A COLLISIONAL 2.67 GA TECTONIC BOUNDARY IN THE TETON RANGE, WYOMING

Recent work in the Archean of the Teton Range, Wyoming, suggests that a tectonically active, continental growth margin persisted along the western Wyoming Cratonic margin from at least 2.71 to 2.66 Ga. High-pressure (HP) granulites, within a layered gneiss unit in the northwestern Teton range, contain garnet-bearing leucosomes in the mafic granulites and Kyanite + K-Feldspar assemblages in the metapelites.  These granulites require P-T conditions of >850 ºC and >12 kbar, corresponding to a crustal depth of ~40 km. The undeformed, cross-cutting 2.55 Ga Mount Owen Quartz Monzonite constrains this metamorphism to the Archean. The presence of supracrustal HP granulites require that crustal material was subducted to mantle depths during the Late Archean, likely during a collision along a continental margin. These granulite facies metasedimentary rocks, have an ε_Nd at 2.71 Ga of -4.0 to -0.6 with Nd crustal residence ages of 3.1-3.4 Ga.

Younger layered gneiss (Bio-Plag-Qtz gneisses and Hbl gneisses) found further East, with ε_Nd at 2.71 Ga of -0.6 to 2.1  (crustal residence ages of 2.9-3.1 Ga) and tonalite gneisses ε_Nd at 2.71 Ga from -0.5 to 2.3 (crustal residence age of 2.8-3.2 Ga) require input from juvenile sources. The youngest layered gneisses, granitic and granodiorite sills, have the most juvenile Nd isotopic compositions (ε_Nd at 2.67 Ga of 2.5 to 3.8; crustal residence age of 2.7-2.8 Ga) found in the Archean Wyoming Province. The mixed provenance of the layered gneiss suggests that juvenile arc material was added to preexisting crust at this location.

In addition, the ~2.7 Ga augen gneiss, 2.67 Ga Webb Canyon gneiss, and 2.67 Ga Rendezvous Gabbro (all at amphibolite facies) show a calc-alkaline affinity. Given the abundance of similar aged calc-alkalic granites in the northern Wind River Range, as well as a 2.67 Ga tholeiitic dike swarm (interpreted as back-arc magmatism) to the east in the Owl Creek Mountains, we propose that the Teton Range was located near an active plate margin at that time. We suggest that there was west to east subduction of a juvenile arc situated near the Teton Range, accompanied by back-arc spreading in the Owl Creek Mountains. It is possible that the HP granulites were juxtaposed with the amphibolite facies gray gneisses at the closing stages of this orogeny.