ABSTRACT

Th and U distributions in Archean granulite-facies rocks from Bahia (Brazil) have been investigated by means of gamma-ray spectrometry. Thorium shows a very large range of values (from < 0.3 to 72.2ppm) with high concentrations in only a few samples. Uranium is more homogeneously distributed at abundance levels below 1 ppm. The Th/U ratios vary irregularly depending on the variable Th contents but are normally much higher than averages for magmatic and metamorphic rocks or crustal averages. Differential behaviour of Th and U during metamorphism is considered as the main cause of the high Th/U values observed. Selective depletion of uranium in metamorphic fluids may have resulted from an early event as documented by other very old crustal materials. Surface heat production data indicate that the Bahia granulitic block is a low-heat production region (average H.P.U. 3.32). Emphasis is placed on the need for a future study of the vertical compositional profile of the crust in shield areas with exposed high-grade metamorphic rocks using combined heat production rate and heat flow data.