Volume 150 Celebration Paper

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 developments 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 evolutionary 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/3/447/4892344/gsjgs.150.3.0447.pdf by guest on 30 September 2021 448 G. ROGERS & R. J. PANKHURST 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.

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