Petrology and Mineral Compositions of the Middle Banded Series of the Stillwater Complex, Montana

The two olivine-bearing zones of the Middle Banded series ofthe Stillwater Complex are characterized by an increase in thenumber of cumulus minerals with height. In each, anorthositeand anorthositic troctolite dominate the lower part whereasolivine gabbro and gabbronorite form much of the upper portions.Electron microprobe analyses of cumulus minerals indicate littleor no variation of average mineral compositions with height.In addition, no significant lateral variations in cumulus mineralcompositions occur along 8 km of section. Plagioclase from throughoutthese zpnes shows the complex, reverse and oscillatory zonationpatterns also seen in plagioclase from the thick (>500 m)anorthosites that sandwich the zones. The data suggest thatthe entire Middle Banded series is genetically related and thereforerequires models for the origin of the thick anorthosites toalso explain the olivine-bearing rocks between them. However,textural features such as discordant troctolites, pegmatoids,slump structures and variably developed mineral laminations,and chemical features such as zonation in clinopyroxene producedby intergranular exchange with orthopyroxene and large within-samplevariations in the mg-number of olivine in low-olivine troctolites,indicate that significant modification of these rocks by postcumulusprocesses has taken place, thereby obscuring evidence of theirgenesis. KEY WORDS: Stillwater Complex; Middle Banded series; layered intrusions; mineral chemistry; postcumulus processes ^*Corresponding author. Telephone: (919) 681-8169. Fax: (919) 684-5833. e-mail: wpm{at}roguegeo.dukeedu