Investigations in the Stillwater Complex: Part II. Petrology and Petrogenesis of the Ultramafic Series

Abstract

The Ultramafic series of the Stillwater Complex has been divided into two major zones: a Peridotite zone formed of 20 macro-rhythmic units of dunite-harzburgite-orthopyroxenite, and an overlying Orthopyroxenite zone. The stratigraphic section has been determined at Mountain View (2065 m) and at Chrome Mountain (840 m). The Mountain View section apparently formed in a subsiding basin whereas the rocks at Chrome Mountain accumulated in a relatively stable, higher area of the chamber floor. In both sections, Mg/(Mg + Fe) in cumulus mafic minerals increases with stratigraphic height in the lower 400 m, then remains relatively constant through the rest of the series. The base of the series is marked by the first appearance of laterally extensive olivine-rich cumulates. The accretion of the cumulates and the growth of the chamber proceeded through periodic injections of olivine-saturated mafic magma. The lower contact of the cycles represents a hiatus in crystallization and a return to a more primitive magma composition. Although hotter, the primitive magma was more dense, so it entered the chamber at or near the floor and did not immediately mix with the more differentiated orthopyroxene-saturated magma already present. As it cooled by transfer of heat across its upper surface, the primitive magma crystallized olivine and differentiated in situ to form the lower dunite. With the accumulation of olivine near the base, the crystal/liquid ratio, and thus the density, decreased at the top of the layer eventually resulting in mixing and the formation of harzburgite. After removal of olivine by resorption and settling from the hybrid magma, orthopyroxene alone crystallized forming an orthopyroxenite. Chromitite layers probably formed by the mixing of primitive olivine± chromite-saturated magma and narrow layers of orthopyroxene-saturated magma trapped underneath. The Mg-enrichment trend in the lower 400 m resulted from reaction of cumulus olivine and/or orthopyroxene with progressively decreasing volumes of intercumulus liquid. As heat loss through the floor decreased, accumulation rate approached a steady state, the fraction of trapped liquid remained more or less constant and variation in Mg/(Mg + Fe) was governed dominantly by cumulus processes. The constant NiO abundances in olivine throughout the section are consistent with the model for the formation of the macro-rhythmic units. Depletion of NiO was dampened by repeated additions of parental magma, localized equilibrium crystallization, mixing, and the effect of postcumulus equi-libration with varied amounts of trapped liquid. Discordant dunite bodies, which are common at Chrome Mountain, formed by the replacement by olivine of earlier formed cumulates. The replacement involved the incongruent dissolution of ortho-pyroxene at near-solidus temperatures by a late-stage, hydrous vapor probably derived from the magma. The vapor phase migrated along fractures formed by the readjustment of the cumulate pile.