A radiogeologic study of the granites of SW. England

Abstract

SUMSixty-four samples from various sites along the granite batholith of SW. England, from the surrounding country rock, and from Lundy Island have been analyzed for the radiogenic elements potassium, uranium and thorium. An overall picture emerges of a radioactive batholith, uniform along its length, emplaced in a normal and less radioactive country rock. Weathered and kaolinized granites are depleted in U and Th but not K, relative to fresh granite. A proportion of the uranium mineralization found in the region, as well as the many different ages determined for uraninite and coffinite mineralization, are probably caused by the remobilization of the MARY uranium which has been leached from the granites. A traverse across the Dartmoor pluton revealed no marked differences between the megacrystic and poorly megacrystic granites. In contrast, the radiogenic concentrations of the fine grained granites which comprise parts of the Land's End and Carnmenellis plutons are atypical and are possibly rafts of assimilated country rock. Assuming that the radiogenic elements are distributed in exponentially decreasing amounts with depth, then the measured data are consistent with the formation of the batholith near the bast. of an originally normal crust with a subsequent high-level emplacement.THE PURPOSE OF THE STUDY was to utilize the radioactive, heat-producing elements K, U and Th to investigate the uniformity of the various plutons and to study the differences with the country rock. It was hoped that the investigation would assist in the understanding of the geology and petrogenesis of the granite as well as in explaining the uranium mineralization of the region. Radiogeology generally consists of detailed radiometric mapping with scintillation techniques, usually by air or car. Comprehensive radiometric investigations of this type have been made in SW. England, and throughout much of Great Britain, by the Institute of Geological Sciences (Bowie et al. 1973, I.G.S. Annual Reports 195i-7 2) but remain virtually unpublished. The type of survey reported here makes amends for its discrete, less complete areal coverage by yielding absolute, as opposed to relative, elemental concentrations; it is the first of its kind in the British Isles.

The Cornubian Batholith: an Example of Magmatic Fractionation on a Crustal Scale

“…They state that they carried out interlaboratory checks of U by INAA and that the results were in reasonable agreement in all cases and consequently they felt that the U results are accurate to ± 5 %. The agreement with other methods that Tammemagi and Smith (1975) reported, which we have also found less in a less direct way, is important since γ-ray counting determines U concentrations indirectly by measuring the activity of 214 Bi in the 238 U decay series. We here use all of the data of these authors.…”

“…This receives some support from the data of Wheildon et al (1981) for which the mean values of standard deviation in U contents are highest for the Dartmoor Granite, followed by the Bodmin Moor Granite. Tammemagi and Smith (1975) analyzed samples from all plutons and found good agreement between U determinations by γ-ray spectrometry and by INAA, showing that the 238 U decay series is in equilibrium. That observation of Tammemagi and Smith (1975) is supported by the good general agreement between their measurements and those of Wheildon et al (1981), also measured by that technique, and data from other sources that were all obtained by different analytical methods.…”

“…Tammemagi and Smith (1975) analyzed samples from all plutons and found good agreement between U determinations by γ-ray spectrometry and by INAA, showing that the 238 U decay series is in equilibrium. That observation of Tammemagi and Smith (1975) is supported by the good general agreement between their measurements and those of Wheildon et al (1981), also measured by that technique, and data from other sources that were all obtained by different analytical methods. This implies that the 238 U decay series is in equilibrium and suggests that the local variation in U content is not a very recent event.…”

“…We have attempted to make an assessment of previous analyses, and in the Appendix the chemical data provided by various analysts on these granites during the last eighty years is reviewed, so that a comprehensive set of good chemical data can be assembled for a discussion of the petrogenesis of these granites. We have made this review as comprehensive as possible and have examined data from Ghosh (1927Ghosh ( , 1934, Brammall and Harwood (1932), Edmonds et al (1968), Fuge and Power (1969), Harding and Hawkes (1971), Edmondson (1972), Wilson (1972), Wilson and Floyd (1974), Tammemagi and Smith (1975), Al-Turki and Stone (1978), Simpson et al (1979), Alderton et al (1980), Alderton and Moore (1981), Dangerfield et al (1980), Wheildon et al (1981), Thompson et al (1984), Watson et al (1984), Jefferies (1985a), Darbyshire and Shepherd (1985), Webb et al (1985), Charoy (1986), Hall (1990), Shaw and Guilbert (1990), Ward et al (1992) and Stimac et al (1995). The average composition (CNG) obtained from our review of these publications is given in Table 3.…”

“…We use all of those data in this paper. Tammemagi and Smith (1975) analyzed samples from the CNB for U, Th and K by γ-ray spectrometry and they provided average values for the five main plutons. Their average Th concentrations are very close to those of Charoy (1986), but higher than those of this paper, and of Darbyshire and Shepherd (1985) and Wheildon et al quoted in Webb et al (1985).…”