Abstract

Thirty-four Rb-Sr isotope dilution analyses on muscovite books as well as four U-Pb zircon and monazite analyses demonstrate two distinct episodes of pegmatite emplacement in the Moines at 730 ± 20 m.y. and 450 ±10 m.y. (λ87Rb = 1.39 x 10−11y−1). Confirmation of the Precambrian episode comes from a detailed Rb-Sr study on muscovite books from the Carn Gorm pegmatite in Central Ross-shire and from the concordance of Rb-Sr muscovite and U-Pb zircon ages from two pre-F2 pegmatites from the hornblende anorthite zone of Kennedy (1949) 11 km west of Glenfinnan in western Inverness-shire. Palaeozoic pegmatites were measured from Loch Monar in central Ross-shire and from occurences in the Moines east of the western tip of Loch Eilt in the Glenfinnan area.

Near Glenfinnan Rb-Sr analyses on cross-cutting pegmatites yield an age of 445 ± 10 m.y. for a suite of post F3 lamprophyres. An older age limit for the F3 deformation is provided by a 450 ±10 m.y. monazite age from a post F2 to pre-F3 pegmatite. Both isotopic and geological evidence is presented for a post Arenig pre-450 ± 10 m.y. age for the second deformation in the Moines. Structural correlations in the Moines are briefly discussed as well as the evidence for two orogenic events corresponding in age to the episodes of pegmatite emplacement.

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