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Paper No. 296-7
Presentation Time: 9:30 AM
POSSIBLE PARENT MAGMA AND ORIGIN OF LAYERING FOR THE PERIDOTITE ZONE, STILLWATER COMPLEX, MONTANA
We examine the nature of the layering in the ultramafic rocks of the Stillwater Complex of Montana, USA. A fine-grained, high-magnesium dike in the Mountain View area of the complex is a plausible parental magma for the layered cumulate rocks in the Ultramafic series. Modeled crystal fractionation of a liquid with the composition of the dike produces cumulus olivine (Fo84-87) and chromite (Cr#=0.55-0.63) consistent with the mineral chemistry of cumulus phases in the Peridotite zone. The composition of the dike is consistent with contamination of mantle-derived primary komatiitic magma by 21% contamination of Archean granodiorite crust. Models of fractional crystallization to form the limited range of mineral and bulk rock compositions observed in the Ultramafic series requires that these rocks accumulated from much larger volumes of magma. The modeled crystal fractionation of the proposed parental magma also demonstrates that relative to olivine, only a small amounts of cumulus chromite crystallizes with cotectic ratios of around 100:2 to 100:2.75 olivine to chromite. The modal abundance of chromite in the olivine-bearing cumulates from the Mountain View area, varies between 0.1 and 80 vol % with an average ratio of 100:9.2 olivine to chromite. The observed range of modal proportions is not consistent with alternation between chromite-only and chromite+olivine cotectic crystallization. This in turn suggests that if olivine and chromite precipitated in cotectic proportions, the crystals have subsequently been sorted to variable extents producing the range of observed modal proportions of chromite in the cumulates. This leads us to challenge the purported cyclicity of the cumulate rocks in the Peridotite zone; simple contact-relationship statistics from logged drill core at various locations in the complex show considerable departure from the suggested regular cycles of cumulate layers. We suggest that the supply of crystal laden magma transported into the magma chamber may have varied randomly leading to the formation of layered rocks that do not conform to the previously accepted cyclic unit model of settled liquidus minerals, and we find no support for the concept that units formed by closed system fractional crystallization from mixed magmas following a succession of discrete magma recharge events.