ABSTRACT

The core of the northern Teton Range is composed of an Archean gneiss complex consisting of heterogeneous layered gneiss, interleaved homogeneous granitic gneisses, and cross-cutting Mount Owen Quartz Monzomte. Diverse whole rock chemistry and rare earth element (REE) suggest the layered gneiss has a mixed supracrustal and igneous origin. The homogeneous granitic Webb Canyon gneiss and augen gneiss are petrographically and chemically distinct units. REE patterns suggest derivation as partial melts of pre-existing felsic crust. Amphibolitic lenses contained within all gneissic units have flat REE patterns with low total values, suggesting derivation from the shallow mantle.

The gneissic units are dominated by amphibolite facies assemblages with garnet-biotite temperatures averaging 600° C. Areas of relict granulite facies assemblages with two pyroxenes, garnet and rare kyanite indicate a previous granuhte facies event. The kyanite occurs as relict grains with cordierite reaction rims, suggesting a minimum pressure of 8 kilobars, which is anomalously high for Archean terranes.

The northern Teton Range has undergone at least three major Precambrian deformations. Compositional layering in the layered gneiss is probably a combination of original bedding and subsequent metamorphic segregation, complicated by rootless isoclinal folding of the first deformation event (Di). This fabric was overprinted and folded during a second deformation, which resulted in isoclinal to open folding of various scales, transposition of (Di) foliations and formation of a secondary amphibolite facies foliation. (D₂) mineral growth lineations parallel mineral stretching lineations and recumbent fold axes, suggesting that (D₂) was characterized by sheath folding concurrent with retrograde metamorphism. The subsequent, post-orogenic intrusion of the Mount Owen Quartz Monzonite and satellite pegmatites was followed by the last major deformation, a greenschist facies shearing event, which formed mylonite zones as much as 45 meters wide.