Pb isotopic composition of feldspars from Scottish Caledonian Granites, and the nature of the underlying crust

Abstract

Synopsis Common Pb isotopic ratios for 11 feldspar separates from Caledonian granite intrusions (400 Ma old) show a linear alignment on a 207 Pb/ 204 Pb vs 206 Pb/ 204 Pb diagram, the best-fit regression line suggesting a lower intercept age of 2700 Ma, using conventional growth-curve models. This accords well with previously published Pb ages for the Lewisian basement in north-west Scotland ( Chapman and Moorbath, 1977 ), and strongly suggests a Lewisian Pb component in the Caledonian granites. Furthermore, the feldspars from the Southern Uplands granites are significantly more radiogenic than those from the Highlands and Midland Valley; this relationship is consistent with the presence of an older crustal layer beneath the more northerly regions which does not extend southwards into the Southern Uplands.

On the sources of uranium in some Scottish Caledonian granites

“…For example a more complex process of mixing of Pb from crustal rocks of differing ages and histories which may have carried a Pb isotope memory in the form of Lewisian-derived sedimentary detritus may be more accurate. However, the data do show (and Blaxland et al (1979) pointed this out) that the Southern Uplands granites contain Pb that is more radiogenic than that found further north, which lends support to the interpretation of seismic evidence that there is a fundamental difference between the basement on either side of the Southern Uplands Fault (Bamford et al, 1978).…”

“…Caledonian granites (open symbols) (from Blaxland et al, 1979, and and for Lewisian samples (filled circles) (from Chapman and Moorbath, 1977) plotted relative to two-stage terrestrial Pb isotope evolution curve of Stacey and Kramers (1975). The curve is subdivided at 0.5 Ga intervals.…”

“…First, the rocks are mainly post-tectonic and have not been affected by subsequent alteration or metamorphism. Secondly, Blaxland et al (1979) reported that one sample (the Loch Doon separate with quite radiogenic Pb) was given a thorough acid wash treatment which resulted in no significant change in Pb-isotopic composition. Thirdly, the Pb in the zircons varies greatly in 2~176 because some have marked inherited (Precambrian) isotopic components.…”

“…Blaxland et al (1979) published the first common lead isotope work on the Scottish Caledonian granites, and this study relies heavily on these data plus those of Pidgeon and Aftalion (1978), , and Halliday et al (1979).…”

The genesis of gold mineralisation hosted by orogenic belts: A lead isotope investigation of Irish gold deposits

“…In contrast to the SET, the Grampian/Lewisian NWT basement is characterised by unradiogenic Pb isotope signatures (e.g. Blaxland et al, 1979;Marcantonio et al, 1988;Whitehouse, 1993;Whitehouse and Moorbath, 1986) that are typically less radiogenic than most Irish gold mineralisation. This basement has a Nd model age of ~2.6 Ga (see section 5.2), and so we infer that the NWT end-member is dominated by crustal, rather than upper mantle source rocks, and that its Pb isotope ratios evolved since crust formation at 2.6 Ga. Sedimentological and petrological evidence suggests that Silurian (or later) lithologies could be formed from both SET-and NWT-derived material (Hutton and Murphy, 1987;Morris, 1987).…”