ARCHEAN CHROMITITES FROM THE EASTERN BEARTOOTH MOUNTAINS, MT, USA: INSIGHTS INTO ARCHEAN CRUSTAL GENESIS THROUGH CHROMITE GEOCHEMISTRY

Abstract

Chromites from ultramafic enclaves, including chromitites, in the eastern Beartooth Mountains, Montana (USA) provide a window into the tectonic evolution of the Archean northern Wyoming Province. 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, but the chromitite pods (ultramafic rocks with >50% chromite) contain euhedral chromite grains that retain their igneous chemistry and textures. Major and trace element analyses of chromites by electron microprobe and laser ablation ICP-MS show chromites have minor compositional zoning with the rims being slightly enriched in Cr and Fe2+ at the expense of Al and Mg. Chromites have Cr # [100*Cr/(Cr+Al)] from 73-79 and Mg# [100*Mg/(Mg+Fe2+)] from 34-45, consistent with derivation from an ophiolite sequence. REEs for chromites and ultramafic host rocks are most consistent with a boninitic source based on their U-shaped chondrite normalized REE patterns. Chondrite-normalized chromite PGE concentrations (Ru, Rh, Pd, Ag, Re, and Pt) are low: Rh 0.01-0.4 ppm, Ru 0.06 to 0.1 ppm, Re up to 0.03 ppm, Pt up to 0.007 ppm, and Ag up to 0.02 ppm . Major element analyses (XRF, wt. %) of 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. As expected, Cr contents of non-chromitites are high (1500-6500 ppm, ICP). Akin to the chromites, chondrite-normalized REE abundances exhibit arc-basalt and boninitic characteristics, with positive and negative Eu anomalies. Primitive mantle normalized spider diagrams show arc characteristics, e.g., enrichments and depletions in LILE, a positive lead anomaly, and negative HFSE anomalies. In conclusion, the boninitic/ophiolitic/arc geochemical signature of the chromites and ultramafic rocks suggest that they formed in the forearc of an early subduction setting. They were intercalated with a variety of lithologies during an episode of plate convergence that preceded the 2.8 Ga TTG plutonism.