, despite uncertainties in absolute closure from about 300°C (Fig. 8). This last stage is, however, temperatures, the mineral dating approach can be used to governed by an apatite fission track age which may reflect explore and elucidate the main elements of orogenic the effect of later re-heating rather than regional evolution. It must be emphasized that a painstaking post-Caledonian cooling. In general, closure temperatures collection of adequate geochronological data must be must depend on a variety of factors, such as mineral combined with good structural control in order to achieve composition, grain-size, cooling rate and fluid interactions. this. Furthermore, Giletti (1991) has claimed that the variations in diffusive exchange rates for Sr are such as to cause major Crystallization ages: dating of specific events errors in the estimated closure temperature for Rb-Sr in biotite. Various closure temperatures have been proposed The indirect approach to dating metamorphic and structural for Sm-Nd garnet systems (e.g.c. 500-700 °C: Humphries & events is based upon the accurate and precise dating of Cliff 1982; 900 °C: Cohen et al. 1988b). Mezger et al. (1992) igneous rocks with structurally-defined relationships, such as was discussed above in relation to the Carn Chuinneag and Glen Dessarry intrusions. Giletti et al. (1961) presented data I I I for two such intrusions within the Dalradian tract, the Zircon pre-metamorphic Ben Vuirich granite (600 + 100 Ma) and ,,..1000 the post-tectonic Galway granite (365 + 10 Ma). Although these ages gave only a very loose constraint for a broad span of Caledonian activity, the technique was seen to be a very 800 promising one, and it was applied extensively thereafter in l hene ~ Dalradian geochronology, as can be seen from the results 600 ~--1 Hornblende listed in Table 2. The age of the Ben Vuirich granite in ~ Muscovite particular became a fascinating problem for subsequent 400 \ Muscovite study, and our erratic approach to the truth--as we now Biotite ~----- .Ap.atite perceive it--is very instructive. The Ben Vuirich granite is regarded as having been 200 Ap-f-t intruded after nappe formation (regional D1/D2) but prior to later, more ductile deformation (regional D3) and the I I I I I I local peak of kyanite-grade metamorphism (Bradbury et al. 400 300 1976; Rogers et al. 1989). Giletti et al. (1961) were able to Age (Ma) show that, although Rb-Sr mica systems in the granite were Fig. 8. Cooling pattern for the Glen Dessarry syenite showing the obviously reset or strongly affected by Caledonian relationship between blocking temperature for each phase and the metamorphism (muscovite and biotite gave ages of c. age determined for that phase. U-Pb ages shown by circles; Rb-Sr 500Ma and 420Ma respectively), whole-rock samples ages, filled stars; K-Ar ages, squares; Fission track age, open star. preserved evidence of a pre-metamorphic history. They Fission track datum represents mean fission track age north of the argued that the closed-system model age of these samples of Great Glen fault (Hurford 1977). All other data from van Breemen c. 800 Ma, calculated assuming a meteoritic initial 87Sr/86Sr et al. (1979). (Figure reproduced from Cliff 1985 with permission ratio, could be regarded as a maximum possible age for from R.A. Cliff). intrusion. Assuming a more typical crustal value of 0.710 for

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520" 4 '520 V. RAPID COOLING lt5-2 5"C/M o

500 r r 500 j l SLOW COOLING ! /'\ 3 T 5"C/McI / 480 480 ! i ~ , / kRb-Sr Mu.c i F ~'- "." / //~ I 460 "\, " " i) " /---v SLOW CooL',NO 460 ",,, /r" K-Ar Musc 2-3"C/MO =E "- RAP, D 'COOL,NO ..~ ..... _t_-?J°~5. :, /

IN~ Biotite SLOW COOL Rb-Sr+ K-Ar 0 440 4 ,-5 C/MaY - 300"C 440 <

C D 420 420 Fig. 9. Summary of the mineral age data B from a NW-SE transect across the Dalradian of Angus, showing approxim- 400 400 ate cooling rates and assumed closure temperatures. A, B, C and D are different structural units. (Figure re- %~£. *% %. produced from Dempster 1985 with permission from T.J. Dempster).

