Crystal Size Distributions and Trace Element Compositions of the Fluorapatite from the Bijigou Fe–Ti Oxide-Bearing Layered Intrusion, Central China: Insights for the Expulsion Processes of Interstitial Liquid from Crystal Mush

Abstract

Abstract The Neoproterozoic Bijigou intrusion is one of the largest and well-differentiated Fe–Ti oxide-bearing layered intrusion in Central China, and hosts Fe–Ti oxide ore layers in the middle zone with a total thickness of ∼112 m. In order to examine the role of compaction and compositional convection on the solidification of a layered intrusion associated with the crystallization of large amounts of Fe–Ti oxides, we collected the samples from a drill core profile of the apatite-oxide gabbronorite unit above the main Fe–Ti oxide layer in the middle zone of the Bijigou intrusion and carried out a detailed study on the crystal size distributions (CSDs) and trace element compositions of the fluorapatite in the samples. The apatite-oxide gabbronorite unit is mainly composed of pyroxene and plagioclase with Fe–Ti oxides and fluorapatite interstitial to the silicates, and can be further divided into the lower and upper sections in terms of grain size, rare earth element (REE) concentrations of fluorapatite and stress deformation of minerals. In the lower section, the plagioclase and pyroxene of the rocks are often bent, fluorapatite crystals have grain sizes ranging from ∼0·10 × 0·30 mm to ∼1·00 × 2·50 mm and the average Ce concentration of the fluorapatite of each sample varies from 230 to 387 μg/g. In contrast, the plagioclase and pyroxene of the rocks from the upper section are sparsely bent, fluorapatite crystals range in size from ∼0·05 × 0·05 mm to ∼0·15 × 0·40 mm, and the average Ce concentration of the fluorapatite of each sample varies from 468 to 704 μg/g. Modeling results show that the fraction of trapped liquid (FTL) is ∼7% in the lower section and ∼15% in the upper section, and relatively elevated REE (e.g. Ce) concentrations of the fluorapatite of the upper section are thus likely attributed to the trapped liquid shift (TLS) effect. The TLS effect may have also enhanced the textural coarsening of the fluorapatite of the upper section, which is illustrated by a convex-upward curve for <0·1 mm crystals and a counter-clockwise rotation around a fixed point in the CSDs of the fluorapatite. The CSDs of the fluorapatite of the lower section, however, change from a steep slope for <0·25 mm crystals to a gentle slope for >0·25 mm crystals with a kinked trend akin to mixed crystal populations, which is interpreted as the exchange of interstitial liquid with the main magma body due to compositional convection. The different FTL and fluorapatite CSDs of the lower and upper sections indicate that the interstitial liquid may have been expelled from the crystal mush of the lower section more efficiently than from the upper section, which is likely controlled by both compaction and compositional convection. However, it was the compositional convection that dominated the expulsion of interstitial liquid in the whole apatite-oxide gabbronorite unit, indicating that compositional convection may prevail after the crystallization of large amounts of Fe–Ti oxides from interstitial liquid and weaken the role of compaction.