Formation Mechanism of Plagioclase–Amphibole and Amphibole–Spinel Symplectites in the Bijigou Layered Intrusion: Insights from Mineralogical and Crystallographic Constraints

The Bijigou layered intrusion is located in the northern margin of the Yangtze block. Based on cumulus mineral assemblages, the intrusion is divided into three major units from the base upwards: the lower zone (LZ), dominated by olivine gabbro; the middle zone (MZ), composed of gabbro and Fe-Ti oxide ore layers; and the upper zone (UZ), characterized by (quartz) diorite. Previous studies reported various vermicular symplectite textures in layered intrusions, which are thought to be related to the magmatic evolution of the layered intrusions and the mineralization of vanadium–titanium magnetite. However, detailed studies on the specific reaction mechanism of those symplectites are lacking. In this study, the characteristics, mineral compositions, and crystal orientation relationships of minerals in symplectites from Fe-Ti oxide Fe-Ti oxide-rich gabbro are in the Bijigou layered intrusion investigated by an Electron Probe Microanalyzer (EPMA) and Electron Backscattered Diffraction (EBSD) to reveal the formation process of symplectites in gabbros. In the Fe-Ti oxide-rich gabbro, abundant amphibole + spinel (Amp1 + Spl) symplectite and amphibole + plagioclase (Pl2 + Amp2) symplectite are developed between the primocryst plagioclase (Pl1) and Fe-Ti oxide; Pl2 had significantly higher An contents (An92–97) relative to Pl1. The Mg # for Amp1 and Amp2 was 0.78–1 and 0.6–0.84, respectively. Amphibole geothermometer calculations show Amp1 and Amp2 at 934–953 °C and 834–914 °C, suggesting that these symplectites crystallized at a late stage of magmatic evolution. The crystallographic orientation relationship between Amp1 and Spl varies in different areas, and Spl has a particular orientation relationship with the external Ilm. Pl2 and Amp2 inherit the crystallographic orientation of Amp1 and Pl1, respectively. We speculate that in the Bijigou layered intrusions, Amp1 + Spl and Pl2 + Amp2 were formed in two stages: Amp1 + Spl symplectite due to Ilm epitaxial growth as a result of supersaturation and rapid nucleation; and Pl2 + Amp2 symplectite due to dissolution–precipitation.