Table 2. Geochronology of intrusions with known structural ages

Rocks Age (Ma) Method Reference

!. Ben Vuirich granite 600 5= 100" Rb-Sr WR model ages (1) 497 ± 37 Rb-Sr WR, with other (2) Older granites 552 ± 24 Rb-Sr errorchron (3) 514+76 U-Pb zircon population (3) lower intercept 590 ± 2 U-Pb zircon abraded needles (4) 597 ± 11 U-Pb zircon, ion microprobe (5) !i. Other key intrusions a. Pre-/syn-tectonic intrusions Portsoy granite gneiss 655 5= 17 Rb-Sr WR (6) Shetland migmatites 519 d= 25 Rb-Sr WR (7) Dunfallandy Hill granite 481 + 15 Rb-Sr WR (3) Insch Upper Zone gabbros 492 5= 26 Rb-Sr WR (8) Haddo House aureole 487 5= 23 Rb-Sr WR (8) Minor foliated intrusions 482 + 12 Rb-Sr WR (8) Cashel-Loch Wheelaun 490 + 1 U-Pb zircon (9) Slieve Gamph granite 477 5= 6 Rb-Sr WR (10) Ox Mountains granodiorite 489 5= 18 Rb-Sr WR (11) Glen Dessarry syenite 546 5= 5 U-Pb zircon (12) Glen Kyllachy pegmatites 444 5= 4 Rb-Sr muscovite (13)

b. Post-tectonic intrusionst Kennethmont granite 453 -t- 4 Rb-Sr WR (6) Belhelvie pegmatite 463 + 5 Rb-Sr muscovite (14) Strichen granite 475 + 5 U-Pb monazite (15)

* As reported: all other Rb-Sr ages recalculated with A87Rb = 1.42 x 10 -11 a -1. t Oldest intrusions only; the majority of Newer Granites have given ages of 390-430 Ma by a variety of methods. WR, Whole rock. References: (1) Giletti et ai. (1961); (2) Bell (1968); (3) Pankhurst & Pidgeon (1976); (4) Rogers et al. (1989); (5) Pidgeon & Compston (1992); (6) Pankhurst (1974); (7) Flinn & Pringle (1976); (8) Pankhurst (1970); Jagger et al. (1988); (10) Pankhurst et al. (1976); (11) Max et al. (1976); (12) van Breemen et al. 1979; (13) van Breemen & Piasecki (1983); (14) van Breemen & Boyd (1972); (15) Pidgeon & Aftalion (1978).

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the initial 87Sr/S°Sr ratio, the growth period of radiogenic Sr peak of metamorphism and/or due to variation in initial in the granite would have been about 500 Ma, and so they 87Sr/86Sr ratios during emplacement. The best age that could proposed emplacement at 6004-100Ma. This, therefore, be obtained from the data was 552 + 24 Ma, must also be a minimum age for Dalradian sedimentation. The geochronological method now most widely used to The procedure that they used is closely analogous to the obtain pre-metamorphic ages is U-Pb dating of zircon. This calculation of TDMNd and TCHURNO model ages in the Sm-Nd mineral is highly resistant to open-system behaviour, and in system discussed earlier, although the assumption of theory even a single, brief episode involving U mobility or insignificant parent-daughter fractionation during meta- Pb loss could be corrected for by extrapolation of linear morphism or crustal anatexis is less obviously justified in the discordance patterns in the concordia diagram. Zircon case of Rb-Sr. fractions were often separated according to their grain size The proper treatment of this problem is, of course, the and magnetic susceptibility to obtain a spread of data in this establishment of a whole-rock isochron for several diagram. The Ben Vuirich zircons analysed in this way by whole-rock samples with different Rb/Sr ratios, which Pankhurst & Pidgeon (1976) were unusual in that they potentially defines both the age of crystallization and the defined a statistically excellent discordia line with the lower true initial 87Sr/86Sr ratio. The isochron method was still intercept (514_+6Ma) being interpreted as the age of being developed at the time of Giletti et al. (Nicolaysen emplacement and the upper intercept of 1316_2s+26 Ma 1961), and its application in the Scottish Highlands followed apparently reflecting the age of the source material (Fig. within a year or two (Long & Lambert 1963; Long 1964; see 10). This type of reverse discordia, due to inheritance of above). In the case of the Ben Vuirich granite this approach pre-magmatic zircons, had already been found in the Carn was complicated by a lack of variation in Rb/Sr (as well as Chuinneag granite cutting the Moinian Supergroup (see the relative difficulty of collecting a good suite of whole-rock above). It was subsequently demonstrated to be a general samples from a rather remote locality!). High-precision feature of Caledonian granites north of the Highland mass-spectrometry and strict statistical treatment are also Boundary fault by Pidgeon & Aftalion (1978), who ascribed essential to the technique, and both were increasingly this to underlying Proterozoic crystalline basement in developed through the late 1960s (e.g. York 1969). Even contrast to Palaeozoic basement further south. Thus, at this when this was applied to the Ben Vuirich granite (Pankhurst time, it was believed that the Ben Vuirich granite was & Pidgeon 1976), it was found that the isochron model emplaced only shortly before the climax of metamorphism, broke down, either due to open system behaviour during the probably in latest Cambrian-early Ordovician times.

