Magma evolution in the Stillwater Complex, Montana; II, Rare earth element evidence for the formation of the J-M Reef

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 Stillwater 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 plagioclase-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 Al 2 O 3 content, (Ce/Yb) n of 3.7, (Nd/Sm) n of 1.5, and (Dy/Yb) n of approximately 1.0, similar to group I gabbronorite dikes in the footwall of the Stillwater Complex (Helz, 1985). This high Al 2 O 3 magma was most likely derived by partial melting of a source of tholeiitic composition in the upper mantle or lower crust. The higher Al 2 O 3 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 average sulfur concentrations in the group I gabbronorite dikes.Rare earth element data for cumulus orthopyroxenes from the Ultramafic series presented in the first paper of this two-part series (Lambert and Simmons, 1987) suggest that magmas parental to cumulates below Troctolite-Anorthosite zone I were derived by multistage dynamic partial melting of a deeper upper mantle source. Rare earth element and sulfur data for group 2 high MgO gabbronorite dikes also in the footwall of the Stillwater Complex suggest that these liquid compositions may be similar to magmas parental to the Ultramafic series. Dynamic partial melting of a platinum-group element-rich source may have permitted enrichment of the platinum-group elements in the Stillwater magma chamber during the formation of the Ultramafic series. Perturbation of the tholeiitic trend of fractional crystallization observed in the Ultramafic series (olivine followed by orthopyroxene) due to the introduction of an Al 2 O 3 -rich, sulfur-saturated magma into platinum-group element-enriched residual magma remaining in the chamber may have produced the cyclic modal and rare earth element cryptic layering in Troctolite-Anorthosite zone I. The identification of feldspars in Troctolite-Anorthosite zone I below the J-M reef which have rare earth element affinities with the Ultramafic series suggests that progressive mixing of these two liquids was accomplished in a system which preserved the integrity of geochemically different liquid layers. Double-diffusive convective mixing (Irvine et al., 1983) of the two Stillwater parental magmas is therefore preferred over mixing via a turbulent plume (Campbell et al., 1983). Rare earth element data suggest that complete mixing first occurred with the formation of the sixth olivine-bearing member directly below the J-M reef in Troctolite-Anorthosite zone I (O 5B of Todd et al., 1982). Complete mixing of these two geochemically distinct magmas produced sulfur-saturated hybrid magmas parental to the first laterally continuous troctolite in Troctolite-Anorthosite zone I and the platinum-group element-enriched J-M reef.