Rb-Sr isochrons obtained for locally extruded siliceous volcanic rocks in units in the Excelsior and Pilot Mountains, western Nevada, indicate ages, respectively, of 103 ± 5.7 m.y. and 142 ± 17 m.y. Both dated units occur in a complex of terrigenous and volcanic rocks previously thought to be entirely of early Mesozoic age. K-Ar ages of granitic rocks of the Excelsior Mountains region indicate that plutonism occurred mainly between about 70 and 100 m.y. B.P.

The newly dated volcanic rocks of the Excelsior Mountains provide the first recognition of a volcanic center of Cretaceous age in the Great Basin and evidence that at least some of the thrusting and folding in the Excelsior Mountains region occurred in the Cretaceous or later time. Together, the two new Rb-Sr ages indicate a more prolonged history of Mesozoic sedimentation and extrusion than previously realized. The dated volcanic rocks of the Excelsior Mountains are generally contemporaneous with recently discovered Cretaceous extrusive rocks in the central Sierra Nevada, suggesting that volcanism may have been widespread in the Cretaceous in eastern California and western Nevada between 37° and 38°N. K-Ar ages of granitic plutons in the Excelsior Mountains region suggest that the Cretaceous volcanic rocks vented near the beginning of a 20-m.y.-long pulse of intrusion. Moreover, the pluton ages indicate a time lag between Cretaceous volcanism and plutonism similar to that in the Sierra Nevada.

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