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Geological Society, London, Special Publications; 1991; v. 57; p. 161-172;
DOI: 10.1144/GSL.SP.1991.057.01.13
© 1991 Geological Society of London

Isotopic characteristics of Ordovician greywacke provenance in the Southern Uplands of Scotland

Jane A. Evans1, Philip Stone2 & James D. Floyd2

1 NERC Isotope Geoscience Laboratories, Keyworth, Nottingham NG12 5GG, UK
2 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK

Interbedded Ordovician greywackes in the Southern Uplands are of markedly different composition. An integration of Sm-Nd isotope data, petrography of detrital clasts, and palaeocurrent flow analyses allows the likely provenance character and distribution to be deduced. The Portpatrick and Galdenoch formations contain detritus from volcanic provenances: andesite dominates the composition of Caradoc-Ashgill greywacke from the Portpatrick Formation which have {varepsilon}Nd445 (time of deposition) between –2.3 and –2.9, the Llandeilo-Caradoc Galdenoch Formation is richer in hornblende and has a slightly lower range of {varepsilon}Nd445 between –3.4 and –4.4. These values contrast with the more isotopically depleted signature of the Llandeilo-Caradoc Kirkcolm Formation greywackes which contain mainly quartz and feldspar clasts and have {varepsilon}Nd445 as low as –11.2. The source of the volcanic rocks, deduced from palaeocurrent analysis, lay on side of the depositional basin (south in terms of modern geography) and the isotopic composition of the volcanic rocks is consistent with a calc-alkaline arc or back-arc assemblage, founded on continental crust. A Proterozoic terrane, represented by detritus from the Kirkcolm Formation, lay on the opposite side of the depositional basin. Dalradian metamorphic rocks currently exposed to the north of the Southern Uplands display a less radiogenic signature than Kirkcolm Formation greywackes and seem unlikely to have acted as their source. This may support interpretations of the Southern Upland Fault as a locus of major sinistral strike-slip movement.