Journal of Petrology | Volume 27 | Number 4 | Pages 791-825 | 1986
© Oxford University Press 1986
research-article |
Geochemistry of the J-M Reef of the Stillwater Complex, Minneapolis Adit Area II. Silicate Mineral Chemistry and Petrogenesis
Department of Geology, University of Toronto Toronto, Ontario, Canada, M5S 1Al
Received June 22, 1984; Revised typescript accepted October 23, 1985
ABSTRACT
The platinum group element (PGE) rich J-M Reef of the Stillwater Complex is associated with the reappearance of olivine and chromite as cumulus phases within a sequence of gabbroic and noritic cumulates. The Olivine-Bearing Subzone (OBZ I) which hosts the Reef is interpreted as the result of fresh influxes of magma into the Stillwater chamber.
Cumulus pyroxenes from the Norite Subzone (No II) overlying OBZ I are enriched in Fe over Mg, depleted in Cr and have similar Ni contents relative to pyroxenes from the underlying GNo I. The most primitive pyroxene compositions are found in PBc and Bc layers within OBZ I, and the most evolved as oikocrysts in OBZ I anorthositic layers. Plagioclase An content correlates closely with pyroxene MgO/FeO, except within OBZ I where a vertical An/En trend is observed.
These observations may be synthesized into a model involving replenishment of the chamber by high pressure differentiates of a liquid P, representing the parental magma to the Ultramafic Series of the Stillwater. Liquid P crystallizes olivine at the pressure of the Stillwater chamber, but owing to migration of the olivine-orthopyroxene phase boundary is saturated with bronzite at higher pressures. The spectrum of cumulus mineralogy and mineral compositions within and above OBZ I may be generated by a series of magma influxes having compositions derived by various degrees of high pressure fractionation from liquid P.
The characteristic textures of the heterogenous plagioclase-olivine cumulates of OBZ I may be explained by mixing of P-type replenishing liquid with a fractionated resident liquid containing suspended plagioclase crystals. Anorthositic layers within OBZ I formed from influxes of liquid derived by approximately 30 per cent fractionation of P.
Model density calculations indicate that the liquids should have entered the chamber as a serious of buoyant plumes. Lateral variations in OBZ I stratigraphy arise from pulses of different size spreading to varying distances from the feeder axis. The Reef formed from a single layer transgressing earlier facies transitions.
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