MULTIPLE TERRANES IN THE TETON RANGE, WYOMING: EVIDENCE FOR TECTONIC ASSEMBLY AT 2.68 AND 2.62 GA

The Archean rocks of Wyoming’s Teton Range consist of three terranes that have been intruded by the 2,547 Ma Mount Owen granodiorite. North of Mount Moran are two terranes with distinct metamorphic and structural histories. The northwestern terrane is composed of migmatites, kyanite-bearing metapelite, and garnet amphibolite. It was metamorphosed to 950° and12 kb at 2.70 Ga. The northeastern terrane records lower temperatures and pressures, migmatites are rare or absent, scarce pelites contain sillimanite without kyanite, and all but extremely iron-rich amphibolites lack garnet. Lineations in the northeast are folded about the average hinge orientation ~35°/126° and lineations in the northwest define folding with average hinge orientations ~44°/257°. ε_Nd values from the northeast terrane are positive and those from the northwestern terrane are negative, indicating that the two terranes have different sources. At 2685-2670 Ma the northeastern and northwestern terranes were juxtaposed in a Himalayan-type orogeny and leucogranites intruded the suture zone.

A third metamorphic terrane occurs south of Mt. Moran. 2.80 Ga granodioritic orthogneisses in this terrane were intruded by the weakly deformed Rendezvous Mountain metagabbro before 2.70 Ga. Paragneiss contains detrital zircons with U-Pb ages at 2.71, 2.69, 2.66, and 2.64 Ga. Pelitic paragneisses from this terrane carry the assemblage Qtz-Bt-Grt-Cord with relict St, and the Qtz-Bt-Grt-Cord assemblage yields equilibration conditions of 675^o and 4.8 kb. There is no evidence that the southern terrane experienced the high-P granulite metamorphism recorded in the northeastern part of the range. A tectonic boundary in the center of the Teton Range must separate the southern metamorphic terrane from those to the north.

Pale sphene from a sheared Rendezvous metagabbro sample in the southern terrane yields a date of 2623.1 ± 1.4 Ma; dark sphene gives 2616.5 ± 1.1 Ma. Sphene from a boudined mafic dike in northeastern terrane yields a date of 2622.8 ± 2.5 Ma. The sphene ages may reflect the time of amphibolite facies metamorphism associated with assembly of the southern with the northern terranes. A number of terranes were accreted to the southern margin of the Wyoming province between 2.65 – 2.62 Ga; the Archean rocks of the Teton Range also record this event.