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

Publisher Website
Google Scholar
Abstract
A 525-m-long drill core (DDH-221) through the Partridge River intrusion has been divided into four zones on the basis of changes in mineral abundances, compositions and grain size. The igneous rocks in the core consist of cumulate gabbro, troctolite and olivine gabbronorite, in which the original cumulate framework of plagioclase and olivine contained varying amounts of trapped intercumulus (pore) liquid. The compositions of the unzoned olivine (Fo31–71) have been modified by reaction with Fe-rich in situ intercumulus liquid, but the plagioclase cores (An59–73) have not. The compositions of postcumulus Ca-rich pyroxene, restricted to En36–44, and the more variable Ca-poor pyroxene (En45–74), follow a downward Fe-enrichment trend similar to the Fe-enrichment in the olivine. The cumulus olivine expected to be in equilibrium with plausible parental magmas to these rocks was not preserved in the drill core, nor is the chilled margin to the intrusion sufficiently primitive to account for all the olivine. Revised mass balance estimates of the primary magmatic compositions of olivine are Fo67–85. The new limiting value for the primary olivine is similar to the Fo83–85 olivine expected to crystallize from the chilled margin to the nearby Pigeon Point olivine diabase sill under equilibrium conditions. The changes in the mineral compositions in core DDH-221 do not adequately describe the behavior of parental melts on an equilibrium cooling path, implying that the cumulus plagioclase and olivine crystallized elsewhere, and were mixed with varying amounts of intercumulus liquid before introduction to the present crustal site of the Partridge River intrusion. Rock density increases with depth from 2·76 to 3·21, with a mean of 2·98 g/cm3. Estimated trapped liquid densities range from 2·56 to 2·92 g/cm3 at high temperatures. This is interpreted to mean that the intercumulus liquid could not have been expelled upward by compaction of the cumulate pile. The dense intercumulus liquid increased downward in abundance to form a series of rocks that range continuously from variously packed framework cumulates to chilled non-cumulate rocks in the basal zone. In situ crystallization is concluded to be the dominant mode of solidification of the Partridge River intrusion, in which infiltration metasomatism is precluded by the high liquid density.

This publication has 0 references indexed in Scilit: