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Journal of the Geological Society

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Discussion on detrital mineral ages from the Southern Uplands using ⁴⁰Ar-³⁹Ar laser probe

C. Elders writes: The value of using radiometric dating techniques in sediment provenance studies has once again been shown by Kelley & Bluck (1989) who report ages between 458 and 502 Ma for detrital muscovites from Late Ordovician greywackes in the Northern Belt of the Southern Uplands of Scotland. These results contrast with Rb-Sr whole-rock isochron ages of c. 1200 Ma, 600-700 Ma and 475-491 Ma obtained from granite clasts from conglomerates of similar stratigraphic age. The ages of the clasts suggest that the Southern Uplands represents an allocthonous terrane in the Caledonian orogenic belt (Elders 1987a).

Kelley & Bluck argue that the greywackes containing the dated muscovites were derived from the same source as the conglomerates containing the granite clasts. The absence of older ages from their data led them to question the validity of the ages reported from the clasts. However, the conglomerates are associated with greywackes of a different composition, belonging to different facies associations and showing different palaeocurrent patterns to the greywackes containing the dated muscovites. This implies that the conglomerates were deposited by separate sedimentary systems from the greywackes sampled by Kelley & Bluck, and that the discrepancy between the ages reported in the two studies reflects derivation of detritus from different sources.

The interaction between sedimentary systems of contrasting provenance is illustrated by the relationship between the Glen Afton conglomerate from West Nithsdale and the Kirkcolm Formation of the Rhinns of Galloway. At each locality greywackes rich in quartzo-feldspathic detritus are overlain by greywackes

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