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Geological Society, London, Special Publications; 1991; v. 57; p. 137-159;
DOI: 10.1144/GSL.SP.1991.057.01.12
© 1991 Geological Society of London

Sourcelands for the Carboniferous Pennine river system: constraints from sedimentary evidence and U-Pb geochronology using zircon and monazite

R. A. Cliff, S. E. Drewery & M. R. Leeder

Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK

Petrographic and U-Pb isotopic data are presented for Carboniferous sandstones from the Pennine basin ranging from Chadian to Westphalian B in age. Variations in the petrography of these samples, particularly the greater abundance of feldspar in Namurian sandstones reported qualitatively by previous authors, are confirmed by modal analyses. The influx of coarse feldspathic sandstones into the Pennine and Southern North Sea basins in the early Namurian is attributed to a marked increase of sediment yields and discharge due to increased precipitation in the hinterlands. The same detrital heavy mineral species occur throughout the Carboniferous. Zircons are petrographically variable both in colour and degree of rounding. Two major sub-populations are recognized; pink-purple grains and colourless-brown grains. U-Pb isotopic analyses of multi-grain fractions of zircon from five of the samples show very similar age patterns; the sixth sample, a Yoredale sandstone appears to be distinct.

Single grain analyses of zircons were used to investigate one sample of late Namurian (Rough Rock) sandstone in detail. Concordant or slightly discordant results demonstrate the presence of: rounded brown grains > 3.5 Ga, rounded purple grains 2.8–3.2 Ga, euhedral purple grains ≥2.9 Ga and clear euhedral grains c. 0.41–0.43 Ga.

Monazites are shown to be very suitable for single grain analysis, yielding concordant results more often than zircon. Most of the monazite in the Rough Rock and a Westphalian sandstone are Caledonian (0.41–0.43 Ga) but two individual grains have ages of 0.947 and 2.70 Ga.

The isotopic results appear to exclude large areas of the North Atlantic region as potential sourcelands and no presently exposed rocks provide a close match with the inferred sourceland geology; rocks now under the northern North sea and/or between Scotland and Greenland are possible, as yet untested, sources.





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Geological Society, London, Memoirs, 2003; 28: 73 - 76.
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