Radiogenic heat production in prebatholithic rocks of the central Sierra Nevada

Abstract

Likely source material for the Sierra Nevada batholith is represented by exposures of Paleozoic rocks on the western flanks of the Sierra Nevada, Paleozoic sedimentary rocks in roof pendants east of the Sierran crest, and both Precambrian and Paleozoic rocks in the western portion of the Basin and Range province. The radiogenic heat production of these rocks has been calculated from γ-radiometric determinations of their U, Th, and K contents. The abundances of these radioisotopes in carbonate rocks are roughly an order of magnitude lower than those in adjacent siliceous sedimentary rocks; therefore, the radiogenic heat production in the Paleozoics depends mainly on the relative amounts of carbonate and siliceous material. Heat production varies from east to west, with weighted values (expressed in units of μcal/g yr) of 3.7 in Precambrian rocks of the Death Valley area, 3.5 in Paleozoics of the Inyo and White mountains, 3.8 in Paleozoic pendants, and 2.1 in rocks of the Calaveras formation on the western slope. A difference, similar to that in the Paleozoics, is observed between Mesozoic rocks exposed near the crest (4.4) and the Mesozoics of the western foothills (2.4). The geographic distribution of heat production matches (though not proportionally) that in the batholithic rocks. This suggests that there was a regional distribution of radioactivity in likely source material for the granitic rocks that now, strongly accentuated by the effects of erosion, is reflected in the batholith.