Abstract

Petrographic and chemical data on five types of intrusive rocks of the so-called Pioneer batholith in the northeastern Pioneer Mountains of southwest Montana show that the rocks range from quartz diorite to granite. The volumetrically most important body is a coarse-grained biotite-hornblende “granite” that superficially resembles the Butte Quartz Monzonite of the Boulder batholith, some 60 km to the northeast. Their K2O content puts the rocks into Tilling's “sodic series” of the Boulder batholith. K-Ar dating on biotite and hornblende shows that the date of intrusion of all but the quartz diorite is about 70 m.y. The hornblende age of the quartz diorite indicates that it was intruded 76.5 m.y. ago. Its biotite age of 70.5 m.y. agrees with those of the younger intrusions. High-grade contact metamorphic rocks of the Silver Hill Formation (Cambrian) at one locality yielded the same 70-m.y. age on biotite porphyroblasts. These ages compare closely with those of the Boulder and Philipsburg batholiths and show the same tendency for the less mafic rocks to be younger.

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