0"20 a __ ...~ 0-30 F ~ 1 i i ...... i BEN VUIRICH 1 ooo~/ oo°L b vu,.,c. 0"15 Euhedral grains "'"" r Subhedral grains ~1.~ "' 206pb 8°°~ ~'1 0 22t" 206Pb 12s°/~/ 238U ' i 23 -U-U 0"10 ~ o.181 - ,

0 • 141-16"1 , =/~'; 15"1 / A_qe+_2o 0"05 ~ zoo.,,/ ~ 597+-11Ma 0.10 o • ~4" 2o7pb/235U 207pb/235U / 0 06 0"00 I ! • , , . , , , 0"00 0"50 1.00 1-50 0-6 1-2 1'8 2-4 3-0 3"6 4"2

BEN VUIRICH 0"18 C 1000~J

0 '16

Fig. 10. Concordia diagrams for zircons from the Ben Vuirich 0'14 8oo ,/ / _ granite. SHRIMP analyses showing (a) the cluster of data points 7J /0ooj around 600 Ma for euhedral grains and (b) the spread of analyses for subhedral grains to higher ages. Error boxes on (a) are drawn at 0'12 the lo level whereas on (b) they are 20. Also shown on (b) as filled circles are the bulk fraction analyses of Pankhurst & Pidgeon (1976). (Figure adaptation reproduced by permission of the Royal 0 10 Society of Edinburgh and R.T. Pidgeon from Pidgeon & Compston 1992). (e) High precision, selected grain analyses with the inset //~2 - 207pb/235U showing the points for abraded, high-integrity needles giving an age o .08 I | I I I of 590 + 2 Ma. Error ellipses are drawn at the 20 level. (Figure 0"60 0'85 1 '10 1"35 1-60 1"85 reproduced from Rogers et al. 1989).

