The relationship between the petrology and the thorium and uranium contents of some granitic rocks

Thorium and uranium contents of granitic rocks are intimately related to modal compositions and general petrologic features. Correlations are quite distinct between thorium content and common indices of general petrogenetic evolution, such as amount of dark minerals, percentage of anorthite in plagioclase, and ratio of potassium feldspar to plagioclase. Thorium content increases regularly toward the more acidic rocks, and the increase is most pronounced in the most highly alkalic samples. Uranium content generally shows little, if any, relationship to modal composition or other petrologic features, and the increase in abundance of uranium toward the more acidic rocks is irregular. The greater petrogenetic control of thorium than of uranium content may be explained on the basis of oxidation and repeated loss of uranium from magmas during the later stages of their differentiation. Such an explanation assumes that magmas are originally derived from a relatively homogeneous source; remobilization, however, of different types of sedimentary or other rocks might provide granitic magmas of widely different initial thorium and uranium contents. The possibility that thorium is added hydrothermally to granites is partly supported by unusually high abundance of thorium in some red, porphyritic, allanite-bearing rocks, but the general petrologic control of thorium abundances argues against major secondary addition of material.