Radiogeologic studies in the central part of the Sierra Nevada Batholith, California

Radioactivity in granitic rocks of the central Sierra Nevada varies both regionally and with rock type. It is lowest in the western foothills and increases eastward to the crest of the range. East of the crest radioactivity is generally lower than in the crestal region, but south of Big Pine Creek it increases eastward from low values in the Inconsolable granodiorite to high values (similar to those along the Sierra crest) in the Tinemaha granodiorite of the Poverty Hills. Within any given area radioactivity varies with silica and the alkalis; it is lowest in diorite and gabbro and progressively higher in quartz diorite, granodiorite, and quartz monzonite. On the west slope of the Sierra Nevada, the isotopic ages of the rocks increase westward, opposite to the direction of increase of radioactivity. This relation and preliminary heat flow values (which indicate a westward decrease in heat flow) are consistent with the concept of a vertically fractionated batholith in which the heat sources were concentrated in the upper parts. According to this concept, the oldest rocks have the lowest heat production because they have been eroded to the deepest levels. This concept fits less well with the relations east of the crest, where the isotopic ages of the granitic rocks are even older than those in the western foothills. The picture may be complicated by an original inhomogeneous distribution of the radioactive minerals in the source rocks from which the granitic magmas were derived. The proportions of U, Th, and K are generally constant within any given pluton and in plutons that are compositionally similar and of the same age, but they may be significantly different in plutons that are compositionally and temporally unrelated. Different proportions of the radioactive elements in three pairs of plutons that had been correlated on the basis of petrographic similarity suggested faulty correlations. Recent geologic work and isotopic age dating have shown that the correlations were incorrect in two of the cases. A limited study in fission-track autoradiography suggests the U is mainly contained in biotite in rocks rich in biotite and in the non-magnetic accessory minerals in rocks that contain little or no biotite.