Fission-Track Ages of Accessory Minerals from Granitic Rocks of the Central Sierra Nevada Batholith, California

Abstract

Ages of apatite, sphene, allanite, epidote, and garnet from plutonic rocks of the central Sierra Nevada and Inyo Mountains have been determined by the fission-track method. Ages of 44 specimens of apatite range from 54 to 128 m.y. Oldest apatites generally occur in rocks from the western portion of the batholith; youngest are from granitic rocks along the eastern slope of the Sierra Nevada. Thirty-four specimens of sphene have been dated and range in age from 71 to 118 m.y. Within experimental error, all sphenes are as old as, or older than, coexisting apatites. The oldest sphene is from granitic rock of the Inyo Mountains; however, sampled rocks from the western Sierra Nevada do not contain sphene. The youngest sphenes are generally from rocks slightly west of the Sierran crest. Fission-track ages of two allanites are 86 and 87 ± 9 m.y. Two epidotes yielded ages of 84 ± 8 m.y. and a third is 140 ± 14 m.y. Age of a garnet was determined to be 114 ± 11 m.y. Several of the apatites and sphenes used for fission-track dating are from the same samples that previous workers had used for potassium-argon dating of biotites and hornblendes. Hornblende K-Ar ages appear to be the most insensitive to heat, apatite fission-track ages, the least, and biotite K-Ar and sphene fission-track ages fall between the two extremes and show the closest concordance of all pairs.