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Following the work of Krogh (1982a, b), more emphasis dates could potentially correspond to metamorphic re- was placed on the selection, cleaning and abrading of only crystallization events in the underlying crust at Grenvillian the most perfect, uncracked, euhedral zircons that had and Laxfordian times respectively, regardless of whether the clearly formed during igneous crystallization. Selection of Ben Vuirich magma were derived by anatexis of such crust just one or a few such high-integrity grains, although or had merely assimilated it. Alternatively, grains of these requiring demanding low-blank chemistry and very sensitive ages may have been eroded into sediments (e.g. Dalradian mass-spectrometry, was usually found to yield concordant Supergroup) and then entrained by the Ben Vuirich magma data points, with a consequent minimization of ambiguity in either at source or en route through the crust. The data do interpretation. not provide unequivocal evidence for the presence of crust Even though the discordia extrapolation involved in the of these ages beneath the Central Highlands. The upper Ben Vuirich granite data of Pankhurst & Pidgeon (1976) intercept of the discordia line of Pankhurst & Pidgeon was relatively short, and apparently well-controlled, the (1976) may be envisaged as the 'weighted mean' of such a zircons which they had analysed consisted of a mixture of at mixed population, modified by the effects of Pb loss. The least two populations: rounded or stubby subhedral crystals erroneous lower intercept may similarly represent a (inherited) and a few clear needles (melt-precipitated). In compromise between the times of igneous crystallization, order to eliminate the effects of inheritance and obtain an peak metamorphism, multi-aged inheritance and Pb loss accurate and precise date for igneous crystallization, Rogers (see also Rogers et al. 1989 for a discussion of this). The et al. (1989) carried out U-Pb analyses of only high-integrity perfect linear alignment of different fractions along the zircons, especially the clearest, needle-shaped, melt- discordia line is still hard to explain, but must be related to precipitated crystals. The purest, most strongly abraded the way in which this complex mixture of zircons fraction analysed was indeed concordant, with two others fractionated during the mineral concentration procedures. defining a small degree of normal Pb-loss discordance (Fig. Nevertheless, the warning is very clear: only genuinely 10). No evidence of inherited U-Pb systematics was detected concordant U-Pb zircon data may be used to define in these fractions, and so the age was determined accurately crystallization ages in disturbed or polygenetic systems. and precisely at 590+2Ma. This result is extremely This experience should lead us to caution in interpreting significant, both for the timing of Dalradian events and for the data listed in the second part of Table 2, almost all of the methodology of determining such events in other which are from pre-1980 studies, even though they support a metamorphic belts. Firstly, it requires that all preceding consistent story. Many of these rocks are key event- Dalradian events (i.e. sedimentation and nappe formation) markers, some of which are the subjects of on-going precise must have been Precambrian, and that the fossiliferous modern geochronological analysis. The 655 ± 17 Ma Rb-Sr rocks of the Leny Limestone, previously considered to be whole-rock isochron for the Portsoy granite gneiss part of the Dalradian Supergroup, and as such to link the (Pankhurst 1974), another pre-metamorphic 'Older Granite' Dalradian to the Laurentian margin, could no longer be that gave a 440 Ma Rb-Sr biotite age in Giletti et al. (1961), regarded as part of the Dalradian (see also Harris, in seems to have survived the later metamorphic overprinting, discussion of Rogers et al. 1989). Secondly, it cast even and, at face value, is fully compatible with late Precambrian more doubt on the general ability of other methods to Dalradian sedimentation and deformation. However, determine pre-metamorphic ages with any reliability. Hall±day et al. (1989) reported a bulk fraction U-Pb zircon The validity of the age determined by Rogers et al. age of 595 ± 5 Ma from the Tayvallich volcanic sequence in (1989) has received support from the work of Pidgeon & the SW Highlands, at the top of the Middle Dalradian and Compston (1992), which has also gone some way towards supposedly correlative with the Portsoy beds. This is thus quantitatively explaining the failure of the earlier attempt at inconsistent with the Portsoy Rb-Sr age, and only consistent zircon dating. These authors have used a highly specialized with the new age of the Ben Vuirich granite if deposition of and still newly-developed method of dating based on the Upper Dalradian and nappe formation were all ion-microprobe analysis---direct sputtering of ions from the accomplished within less than 12Ma prior to granite sample, followed by high-resolution mass-spectrometry emplacement. (Compston et al. 1984). The sensitivity and spatial The data of Pankhurst (1970) for the basic intrusions in resolution of this instrument (SHRIMP), which can measure the NE Grampians have been very influential in ascribing an the Pb-isotopic composition of material excavated from a age of about 490 Ma to the peak of metamorphism in the volume of about 10-6mm 3, allows the discrimination of Buchan area of high-T, low-P metamorphism. This date is U-Pb ages between the growth zones of individual zircons. supported by an age of 490 Ma--being the mean 2°Tpb/E°rpb It is thus a unique tool for potentially obtaining concordant age of six magnetic fractions of discordant zircon--from the (crystallization) ages for both the younger igneous and Cashel-Lough Wheelaun intrusion in Connemara which inherited components in a zircon population. The data occupies a similar structural position (Jagger et al. 1988). produced by analysis of zircon grains from the concentrates The age for these intrusions compared with that for the originally used by Pankhurst & Pidgeon (1976) results in two Glen Dessarry syenite (456±5 Ma) has been taken as major observations (Fig. 10). Firstly, euhedral grains cluster indicating diachronism of Caledonian metamorphism and around concordia at a weighted mean age of 597 + 11 Ma, deformation across the orogen with younger times in the within error of the more precise value of 590+2Ma west (e.g. van Breemen et al. 1979; Powell & Phillips 1985). obtained by selected-grain zircon analysis. Secondly, the Van Breemen & Piasecki (1983) reported an average Rb-Sr subhedral grains exhibit two separate, older groups of muscovite age of 444 + 4 Ma from late pegmatites associated concordant or near-concordant ages, interpreted as the with the Glen Kyllachy granite in the NW Grampian primary crystallization ages of zircons inherited by the Highlands which they said was intruded late in the local F3 younger magma; three grains form a group at 950-1100 Ma stress field. They argued that this indicated that there was and two analyses of one grain are close to 1700 Ma. These no significant difference between the timing of deformation

