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Geological Society, London, Special Publications; 1989; v. 43; p. 149-160;
DOI: 10.1144/GSL.SP.1989.043.01.09
© 1989 Geological Society of London

A laser-microprobe study of argon isotopes in deformed pegmatites from the Northern Highlands of Scotland

James J. Irwin & Charles Kirschbaum

Department of Physics, University of California, Berkeley, CA 94720, USA

Tek. H. Lim

Department of Nuclear Engineering, University of California, Berkeley, CA 94720, USA

Derek Powell

Department of Geology, Royal Holloway and Bedford New College, University of London, Egham Hill TW20 0EX, UK

William E. Glassley

Lawrence Livermore National Laboratory, Livermore, CA 94550, USA

In the Highlands of Scotland, the Moine schists are cut by pegmatites that were deformed and metamorphosed after emplacement. The 40Ar/39Ar{kappa} ages of laser ‘pits’, in muscovite, feldspar, and some garnet and quartz crystals in two of these pegmatites are consistent with regionally well-established ages of metamorphism of the Moine. The ages of cores, rims and of different sizes of muscovite crystals are the same within errors. Individual crystals of garnet and quartz in the Ardnish pegmatite are heterogeneous in their argon isotopic composition, indicating that the last episode of metamorphism did not equilibrate the noble gas isotopes. Some quartz and garnet crystals from a pegmatite at Ardnish contain radiogenic 40Ar generated in the last c. 420 Ma and a trapped component having a 40Ar/36Ar ratio close to modern atmosphere. A few garnets from this same pegmatite also appear to contain t least one trapped component of 40Ar with an 40Ar/36Ar ratio > 10000.