38-5 Geochemical Characterization of Neoarchean Metasediments in the Gravelly Range, Montana, USA

Session: Petrology and Geochemistry (Posters)

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Abstract:

We present new major and trace element, mineral composition, radiogenic isotope, and metamorphic facies data on metasupracrustal rocks of probable Neoarchean age from the Gravelly Range of the northern Wyoming Province in southwestern Montana, USA. O'Neill and Christiansen (2004) describe the Precambrian rocks in the study area as weakly metamorphosed folded metasedimentary rocks with minor metavolcanic rocks intruded by now gneissic granitoids and overlain by alluvium and Tertiary sedimentary rocks. Lithologies include phyllite, micaceous quartzite, banded iron formation, biotite plagioclase schist, and epidote-actinolite amphibolite. Chemical and mineral characterization of the metasedimentary rocks referred to as the "Cherry Creek Group" in their type locality in the Gravelly Range improves our understanding of their relationship to other rocks also described as the Cherry Creek Group throughout the Montana Metasedimentary Terrane (MMT) in the Wyoming Province. Petrographic, SEM, XRF, and ICPMS analysis data are used to assess the distinctiveness of the type locality's composition, provenance of metapelites, metamorphic grade, and geologic context.

Rare earth element patterns of the Gravelly Range metapelites are consistent with the Archean Upper Crust pattern and have negative europium anomalies that indicate the source rocks for their sediments were not igneous rocks derived from depleted mantle. The metapelite Sm/Nd concentrations (0.18–0.22 ppm) are similar to Post-Archean Australian Shale (0.18) (Taylor and McLennan, 1985) and suggest protoliths were predominantly felsic. However, the metapelites have average FeOt (8.72 ±7.96 wt%) 1-3 wt% greater than most shale reference standards except for Late Archean Mudstone (FeO 9.5 wt%). Average Th/U in Cherry Creek metasedimentary rocks in the Gravelly Range is more than twice the estimated average in Archean shale (Th/U 1.7 ±0.3 ppm) (Taylor and McLennan, 1985). Although the Th/U indicates oxidation that may be attributed to weathering, this proxy does not correlate with the results of weathering indices such as chemical index of alteration (CIA) and chemical index of weathering (CIW) that suggest only slight to moderate weathering occurred in these rocks. This work contributes to a better understanding of the Cherry Creek type locality, the MMT in the Wyoming Province regionally, and Neoarchean metasedimentary rocks globally.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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