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in the NW Grampian Highlands and the Northern lies in detailed mineral studies, allied, of course, to sound Highlands, and that any major diachronism occurred field and textural control of samples. between the NW Grampians and the other parts of the Dalradian block. However, a number of the late- F3 The manuscript benefitted from comments by S. Moorbath, R.A. Cliff and M.J. Whitehouse pegmatites analysed by van Breemen & Piasecki (1983) gave ages down to 422 Ma which they interpreted as being due to References resetting by the later Findhorn granite: this raises doubts as to whether the other pegmatite dates have also been AFTALION, M. & VAN BREEMEN, O. 1980. U-Pb zircon, monazite and Rb-Sr whole rock systematics of granitic gneiss and psammitic to semi-pelitic partially reset. Moreover, as there is no evidence in the host gneiss from Glenfinnan, northwestern Scotland. Contributions to Northern Highlands for the c. 600 Ma deformation and Mineralogy and Petrology, 72, 87-98. metamorphism (Grampian orogeny) which affected the ALLEN, P., SUT]~ON J. & WATSON, J.V. 1974. Torridonian tourmaline-quartz Dalradian block (Rogers et al. 1989), correlation across the pebbles and the Precambrian crust northwest of Britain. Journal of the Geological Society, London, 130, 85-91. Great Glen must still be considered doubtful. If the Glen ARNDT, N.T. & GOLDSTEIN, S.L. 1987. Use and abuse of crust-formation Kyllachy granite really is 444 Ma old, then the fact that the ages. Geology, 15, 893-895. post-tectonic Strichen granite from the NE Grampians has BARR, D., ROBERTS, A.M., HIGHTON, A.J., PARSON, L.M, & HARRIS, A.L. given a concordant U-Pb monazite age of 475 +5MR 1985. Structural setting and geochronological significance of the West Highland Granite Gneiss, a deformed early granite within Proterozoic, (Pidgeon & Aftalion 1978) suggests that there may be lateral Moine rocks of NW Scotland. Journal of the Geological Society, London, diachroneity within the Dalradian block. As pointed out by 142, 663-675. Dempster (1985), however, such observations could also be BELL, K. 1968. Age relations and provenance of the Dalradian series of due vertical diachronism exposed by differential uplift. Scotland. Bulletin of the Geological Society of America, 79, 1167-1194. Consequently, in order to assess the relationships BowLs, D.R. 1968. The absolute time scale and the subdivision of Precambrian rocks in Scotland. Geologiska Fi~reningens i Stockholm between the timing of deformation and metamorphism in Fi~rhandlingar , 90, 175-188. differing areas a body of reliable geochronological data is 1978. Shield formation in early Precambrian times: the Lewisian required based on sound field relationships. Defining the full complex. In: BOWLS, D.R. & LEAKE, B.E. (eds) Crustal evolution in chronicle of events during orogeny is still not a simple northwestern Britain and adjacent regions. Geological Journal Special Issue, 10, 39-80. matter, more than 30 years after the work of Giletti et al. -- & KHOURY, S.G. 1965. Successive periods of basic dyke emplacement in the Lewisian complex south of Scourie, Sutherland. Scottish Journal of Geology, 1, 295-299. Future developments BRADBURY, H.J., SMITH, R.A. & HARRIS, A.L. 1976. 