Abstract

Dr R. S. Thorpe writes: Since first proposed by Greenly, it has frequently been argued that the Monian gneisses are older than, and hence form a floor to the ‘Bedded Succession’. In spite of Shackleton’s cogent arguments that the gneisses are metamorphosed equivalents of part of the lower-grade Bedded Succession, the authors have accepted Greenly’s view and suggest that the gneisses are Archaean in age and possibly equivalent to the Rosslare Complex. New Rb-Sr whole-rock isochron ages for Monian gneisses at Gaerwen Quarry enable a clear distinction to be made between these conflicting hypotheses (Beckin-sale & Thorpe 1979). The Rb-Sr data indicate that a major metamorphic gneiss-forming episode, involving paragneiss and amphibolite at 595 ± 12 Ma (initial 87Sr/86Sr = 0.7061 ± 3) was followed by emplacement of tonalite, granodiorite and granite gneiss at 562 ± 13 Ma (initial 87Sr/86Sr = 0.7081 ± 8). These data are clearly inconsistent with an Archaean age for the gneisses. A new Rb-Sr whole-rock isochron indicates that the Coedana granite crystallized at 603 ± 34 Ma (initial ratio = 0.7086 ± 9). In view of the relatively low initial 87Sr/86Sr ratios of the older gneisses (which precludes a long earlier crustal history) and the clear evidence for a late Precambrian metamorphic-igneous cycle at c. 600 Ma, it is felt that the available evidence indicates that the Monian gneisses must be metamorphosed equivalents of the Bedded Succession. This view is in full agreement with the interpretation of Shackleton.

This interpretation has implications for the age

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