Abstract

Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement.

The oldest rocks in the Teton Range are complexly deformed interlayered biotite gneiss, plagioclase gneiss, amphibole gneiss, and amphibolite. Also, within these rocks, there are concordant bodies of strongly lineated quartz monzonite gneiss, here named the Webb Canyon Gneiss, which may be of volcanic origin. Coarse metagabbro, here named the Rendezvous Metagabbro, is intrusive into the layered gneiss sequence and was metamorphosed and deformed along with the enclosing rocks.

These older rocks are cut by discordant plutons and swarms of undeformed dikes of quartz monzonite and associated pegmatite. The quartz monzonite, which makes up much of the central part of the Teton Range, is here named the Mount Owen Quartz Monzonite.

The youngest Precambrian rocks are undeformed dikes of slightly metamorphosed tholeiitic diabase.

A Rb-Sr whole-rock isochron on the Webb Canyon Gneiss and the Rendezvous Metagabbro indicates that these rocks were metamorphosed 2,875 ± 150 m.y. ago. The initial Sr. ratio of 0.700 suggests that the original rocks are probably not significantly older than the metamorphism. The Mount Owen Quartz Monzonite has a whole-rock isochron age of 2,495 ± 75 m.y. and an unusually high initial ratio of 0.732. Plagioclase-microcline isochrons from two samples of the quartz monzonite indicate partial re-equilibration of the Rb-Sr system during a thermal event 1,800 m.y. ago.

The age of the diabase dikes has not been definitely determined, but biotite in the wall rocks of one major dike has a K-Ar age of 1,450 m.y. This suggests that the dike was emplaced during or prior to a thermal event 1,300 to 1,500 m.y. ago that was responsible for resetting many of the previously reported K-Ar mineral ages throughout the range.

The geochronologic record in the Teton Range is very similar to that elsewhere in the Wyoming Precambrian province. Major metamorphic events with ages between 2,700 and 2,900 m.y. have been identified in the Bighorn, Beartooth, Little Belt, and Granite Mountains. Post-tectonic granitic rocks with ages of 2,500 to 2,700 m.y. have been found in the Wind River Range and the Granite Mountains. Later thermal events have affected Rb-Sr systematics of rocks in the Beartooth Mountains, Wind River Range, and Granite Mountains, as well as in the Teton Range at about the same time as major episodes of regional metamorphism in terranes flanking the Wyoming province in southwestern Montana and in the Front Range in Colorado.

First Page Preview

First page PDF preview
You do not currently have access to this article.