A Geochronological Study of the Metamorphic Complexes of the Scottish Highlands

Abstract

Rubidium–strontium age-determinations are presented for minerals and whole rocks from the Lewisian, Moinian and Dalradian metamorphic complexes of Scotland and from the Connemara schists of western Ireland. Age data from the Lewisian complex confirm that it was affected by two major periods of metamorphism. Pegmatites associated with the Scourian part of the Lewisian complex are shown to be at least 2460 m.y. old, whereas the Laxfordian metamorphism occured about 1600 m.y. ago. The effect of the Laxfordian metamorphism on the Scourian pegmatites is to produce a scatter of ages in which coexisting potassium feldspars and biotites show the pattern potassium-feldspar age > biotite age. Six biotites, a microcline and a muscovite from the Moine Series have ages in the range 435 to 405 m.y., showing that a widespread Caledonian (sensu stricto) metamorphism affected the Moine Series 420 ± 15 m.y. ago. Two pegmatites from the Knoydart–Morar area yielded muscovites with ages of 740 m.y. and 665 m.y.; asurvey of the geochemical possibilities and consideration of the geological setting of the pegmatites suggest that the Moine sediments in this area are older than 740 m.y. and may have undergone an early metamorphism before this date. Specimens from the Dalradian Series of Perthshire suggest a major metamorphism at 475 ± 15 m.y. ago, interpreted as Lower or Middle Ordovician in age. Two whole-rock and three mineral analyses from the pre-metamorphic Ben Vuroeh granite-gneiss suggest that the intrusion was formed 600 ± 100 m.y. ago and that a partial reconstitution occurred 415 ± 10 m.y. ago. The Ben Vuroch granite complex as a whole appears to have behaved as a closed system with respect to rubidium and strontium during later metamorphism. Three specimens of muscovite and biotite from the Connemara schists of western Ireland have a mean age of 475 m.y.; this finding tends to support the generally supposed contemporaneity of the Dalradian and Connemara metamorphisms. Biotite from the Galway granite has an age of 365 ± 10 m.y., which suggests that this granite may be contemporaneous with other dated Caledonian granites of the British Isles. Four potassium–argon ages support the conclusions on the age of the Laxfordian and Caledonian–Moinian metamorphisms.

High-precision U–Pb dating of complex zircon from the Lewisian Gneiss Complex of Scotland using an incremental CA-ID-TIMS approach

“…Indeed, some of these outstanding controversies are likely a legacy of the long history of research in this area in that many isotope studies were undertaken when techniques were still in their infancy and have since either been improved upon or disregarded as obsolete (e.g. Giletti et al, 1961;Pidgeon and Aftalion, 1972). Among the key points still disputed are: (1) protolith ages of the gneisses, (2) age of granulite metamorphism in the Assynt block and (3) relative affinities of different fault bounded blocks.…”

Late Precambrian U–Pb titanite age for peak regional metamorphism and deformation (Knoydartian orogeny) in the western Moine, Scotland

“…Giletti et al (1961) and Long & Lambert (1963) obtained the first Precambrian ages of around 740 Ma from the Moine by Rb-Sr dating of large (up to 30 cm across) muscovite crystals from pegmatites from the Knoydart mica mine (location A, Fig. 3), Sgurr Breac and Loch Eilt (both shown in Fig.…”

“…A classical example of this dilemma is provided by the Neoproterozoic Moine block in the Caledonides of western Scotland ( Fig. 1), where pioneering radiometric dating in the 1960s appeared to demonstrate the presence of a Precambrian orogeny at .700 Ma (Giletti et al 1961;Long & Lambert 1963), overprinted by a much younger Caledonian orogenic event. This model was considerably refined during the 1980s (Piasecki & van Breemen 1983;Barr et al 1985Barr et al , 1986Roberts et al 1987), but more recent work has led to an apparently irreconcilable difference of opinion between those who support this hypothesis (Bluck et al 1997;Vance et al 1998;Bluck 2000Bluck , 2001, and those who contend that all of the regionally important structural and metamorphic events that have affected the Moine are of early Ordovician (c. 470 Ma) age (Soper & England 1995;Soper & Harris 1997;Soper et al 1999;Dalziel & Soper 2001).…”

Structure and stratigraphy of the Morar Group in Knoydart, NW Highlands: implications for the history of the Moine Nappe and stratigraphic links between the Moine and Torridonian successions

“…Giletti, 1961;van Breemen et al 1974;Rogers et al 1998;Vance et al 1998;Tanner & Bluck 1999;Oliver 2002;Tanner & Evans 2003;Cutts et al 2010;Cawood et al in press). deformation and magmatism appears to be mainly restricted to the higher Sgurr Beag and Naver nappes (Kinny et al 1999;Cutts et al 2010).…”