Compositional Evolution of Interstitial Liquid After Onset of Abundant Fe–Ti Oxide Crystallization in Crystal Mush: Insights from Late-Stage Microstructures and Mineral Compositions of the Bijigou Layered Intrusion, Central China

Abstract

ABSTRACT Compaction is important to the expulsion and compositional modification of interstitial liquid of crystal mush in the solidification of layered intrusions hosting Fe–Ti oxide mineralization. However, the compaction manner and compositional evolution of interstitial liquid after the onset of abundant Fe–Ti oxide crystallization in crystal mush are still enigmatic. In this study, we collected samples from a drill core profile across the major Fe–Ti oxide-rich segment of the Bijigou intrusion, a large and differentiated layered intrusion in Central China, and carried out a detailed study on the late-stage magmatic microstructures of rocks and crystal size distributions (CSDs) and spatial distribution patterns (SDPs) of clinopyroxene and plagioclase to examine the compaction manner of interstitial liquid. The Fe–Ti oxide-rich segment is composed of, from the base upward, a Fe–Ti oxide ore layer (~80 m), a troctolite unit (~105 m), an oxide gabbro unit (~153 m) and an apatite–oxide–gabbronorite unit (~165 m). The CSD curves for both plagioclase and clinopyroxene grains of the samples from the Fe–Ti oxide ore layer display gentle slopes for small crystals and consistent slopes for large crystals. In addition, there is a positive correlation of the SDP R-value (ratio of observed and predicted nearest neighbor distance of plagioclase/clinopyroxene) and mineral mode except plagioclase/clinopyroxene of the samples. The interstitial liquid was likely expelled by intensive pressure solution concomitant with crystallization of abundant Fe–Ti oxides, evidenced by the positive correlation of aspect ratio and alignment factor of plagioclase and the low fraction of trapped liquid (FTL) of 4–6% for the Fe–Ti oxide ore layer. The cumulus plagioclase of the Fe–Ti oxide ore layer was partially dissolved during pressure-solution compaction, leading to ambient Fe-rich interstitial liquid becoming Al-enriched in local places so that abundant amphibole and hercynite are present in the replacive symplectites adjacent to cumulus plagioclase. Above the Fe–Ti oxide ore layer, plagioclase grains in the samples from the troctolite unit show consistent slopes of the CSD curves and negative correlation of SDP R-value and mineral mode in the SDP plot, indicating that the interstitial liquid was mainly expelled by mechanical compaction, with FTL ranging from 7 to 9%. The CSD curves for clinopyroxene grains of the samples from the oxide gabbro unit have been modified by Ostwald ripening, but the expulsion of interstitial liquid in this unit is also attributed to mechanical compaction due to higher compaction rate than accumulation rate. This study demonstrates that the compaction manner of interstitial liquid can change from pressure solution to mechanical compaction with dramatically diminishing crystallization of Fe–Ti oxides. The pressure-solution compaction accompanying with large amounts of Fe–Ti oxides can remarkably modify the composition of interstitial liquid. This study also demonstrates that a combined measurement of CSDs and SDPs for cumulus minerals can be used to quantify the compaction manner of interstitial liquid during solidification of layered intrusions.