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Thorium and uranium contents in about 150 samples of igneous rocks from the Boulder batholith region generally increase with increasing SiC>2 content and decreasing CaO/(Na2O+K8O) ratio of their host rocks. Prebatholith volcanic rocks (the Upper Cretaceous Elkhorn Mountains Yolcanics and its correlatives) vary in thorium, uranium, and potassium contents from one outcrop area to another: two areas contain rocks with average values of 2.4-2.6 percent K, 6.0-6.6 ppm (parts per million) Th, 2.1-2.9 ppm U, and Th/U ratios between 2.4 and 3.2; a third area contains rocks with average values of 3.5 percent K, 16.6 ppm Th, 3.7 ppm U, and a Th/U ratio of 4.2. The older (and more mafic) rocks of the composite Boulder batholith (also Late Cretaceous) have an average content of 6.2-11.0 ppm Th and 1.5-3.4 ppm U, whereas the younger (and more felsic) rocks have an average content of 15.7-36.3 ppm Th and 4.0-9.2 ppm U. The average radioelement content of the batholith, weighted according to areal abundance of constituent rocks, is 3.3 percent K, 15.4 ppm Th, and 3.9 ppm U. Average Th/U ratios of the batholith samples range from 4.0 to 5.8 and have no apparent correlation with rock composition or position in the intrusive sequence. Postbatholith volcanic rocks (the Lowland Creek Volcanics of early Eocene age and post-Lowland Creek rocks of Pliocene or Miocene age) have thorium, uranium, and potassium contents comparable with those in some of the younger batholith rocks. Comparison of the Boulder batholith with selected igneous suites representative of diverse magmatic provinces shows that its thorium and uranium distribution is typical of that for calcalkalic suites. The thorium and uranium distribution pattern of any comagmatic igneous suite can be grossly correlated with its lime-alkali (Peacock) index, which in turn reflects differences in tectonic setting and (or) magma provinces. Calculated from average thorium, uranium, and potassium contents (using Birch's heat-generation values), the total radiogenic production of the calc-alkalic Boulder batholith (weighted according to rock abundance) is 6.8 microcalories per gram per year, as opposed to a similarly computed value of 2.7 microcalories per gram per year for the calcic Southern California batholith. Available heat-flow measurements from a drill hole and underground workings in the Butte Quartz Monzonite near Butte are about 2.2 microcalories per square centimeter per second. Calculations demonstrate that the observed heat flow can be fully attributed to radiogenic heat produced in a column of surface rocks 25-35 kilometers thick (depending on the average thorium, uranium, and potassium contents used in the calculation), even though limited seismic data indicate that the crust in the Boulder batholith region is about 45 kilometers thick. The apparent discrepancy between radiogenic heat production and heat flow may be resolved by postulating that the content of heat-producing elements in the crust decreases with depth, which has also been suggested by other investigators as part'il explanation for analogous discrepancies between heat flow and radiogenic heat in the Sierra Nevada batholith.

u.s. geological survey professional papers/u.s. geological survey professional paper

Distribution of thorium, uranium, and potassium in igneous rocks of the Boulder batholith region, Montana, and its bearing on radiogenic heat production and heat flow