Uranium and thorium in potash-rich rhyolites from western Bahia (Brazil)

Abstract

The U and Th distribution in K-rich acid metavolcanics affected by postmagmatic processes has been studied. Th (20 ppm) and U (2.41 ppm) average abundances are well within the normal ranges for granitic rocks, but the Th/U values are appreciably higher than most of the values reported. Since the rhyolitic magma is supposed to have been formed by partial melting of pre-existing rocks, the high Th/U ratios may be due: (1) to an extreme magmatic fractionation towards a K-rich trend in the magma chamber or during magma displacement; and (2) to high Th/U ratios in the original material, probably ancient crystalline basemental rocks depleted in U with respect to Th during previous cycles. The possible influence of postmagmatic processes on U and Th distribution is discussed, and evidence of U mobilization and redistribution within the formation is set forth.