The Archean History of the Teton Range

April 16  Presentation: “The Archean History of the Teton Range, Presented by Ron Frost, University of Wyoming.

Take a tour with Ron Frost of the ancient rocks that make up one of the youngest mountain ranges in North America, and learn how data show these rocks to have been formed in part in primeval oceans, and incorporated into mountain building events early in Wyoming’s, and indeed earth’s history.

The Precambrian gneisses of the Teton Range consist of two distinct domains, a northern domain, which is separated from the southern domain by the Moran shear zone.  The Moran shear zone, extends from Thor Peak to Bivouac Peak.  In the south it is truncated near Paintbrush Divide by the 2.54 billion-year old (Ga) Mount Owen batholith whereas in the north it disappears in Snowshoe Canyon.   The northern domain is characterized by remnant metamorphism that reached at least 12 kilobars and 900°C. Partial melts from mafic granulites that formed during this event contain zircon that gives a U-Pb age of 2.70 Ga., making these the oldest Hi-Temp – Hi-Pressure rocks in the world.  These rocks are in tectonic contact with rocks that are petrologically similar to modern ocean-floor rocks, including metaperidotites, amphibolitic metagabbros, and paragneisses, which are probably meta-greywackes. The tectonic assembly of the high-Pressure rocks with the sea-floor assembly occurred at 2.685 Ga. and probably records the oldest Himalayan-type orogeny in Earth History. The Webb Canyon gneiss, an orthogneiss that was emplaced in several events between 2.685 and 2.675 Ga, intrudes this whole sequence of rocks.

            The southern domain consists of a complex sequence of paragneiss and orthogneiss, and metagabbro.  Granodioritic orthogneiss from this domain dates from 2.8 Ga, this age is far older than the oldest age obtained from the northern domain.  Metagabbro at Rendezvous Peak dates from 2.69 Ga, This date overlaps the high-pressure metamorphism of the northern part of the range and indicates that the southern domain did not undergo the HP granulite metamorphism seen in the north. Paragneisses at Paintbrush divide have detrital zircons that range in age from 2.700-2.650 Ga.  These ages clearly indicate that the sedimentary parent to the paragneiss was deposited after the high-pressure metamorphism in the north.  Sphene from the sheared Rendezvous gabbro in the southern domain gives ages of 2.623 Ga. and 2.616 Ga. the older age is identical with sphene age obtained from a dike in the northern domain.  These data imply that the northern and southern domains evolved independently between 2.8 Ga. and 2.65  Ga and were tectonically juxtaposed around 2.62 Ga.  This 2.62 Ga tectonic event is likely the same one that affected the whole southern portion of the Wyoming province at this time.