'Older' granites as time-markers in Dalradian evolution. Journal of the Geological Society, The advent of high-precision mass-spectrometry, coupled London, 132, 677-684. with careful mineral geochronology is clearly reaping BROOK, M., BREWER, M.S. & POWELL, D. 1976. Grenville age for rocks in the significant rewards in our understanding of Highland Moine of north-western Scotland. Nature, 2611, 515-517. BROWN, P.E., MILLER, J.A., SOPER, N.J. & YORK, D. 1965a. evolution, and of orogenic belts elsewhere (e.g. Corfu 1988; Potassium-argon age pattern of the British Caledonides. Proceedings of Mezger et al. 1992). Of particular importance is the advent the Yorkshire Geological Society, 35, 103-138. of multi-isotopic techniques applied to garnet (e.g. Mezger --, YORK, D., SOPER, N.J., MILLER, J.A., MACINTYRE, R.M. & FARRAR, et al. 1989, 1992) as these may potentially be able to relate E. 1965b. Potassium-argon ages of some Dalradian, Moine and related Scottish rocks. Scottish Journal of Geology, 1, 144-151. geochronological information to P-T conditions on the same BURTON, K.W. & O'NIONS, R.K. 1992. Contrasting trace element behaviour sample (but see the note of caution in Mezger et al. 1992). and exceptional age preservation in the Lewisian of north west Scotland. The use of a laser micro-probe for K-Ar and 4°Ar-39Ar LOS, 73, No.14, 332. studies of individual detrital micas (e.g. Kelley & Bluck CHAPMAN, H.J. 1979. 2,390 Myr Rb-Sr whole-rock for the Scourie dykes of north-west Scotland. Nature, 277, 642-643. 1989, 1992) is providing exciting data relating to -- & MOORBATH, S. 1977. Lead isotope measurements from the oldest sedimentary provenance and regional tectonics. As men- recognised Lewisian gneisses of north-west Scotland. Nature, 268, 41-42. tioned earlier, the ability of the SHRIMP instrument to date CLIFF, R.A. 1985. Isotopic dating in metamorphic belts. Journal of the multiple stages of growth within zircon grains provides the Geological Society, London, 142, 97-110. potential for determining the age spectrum of inherited -- & REX, D.C. 1989. Evidence for a 'Grenville' event in the Lewisian of the northern Outer Hebrides. Journal of the Geological Society, London, zircon cores within granitoid magmas; moreover, it may 146, 921-924. prove a relatively rapid technique for assessing the overall --, GRAY, C.M. & HUHMA, H. 1983. A Sm-Nd isotopic study of the South spread of ages in detrital zircons from a given rock, where Harris igneous complex, the Outer Hebrides. Contributions to high precision data are not necessarily required. All these Mineralogy and Petrology, 82, 91-98. COHEN, A.S., O'NIoNs, R.K. & O'HARA, M.J. 1988a The timing and applications should make important contributions to future mechanism of depletion in Lewisian granulites. In: Workshop on the research. growth of continental crust. L.P.I. Technical Report Number 88-02. Lunar and Planetary Institute, Houston. , , SIEGENTHALER, R. & GRIFFEN, W.L. 1988b. Chronology of the Conclusions pressure-temperature history recorded by a granulite terrain. 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Received 28 November 1992; revised typescript accepted 21 December 1992.

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