Hornblendes from Granitic Rocks of the Central Sierra Nevada Batholith, California

Twenty samples of hornblendes from rocks of 14 plutonic unitsin the central Sierra Nevada and Inyo Mountains, California,have been studied in detail. Optical, density, single-crystaland powder X-ray diffraction, and major and minor element chemicaldata are reported. The compositions of the hornblendes show only limited correlationwith the chemistry of the rocks in which they occurred. Hornblendesfrom granitic rocks of the eastern Sierra Nevada and Inyo Mountainshave a wide range of tetrahedral aluminum content which is oftenas low as three-quarters of an atom per formula unit, whereashornblendes from younger granitic rocks elsewhere in the SierraNevada batholith contain more than one atom of tetrahedral aluminumper formula unit. Because an increase of aluminum in tetrahedralco-ordination is considered indicative of higher temperaturesof crystallization, the observed differences in the hornblendessuggest that older plutonic rocks of the batholith may havebeen metamorphosed regionally or may have been affected by widespreadhydrothermal action prior to consolidation of later graniticrocks.