THE STILLWATER-COMPLEX, MONTANA - A SUBVOLCANIC MAGMA CHAMBER

Five magma types occur as sills in the footwall of the Stillwater Comp lex, all of them coeval with the Stillwater in age. Two of the magma t ypes have compositions that suggest they are similar to the magmas fro m which the cumulates of the Stillwater's Basal and Ultramafic series crystallized. Melting experiments performed on samples of these two ma gma types and on a 50-50 mix of the two types show that the crystalliz ation sequence inferred from the Ultramafic series cumulates (olivine > orthopyroxene > plagioclase > augite) is matched only in the mixture of the two sill types at 1.5-3 kbar. Thus, the observed stratigraphy of the Basal and Ultramafic series may result from mixing of two disti nct magmas rather than fractionation of a single magma. The permissibl e pressure range of 1.5-3 kbar implies that the depth of the Stillwate r magma chamber was similar to that of subvolcanic magma chambers such as that of Kilauea Volcano, which raises the possibility that the Sti llwater magma body may also have been a subvolcanic reservoir.