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Isotopic U-Pb ages of zircon from the granitoids of the central Sierra Nevada, California

USGS professional paper1981

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T. W. Stern, Paul Charles Bateman, Benjamin A. Morgan, M. F. Newell, Dallas L. Peck

Abstract

Sixty-two samples from well-established comagmatic granitoid sequences and certain unassigned formations and plutons of the central part of the Sierra Nevada batholith between latitudes 37 and 38 N. have been dated by the isotopic U-Pb method on zircon. The U-Pb ages indicate the following age distribution of the granitoids: (1) The axial part of the batholith is occupied by Cretaceous granitoid sequences that are progressively younger eastward over a 37-m.y. interval extending from about 125 m.y. to about 88 m.y. ago. (2) A single, but extensive, Triassic sequence with an optimum average age of about 210 m.y. is present in the east side of the batholith. (3) Plutons and granitoid sequences of Jurassic age, most of them with U-Pb ages between 186 and 155 m.y., occur in both margins and locally in the interior of the batholith. The distribution of Jurassic ages suggests that prior to the emplacement of the Cretaceous granitoids, Jurassic granitoids were widely distributed across the central Sierra Nevada but were not emplaced in a west-to-east succession as were the Cretaceous granitoids. Few of our ages fall between 155 and 125 m.y. However, a U-Pb age of 144 m.y. has been reported on the Sage Hen Flat pluton in the White Mountains, and U-Pb ages between 134 and 128 m.y. have been reported on remnants of older granitoids farther south in the Sierra Nevada, which are associated with roof pendants and septa. Also, numerous K-Ar ages on hornblende in the range of 152 to 131 m.y. have been reported on samples collected farther north along the west side of the batholith.

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Abstract

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