Abstract

Twenty-seven whole-rock samples of granitic rocks and associated mafic inclusions define a Rb–Sr ispchron of 156 ± 6 m.y. with an initial ratio of 0.70415 ± 0.00017 (95% confidence limits). K–Ar and Rb–Sr data for granodiorite, quartz monzonite, and granite are concordant with this Upper Jurassic age. K–Ar dates suggest that diorite and quartz diorite are distinctly older, 165 to 185 m.y. old. The Rb–Sr data for these more mafic rocks alone (166 ± 53 m.y.) is not accurate enough to confirm this, but is not in conflict. Country rock samples are generally similar to the batholith in Sr-isotopic composition, and nearly all mafic inclusions are equilibrated with their enclosing granitic host. The Cordilleran transition zone between relatively low initial Sr isotope ratios (0.704 and less) and relatively high ratios (0.706 and greater) appears to lie just east of the batholith studied.

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