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A GEOCHRONOLOGICAL STUDY OF THE METAMORPHIC COMPLEXES OF THE SCOTTISH HIGHLANDS

1961; Geological Society of London; Volume: 117; Issue: 1-4 Linguagem: Inglês

10.1144/gsjgs.117.1.0233

2058-105X

Bruno J. Giletti, S. Moorbath, R. St J. Lambert,

Geochemistry and Geologic Mapping

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.