Semester of Graduation
Master of Science (MS)
Geology and Geophysics
Metamorphosed ultramafic rocks and associated chromitite pods in the eastern Beartooth Mountains, Montana (USA) provide a window into the tectonic evolution of the Archean northern Wyoming Province. Meta-ultramafic rocks occur with a variety of metasupracrustal rocks as xenoliths in an extensive 2.8 Ga suite of TTG (meta)plutonic rocks. The origin of the ultramafic rocks is obscured by an upper amphibolite-to-greenschist facies overprint. Major element analyses (XRF, wt. %) of meta-ultramafic host rocks show SiO2 from 40.53-50.37, MgO from 15.33-45.25, TiO2 from 0.07-0.62, Al2O3 from 1.25-10.62, and Fe2O3(t) from 8.06-14.54. Total alkali contents (Na2O+K2O) range from 0.06-3.78 wt.% and plot in the picro-basalt and basalt fields on a TAS diagram and gabbro and peridot-gabbro fields on the plutonic equivalent TAS diagram. Major oxide whole-rock analyses for meta-ultramafic host rocks plot within boninitic and komatiitic fields based on their high MgO and low TiO2 values. Cr contents of non-chromitites are high (1500-6500 ppm). Primitive mantle normalized spider diagrams show arc characteristics, e.g., enrichments and depletions in LILE, a positive lead anomaly, and negative HFSE anomalies.
In contrast, chromitite pods (ultramafic rocks with >75% chromite) contain chromite grains that retain their igneous chemistry and textures. Pull-apart textures, relatively large (>1 mm) to massive anhedral chromite, and poorly developed/tectonically disrupted chromite nodules are inconsistent with derivation from a layered mafic intrusion. Major and trace element analyses of chromites by electron microprobe show chromites have minor compositional zoning with the rims being slightly enriched in Mg and Al at the expense of Cr and Fe2+. Chromites have Cr/(Cr+Al) from 0.69-0.83 and Mg/(Mg+Fe2+) from 0.11-0.56, consistent with derivation from an ophiolite sequence or layered mafic intrusion. Low TiO2 concentrations of chromite from meta-chromitites are inconsistent with derivation from a layered mafic intrusion. Melts calculated to be in equilibrium with chromite are boninitic in character.
In conclusion, the boninitic/ophiolitic/arc geochemical signature of the meta-chromites and meta-ultramafic rocks suggest that they formed in the forearc of an early subduction setting. These rocks were intercalated with a variety of lithologies during an episode of plate convergence that preceded the 2.8 Ga TTG plutonism.
Gnieski, Rachel M., "Archean Metamorphosed Ultramafic Rocks and Chromitites from the Beartooth Mountains, Montana, USA: Insights into Mantle Geochemistry and Tectonics" (2022). LSU Master's Theses. 5683.
Henry, Darrell J.