ARCHEAN CHROMITE, CHROMITITES, AND BONINITES IN THE EASTERN BEARTOOTH MOUNTAINS, MT

Archean ultramafic rocks, including podiform chromitites, and a group of associated high-Mg andesites or boninites occur as xenoliths in the predominant 2.8 Ga calc-alkaline magmatic rocks of the eastern Beartooth Mountains of Montana (USA). Most of the ultramafic and mafic rocks exhibit an upper-amphibolite to greenschist-facies paragenesis commonly characterized by magnesio-hornblende and orthopyroxene in the more aluminous rocks and antigoritic serpentine +/- tremolite in the olivine-rich protoliths. Metamorphism caused exsolution of chromite in the ultramafic rocks, but chromite in the chromitites is commonly euhedral and unzoned. These chromites retain their igneous chemistry and textures. The Cr# [100 x Cr/(Cr + Al)] of chromite ranges from 68 – 83 and Mg# [Mg /(Mg +Fe²⁺)] ranges from 11 – 56. Chromite in chromitites is commonly poikilitic with inclusions of olivine, phlogopite, amphibole, diopside, apatite, calcite, dolomite, titanite, and rutile. The matrix of chromitites consists of partially to completely serpentinized olivine (>Fo₉₅) with intercumulus texture and euhedral phlogopite laths partially replaced by Cr-chlorite.

Whole rock major element compositions are magnesian (>8% MgO) at 45-55% SiO₂. Chondrite-normalized REE abundances exhibit arc-basalt and boninitic characteristics, with positive and negative Eu anomalies. Primitive mantle normalized spider diagrams also show arc characteristics, e.g., enrichment in LILE, positive Pb anomalies, and negative HFSE anomalies. These mineralogic (high Cr# of the chromitites and high Fo olivines) and whole rock geochemical data suggest that the ultramafic rocks, chromitites, and related andesitic rocks formed in a suprasubduction zone (SSZ) setting. The chromitites formed in the forearc during the initiation of subduction by interaction of mantle peridotite and a boninitic magma. Similar occurrences of hydrous inclusions and matrix minerals are found in younger podiform chromitites formed in subduction zone settings and interpreted as ophiolites. These findings indicate that the assemblage of boninitic and chromium-rich ultramafic rocks are parts of a dismembered Archean ophiolite and that modern-style plate tectonics was operational in the early Mesoarchean (2.8 Ga) in the northern Wyoming Province.