Abstract

Sixty-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 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 base. of an originally normal crust with a subsequent high-level emplacement.

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