Plagioclase An Range and Residual Porosity in Igneous Cumulates of the Kiglapait Intrusion


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Abstract

The range in An content of plagioclase in grain mounts of igneous cumulates provides a measure of diversity that is uniquely preserved in plagioclase because of its well-known refractory nature. To a first approximation such data provide, when calibrated, an estimate of the residual porosity or fraction of trapped liquid, in each specimen. The ensemble of specimens then provides a model for the stratigraphic variation of residual porosity. The raw data, however, include pre-cumulus zoning that can be isolated from in situ zoning by textural analysis in thin section. The baseline of residual porosity determinations was earlier determined for the Lower Zone of the Kiglapait intrusion from the content of excluded components in the solid rock compared with their content in the melt as calculated by summation and Rayleigh fractionation. The baseline equation was then used to calibrate the residual porosity obtained from the An range in grain mounts. This calibration is now extended to the remainder of the intrusion. The An range and the calculated residual porosity decrease to zero at 99% solidified (PCS) and then rise to the end of crystallization. The data suggest initial porosities smaller than 0·35. Allowing for pre-cumulus zoning, the data suggest a dominance of adcumulates in the intrusion and these impermeable barriers occupy 75% of the rocks in the Lower Zone. They occur at intervals of 1 to rarely 15 m and thereby restrict the likelihood of compaction over thick mushy zones. Variations in the Fo range of olivine are also observed in grain mounts and they follow those in plagioclase. However, they are in part due to subsolidus equilibration with Fe–Ti oxides and augite. The new calibration is successfully applied to the Skaergaard intrusion to supplement the published results from excluded components, with some interesting contradictions.

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