The Source and Emplacement of Leucogranites in the Teton Range Following High-Pressure Archean Metamorphism

Archean crystalline rocks exposed in the Teton Range, Wyoming, experienced high-pressure metamorphism (~12 kbar) at 2690 Ma. These pressures were likely attained during an Archean continent-continent collision analogous to the modern collision of India with Asia to form the Himalayas. Metamorphism was followed by intrusion of leucogranites, which exhibit the following characteristics:

1) The leucogranite bodies (Webb Canyon Gneiss) do not record evidence of early deformation events experienced by the country rock. The f1 and f2 events describe isoclinal folding and refolding with preferred fold hinge parallel mineral growth orientations. Such fabrics are not seen in the Webb Canyon Gneiss. The final penetrative deformation (f3) is a foliation seen in all lithologic units.

2) U-Pb dating of zircon from the Webb Canyon Gneiss suggests emplacement post-peak metamorphism, from 2685±5 to 2675±3 Ma. The ten million year range in dates describes multiple leucogranitic sills intruding over that time span. Ages recorded are: 2685±5 Ma, 2685± 4 Ma, 2680± 5 Ma, 2675± 7 Ma, and 2675± 3 Ma.

3) Extensive migmatization is recorded in the high-grade gneisses. However, Sm-Nd isotopic analysis of multiple leucogranitic bodies suggests a melt source not from the local, high-pressure pelitic gneisses but from the isotopically distinct Wyoming craton located to the east. The cratonic εNd values range from -0.69 to 4.12 (N=17, AVG=0.45) while the western “Idaho Block” contains values ranging from -0.75 to -4.2 (N=4, AVG=-2.5). The Webb Canyon Gneiss exhibits mainly positive εNd values (N=19, AVG=0.34). The few negative εNd values of the Webb Canyon are from younger, garnet-rich samples suggesting a change in source to include local pelitic gneisses as crustal melting continued.