Infiltration metasomatism in layered intrusions - An example from the Stillwater Complex, Montana
The contact between Anorthosite zone II (AN II) and the overlying troctolite of Olivine-Bearing zone V (OB V) of the Stillwater Complex is characterized by a 5-m-thick, adcumulate anorthosite. This adcumulate anorthosite separates rocks of AN II, characterized by the apparent crystallization sequence plagioclase followed by orthopyroxene+clinopyroxene±quartz±Fe-Ti oxide assemblages, from those of OB V, where the apparent crystallization sequence is olivine+plagioclase followed by orthopyroxene. The grain size and compositional zoning of plagioclase in this transition zone is similar to that in the overlying troctolite and distinct from the coarser, complexly zoned plagioclase that makes up the underlying anorthosite. In contrast, the minor interstitial pyroxene in the transition zone compositionally overlaps that in the underlying anorthosite, and both have a lower molar Mg (Mg+Fe) ratio than the pyroxene of the troctolite. These relationships suggest that the intercumulus liquids of AN II and OB V were compositionally distinct. Model phase diagram considerations suggest that infiltration of any plagioclase-saturated liquids into a troctolite assemblage will result in the assimilation of olivine and, depending on the actual composition of the anorthositic liquid, will also either assimilate or crystallize plagioclase as well. Hence, the monomineralic anorthosite zone at the top of AN II can be attributed to infiltration of interstitial liquids from AN II that were displaced upward into the troctolite during compaction of AN II. This reaction can be either exothermic or endothermic, depending on the actual composition of the infiltrating liquid and the bulk solids composition. Numerical models of the thermal effects of infiltration suggest that heat of reaction can significantly affect the liquid/solid ratio away from the infiltration boundary. © 1992.
Boudreau, AE; McCallum, IS
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