Magma evolution in the Stillwater Complex, Montana; I, Rare-earth element evidence for the formation of the Ultramafic Series

Abstract

Rare earth element data for 20 cumulus plagioclase feldspars from Troctolite-Anorthosite zone I in the Lower Banded series and five cumulus and intercumulus plagioclase feldspars from samples that bracket the Ultramafic series-Lower Banded series contact of the Still-water Complex are consistent with magma mixing as a fundamental process which produced cyclic units in Troctolite-Anorthosite zone I. The association of the J-M reef with plagio-clase-rich cumulates in Troctolite-Anorthosite zone I which have a crystallization order different from that observed in the Ultramafic series suggests that a geochemically distinct type of magma was added to the chamber. This new magma was characterized by a higher A1203 content, (Ce/Yb)n of 3.7, (Nd/Sm)n of 1.5, and (Dy/Yb) • of approximately 1.0, similar to group I gabbronorite dikes in the footwall of the Stillwater Complex (Helz, 1985). This high A1203 magma was most likely derived by partial melting of a source of tholeiitic composition in the upper mantle or lower crust. The higher AI•Oo content of this new magma suggests that it may have been near the sulfur saturation limit for melts of this composition even though the total sulfur concentration may have been low, consistent with