The sub-Palaeozoic basement in central Ireland
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Basement
Xenolith
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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. (λ 87 Rb = 1.39 x 10 −11 y −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.
Pegmatite
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A general consolidation of data relating to reports of Precambrian and lower Paleozoic spores of land plants and hystrichosphaers is presented. Emphasis is placed on the necessity of establishing clear and objective means of separating and classifying these organic remains. Their value lies in the theoretical interpretation of the origin of land plants as well as in the practical applications in stratigraphic correlations of the oldest sedimentary formations. -- F. M. Hueber.
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An occurrence of Lower Palaeozoic rocks within the Precambrian terrain near Sukkertoppen, West Greenland, was discovered in 1965 by Caj Kortman and Jens Gothenborg working under fil. mag. L. Keto for Kryolitselskabet Øresund AIS, Copenhagen. The material was kindly placed at the disposal of the writer for further study.
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Basement
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Phyllite and quartzite, probably Precambrian, trend meridionally and dip steeply in the New River Gorge above the junction with the Salisbury River in southern Tasmania.Further upstream an association of silty dolomite and conglomerate is less deformed and may be younger.Siliceous conglomerate boulders occur in the New River below the gorge.Late Ordovician carbonates of the Gordon Group are exposed in the New River valley just above the junction with the Salisbury River and up the Salisbury to Vanishing Falls, above which a sheet of Jurassic dolerite seems to have been intruded along the unconformity between the Ordovician rocks and almost horizontal Late Palaeozoic sedimentary rocks of the Parmeener Supergroup.
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