Abstract
K-Ar ages of 23 and 26 m.y. for pyroxene and Pb206/U238 ages of 32 m.y. for zircon were determined for monzonite and soda syenite porphyries. A K-Ar age of 55 m.y. for hornblende and a Pb206/U238 age of 494 m.y. for zircon from diorite porphyry are interpreted as due to incomplete degassing of Precambrian hornblende and the presence of Precambrian zircon xenocrysts. Feldspar-lead is similar in isotopic composition to galena from the uranium ore deposits of the Colorado plateau.
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