Petrology of the Partridge River Intrusion, Duluth Complex, Minnesota: 1. Relationships between Mineral Compositions, Density and Trapped Liquid Abundance

A 525-m-long drill core (DDH-221) through the Partridge Riverintrusion has been divided into four zones on the basis of changesin mineral abundances, compositions and grain size. The igneousrocks in the core consist of cumulate gabbro, troctolite andolivine gabbronorite, in which the original cumulate frameworkof plagioclase and olivine contained varying amounts of trappedintercumulus (pore) liquid. The compositions of the unzoned olivine (Fo_31–71) havebeen modified by reaction with Fe-rich in situ intercumulusliquid, but the plagioclase cores (An_59–73) have not.The compositions of postcumulus Ca-rich pyroxene, restrictedto En_36–44, and the more variable Ca-poor pyroxene (En_45–74),follow a downward Fe-enrichment trend similar to the Fe-enrichmentin the olivine. The cumulus olivine expected to be in equilibriumwith plausible parental magmas to these rocks was not preservedin the drill core, nor is the chilled margin to the intrusionsufficiently primitive to account for all the olivine. Revisedmass balance estimates of the primary magmatic compositionsof olivine are Fo_67–85. The new limiting value for theprimary olivine is similar to the Fo_83–85 olivine expectedto crystallize from the chilled margin to the nearby PigeonPoint olivine diabase sill under equilibrium conditions. Thechanges in the mineral compositions in core DDH-221 do not adequatelydescribe the behavior of parental melts on an equilibrium coolingpath, implying that the cumulus plagioclase and olivine crystallizedelsewhere, and were mixed with varying amounts of intercumulusliquid before introduction to the present crustal site of thePartridge River intrusion. Rock density increases with depth from 2?76 to 3?21, with amean of 2?98 g/cm³. Estimated trapped liquid densities rangefrom 2?56 to 2?92 g/cm³ at high temperatures. This is interpretedto mean that the intercumulus liquid could not have been expelledupward by compaction of the cumulate pile. The dense intercumulusliquid increased downward in abundance to form a series of rocksthat range continuously from variously packed framework cumulatesto chilled non-cumulate rocks in the basal zone. In situ crystallizationis concluded to be the dominant mode of solidification of thePartridge River intrusion, in which infiltration metasomatismis precluded by the high liquid density.