The influence of the thick banded series anorthosites on the crystallization of the surrounding rock of the Stillwater Complex, Montana

Abstract

The Banded Series of the Archean-aged Stillwater Complex contains three thick anorthosites overlain by olivine-bearing rocks with apparently different crystallization sequence in that orthopyroxene appears late, if at all, as compared with that of the Ultramafic and Lower Banded series where orthopyroxene appears before plagioclase. Conventional models suggest that the rocks above the anorthosites represent injection of a magma with a different liquid line of descent than the rest of the complex. Alternatively, it has been suggested that the plagioclase + pyroxene mush protolith above the anorthosites was hydrated by the degassing of the underlying thick anorthosite units to produce olivine rather than pyroxene by either expansion of the olivine phase field by addition of H2O, incongruent melting of pyroxene, and/or silica loss to the vapor. To test for hydration melting and a late silica loss, electron microprobe core/rim analyses were done on plagioclase grains from troctolite of Olivine-bearing zone V (OB-V). Approximately a dozen grains were analyzed for each of six thin sections. In total, 69 pairs were reversely zoned, and 22 were normal or unzoned. The most negative An difference was − 8.25, the most positive was 4.32, but the majority of pairs were clustered from − 5 to 1. The average was − 2.1. Core grains had an average An number of 77.8, typical for other plagioclase from the Middle Banded series. A green, Na-rich hornblende is commonly associated with plagioclase where the latter is enclosed in olivine. The prevalence of reversely zoned grains is consistent with silica and sodium loss from pre-existing plagioclase. This evidence, as well as the lack of plagioclase core compositions being reset to more primitive, An-rich compositions, the amoeboidal, poikilitic habit of the olivine, and the presence of amphibole are all consistent with the troctolite and olivine gabbro of the Middle Banded series and lower parts of the Upper Banded series having formed as the result of a combination of processes including hydration, incongruent melting, and element leaching during degassing of the underlying anorthosites.