Petrology and mineral compositions of the Middle Banded series of the Stillwater Complex, Montana

Abstract

The two olivine-bearing zones of the Middle Banded series of the Stillwater Complex are characterized by an increase in the number of cumulus minerals with height. In each, anorthosite and anorthositic troctolite dominate the lower part whereas olishy; vine gabbro and gabbronorite form much of the upper portions. Electron microprobe analyses of cumulus minerals indicate little or no variation of average mineral compositions with height. In addition, no significant lateral variations in cumulus mineral compositions occur along 8 km of section. Plagioclase from throughout these zones shows the complex, reverse and oscillatory zonation patterns also seen in plagioclase from the thick (>500 m) anorthosites that sandwich the zones. The data suggest that the entire Middle Banded series is genetically related and therefore requires models for the origin of the thick anorthosites to also 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 produced by intergranular exchange with orthopyroxene and large within-sample variations in the mg-number of olivine in low-olivine troctolites, indicate that significant modification of these rocks by postcumulus processes has taken place, thereby obscuring evidence of their genesis.